diff --git a/exercises/09_GPS_Time/README.md b/exercises/09_GPS_Time/README.md
new file mode 100644
index 0000000..a8a1a81
--- /dev/null
+++ b/exercises/09_GPS_Time/README.md
@@ -0,0 +1,49 @@
+## Exercise 09: GPS Time (L76K)
+
+This exercise boots the T-Beam Supreme and verifies GPS behavior at startup.
+
+Important sequence note:
+
+- Exercise 10 (`10_Simple_GPS`) should be completed before this exercise.
+- Exercise 10 README contains the detailed pin-configuration explanation and troubleshooting rationale for why explicit GPS pin mapping is critical on this hardware.
+- If GPS behavior is unexpected here, review Exercise 10 README first, then return to Exercise 9.
+
+Implemented behavior:
+
+1. Initializes PMU, OLED, and SD startup watcher (same startup SD path used in Exercise 08).
+2. Probes GPS at startup for NMEA traffic, module identity, satellite count, and UTC time availability.
+   - Uses explicit GPS UART pins and an active startup probe (multi-baud + common GPS query commands), aligned with the approach validated in Exercise 10.
+3. If L76K is detected, normal GPS-time flow continues.
+4. If L76K is not detected and Quectel-style module text is detected, OLED shows a hard TODO error:
+   - Quectel detected
+   - L76K required
+   - Quectel support is TODO
+5. Every minute:
+   - If GPS UTC is valid: shows GPS UTC time and satellites on OLED.
+   - If satellites are seen but UTC is not valid yet: shows that condition and RTC time.
+   - If no satellites: shows:
+     - "Unable to acquire"
+     - "satellites"
+     - "Take me outside so I"
+     - "can see satellites"
+     - plus current RTC time.
+
+Notes:
+
+- GPS time displayed is UTC from NMEA RMC with valid status.
+- Satellite count uses best available from GGA/GSV.
+- RTC fallback reads PCF8563 via Wire1.
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```
diff --git a/exercises/09_GPS_Time/lib/startup_sd/StartupSdManager.cpp b/exercises/09_GPS_Time/lib/startup_sd/StartupSdManager.cpp
new file mode 100644
index 0000000..1e8791c
--- /dev/null
+++ b/exercises/09_GPS_Time/lib/startup_sd/StartupSdManager.cpp
@@ -0,0 +1,360 @@
+#include "StartupSdManager.h"
+
+#include <stdarg.h>
+#include "driver/gpio.h"
+
+StartupSdManager::StartupSdManager(Print& serial) : serial_(serial) {}
+
+bool StartupSdManager::begin(const SdWatcherConfig& cfg, SdStatusCallback callback) {
+  cfg_ = cfg;
+  callback_ = callback;
+
+  forceSpiDeselected();
+  dumpSdPins("very-early");
+
+  if (!initPmuForSdPower()) {
+    return false;
+  }
+
+  cycleSdRail();
+  delay(cfg_.startupWarmupMs);
+
+  bool warmMounted = false;
+  for (uint8_t i = 0; i < 3; ++i) {
+    if (mountPreferred(false)) {
+      warmMounted = true;
+      break;
+    }
+    delay(200);
+  }
+
+  // Some cards need a longer power/settle window after cold boot.
+  // Before declaring ABSENT, retry with extended settle and a full scan.
+  if (!warmMounted) {
+    logf("Watcher: startup preferred mount failed, retrying with extended settle");
+    cycleSdRail(400, 1200);
+    delay(cfg_.startupWarmupMs + 1500);
+    warmMounted = mountCardFullScan();
+  }
+
+  if (warmMounted) {
+    setStateMounted();
+  } else {
+    setStateAbsent();
+  }
+  return true;
+}
+
+void StartupSdManager::update() {
+  const uint32_t now = millis();
+  const uint32_t pollInterval =
+      (watchState_ == SdWatchState::MOUNTED) ? cfg_.pollIntervalMountedMs : cfg_.pollIntervalAbsentMs;
+
+  if ((uint32_t)(now - lastPollMs_) < pollInterval) {
+    return;
+  }
+  lastPollMs_ = now;
+
+  if (watchState_ == SdWatchState::MOUNTED) {
+    if (verifyMountedCard()) {
+      presentVotes_ = 0;
+      absentVotes_ = 0;
+      return;
+    }
+
+    if (mountPreferred(false) && verifyMountedCard()) {
+      presentVotes_ = 0;
+      absentVotes_ = 0;
+      return;
+    }
+
+    absentVotes_++;
+    presentVotes_ = 0;
+    if (absentVotes_ >= cfg_.votesToAbsent) {
+      setStateAbsent();
+      absentVotes_ = 0;
+    }
+    return;
+  }
+
+  bool mounted = mountPreferred(false);
+  if (!mounted && (uint32_t)(now - lastFullScanMs_) >= cfg_.fullScanIntervalMs) {
+    lastFullScanMs_ = now;
+    if (cfg_.recoveryRailCycleOnFullScan) {
+      logf("Watcher: recovery rail cycle before full scan");
+      cycleSdRail(cfg_.recoveryRailOffMs, cfg_.recoveryRailOnSettleMs);
+      delay(150);
+    }
+    logf("Watcher: preferred probe failed, running full scan");
+    mounted = mountCardFullScan();
+  }
+
+  if (mounted) {
+    presentVotes_++;
+    absentVotes_ = 0;
+    if (presentVotes_ >= cfg_.votesToPresent) {
+      setStateMounted();
+      presentVotes_ = 0;
+    }
+  } else {
+    absentVotes_++;
+    presentVotes_ = 0;
+    if (absentVotes_ >= cfg_.votesToAbsent) {
+      setStateAbsent();
+      absentVotes_ = 0;
+    }
+  }
+}
+
+bool StartupSdManager::consumeMountedEvent() {
+  bool out = mountedEventPending_;
+  mountedEventPending_ = false;
+  return out;
+}
+
+bool StartupSdManager::consumeRemovedEvent() {
+  bool out = removedEventPending_;
+  removedEventPending_ = false;
+  return out;
+}
+
+void StartupSdManager::logf(const char* fmt, ...) {
+  char msg[196];
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(msg, sizeof(msg), fmt, args);
+  va_end(args);
+  serial_.printf("[%10lu][%06lu] %s\r\n",
+                 (unsigned long)millis(),
+                 (unsigned long)logSeq_++,
+                 msg);
+}
+
+void StartupSdManager::notify(SdEvent event, const char* message) {
+  if (callback_ != nullptr) {
+    callback_(event, message);
+  }
+}
+
+void StartupSdManager::forceSpiDeselected() {
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  pinMode(tbeam_supreme::imuCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::imuCs(), HIGH);
+}
+
+void StartupSdManager::dumpSdPins(const char* tag) {
+  if (!cfg_.enablePinDumps) {
+    (void)tag;
+    return;
+  }
+
+  const gpio_num_t cs = (gpio_num_t)tbeam_supreme::sdCs();
+  const gpio_num_t sck = (gpio_num_t)tbeam_supreme::sdSck();
+  const gpio_num_t miso = (gpio_num_t)tbeam_supreme::sdMiso();
+  const gpio_num_t mosi = (gpio_num_t)tbeam_supreme::sdMosi();
+  logf("PINS(%s): CS=%d SCK=%d MISO=%d MOSI=%d",
+       tag, gpio_get_level(cs), gpio_get_level(sck), gpio_get_level(miso), gpio_get_level(mosi));
+}
+
+bool StartupSdManager::initPmuForSdPower() {
+  if (!tbeam_supreme::initPmuForPeripherals(pmu_, &serial_)) {
+    logf("ERROR: PMU init failed");
+    return false;
+  }
+  return true;
+}
+
+void StartupSdManager::cycleSdRail(uint32_t offMs, uint32_t onSettleMs) {
+  if (!cfg_.enableSdRailCycle) {
+    return;
+  }
+  if (!pmu_) {
+    logf("SD rail cycle skipped: pmu=null");
+    return;
+  }
+
+  forceSpiDeselected();
+  pmu_->disablePowerOutput(XPOWERS_BLDO1);
+  delay(offMs);
+  pmu_->setPowerChannelVoltage(XPOWERS_BLDO1, 3300);
+  pmu_->enablePowerOutput(XPOWERS_BLDO1);
+  delay(onSettleMs);
+}
+
+bool StartupSdManager::tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, bool verbose) {
+  SD.end();
+  bus.end();
+  delay(10);
+  forceSpiDeselected();
+
+  bus.begin(tbeam_supreme::sdSck(), tbeam_supreme::sdMiso(), tbeam_supreme::sdMosi(), tbeam_supreme::sdCs());
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  delay(2);
+  for (int i = 0; i < 10; i++) {
+    bus.transfer(0xFF);
+  }
+  delay(2);
+
+  if (verbose) {
+    logf("SD: trying bus=%s freq=%lu Hz", busName, (unsigned long)hz);
+  }
+
+  if (!SD.begin(tbeam_supreme::sdCs(), bus, hz)) {
+    if (verbose) {
+      logf("SD: mount failed (possible non-FAT format, power, or bus issue)");
+    }
+    return false;
+  }
+
+  if (SD.cardType() == CARD_NONE) {
+    SD.end();
+    return false;
+  }
+
+  sdSpi_ = &bus;
+  sdBusName_ = busName;
+  sdFreq_ = hz;
+  return true;
+}
+
+bool StartupSdManager::mountPreferred(bool verbose) {
+  return tryMountWithBus(sdSpiH_, "HSPI", 400000, verbose);
+}
+
+bool StartupSdManager::mountCardFullScan() {
+  const uint32_t freqs[] = {400000, 1000000, 4000000, 10000000};
+
+  for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
+    if (tryMountWithBus(sdSpiH_, "HSPI", freqs[i], true)) {
+      logf("SD: card detected and mounted");
+      return true;
+    }
+  }
+  for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
+    if (tryMountWithBus(sdSpiF_, "FSPI", freqs[i], true)) {
+      logf("SD: card detected and mounted");
+      return true;
+    }
+  }
+
+  logf("SD: begin() failed on all bus/frequency attempts");
+  return false;
+}
+
+bool StartupSdManager::verifyMountedCard() {
+  File root = SD.open("/", FILE_READ);
+  if (!root) {
+    return false;
+  }
+  root.close();
+  return true;
+}
+
+const char* StartupSdManager::cardTypeToString(uint8_t type) {
+  switch (type) {
+    case CARD_MMC:
+      return "MMC";
+    case CARD_SD:
+      return "SDSC";
+    case CARD_SDHC:
+      return "SDHC/SDXC";
+    default:
+      return "UNKNOWN";
+  }
+}
+
+void StartupSdManager::printCardInfo() {
+  uint8_t cardType = SD.cardType();
+  uint64_t cardSizeMB = SD.cardSize() / (1024ULL * 1024ULL);
+  uint64_t totalMB = SD.totalBytes() / (1024ULL * 1024ULL);
+  uint64_t usedMB = SD.usedBytes() / (1024ULL * 1024ULL);
+
+  logf("SD type: %s", cardTypeToString(cardType));
+  logf("SD size: %llu MB", cardSizeMB);
+  logf("FS total: %llu MB", totalMB);
+  logf("FS used : %llu MB", usedMB);
+  logf("SPI bus: %s @ %lu Hz", sdBusName_, (unsigned long)sdFreq_);
+}
+
+bool StartupSdManager::ensureDirRecursive(const char* path) {
+  String full(path);
+  if (!full.startsWith("/")) {
+    full = "/" + full;
+  }
+
+  int start = 1;
+  while (start > 0 && start < (int)full.length()) {
+    int slash = full.indexOf('/', start);
+    String partial = (slash < 0) ? full : full.substring(0, slash);
+    if (!SD.exists(partial.c_str()) && !SD.mkdir(partial.c_str())) {
+      logf("ERROR: mkdir failed for %s", partial.c_str());
+      return false;
+    }
+    if (slash < 0) {
+      break;
+    }
+    start = slash + 1;
+  }
+
+  return true;
+}
+
+bool StartupSdManager::rewriteFile(const char* path, const char* payload) {
+  if (SD.exists(path) && !SD.remove(path)) {
+    logf("ERROR: failed to erase %s", path);
+    return false;
+  }
+
+  File f = SD.open(path, FILE_WRITE);
+  if (!f) {
+    logf("ERROR: failed to create %s", path);
+    return false;
+  }
+
+  size_t wrote = f.println(payload);
+  f.close();
+  if (wrote == 0) {
+    logf("ERROR: write failed for %s", path);
+    return false;
+  }
+  return true;
+}
+
+void StartupSdManager::permissionsDemo(const char* path) {
+  logf("Permissions demo: FAT has no Unix chmod/chown, use open mode only.");
+  File r = SD.open(path, FILE_READ);
+  if (!r) {
+    logf("Could not open %s as FILE_READ", path);
+    return;
+  }
+  size_t writeInReadMode = r.print("attempt write while opened read-only");
+  if (writeInReadMode == 0) {
+    logf("As expected, FILE_READ write was blocked.");
+  } else {
+    logf("NOTE: FILE_READ write returned %u (unexpected)", (unsigned)writeInReadMode);
+  }
+  r.close();
+}
+
+void StartupSdManager::setStateMounted() {
+  if (watchState_ != SdWatchState::MOUNTED) {
+    logf("EVENT: card inserted/mounted");
+    mountedEventPending_ = true;
+    notify(SdEvent::CARD_MOUNTED, "SD card mounted");
+  }
+  watchState_ = SdWatchState::MOUNTED;
+}
+
+void StartupSdManager::setStateAbsent() {
+  if (watchState_ == SdWatchState::MOUNTED) {
+    logf("EVENT: card removed/unavailable");
+    removedEventPending_ = true;
+    notify(SdEvent::CARD_REMOVED, "SD card removed");
+  } else if (watchState_ != SdWatchState::ABSENT) {
+    logf("EVENT: no card detected");
+    notify(SdEvent::NO_CARD, "Missing SD card or invalid FAT16/FAT32 format");
+  }
+  SD.end();
+  watchState_ = SdWatchState::ABSENT;
+}
diff --git a/exercises/09_GPS_Time/lib/startup_sd/StartupSdManager.h b/exercises/09_GPS_Time/lib/startup_sd/StartupSdManager.h
new file mode 100644
index 0000000..be9ef27
--- /dev/null
+++ b/exercises/09_GPS_Time/lib/startup_sd/StartupSdManager.h
@@ -0,0 +1,90 @@
+#pragma once
+
+#include <Arduino.h>
+#include <SD.h>
+#include <SPI.h>
+#include <Wire.h>
+#include "tbeam_supreme_adapter.h"
+
+enum class SdWatchState : uint8_t {
+  UNKNOWN = 0,
+  ABSENT,
+  MOUNTED
+};
+
+enum class SdEvent : uint8_t {
+  NO_CARD,
+  CARD_MOUNTED,
+  CARD_REMOVED
+};
+
+using SdStatusCallback = void (*)(SdEvent event, const char* message);
+
+struct SdWatcherConfig {
+  bool enableSdRailCycle = true;
+  bool enablePinDumps = true;
+  bool recoveryRailCycleOnFullScan = true;
+  uint32_t recoveryRailOffMs = 250;
+  uint32_t recoveryRailOnSettleMs = 700;
+  uint32_t startupWarmupMs = 1500;
+  uint32_t pollIntervalAbsentMs = 1000;
+  uint32_t pollIntervalMountedMs = 2000;
+  uint32_t fullScanIntervalMs = 10000;
+  uint8_t votesToPresent = 2;
+  uint8_t votesToAbsent = 5;
+};
+
+class StartupSdManager {
+ public:
+  explicit StartupSdManager(Print& serial = Serial);
+
+  bool begin(const SdWatcherConfig& cfg, SdStatusCallback callback = nullptr);
+  void update();
+
+  bool isMounted() const { return watchState_ == SdWatchState::MOUNTED; }
+  SdWatchState state() const { return watchState_; }
+
+  bool consumeMountedEvent();
+  bool consumeRemovedEvent();
+
+  void printCardInfo();
+  bool ensureDirRecursive(const char* path);
+  bool rewriteFile(const char* path, const char* payload);
+  void permissionsDemo(const char* path);
+
+ private:
+  void logf(const char* fmt, ...);
+  void notify(SdEvent event, const char* message);
+  void forceSpiDeselected();
+  void dumpSdPins(const char* tag);
+  bool initPmuForSdPower();
+  void cycleSdRail(uint32_t offMs = 250, uint32_t onSettleMs = 600);
+  bool tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, bool verbose);
+  bool mountPreferred(bool verbose);
+  bool mountCardFullScan();
+  bool verifyMountedCard();
+  const char* cardTypeToString(uint8_t type);
+  void setStateMounted();
+  void setStateAbsent();
+
+  Print& serial_;
+  SdWatcherConfig cfg_{};
+  SdStatusCallback callback_ = nullptr;
+
+  SPIClass sdSpiH_{HSPI};
+  SPIClass sdSpiF_{FSPI};
+  SPIClass* sdSpi_ = nullptr;
+  const char* sdBusName_ = "none";
+  uint32_t sdFreq_ = 0;
+  XPowersLibInterface* pmu_ = nullptr;
+
+  SdWatchState watchState_ = SdWatchState::UNKNOWN;
+  uint8_t presentVotes_ = 0;
+  uint8_t absentVotes_ = 0;
+  uint32_t lastPollMs_ = 0;
+  uint32_t lastFullScanMs_ = 0;
+  uint32_t logSeq_ = 0;
+
+  bool mountedEventPending_ = false;
+  bool removedEventPending_ = false;
+};
diff --git a/exercises/09_GPS_Time/lib/startup_sd/library.json b/exercises/09_GPS_Time/lib/startup_sd/library.json
new file mode 100644
index 0000000..4978fdd
--- /dev/null
+++ b/exercises/09_GPS_Time/lib/startup_sd/library.json
@@ -0,0 +1,12 @@
+{
+  "name": "startup_sd",
+  "version": "0.1.0",
+  "dependencies": [
+    {
+      "name": "XPowersLib"
+    },
+    {
+      "name": "Wire"
+    }
+  ]
+}
diff --git a/exercises/09_GPS_Time/lib/system_startup/SystemStartup.cpp b/exercises/09_GPS_Time/lib/system_startup/SystemStartup.cpp
new file mode 100644
index 0000000..3a82e1f
--- /dev/null
+++ b/exercises/09_GPS_Time/lib/system_startup/SystemStartup.cpp
@@ -0,0 +1,96 @@
+#include "SystemStartup.h"
+
+#include <Wire.h>
+#include <U8g2lib.h>
+
+#ifndef OLED_SDA
+#define OLED_SDA 17
+#endif
+
+#ifndef OLED_SCL
+#define OLED_SCL 18
+#endif
+
+#ifndef OLED_ADDR
+#define OLED_ADDR 0x3C
+#endif
+
+static const bool kEnableOled = true;
+
+static U8G2_SH1106_128X64_NONAME_F_HW_I2C g_oled(U8G2_R0, /* reset=*/U8X8_PIN_NONE);
+static SystemStartup* g_activeSystemStartup = nullptr;
+
+static void forceSpiDeselectedEarly() {
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  pinMode(tbeam_supreme::imuCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::imuCs(), HIGH);
+}
+
+SystemStartup::SystemStartup(Print& serial) : serial_(serial), sd_(serial) {}
+
+bool SystemStartup::begin(const SystemStartupConfig& cfg, SystemEventCallback callback) {
+  cfg_ = cfg;
+  callback_ = callback;
+  g_activeSystemStartup = this;
+
+  // Match Exercise 05 behavior: deselect SPI devices immediately at startup.
+  forceSpiDeselectedEarly();
+
+  if (kEnableOled) {
+    Wire.begin(OLED_SDA, OLED_SCL);
+    g_oled.setI2CAddress(OLED_ADDR << 1);
+    g_oled.begin();
+  }
+
+  emit(SystemEvent::BOOTING, "System startup booting");
+  oledShow3("System Startup", "Booting...");
+
+  serial_.printf("Sleeping for %lu ms to allow Serial Monitor connection...\r\n",
+                 (unsigned long)cfg_.serialDelayMs);
+  delay(cfg_.serialDelayMs);
+
+  return sd_.begin(cfg_.sd, &SystemStartup::onSdEventThunk);
+}
+
+void SystemStartup::update() {
+  sd_.update();
+}
+
+void SystemStartup::onSdEventThunk(SdEvent event, const char* message) {
+  if (g_activeSystemStartup != nullptr) {
+    g_activeSystemStartup->onSdEvent(event, message);
+  }
+}
+
+void SystemStartup::onSdEvent(SdEvent event, const char* message) {
+  if (event == SdEvent::NO_CARD) {
+    oledShow3("SD missing or", "invalid FAT16/32", "Insert/format card");
+    emit(SystemEvent::SD_MISSING, message);
+  } else if (event == SdEvent::CARD_MOUNTED) {
+    oledShow3("SD card ready", "Mounted OK");
+    emit(SystemEvent::SD_READY, message);
+  } else if (event == SdEvent::CARD_REMOVED) {
+    oledShow3("SD card removed", "Please re-insert");
+    emit(SystemEvent::SD_REMOVED, message);
+  }
+}
+
+void SystemStartup::emit(SystemEvent event, const char* message) {
+  serial_.printf("[SYSTEM] %s\r\n", message);
+  if (callback_ != nullptr) {
+    callback_(event, message);
+  }
+}
+
+void SystemStartup::oledShow3(const char* l1, const char* l2, const char* l3) {
+  if (!kEnableOled) {
+    return;
+  }
+  g_oled.clearBuffer();
+  g_oled.setFont(u8g2_font_6x10_tf);
+  if (l1) g_oled.drawUTF8(0, 16, l1);
+  if (l2) g_oled.drawUTF8(0, 32, l2);
+  if (l3) g_oled.drawUTF8(0, 48, l3);
+  g_oled.sendBuffer();
+}
diff --git a/exercises/09_GPS_Time/lib/system_startup/SystemStartup.h b/exercises/09_GPS_Time/lib/system_startup/SystemStartup.h
new file mode 100644
index 0000000..cd83857
--- /dev/null
+++ b/exercises/09_GPS_Time/lib/system_startup/SystemStartup.h
@@ -0,0 +1,46 @@
+#pragma once
+
+#include <Arduino.h>
+#include "StartupSdManager.h"
+
+// Convenience alias so sketches can use System.println(...) style logging.
+// Arduino exposes Serial, not System, so map System -> Serial.
+#ifndef System
+#define System Serial
+#endif
+
+enum class SystemEvent : uint8_t {
+  BOOTING = 0,
+  SD_MISSING,
+  SD_READY,
+  SD_REMOVED
+};
+
+using SystemEventCallback = void (*)(SystemEvent event, const char* message);
+
+struct SystemStartupConfig {
+  uint32_t serialDelayMs = 5000;
+  SdWatcherConfig sd{};
+};
+
+class SystemStartup {
+ public:
+  explicit SystemStartup(Print& serial = Serial);
+
+  bool begin(const SystemStartupConfig& cfg = SystemStartupConfig{}, SystemEventCallback callback = nullptr);
+  void update();
+
+  bool isSdMounted() const { return sd_.isMounted(); }
+  StartupSdManager& sdManager() { return sd_; }
+
+ private:
+  static void onSdEventThunk(SdEvent event, const char* message);
+  void onSdEvent(SdEvent event, const char* message);
+  void emit(SystemEvent event, const char* message);
+  void oledShow3(const char* l1, const char* l2 = nullptr, const char* l3 = nullptr);
+
+  Print& serial_;
+  SystemStartupConfig cfg_{};
+  SystemEventCallback callback_ = nullptr;
+  StartupSdManager sd_;
+};
diff --git a/exercises/09_GPS_Time/lib/system_startup/library.json b/exercises/09_GPS_Time/lib/system_startup/library.json
new file mode 100644
index 0000000..0c2c1cf
--- /dev/null
+++ b/exercises/09_GPS_Time/lib/system_startup/library.json
@@ -0,0 +1,15 @@
+{
+  "name": "system_startup",
+  "version": "0.1.0",
+  "dependencies": [
+    {
+      "name": "startup_sd"
+    },
+    {
+      "name": "U8g2"
+    },
+    {
+      "name": "Wire"
+    }
+  ]
+}
diff --git a/exercises/09_GPS_Time/platformio.ini b/exercises/09_GPS_Time/platformio.ini
new file mode 100644
index 0000000..fff0d30
--- /dev/null
+++ b/exercises/09_GPS_Time/platformio.ini
@@ -0,0 +1,41 @@
+; 20260213 ChatGPT
+; $Id$
+; $HeadURL$
+
+[platformio]
+default_envs = node_a
+
+[env]
+platform = espressif32
+framework = arduino
+board = esp32-s3-devkitc-1
+monitor_speed = 115200
+lib_deps =
+  lewisxhe/XPowersLib@0.3.3
+  Wire
+  olikraus/U8g2@^2.36.4
+
+; SD pins based on T-Beam S3 core pin mapping
+build_flags =
+  -I ../../shared/boards
+  -I ../../external/microReticulum_Firmware
+  -D BOARD_MODEL=BOARD_TBEAM_S_V1
+  -D OLED_SDA=17
+  -D OLED_SCL=18
+  -D OLED_ADDR=0x3C
+  -D GPS_RX_PIN=9
+  -D GPS_TX_PIN=8
+  -D GPS_WAKEUP_PIN=7
+  -D GPS_1PPS_PIN=6
+  -D ARDUINO_USB_MODE=1
+  -D ARDUINO_USB_CDC_ON_BOOT=1
+
+[env:node_a]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"A\"
+
+[env:node_b]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"B\"
diff --git a/exercises/09_GPS_Time/src/main.cpp b/exercises/09_GPS_Time/src/main.cpp
new file mode 100644
index 0000000..36b0624
--- /dev/null
+++ b/exercises/09_GPS_Time/src/main.cpp
@@ -0,0 +1,556 @@
+// 20260217 ChatGPT
+// $Id$
+// $HeadURL$
+
+#include <Arduino.h>
+#include <Wire.h>
+#include <U8g2lib.h>
+
+#include "StartupSdManager.h"
+#include "tbeam_supreme_adapter.h"
+
+#ifndef OLED_SDA
+#define OLED_SDA 17
+#endif
+
+#ifndef OLED_SCL
+#define OLED_SCL 18
+#endif
+
+#ifndef OLED_ADDR
+#define OLED_ADDR 0x3C
+#endif
+
+#ifndef RTC_I2C_ADDR
+#define RTC_I2C_ADDR 0x51
+#endif
+
+#ifndef GPS_BAUD
+#define GPS_BAUD 9600
+#endif
+
+static const uint32_t kSerialDelayMs = 5000;
+static const uint32_t kGpsStartupProbeMs = 20000;
+static const uint32_t kMinuteMs = 60000;
+
+static XPowersLibInterface* g_pmu = nullptr;
+static StartupSdManager g_sd(Serial);
+static U8G2_SH1106_128X64_NONAME_F_HW_I2C g_oled(U8G2_R0, /* reset=*/U8X8_PIN_NONE);
+static HardwareSerial g_gpsSerial(1);
+
+static uint32_t g_logSeq = 0;
+static uint32_t g_lastMinuteReportMs = 0;
+static uint32_t g_gpsBaud = GPS_BAUD;
+
+static bool g_prevHadSatellites = false;
+static bool g_prevHadValidUtc = false;
+static bool g_satellitesAcquiredAnnounced = false;
+static bool g_timeAcquiredAnnounced = false;
+
+static char g_gpsLine[128];
+static size_t g_gpsLineLen = 0;
+
+enum class GpsModuleKind : uint8_t {
+  UNKNOWN = 0,
+  L76K,
+  QUECTEL_TODO
+};
+
+struct RtcDateTime {
+  uint16_t year;
+  uint8_t month;
+  uint8_t day;
+  uint8_t hour;
+  uint8_t minute;
+  uint8_t second;
+};
+
+struct GpsState {
+  GpsModuleKind module = GpsModuleKind::UNKNOWN;
+  bool sawAnySentence = false;
+
+  uint8_t satsUsed = 0;
+  uint8_t satsInView = 0;
+
+  bool hasValidUtc = false;
+  uint16_t utcYear = 0;
+  uint8_t utcMonth = 0;
+  uint8_t utcDay = 0;
+  uint8_t utcHour = 0;
+  uint8_t utcMinute = 0;
+  uint8_t utcSecond = 0;
+};
+
+static GpsState g_gps;
+
+static void logf(const char* fmt, ...) {
+  char msg[220];
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(msg, sizeof(msg), fmt, args);
+  va_end(args);
+  Serial.printf("[%10lu][%06lu] %s\r\n", (unsigned long)millis(), (unsigned long)g_logSeq++, msg);
+}
+
+static void oledShowLines(const char* l1,
+                          const char* l2 = nullptr,
+                          const char* l3 = nullptr,
+                          const char* l4 = nullptr,
+                          const char* l5 = nullptr) {
+  g_oled.clearBuffer();
+  g_oled.setFont(u8g2_font_5x8_tf);
+  if (l1) g_oled.drawUTF8(0, 12, l1);
+  if (l2) g_oled.drawUTF8(0, 24, l2);
+  if (l3) g_oled.drawUTF8(0, 36, l3);
+  if (l4) g_oled.drawUTF8(0, 48, l4);
+  if (l5) g_oled.drawUTF8(0, 60, l5);
+  g_oled.sendBuffer();
+}
+
+static uint8_t fromBcd(uint8_t b) {
+  return ((b >> 4U) * 10U) + (b & 0x0FU);
+}
+
+static bool rtcRead(RtcDateTime& out, bool& lowVoltageFlag) {
+  Wire1.beginTransmission(RTC_I2C_ADDR);
+  Wire1.write(0x02);
+  if (Wire1.endTransmission(false) != 0) {
+    return false;
+  }
+
+  const uint8_t need = 7;
+  uint8_t got = Wire1.requestFrom((int)RTC_I2C_ADDR, (int)need);
+  if (got != need) {
+    return false;
+  }
+
+  uint8_t sec = Wire1.read();
+  uint8_t min = Wire1.read();
+  uint8_t hour = Wire1.read();
+  uint8_t day = Wire1.read();
+  (void)Wire1.read();  // weekday
+  uint8_t month = Wire1.read();
+  uint8_t year = Wire1.read();
+
+  lowVoltageFlag = (sec & 0x80U) != 0;
+  out.second = fromBcd(sec & 0x7FU);
+  out.minute = fromBcd(min & 0x7FU);
+  out.hour = fromBcd(hour & 0x3FU);
+  out.day = fromBcd(day & 0x3FU);
+  out.month = fromBcd(month & 0x1FU);
+  uint8_t yy = fromBcd(year);
+  bool century = (month & 0x80U) != 0;
+  out.year = century ? (1900U + yy) : (2000U + yy);
+  return true;
+}
+
+static String formatRtcNow() {
+  RtcDateTime now{};
+  bool lowV = false;
+  if (!rtcRead(now, lowV)) {
+    return "RTC: read failed";
+  }
+
+  char buf[48];
+  snprintf(buf,
+           sizeof(buf),
+           "RTC %04u-%02u-%02u %02u:%02u:%02u%s",
+           (unsigned)now.year,
+           (unsigned)now.month,
+           (unsigned)now.day,
+           (unsigned)now.hour,
+           (unsigned)now.minute,
+           (unsigned)now.second,
+           lowV ? " !LOWV" : "");
+  return String(buf);
+}
+
+static String gpsModuleToString(GpsModuleKind kind) {
+  if (kind == GpsModuleKind::L76K) {
+    return "L76K";
+  }
+  if (kind == GpsModuleKind::QUECTEL_TODO) {
+    return "Quectel/TODO";
+  }
+  return "Unknown";
+}
+
+static bool parseUInt2(const char* s, uint8_t& out) {
+  if (!s || !isdigit((unsigned char)s[0]) || !isdigit((unsigned char)s[1])) {
+    return false;
+  }
+  out = (uint8_t)((s[0] - '0') * 10 + (s[1] - '0'));
+  return true;
+}
+
+static void detectModuleFromText(const char* text) {
+  if (!text || text[0] == '\0') {
+    return;
+  }
+
+  String t(text);
+  t.toUpperCase();
+
+  if (t.indexOf("L76K") >= 0) {
+    g_gps.module = GpsModuleKind::L76K;
+    return;
+  }
+
+  if (t.indexOf("QUECTEL") >= 0 || t.indexOf("2021-10-20") >= 0 || t.indexOf("2021/10/20") >= 0) {
+    if (g_gps.module != GpsModuleKind::L76K) {
+      g_gps.module = GpsModuleKind::QUECTEL_TODO;
+    }
+  }
+}
+
+static void parseGga(char* fields[], int count) {
+  if (count <= 7) {
+    return;
+  }
+  int sats = atoi(fields[7]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsUsed = (uint8_t)sats;
+  }
+}
+
+static void parseGsv(char* fields[], int count) {
+  if (count <= 3) {
+    return;
+  }
+  int sats = atoi(fields[3]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsInView = (uint8_t)sats;
+  }
+}
+
+static void parseRmc(char* fields[], int count) {
+  if (count <= 9) {
+    return;
+  }
+
+  const char* utc = fields[1];
+  const char* status = fields[2];
+  const char* date = fields[9];
+
+  if (!status || status[0] != 'A') {
+    return;
+  }
+
+  if (!utc || strlen(utc) < 6 || !date || strlen(date) < 6) {
+    return;
+  }
+
+  uint8_t hh = 0, mm = 0, ss = 0;
+  uint8_t dd = 0, mo = 0, yy = 0;
+  if (!parseUInt2(utc + 0, hh) || !parseUInt2(utc + 2, mm) || !parseUInt2(utc + 4, ss)) {
+    return;
+  }
+  if (!parseUInt2(date + 0, dd) || !parseUInt2(date + 2, mo) || !parseUInt2(date + 4, yy)) {
+    return;
+  }
+
+  g_gps.utcHour = hh;
+  g_gps.utcMinute = mm;
+  g_gps.utcSecond = ss;
+  g_gps.utcDay = dd;
+  g_gps.utcMonth = mo;
+  g_gps.utcYear = (uint16_t)(2000U + yy);
+  g_gps.hasValidUtc = true;
+}
+
+static void parseTxt(char* fields[], int count) {
+  if (count <= 4) {
+    return;
+  }
+  detectModuleFromText(fields[4]);
+}
+
+static void processNmeaLine(char* line) {
+  if (!line || line[0] != '$') {
+    return;
+  }
+  g_gps.sawAnySentence = true;
+
+  char* star = strchr(line, '*');
+  if (star) {
+    *star = '\0';
+  }
+
+  char* fields[24] = {0};
+  int count = 0;
+  char* saveptr = nullptr;
+  char* tok = strtok_r(line, ",", &saveptr);
+  while (tok && count < 24) {
+    fields[count++] = tok;
+    tok = strtok_r(nullptr, ",", &saveptr);
+  }
+  if (count <= 0 || !fields[0]) {
+    return;
+  }
+
+  const char* header = fields[0];
+  size_t n = strlen(header);
+  if (n < 6) {
+    return;
+  }
+  const char* type = header + (n - 3);
+
+  if (strcmp(type, "GGA") == 0) {
+    parseGga(fields, count);
+  } else if (strcmp(type, "GSV") == 0) {
+    parseGsv(fields, count);
+  } else if (strcmp(type, "RMC") == 0) {
+    parseRmc(fields, count);
+  } else if (strcmp(type, "TXT") == 0) {
+    parseTxt(fields, count);
+  }
+}
+
+static void pollGpsSerial() {
+  while (g_gpsSerial.available() > 0) {
+    char c = (char)g_gpsSerial.read();
+    if (c == '\r') {
+      continue;
+    }
+    if (c == '\n') {
+      if (g_gpsLineLen > 0) {
+        g_gpsLine[g_gpsLineLen] = '\0';
+        processNmeaLine(g_gpsLine);
+        g_gpsLineLen = 0;
+      }
+      continue;
+    }
+
+    if (g_gpsLineLen + 1 < sizeof(g_gpsLine)) {
+      g_gpsLine[g_gpsLineLen++] = c;
+    } else {
+      g_gpsLineLen = 0;
+    }
+  }
+}
+
+static void startGpsUart(uint32_t baud) {
+  g_gpsSerial.end();
+  delay(20);
+  g_gpsSerial.setRxBufferSize(1024);
+  g_gpsSerial.begin(baud, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+  g_gpsBaud = baud;
+}
+
+static bool collectGpsTraffic(uint32_t windowMs, bool updateSd) {
+  uint32_t start = millis();
+  size_t bytesSeen = 0;
+  while ((uint32_t)(millis() - start) < windowMs) {
+    while (g_gpsSerial.available() > 0) {
+      (void)g_gpsSerial.read();
+      bytesSeen++;
+    }
+    pollGpsSerial();
+    if (updateSd) {
+      g_sd.update();
+    }
+    delay(2);
+  }
+  return bytesSeen > 0 || g_gps.sawAnySentence;
+}
+
+static bool probeGpsAtBaud(uint32_t baud) {
+  startGpsUart(baud);
+  logf("Probing GPS at %lu baud...", (unsigned long)baud);
+  if (collectGpsTraffic(700, true)) {
+    return true;
+  }
+
+  // Common commands for MTK/L76K and related chipsets.
+  g_gpsSerial.write("$PCAS06,0*1B\r\n");
+  g_gpsSerial.write("$PMTK605*31\r\n");
+  g_gpsSerial.write("$PQTMVERNO*58\r\n");
+  g_gpsSerial.write("$PMTK353,1,1,1,1,1*2A\r\n");
+  g_gpsSerial.write("$PMTK314,0,1,0,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29\r\n");
+
+  return collectGpsTraffic(1200, true);
+}
+
+static void initialGpsProbe() {
+  const uint32_t bauds[] = {GPS_BAUD, 115200, 38400, 57600, 19200};
+  for (size_t i = 0; i < sizeof(bauds) / sizeof(bauds[0]); ++i) {
+    if (probeGpsAtBaud(bauds[i])) {
+      logf("GPS traffic detected at %lu baud", (unsigned long)g_gpsBaud);
+      return;
+    }
+  }
+  logf("No GPS traffic detected during startup probe");
+}
+
+static uint8_t bestSatelliteCount() {
+  return (g_gps.satsUsed > g_gps.satsInView) ? g_gps.satsUsed : g_gps.satsInView;
+}
+
+static bool isUnsupportedQuectelMode() {
+  return g_gps.module == GpsModuleKind::QUECTEL_TODO;
+}
+
+static void reportStatusToSerial() {
+  uint8_t sats = bestSatelliteCount();
+  logf("GPS module: %s", gpsModuleToString(g_gps.module).c_str());
+  logf("GPS sentences seen: %s", g_gps.sawAnySentence ? "yes" : "no");
+  logf("GPS satellites: used=%u in-view=%u best=%u",
+       (unsigned)g_gps.satsUsed,
+       (unsigned)g_gps.satsInView,
+       (unsigned)sats);
+  logf("GPS can provide time from satellites: %s", g_gps.hasValidUtc ? "YES" : "NO");
+}
+
+static void maybeAnnounceGpsTransitions() {
+  if (isUnsupportedQuectelMode()) {
+    return;
+  }
+
+  uint8_t sats = bestSatelliteCount();
+  bool hasSats = sats > 0;
+  bool hasUtc = g_gps.hasValidUtc;
+
+  if (!g_satellitesAcquiredAnnounced && !g_prevHadSatellites && hasSats) {
+    String rtc = formatRtcNow();
+    char l2[28];
+    snprintf(l2, sizeof(l2), "Satellites: %u", (unsigned)sats);
+    oledShowLines("GPS acquired", l2, "Satellite lock found", "Waiting for UTC...", rtc.c_str());
+    logf("Transition: satellites acquired (%u)", (unsigned)sats);
+    g_satellitesAcquiredAnnounced = true;
+  }
+
+  if (!g_timeAcquiredAnnounced && !g_prevHadValidUtc && hasUtc) {
+    char line2[40];
+    char line3[28];
+    snprintf(line2,
+             sizeof(line2),
+             "%04u-%02u-%02u %02u:%02u:%02u",
+             (unsigned)g_gps.utcYear,
+             (unsigned)g_gps.utcMonth,
+             (unsigned)g_gps.utcDay,
+             (unsigned)g_gps.utcHour,
+             (unsigned)g_gps.utcMinute,
+             (unsigned)g_gps.utcSecond);
+    snprintf(line3, sizeof(line3), "Satellites: %u", (unsigned)sats);
+    oledShowLines("GPS UTC acquired", line2, line3, "Source: L76K");
+    logf("Transition: GPS UTC acquired: %s", line2);
+    g_timeAcquiredAnnounced = true;
+  }
+
+  g_prevHadSatellites = hasSats;
+  g_prevHadValidUtc = hasUtc;
+}
+
+static void drawMinuteStatus() {
+  if (isUnsupportedQuectelMode()) {
+    oledShowLines("GPS module mismatch", "Quectel detected", "L76K required", "TODO: implement", "Quectel support");
+    logf("GPS module mismatch: Quectel detected but this exercise currently supports only L76K (TODO)");
+    return;
+  }
+
+  uint8_t sats = bestSatelliteCount();
+  if (g_gps.hasValidUtc) {
+    char line2[40];
+    char line3[28];
+    snprintf(line2,
+             sizeof(line2),
+             "%04u-%02u-%02u %02u:%02u:%02u",
+             (unsigned)g_gps.utcYear,
+             (unsigned)g_gps.utcMonth,
+             (unsigned)g_gps.utcDay,
+             (unsigned)g_gps.utcHour,
+             (unsigned)g_gps.utcMinute,
+             (unsigned)g_gps.utcSecond);
+    snprintf(line3, sizeof(line3), "Satellites: %u", (unsigned)sats);
+    oledShowLines("GPS time (UTC)", line2, line3, "Source: L76K");
+    logf("GPS time (UTC): %s  satellites=%u", line2, (unsigned)sats);
+    return;
+  }
+
+  String rtc = formatRtcNow();
+  if (sats > 0) {
+    char line2[28];
+    snprintf(line2, sizeof(line2), "Satellites: %u", (unsigned)sats);
+    oledShowLines("GPS signal detected", line2, "GPS UTC not ready", "yet, using RTC", rtc.c_str());
+    logf("Satellites detected (%u) but GPS UTC not ready. %s", (unsigned)sats, rtc.c_str());
+  } else {
+    oledShowLines("Unable to acquire", "satellites", "Take me outside so I", "can see satellites", rtc.c_str());
+    logf("Unable to acquire satellites. %s", rtc.c_str());
+  }
+}
+
+void setup() {
+  Serial.begin(115200);
+  delay(kSerialDelayMs);
+
+  Serial.println("\r\n==================================================");
+  Serial.println("Exercise 09: GPS Time");
+  Serial.println("==================================================");
+
+  if (!tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial)) {
+    logf("PMU init failed");
+  }
+
+  Wire.begin(OLED_SDA, OLED_SCL);
+  g_oled.setI2CAddress(OLED_ADDR << 1);
+  g_oled.begin();
+  oledShowLines("GPS Time exercise", "Booting...");
+
+  SdWatcherConfig sdCfg{};
+  if (!g_sd.begin(sdCfg, nullptr)) {
+    logf("SD startup manager begin() failed");
+  }
+
+#ifdef GPS_WAKEUP_PIN
+  // Keep wake pin neutral; avoid forcing an unknown standby state.
+  pinMode(GPS_WAKEUP_PIN, INPUT);
+#endif
+
+#ifdef GPS_1PPS_PIN
+  pinMode(GPS_1PPS_PIN, INPUT);
+#endif
+
+  startGpsUart(GPS_BAUD);
+  logf("GPS UART started: RX=%d TX=%d baud=%lu", GPS_RX_PIN, GPS_TX_PIN, (unsigned long)g_gpsBaud);
+  initialGpsProbe();
+
+  oledShowLines("GPS startup probe", "Checking satellites", "and GPS time...");
+
+  uint32_t probeStart = millis();
+  uint32_t lastProbeUiMs = 0;
+  while ((uint32_t)(millis() - probeStart) < kGpsStartupProbeMs) {
+    pollGpsSerial();
+    g_sd.update();
+
+    uint32_t now = millis();
+    if ((uint32_t)(now - lastProbeUiMs) >= 1000) {
+      lastProbeUiMs = now;
+      char l3[28];
+      char l4[30];
+      char l5[24];
+      snprintf(l3, sizeof(l3), "Sats: %u", (unsigned)bestSatelliteCount());
+      snprintf(l4, sizeof(l4), "Module: %s", gpsModuleToString(g_gps.module).c_str());
+      snprintf(l5, sizeof(l5), "NMEA:%s %lu", g_gps.sawAnySentence ? "yes" : "no", (unsigned long)g_gpsBaud);
+      oledShowLines("GPS startup probe", "Checking satellites", l3, l4, l5);
+    }
+    delay(10);
+  }
+
+  reportStatusToSerial();
+  g_prevHadSatellites = (bestSatelliteCount() > 0);
+  g_prevHadValidUtc = g_gps.hasValidUtc;
+  drawMinuteStatus();
+  g_lastMinuteReportMs = millis();
+}
+
+void loop() {
+  pollGpsSerial();
+  g_sd.update();
+  maybeAnnounceGpsTransitions();
+
+  uint32_t now = millis();
+  if ((uint32_t)(now - g_lastMinuteReportMs) >= kMinuteMs) {
+    g_lastMinuteReportMs = now;
+    drawMinuteStatus();
+  }
+}
diff --git a/exercises/10_Simple_GPS/README.md b/exercises/10_Simple_GPS/README.md
new file mode 100644
index 0000000..6fc5c73
--- /dev/null
+++ b/exercises/10_Simple_GPS/README.md
@@ -0,0 +1,98 @@
+## Exercise 10: Simple GPS (No SD)
+
+Goal: verify GPS satellite and UTC time acquisition on T-Beam Supreme using OLED-only status updates.
+
+## Current behavior
+
+1. Boots PMU, OLED, RTC, and GPS UART.
+2. Runs an active startup GPS probe (multi-baud + query commands) to detect GPS serial traffic.
+3. Every 30 seconds:
+   - Shows `Trying to locate satellites` + `NMEA seen: yes/no` + current RTC time.
+   - Continues parsing GPS NMEA data.
+   - If GPS UTC is valid, shows GPS UTC + satellite count + `NMEA seen: yes/no`.
+   - Otherwise shows `Take me outside` + `NMEA seen: yes/no` + RTC.
+4. No SD card logic is used in this exercise.
+
+## Walk-through: original approach and why
+
+Initial implementation used a minimal/simple GPS strategy:
+
+1. Power up PMU rails using the existing T-Beam adapter.
+2. Start `Serial1` at 9600 baud.
+3. Parse incoming NMEA (`GGA/GSV/RMC`) passively.
+4. Show periodic OLED status every 30 seconds.
+
+Why this was chosen:
+
+- It is the smallest path to validate basic GPS lock/time behavior.
+- It avoids introducing SD complexity while isolating GPS.
+- It is easy for field testing (OLED-first, battery-powered).
+
+## What was discovered by comparing with Meshtastic
+
+Meshtastic GPS handling is more defensive and hardware-aware in principle:
+
+1. It uses a board variant that provides explicit GPS pin mapping for the T-Beam Supreme path.
+2. It initializes GPS serial with explicit RX/TX pins and larger receive buffers.
+3. It performs active startup probing (commands + response checks), not only passive listening.
+4. It attempts detection across known module families and may try multiple serial settings.
+5. It manages GNSS-related power/standby states deliberately (rather than assuming default UART traffic immediately appears).
+
+## What differed in this exercise and likely caused the issue
+
+The first Exercise 10 version was built on `esp32-s3-devkitc-1` with conditional pin usage.
+
+- If GPS pin macros are not present, `Serial1` can start on default pins.
+- That can produce `NMEA seen: no` forever even outdoors, because firmware is listening on the wrong UART pins.
+
+## Corrections applied after Meshtastic review
+
+1. Added explicit GPS pin defines in `platformio.ini`:
+   - `GPS_RX_PIN=9`
+   - `GPS_TX_PIN=8`
+   - `GPS_WAKEUP_PIN=7`
+   - `GPS_1PPS_PIN=6`
+2. Forced UART startup using explicit RX/TX pins.
+3. Added startup multi-baud active probe and common GPS query commands.
+4. Added OLED `NMEA seen: yes/no` so field tests distinguish:
+   - `no sky fix yet` vs
+   - `no GPS serial traffic at all`.
+
+## Field Test Checklist
+
+1. Flash and reboot outdoors with clear sky view.
+2. Confirm the OLED updates every 30 seconds.
+3. Watch for this expected progression:
+   - `Trying to locate satellites` + `NMEA seen: no`
+   - then `Trying to locate satellites` + `NMEA seen: yes`
+   - then either:
+     - `GPS lock acquired` with UTC and satellite count, or
+     - `Take me outside` if no fix yet.
+4. Keep unit stationary for 2-5 minutes for first lock after cold start.
+
+Interpretation guide:
+
+- `NMEA seen: no`: likely UART/pin/baud/module-power communication issue.
+- `NMEA seen: yes` + no lock: GPS is talking, but no valid fix yet (sky view/time-to-first-fix issue).
+- `GPS lock acquired`: fix is valid; UTC and satellites are available from GPS.
+- RTC line updates every 30 seconds: loop is alive and retry cycle is running.
+
+If still failing:
+
+1. Capture serial log from boot through at least 2 full 30-second cycles.
+2. Note whether `NMEA seen` ever changes from `no` to `yes`.
+3. Record whether GPS startup probe reports traffic at any baud rate.
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```
diff --git a/exercises/10_Simple_GPS/platformio.ini b/exercises/10_Simple_GPS/platformio.ini
new file mode 100644
index 0000000..a183c3b
--- /dev/null
+++ b/exercises/10_Simple_GPS/platformio.ini
@@ -0,0 +1,40 @@
+; 20260217 ChatGPT
+; $Id$
+; $HeadURL$
+
+[platformio]
+default_envs = node_a
+
+[env]
+platform = espressif32
+framework = arduino
+board = esp32-s3-devkitc-1
+monitor_speed = 115200
+lib_deps =
+  lewisxhe/XPowersLib@0.3.3
+  Wire
+  olikraus/U8g2@^2.36.4
+
+build_flags =
+  -I ../../shared/boards
+  -I ../../external/microReticulum_Firmware
+  -D BOARD_MODEL=BOARD_TBEAM_S_V1
+  -D OLED_SDA=17
+  -D OLED_SCL=18
+  -D OLED_ADDR=0x3C
+  -D GPS_RX_PIN=9
+  -D GPS_TX_PIN=8
+  -D GPS_WAKEUP_PIN=7
+  -D GPS_1PPS_PIN=6
+  -D ARDUINO_USB_MODE=1
+  -D ARDUINO_USB_CDC_ON_BOOT=1
+
+[env:node_a]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"A\"
+
+[env:node_b]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"B\"
diff --git a/exercises/10_Simple_GPS/src/main.cpp b/exercises/10_Simple_GPS/src/main.cpp
new file mode 100644
index 0000000..7456125
--- /dev/null
+++ b/exercises/10_Simple_GPS/src/main.cpp
@@ -0,0 +1,431 @@
+// 20260217 ChatGPT
+// $Id$
+// $HeadURL$
+
+#include <Arduino.h>
+#include <Wire.h>
+#include <U8g2lib.h>
+
+#include "tbeam_supreme_adapter.h"
+
+#ifndef OLED_SDA
+#define OLED_SDA 17
+#endif
+
+#ifndef OLED_SCL
+#define OLED_SCL 18
+#endif
+
+#ifndef OLED_ADDR
+#define OLED_ADDR 0x3C
+#endif
+
+#ifndef RTC_I2C_ADDR
+#define RTC_I2C_ADDR 0x51
+#endif
+
+#ifndef GPS_BAUD
+#define GPS_BAUD 9600
+#endif
+
+static const uint32_t kSerialDelayMs = 5000;
+static const uint32_t kReportIntervalMs = 30000;
+
+static XPowersLibInterface* g_pmu = nullptr;
+static U8G2_SH1106_128X64_NONAME_F_HW_I2C g_oled(U8G2_R0, /* reset=*/U8X8_PIN_NONE);
+static HardwareSerial g_gpsSerial(1);
+
+static uint32_t g_logSeq = 0;
+static uint32_t g_lastReportMs = 0;
+static uint32_t g_gpsBaud = GPS_BAUD;
+
+static char g_gpsLine[128];
+static size_t g_gpsLineLen = 0;
+
+struct RtcDateTime {
+  uint16_t year;
+  uint8_t month;
+  uint8_t day;
+  uint8_t hour;
+  uint8_t minute;
+  uint8_t second;
+};
+
+struct GpsState {
+  bool sawAnySentence = false;
+  uint8_t satsUsed = 0;
+  uint8_t satsInView = 0;
+
+  bool hasValidUtc = false;
+  uint16_t utcYear = 0;
+  uint8_t utcMonth = 0;
+  uint8_t utcDay = 0;
+  uint8_t utcHour = 0;
+  uint8_t utcMinute = 0;
+  uint8_t utcSecond = 0;
+};
+
+static GpsState g_gps;
+
+static void logf(const char* fmt, ...) {
+  char msg[220];
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(msg, sizeof(msg), fmt, args);
+  va_end(args);
+  Serial.printf("[%10lu][%06lu] %s\r\n", (unsigned long)millis(), (unsigned long)g_logSeq++, msg);
+}
+
+static void oledShowLines(const char* l1,
+                          const char* l2 = nullptr,
+                          const char* l3 = nullptr,
+                          const char* l4 = nullptr,
+                          const char* l5 = nullptr) {
+  g_oled.clearBuffer();
+  g_oled.setFont(u8g2_font_5x8_tf);
+  if (l1) g_oled.drawUTF8(0, 12, l1);
+  if (l2) g_oled.drawUTF8(0, 24, l2);
+  if (l3) g_oled.drawUTF8(0, 36, l3);
+  if (l4) g_oled.drawUTF8(0, 48, l4);
+  if (l5) g_oled.drawUTF8(0, 60, l5);
+  g_oled.sendBuffer();
+}
+
+static uint8_t fromBcd(uint8_t b) {
+  return ((b >> 4U) * 10U) + (b & 0x0FU);
+}
+
+static bool rtcRead(RtcDateTime& out, bool& lowVoltageFlag) {
+  Wire1.beginTransmission(RTC_I2C_ADDR);
+  Wire1.write(0x02);
+  if (Wire1.endTransmission(false) != 0) {
+    return false;
+  }
+
+  const uint8_t need = 7;
+  uint8_t got = Wire1.requestFrom((int)RTC_I2C_ADDR, (int)need);
+  if (got != need) {
+    return false;
+  }
+
+  uint8_t sec = Wire1.read();
+  uint8_t min = Wire1.read();
+  uint8_t hour = Wire1.read();
+  uint8_t day = Wire1.read();
+  (void)Wire1.read();
+  uint8_t month = Wire1.read();
+  uint8_t year = Wire1.read();
+
+  lowVoltageFlag = (sec & 0x80U) != 0;
+  out.second = fromBcd(sec & 0x7FU);
+  out.minute = fromBcd(min & 0x7FU);
+  out.hour = fromBcd(hour & 0x3FU);
+  out.day = fromBcd(day & 0x3FU);
+  out.month = fromBcd(month & 0x1FU);
+  uint8_t yy = fromBcd(year);
+  bool century = (month & 0x80U) != 0;
+  out.year = century ? (1900U + yy) : (2000U + yy);
+  return true;
+}
+
+static String formatRtcNow() {
+  RtcDateTime now{};
+  bool lowV = false;
+  if (!rtcRead(now, lowV)) {
+    return "RTC read failed";
+  }
+
+  char buf[48];
+  snprintf(buf,
+           sizeof(buf),
+           "RTC %04u-%02u-%02u %02u:%02u:%02u%s",
+           (unsigned)now.year,
+           (unsigned)now.month,
+           (unsigned)now.day,
+           (unsigned)now.hour,
+           (unsigned)now.minute,
+           (unsigned)now.second,
+           lowV ? " !LOWV" : "");
+  return String(buf);
+}
+
+static bool parseUInt2(const char* s, uint8_t& out) {
+  if (!s || !isdigit((unsigned char)s[0]) || !isdigit((unsigned char)s[1])) {
+    return false;
+  }
+  out = (uint8_t)((s[0] - '0') * 10 + (s[1] - '0'));
+  return true;
+}
+
+static void parseGga(char* fields[], int count) {
+  if (count <= 7) {
+    return;
+  }
+  int sats = atoi(fields[7]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsUsed = (uint8_t)sats;
+  }
+}
+
+static void parseGsv(char* fields[], int count) {
+  if (count <= 3) {
+    return;
+  }
+  int sats = atoi(fields[3]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsInView = (uint8_t)sats;
+  }
+}
+
+static void parseRmc(char* fields[], int count) {
+  if (count <= 9) {
+    return;
+  }
+
+  const char* utc = fields[1];
+  const char* status = fields[2];
+  const char* date = fields[9];
+
+  if (!status || status[0] != 'A') {
+    return;
+  }
+
+  if (!utc || strlen(utc) < 6 || !date || strlen(date) < 6) {
+    return;
+  }
+
+  uint8_t hh = 0, mm = 0, ss = 0;
+  uint8_t dd = 0, mo = 0, yy = 0;
+  if (!parseUInt2(utc + 0, hh) || !parseUInt2(utc + 2, mm) || !parseUInt2(utc + 4, ss)) {
+    return;
+  }
+  if (!parseUInt2(date + 0, dd) || !parseUInt2(date + 2, mo) || !parseUInt2(date + 4, yy)) {
+    return;
+  }
+
+  g_gps.utcHour = hh;
+  g_gps.utcMinute = mm;
+  g_gps.utcSecond = ss;
+  g_gps.utcDay = dd;
+  g_gps.utcMonth = mo;
+  g_gps.utcYear = (uint16_t)(2000U + yy);
+  g_gps.hasValidUtc = true;
+}
+
+static void processNmeaLine(char* line) {
+  if (!line || line[0] != '$') {
+    return;
+  }
+
+  g_gps.sawAnySentence = true;
+
+  char* star = strchr(line, '*');
+  if (star) {
+    *star = '\0';
+  }
+
+  char* fields[24] = {0};
+  int count = 0;
+  char* saveptr = nullptr;
+  char* tok = strtok_r(line, ",", &saveptr);
+  while (tok && count < 24) {
+    fields[count++] = tok;
+    tok = strtok_r(nullptr, ",", &saveptr);
+  }
+
+  if (count <= 0 || !fields[0]) {
+    return;
+  }
+
+  const char* header = fields[0];
+  size_t n = strlen(header);
+  if (n < 6) {
+    return;
+  }
+
+  const char* type = header + (n - 3);
+  if (strcmp(type, "GGA") == 0) {
+    parseGga(fields, count);
+  } else if (strcmp(type, "GSV") == 0) {
+    parseGsv(fields, count);
+  } else if (strcmp(type, "RMC") == 0) {
+    parseRmc(fields, count);
+  }
+}
+
+static void pollGpsSerial() {
+  while (g_gpsSerial.available() > 0) {
+    char c = (char)g_gpsSerial.read();
+    if (c == '\r') {
+      continue;
+    }
+    if (c == '\n') {
+      if (g_gpsLineLen > 0) {
+        g_gpsLine[g_gpsLineLen] = '\0';
+        processNmeaLine(g_gpsLine);
+        g_gpsLineLen = 0;
+      }
+      continue;
+    }
+
+    if (g_gpsLineLen + 1 < sizeof(g_gpsLine)) {
+      g_gpsLine[g_gpsLineLen++] = c;
+    } else {
+      g_gpsLineLen = 0;
+    }
+  }
+}
+
+static void startGpsUart(uint32_t baud) {
+  g_gpsSerial.end();
+  delay(20);
+  g_gpsSerial.setRxBufferSize(1024);
+  g_gpsSerial.begin(baud, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+  g_gpsBaud = baud;
+}
+
+static bool collectGpsTraffic(uint32_t windowMs) {
+  uint32_t start = millis();
+  size_t bytesSeen = 0;
+  while ((uint32_t)(millis() - start) < windowMs) {
+    while (g_gpsSerial.available() > 0) {
+      (void)g_gpsSerial.read();
+      bytesSeen++;
+    }
+    pollGpsSerial();
+    delay(2);
+  }
+  return bytesSeen > 0 || g_gps.sawAnySentence;
+}
+
+static bool probeGpsAtBaud(uint32_t baud) {
+  startGpsUart(baud);
+  logf("Probing GPS at %lu baud...", (unsigned long)baud);
+  if (collectGpsTraffic(700)) {
+    return true;
+  }
+
+  // Try common query/wake commands used by MTK/L76K and related chipsets.
+  g_gpsSerial.write("$PCAS06,0*1B\r\n");     // Request module SW text
+  g_gpsSerial.write("$PMTK605*31\r\n");      // MTK firmware query
+  g_gpsSerial.write("$PQTMVERNO*58\r\n");    // Quectel LC86 query
+  g_gpsSerial.write("$PMTK353,1,1,1,1,1*2A\r\n");
+  g_gpsSerial.write("$PMTK314,0,1,0,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29\r\n");
+
+  return collectGpsTraffic(1200);
+}
+
+static void initialGpsProbe() {
+  const uint32_t bauds[] = {GPS_BAUD, 115200, 38400, 57600, 19200};
+  for (size_t i = 0; i < sizeof(bauds) / sizeof(bauds[0]); ++i) {
+    if (probeGpsAtBaud(bauds[i])) {
+      logf("GPS traffic detected at %lu baud", (unsigned long)g_gpsBaud);
+      return;
+    }
+  }
+  logf("No GPS traffic detected during startup probe");
+}
+
+static uint8_t bestSatelliteCount() {
+  return (g_gps.satsUsed > g_gps.satsInView) ? g_gps.satsUsed : g_gps.satsInView;
+}
+
+static void showTryingMessage() {
+  String rtc = formatRtcNow();
+  oledShowLines("Trying to locate",
+                "satellites",
+                g_gps.sawAnySentence ? "NMEA seen: yes" : "NMEA seen: no",
+                rtc.c_str());
+  logf("Trying to locate satellites. %s", rtc.c_str());
+}
+
+static void showStatusMessage() {
+  uint8_t sats = bestSatelliteCount();
+
+  if (g_gps.hasValidUtc) {
+    char line2[40];
+    char line3[28];
+    snprintf(line2,
+             sizeof(line2),
+             "GPS UTC %04u-%02u-%02u",
+             (unsigned)g_gps.utcYear,
+             (unsigned)g_gps.utcMonth,
+             (unsigned)g_gps.utcDay);
+    snprintf(line3,
+             sizeof(line3),
+             "%02u:%02u:%02u  sats:%u",
+             (unsigned)g_gps.utcHour,
+             (unsigned)g_gps.utcMinute,
+             (unsigned)g_gps.utcSecond,
+             (unsigned)sats);
+    oledShowLines("GPS lock acquired",
+                  line2,
+                  line3,
+                  g_gps.sawAnySentence ? "NMEA seen: yes" : "NMEA seen: no");
+    logf("GPS lock acquired. %s  sats=%u", line3, (unsigned)sats);
+    return;
+  }
+
+  String rtc = formatRtcNow();
+  oledShowLines("Take me outside",
+                "No GPS time/sats yet",
+                g_gps.sawAnySentence ? "NMEA seen: yes" : "NMEA seen: no",
+                rtc.c_str());
+  logf("Take me outside. sats=%u, has_utc=%s, nmea_seen=%s. %s",
+       (unsigned)sats,
+       g_gps.hasValidUtc ? "yes" : "no",
+       g_gps.sawAnySentence ? "yes" : "no",
+       rtc.c_str());
+}
+
+void setup() {
+  Serial.begin(115200);
+  delay(kSerialDelayMs);
+
+  Serial.println("\r\n==================================================");
+  Serial.println("Exercise 10: Simple GPS (No SD)");
+  Serial.println("==================================================");
+
+  if (!tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial)) {
+    logf("PMU init failed");
+  }
+
+  Wire.begin(OLED_SDA, OLED_SCL);
+  g_oled.setI2CAddress(OLED_ADDR << 1);
+  g_oled.begin();
+  oledShowLines("Simple GPS", "Booting...");
+
+#ifdef GPS_1PPS_PIN
+  pinMode(GPS_1PPS_PIN, INPUT);
+#endif
+
+#ifdef GPS_WAKEUP_PIN
+  // Keep wake pin in a neutral state similar to Meshtastic behavior.
+  pinMode(GPS_WAKEUP_PIN, INPUT);
+#endif
+
+  startGpsUart(GPS_BAUD);
+  logf("GPS UART started: RX=%d TX=%d baud=%lu", GPS_RX_PIN, GPS_TX_PIN, (unsigned long)g_gpsBaud);
+  initialGpsProbe();
+
+  showTryingMessage();
+  g_lastReportMs = millis();
+}
+
+void loop() {
+  pollGpsSerial();
+
+  uint32_t now = millis();
+  if ((uint32_t)(now - g_lastReportMs) >= kReportIntervalMs) {
+    g_lastReportMs = now;
+
+    showTryingMessage();
+    uint32_t start = millis();
+    while ((uint32_t)(millis() - start) < 2000) {
+      pollGpsSerial();
+      delay(5);
+    }
+
+    showStatusMessage();
+  }
+}
diff --git a/exercises/11_Set_RTC2GPS/README.md b/exercises/11_Set_RTC2GPS/README.md
new file mode 100644
index 0000000..462c2d2
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/README.md
@@ -0,0 +1,33 @@
+## Exercise 11: Set RTC to GPS (1PPS Discipline)
+
+This exercise extends Exercise 9 behavior (GPS + SD + OLED) and disciplines the onboard RTC from GPS UTC using the GPS `1PPS` (pulse-per-second) timing signal.
+
+Implemented behavior:
+
+1. Boots PMU, OLED, SD watcher, and GPS UART using the same T-Beam Supreme pin mapping from prior exercises.
+2. Parses NMEA (`RMC`, `GGA`, `GSV`) to track UTC validity and satellite counts.
+3. Every 1 minute, attempts to set RTC from GPS:
+   - Uses latest valid GPS UTC.
+   - Waits for next `1PPS` rising edge.
+   - Sets RTC to GPS time aligned to that edge (UTC + 1 second).
+4. Appends event records to SD file:
+   - Path: `/gps/discipline_rtc.log`
+   - Append-only writes (`FILE_APPEND`)
+   - Format:
+     - `YYYYMMDD_HH24MISS_z\t set RTC to GPS using 1PPS pulse-per-second discipline\trtc-gps drift=+/-Ns`
+5. OLED success message shows RTC disciplined confirmation and timestamp.
+6. If GPS time cannot be determined (or 1PPS edge is not seen in timeout), OLED shows failure status and the loop delays 30 seconds before retry.
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```
diff --git a/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.cpp b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.cpp
new file mode 100644
index 0000000..1e8791c
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.cpp
@@ -0,0 +1,360 @@
+#include "StartupSdManager.h"
+
+#include <stdarg.h>
+#include "driver/gpio.h"
+
+StartupSdManager::StartupSdManager(Print& serial) : serial_(serial) {}
+
+bool StartupSdManager::begin(const SdWatcherConfig& cfg, SdStatusCallback callback) {
+  cfg_ = cfg;
+  callback_ = callback;
+
+  forceSpiDeselected();
+  dumpSdPins("very-early");
+
+  if (!initPmuForSdPower()) {
+    return false;
+  }
+
+  cycleSdRail();
+  delay(cfg_.startupWarmupMs);
+
+  bool warmMounted = false;
+  for (uint8_t i = 0; i < 3; ++i) {
+    if (mountPreferred(false)) {
+      warmMounted = true;
+      break;
+    }
+    delay(200);
+  }
+
+  // Some cards need a longer power/settle window after cold boot.
+  // Before declaring ABSENT, retry with extended settle and a full scan.
+  if (!warmMounted) {
+    logf("Watcher: startup preferred mount failed, retrying with extended settle");
+    cycleSdRail(400, 1200);
+    delay(cfg_.startupWarmupMs + 1500);
+    warmMounted = mountCardFullScan();
+  }
+
+  if (warmMounted) {
+    setStateMounted();
+  } else {
+    setStateAbsent();
+  }
+  return true;
+}
+
+void StartupSdManager::update() {
+  const uint32_t now = millis();
+  const uint32_t pollInterval =
+      (watchState_ == SdWatchState::MOUNTED) ? cfg_.pollIntervalMountedMs : cfg_.pollIntervalAbsentMs;
+
+  if ((uint32_t)(now - lastPollMs_) < pollInterval) {
+    return;
+  }
+  lastPollMs_ = now;
+
+  if (watchState_ == SdWatchState::MOUNTED) {
+    if (verifyMountedCard()) {
+      presentVotes_ = 0;
+      absentVotes_ = 0;
+      return;
+    }
+
+    if (mountPreferred(false) && verifyMountedCard()) {
+      presentVotes_ = 0;
+      absentVotes_ = 0;
+      return;
+    }
+
+    absentVotes_++;
+    presentVotes_ = 0;
+    if (absentVotes_ >= cfg_.votesToAbsent) {
+      setStateAbsent();
+      absentVotes_ = 0;
+    }
+    return;
+  }
+
+  bool mounted = mountPreferred(false);
+  if (!mounted && (uint32_t)(now - lastFullScanMs_) >= cfg_.fullScanIntervalMs) {
+    lastFullScanMs_ = now;
+    if (cfg_.recoveryRailCycleOnFullScan) {
+      logf("Watcher: recovery rail cycle before full scan");
+      cycleSdRail(cfg_.recoveryRailOffMs, cfg_.recoveryRailOnSettleMs);
+      delay(150);
+    }
+    logf("Watcher: preferred probe failed, running full scan");
+    mounted = mountCardFullScan();
+  }
+
+  if (mounted) {
+    presentVotes_++;
+    absentVotes_ = 0;
+    if (presentVotes_ >= cfg_.votesToPresent) {
+      setStateMounted();
+      presentVotes_ = 0;
+    }
+  } else {
+    absentVotes_++;
+    presentVotes_ = 0;
+    if (absentVotes_ >= cfg_.votesToAbsent) {
+      setStateAbsent();
+      absentVotes_ = 0;
+    }
+  }
+}
+
+bool StartupSdManager::consumeMountedEvent() {
+  bool out = mountedEventPending_;
+  mountedEventPending_ = false;
+  return out;
+}
+
+bool StartupSdManager::consumeRemovedEvent() {
+  bool out = removedEventPending_;
+  removedEventPending_ = false;
+  return out;
+}
+
+void StartupSdManager::logf(const char* fmt, ...) {
+  char msg[196];
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(msg, sizeof(msg), fmt, args);
+  va_end(args);
+  serial_.printf("[%10lu][%06lu] %s\r\n",
+                 (unsigned long)millis(),
+                 (unsigned long)logSeq_++,
+                 msg);
+}
+
+void StartupSdManager::notify(SdEvent event, const char* message) {
+  if (callback_ != nullptr) {
+    callback_(event, message);
+  }
+}
+
+void StartupSdManager::forceSpiDeselected() {
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  pinMode(tbeam_supreme::imuCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::imuCs(), HIGH);
+}
+
+void StartupSdManager::dumpSdPins(const char* tag) {
+  if (!cfg_.enablePinDumps) {
+    (void)tag;
+    return;
+  }
+
+  const gpio_num_t cs = (gpio_num_t)tbeam_supreme::sdCs();
+  const gpio_num_t sck = (gpio_num_t)tbeam_supreme::sdSck();
+  const gpio_num_t miso = (gpio_num_t)tbeam_supreme::sdMiso();
+  const gpio_num_t mosi = (gpio_num_t)tbeam_supreme::sdMosi();
+  logf("PINS(%s): CS=%d SCK=%d MISO=%d MOSI=%d",
+       tag, gpio_get_level(cs), gpio_get_level(sck), gpio_get_level(miso), gpio_get_level(mosi));
+}
+
+bool StartupSdManager::initPmuForSdPower() {
+  if (!tbeam_supreme::initPmuForPeripherals(pmu_, &serial_)) {
+    logf("ERROR: PMU init failed");
+    return false;
+  }
+  return true;
+}
+
+void StartupSdManager::cycleSdRail(uint32_t offMs, uint32_t onSettleMs) {
+  if (!cfg_.enableSdRailCycle) {
+    return;
+  }
+  if (!pmu_) {
+    logf("SD rail cycle skipped: pmu=null");
+    return;
+  }
+
+  forceSpiDeselected();
+  pmu_->disablePowerOutput(XPOWERS_BLDO1);
+  delay(offMs);
+  pmu_->setPowerChannelVoltage(XPOWERS_BLDO1, 3300);
+  pmu_->enablePowerOutput(XPOWERS_BLDO1);
+  delay(onSettleMs);
+}
+
+bool StartupSdManager::tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, bool verbose) {
+  SD.end();
+  bus.end();
+  delay(10);
+  forceSpiDeselected();
+
+  bus.begin(tbeam_supreme::sdSck(), tbeam_supreme::sdMiso(), tbeam_supreme::sdMosi(), tbeam_supreme::sdCs());
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  delay(2);
+  for (int i = 0; i < 10; i++) {
+    bus.transfer(0xFF);
+  }
+  delay(2);
+
+  if (verbose) {
+    logf("SD: trying bus=%s freq=%lu Hz", busName, (unsigned long)hz);
+  }
+
+  if (!SD.begin(tbeam_supreme::sdCs(), bus, hz)) {
+    if (verbose) {
+      logf("SD: mount failed (possible non-FAT format, power, or bus issue)");
+    }
+    return false;
+  }
+
+  if (SD.cardType() == CARD_NONE) {
+    SD.end();
+    return false;
+  }
+
+  sdSpi_ = &bus;
+  sdBusName_ = busName;
+  sdFreq_ = hz;
+  return true;
+}
+
+bool StartupSdManager::mountPreferred(bool verbose) {
+  return tryMountWithBus(sdSpiH_, "HSPI", 400000, verbose);
+}
+
+bool StartupSdManager::mountCardFullScan() {
+  const uint32_t freqs[] = {400000, 1000000, 4000000, 10000000};
+
+  for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
+    if (tryMountWithBus(sdSpiH_, "HSPI", freqs[i], true)) {
+      logf("SD: card detected and mounted");
+      return true;
+    }
+  }
+  for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
+    if (tryMountWithBus(sdSpiF_, "FSPI", freqs[i], true)) {
+      logf("SD: card detected and mounted");
+      return true;
+    }
+  }
+
+  logf("SD: begin() failed on all bus/frequency attempts");
+  return false;
+}
+
+bool StartupSdManager::verifyMountedCard() {
+  File root = SD.open("/", FILE_READ);
+  if (!root) {
+    return false;
+  }
+  root.close();
+  return true;
+}
+
+const char* StartupSdManager::cardTypeToString(uint8_t type) {
+  switch (type) {
+    case CARD_MMC:
+      return "MMC";
+    case CARD_SD:
+      return "SDSC";
+    case CARD_SDHC:
+      return "SDHC/SDXC";
+    default:
+      return "UNKNOWN";
+  }
+}
+
+void StartupSdManager::printCardInfo() {
+  uint8_t cardType = SD.cardType();
+  uint64_t cardSizeMB = SD.cardSize() / (1024ULL * 1024ULL);
+  uint64_t totalMB = SD.totalBytes() / (1024ULL * 1024ULL);
+  uint64_t usedMB = SD.usedBytes() / (1024ULL * 1024ULL);
+
+  logf("SD type: %s", cardTypeToString(cardType));
+  logf("SD size: %llu MB", cardSizeMB);
+  logf("FS total: %llu MB", totalMB);
+  logf("FS used : %llu MB", usedMB);
+  logf("SPI bus: %s @ %lu Hz", sdBusName_, (unsigned long)sdFreq_);
+}
+
+bool StartupSdManager::ensureDirRecursive(const char* path) {
+  String full(path);
+  if (!full.startsWith("/")) {
+    full = "/" + full;
+  }
+
+  int start = 1;
+  while (start > 0 && start < (int)full.length()) {
+    int slash = full.indexOf('/', start);
+    String partial = (slash < 0) ? full : full.substring(0, slash);
+    if (!SD.exists(partial.c_str()) && !SD.mkdir(partial.c_str())) {
+      logf("ERROR: mkdir failed for %s", partial.c_str());
+      return false;
+    }
+    if (slash < 0) {
+      break;
+    }
+    start = slash + 1;
+  }
+
+  return true;
+}
+
+bool StartupSdManager::rewriteFile(const char* path, const char* payload) {
+  if (SD.exists(path) && !SD.remove(path)) {
+    logf("ERROR: failed to erase %s", path);
+    return false;
+  }
+
+  File f = SD.open(path, FILE_WRITE);
+  if (!f) {
+    logf("ERROR: failed to create %s", path);
+    return false;
+  }
+
+  size_t wrote = f.println(payload);
+  f.close();
+  if (wrote == 0) {
+    logf("ERROR: write failed for %s", path);
+    return false;
+  }
+  return true;
+}
+
+void StartupSdManager::permissionsDemo(const char* path) {
+  logf("Permissions demo: FAT has no Unix chmod/chown, use open mode only.");
+  File r = SD.open(path, FILE_READ);
+  if (!r) {
+    logf("Could not open %s as FILE_READ", path);
+    return;
+  }
+  size_t writeInReadMode = r.print("attempt write while opened read-only");
+  if (writeInReadMode == 0) {
+    logf("As expected, FILE_READ write was blocked.");
+  } else {
+    logf("NOTE: FILE_READ write returned %u (unexpected)", (unsigned)writeInReadMode);
+  }
+  r.close();
+}
+
+void StartupSdManager::setStateMounted() {
+  if (watchState_ != SdWatchState::MOUNTED) {
+    logf("EVENT: card inserted/mounted");
+    mountedEventPending_ = true;
+    notify(SdEvent::CARD_MOUNTED, "SD card mounted");
+  }
+  watchState_ = SdWatchState::MOUNTED;
+}
+
+void StartupSdManager::setStateAbsent() {
+  if (watchState_ == SdWatchState::MOUNTED) {
+    logf("EVENT: card removed/unavailable");
+    removedEventPending_ = true;
+    notify(SdEvent::CARD_REMOVED, "SD card removed");
+  } else if (watchState_ != SdWatchState::ABSENT) {
+    logf("EVENT: no card detected");
+    notify(SdEvent::NO_CARD, "Missing SD card or invalid FAT16/FAT32 format");
+  }
+  SD.end();
+  watchState_ = SdWatchState::ABSENT;
+}
diff --git a/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.h b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.h
new file mode 100644
index 0000000..be9ef27
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.h
@@ -0,0 +1,90 @@
+#pragma once
+
+#include <Arduino.h>
+#include <SD.h>
+#include <SPI.h>
+#include <Wire.h>
+#include "tbeam_supreme_adapter.h"
+
+enum class SdWatchState : uint8_t {
+  UNKNOWN = 0,
+  ABSENT,
+  MOUNTED
+};
+
+enum class SdEvent : uint8_t {
+  NO_CARD,
+  CARD_MOUNTED,
+  CARD_REMOVED
+};
+
+using SdStatusCallback = void (*)(SdEvent event, const char* message);
+
+struct SdWatcherConfig {
+  bool enableSdRailCycle = true;
+  bool enablePinDumps = true;
+  bool recoveryRailCycleOnFullScan = true;
+  uint32_t recoveryRailOffMs = 250;
+  uint32_t recoveryRailOnSettleMs = 700;
+  uint32_t startupWarmupMs = 1500;
+  uint32_t pollIntervalAbsentMs = 1000;
+  uint32_t pollIntervalMountedMs = 2000;
+  uint32_t fullScanIntervalMs = 10000;
+  uint8_t votesToPresent = 2;
+  uint8_t votesToAbsent = 5;
+};
+
+class StartupSdManager {
+ public:
+  explicit StartupSdManager(Print& serial = Serial);
+
+  bool begin(const SdWatcherConfig& cfg, SdStatusCallback callback = nullptr);
+  void update();
+
+  bool isMounted() const { return watchState_ == SdWatchState::MOUNTED; }
+  SdWatchState state() const { return watchState_; }
+
+  bool consumeMountedEvent();
+  bool consumeRemovedEvent();
+
+  void printCardInfo();
+  bool ensureDirRecursive(const char* path);
+  bool rewriteFile(const char* path, const char* payload);
+  void permissionsDemo(const char* path);
+
+ private:
+  void logf(const char* fmt, ...);
+  void notify(SdEvent event, const char* message);
+  void forceSpiDeselected();
+  void dumpSdPins(const char* tag);
+  bool initPmuForSdPower();
+  void cycleSdRail(uint32_t offMs = 250, uint32_t onSettleMs = 600);
+  bool tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, bool verbose);
+  bool mountPreferred(bool verbose);
+  bool mountCardFullScan();
+  bool verifyMountedCard();
+  const char* cardTypeToString(uint8_t type);
+  void setStateMounted();
+  void setStateAbsent();
+
+  Print& serial_;
+  SdWatcherConfig cfg_{};
+  SdStatusCallback callback_ = nullptr;
+
+  SPIClass sdSpiH_{HSPI};
+  SPIClass sdSpiF_{FSPI};
+  SPIClass* sdSpi_ = nullptr;
+  const char* sdBusName_ = "none";
+  uint32_t sdFreq_ = 0;
+  XPowersLibInterface* pmu_ = nullptr;
+
+  SdWatchState watchState_ = SdWatchState::UNKNOWN;
+  uint8_t presentVotes_ = 0;
+  uint8_t absentVotes_ = 0;
+  uint32_t lastPollMs_ = 0;
+  uint32_t lastFullScanMs_ = 0;
+  uint32_t logSeq_ = 0;
+
+  bool mountedEventPending_ = false;
+  bool removedEventPending_ = false;
+};
diff --git a/exercises/11_Set_RTC2GPS/lib/startup_sd/library.json b/exercises/11_Set_RTC2GPS/lib/startup_sd/library.json
new file mode 100644
index 0000000..4978fdd
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/lib/startup_sd/library.json
@@ -0,0 +1,12 @@
+{
+  "name": "startup_sd",
+  "version": "0.1.0",
+  "dependencies": [
+    {
+      "name": "XPowersLib"
+    },
+    {
+      "name": "Wire"
+    }
+  ]
+}
diff --git a/exercises/11_Set_RTC2GPS/platformio.ini b/exercises/11_Set_RTC2GPS/platformio.ini
new file mode 100644
index 0000000..a91b0bc
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/platformio.ini
@@ -0,0 +1,42 @@
+; 20260213 ChatGPT
+; $Id$
+; $HeadURL$
+
+[platformio]
+default_envs = node_a
+
+[env]
+platform = espressif32
+framework = arduino
+board = esp32-s3-devkitc-1
+monitor_speed = 115200
+extra_scripts = pre:scripts/set_build_epoch.py
+lib_deps =
+  lewisxhe/XPowersLib@0.3.3
+  Wire
+  olikraus/U8g2@^2.36.4
+
+; SD pins based on T-Beam S3 core pin mapping
+build_flags =
+  -I ../../shared/boards
+  -I ../../external/microReticulum_Firmware
+  -D BOARD_MODEL=BOARD_TBEAM_S_V1
+  -D OLED_SDA=17
+  -D OLED_SCL=18
+  -D OLED_ADDR=0x3C
+  -D GPS_RX_PIN=9
+  -D GPS_TX_PIN=8
+  -D GPS_WAKEUP_PIN=7
+  -D GPS_1PPS_PIN=6
+  -D ARDUINO_USB_MODE=1
+  -D ARDUINO_USB_CDC_ON_BOOT=1
+
+[env:node_a]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"A\"
+
+[env:node_b]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"B\"
diff --git a/exercises/11_Set_RTC2GPS/scripts/set_build_epoch.py b/exercises/11_Set_RTC2GPS/scripts/set_build_epoch.py
new file mode 100644
index 0000000..3011129
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/scripts/set_build_epoch.py
@@ -0,0 +1,12 @@
+import time
+Import("env")
+
+epoch = int(time.time())
+utc_tag = time.strftime("%Y%m%d_%H%M%S_z", time.gmtime(epoch))
+
+env.Append(
+    CPPDEFINES=[
+        ("FW_BUILD_EPOCH", str(epoch)),
+        ("FW_BUILD_UTC", '\\"%s\\"' % utc_tag),
+    ]
+)
diff --git a/exercises/11_Set_RTC2GPS/src/main.cpp b/exercises/11_Set_RTC2GPS/src/main.cpp
new file mode 100644
index 0000000..9e54ea7
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/src/main.cpp
@@ -0,0 +1,786 @@
+// 20260217 ChatGPT
+// $Id$
+// $HeadURL$
+
+#include <Arduino.h>
+#include <Wire.h>
+#include <SD.h>
+#include <U8g2lib.h>
+
+#include "StartupSdManager.h"
+#include "tbeam_supreme_adapter.h"
+
+#ifndef OLED_SDA
+#define OLED_SDA 17
+#endif
+
+#ifndef OLED_SCL
+#define OLED_SCL 18
+#endif
+
+#ifndef OLED_ADDR
+#define OLED_ADDR 0x3C
+#endif
+
+#ifndef RTC_I2C_ADDR
+#define RTC_I2C_ADDR 0x51
+#endif
+
+#ifndef GPS_BAUD
+#define GPS_BAUD 9600
+#endif
+
+#ifndef FILE_APPEND
+#define FILE_APPEND FILE_WRITE
+#endif
+
+#ifndef FW_BUILD_EPOCH
+#define FW_BUILD_EPOCH 0
+#endif
+
+#ifndef FW_BUILD_UTC
+#define FW_BUILD_UTC "unknown"
+#endif
+
+static const uint32_t kSerialDelayMs = 5000;
+static const uint32_t kLoopMsDiscipline = 60000;
+static const uint32_t kNoTimeDelayMs = 30000;
+static const uint32_t kGpsStartupProbeMs = 20000;
+static const uint32_t kPpsWaitTimeoutMs = 1500;
+
+static XPowersLibInterface* g_pmu = nullptr;
+static StartupSdManager g_sd(Serial);
+static U8G2_SH1106_128X64_NONAME_F_HW_I2C g_oled(U8G2_R0, /* reset=*/U8X8_PIN_NONE);
+static HardwareSerial g_gpsSerial(1);
+
+static uint32_t g_logSeq = 0;
+static uint32_t g_nextDisciplineMs = 0;
+static bool g_gpsPathReady = false;
+
+static char g_gpsLine[128];
+static size_t g_gpsLineLen = 0;
+
+static volatile uint32_t g_ppsEdgeCount = 0;
+
+struct DateTime {
+  uint16_t year;
+  uint8_t month;
+  uint8_t day;
+  uint8_t hour;
+  uint8_t minute;
+  uint8_t second;
+};
+
+struct GpsState {
+  bool sawAnySentence = false;
+  bool hasValidUtc = false;
+  bool hasValidPosition = false;
+  bool hasValidAltitude = false;
+  uint8_t satsUsed = 0;
+  uint8_t satsInView = 0;
+  float hdop = -1.0f;
+  float altitudeM = 0.0f;
+  double latitudeDeg = 0.0;
+  double longitudeDeg = 0.0;
+  DateTime utc{};
+  uint32_t lastUtcMs = 0;
+};
+
+static GpsState g_gps;
+
+static void logf(const char* fmt, ...) {
+  char msg[240];
+  va_list args;
+  va_start(args, fmt);
+  vsnprintf(msg, sizeof(msg), fmt, args);
+  va_end(args);
+  Serial.printf("[%10lu][%06lu] %s\r\n", (unsigned long)millis(), (unsigned long)g_logSeq++, msg);
+}
+
+static void oledShowLines(const char* l1,
+                          const char* l2 = nullptr,
+                          const char* l3 = nullptr,
+                          const char* l4 = nullptr,
+                          const char* l5 = nullptr) {
+  g_oled.clearBuffer();
+  g_oled.setFont(u8g2_font_5x8_tf);
+  if (l1) g_oled.drawUTF8(0, 12, l1);
+  if (l2) g_oled.drawUTF8(0, 24, l2);
+  if (l3) g_oled.drawUTF8(0, 36, l3);
+  if (l4) g_oled.drawUTF8(0, 48, l4);
+  if (l5) g_oled.drawUTF8(0, 60, l5);
+  g_oled.sendBuffer();
+}
+
+static uint8_t toBcd(uint8_t v) {
+  return (uint8_t)(((v / 10U) << 4U) | (v % 10U));
+}
+
+static uint8_t fromBcd(uint8_t b) {
+  return (uint8_t)(((b >> 4U) * 10U) + (b & 0x0FU));
+}
+
+static bool rtcRead(DateTime& out, bool& lowVoltageFlag) {
+  Wire1.beginTransmission(RTC_I2C_ADDR);
+  Wire1.write(0x02);
+  if (Wire1.endTransmission(false) != 0) {
+    return false;
+  }
+
+  const uint8_t need = 7;
+  uint8_t got = Wire1.requestFrom((int)RTC_I2C_ADDR, (int)need);
+  if (got != need) {
+    return false;
+  }
+
+  uint8_t sec = Wire1.read();
+  uint8_t min = Wire1.read();
+  uint8_t hour = Wire1.read();
+  uint8_t day = Wire1.read();
+  (void)Wire1.read();
+  uint8_t month = Wire1.read();
+  uint8_t year = Wire1.read();
+
+  lowVoltageFlag = (sec & 0x80U) != 0;
+  out.second = fromBcd(sec & 0x7FU);
+  out.minute = fromBcd(min & 0x7FU);
+  out.hour = fromBcd(hour & 0x3FU);
+  out.day = fromBcd(day & 0x3FU);
+  out.month = fromBcd(month & 0x1FU);
+  uint8_t yy = fromBcd(year);
+  bool century = (month & 0x80U) != 0;
+  out.year = century ? (1900U + yy) : (2000U + yy);
+  return true;
+}
+
+static bool rtcWrite(const DateTime& dt) {
+  Wire1.beginTransmission(RTC_I2C_ADDR);
+  Wire1.write(0x02);
+  Wire1.write(toBcd(dt.second & 0x7FU));
+  Wire1.write(toBcd(dt.minute));
+  Wire1.write(toBcd(dt.hour));
+  Wire1.write(toBcd(dt.day));
+  Wire1.write(0x00);
+
+  uint8_t monthReg = toBcd(dt.month);
+  if (dt.year < 2000U) {
+    monthReg |= 0x80U;
+  }
+  Wire1.write(monthReg);
+  Wire1.write(toBcd((uint8_t)(dt.year % 100U)));
+
+  return Wire1.endTransmission() == 0;
+}
+
+static bool isLeapYear(uint16_t y) {
+  return ((y % 4U) == 0U && (y % 100U) != 0U) || ((y % 400U) == 0U);
+}
+
+static uint8_t daysInMonth(uint16_t year, uint8_t month) {
+  static const uint8_t kDays[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
+  if (month == 2) {
+    return (uint8_t)(isLeapYear(year) ? 29 : 28);
+  }
+  if (month >= 1 && month <= 12) {
+    return kDays[month - 1];
+  }
+  return 30;
+}
+
+static bool isValidDateTime(const DateTime& dt) {
+  if (dt.year < 2000U || dt.year > 2099U) return false;
+  if (dt.month < 1 || dt.month > 12) return false;
+  if (dt.day < 1 || dt.day > daysInMonth(dt.year, dt.month)) return false;
+  if (dt.hour > 23 || dt.minute > 59 || dt.second > 59) return false;
+  return true;
+}
+
+static int64_t daysFromCivil(int y, unsigned m, unsigned d) {
+  y -= (m <= 2);
+  const int era = (y >= 0 ? y : y - 399) / 400;
+  const unsigned yoe = (unsigned)(y - era * 400);
+  const unsigned doy = (153 * (m + (m > 2 ? (unsigned)-3 : 9)) + 2) / 5 + d - 1;
+  const unsigned doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+  return era * 146097 + (int)doe - 719468;
+}
+
+static int64_t toEpochSeconds(const DateTime& dt) {
+  int64_t days = daysFromCivil((int)dt.year, dt.month, dt.day);
+  return days * 86400LL + (int64_t)dt.hour * 3600LL + (int64_t)dt.minute * 60LL + (int64_t)dt.second;
+}
+
+static bool fromEpochSeconds(int64_t sec, DateTime& out) {
+  if (sec < 0) {
+    return false;
+  }
+
+  int64_t days = sec / 86400LL;
+  int64_t rem = sec % 86400LL;
+  if (rem < 0) {
+    rem += 86400LL;
+    days -= 1;
+  }
+
+  out.hour = (uint8_t)(rem / 3600LL);
+  rem %= 3600LL;
+  out.minute = (uint8_t)(rem / 60LL);
+  out.second = (uint8_t)(rem % 60LL);
+
+  days += 719468;
+  const int era = (days >= 0 ? days : days - 146096) / 146097;
+  const unsigned doe = (unsigned)(days - era * 146097);
+  const unsigned yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
+  int y = (int)yoe + era * 400;
+  const unsigned doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+  const unsigned mp = (5 * doy + 2) / 153;
+  const unsigned d = doy - (153 * mp + 2) / 5 + 1;
+  const unsigned m = mp + (mp < 10 ? 3 : (unsigned)-9);
+  y += (m <= 2);
+
+  out.year = (uint16_t)y;
+  out.month = (uint8_t)m;
+  out.day = (uint8_t)d;
+  return isValidDateTime(out);
+}
+
+static void addOneSecond(DateTime& dt) {
+  int64_t t = toEpochSeconds(dt);
+  DateTime out{};
+  if (fromEpochSeconds(t + 1, out)) {
+    dt = out;
+  }
+}
+
+static void formatUtcCompact(const DateTime& dt, char* out, size_t outLen) {
+  snprintf(out,
+           outLen,
+           "%04u%02u%02u_%02u%02u%02u_z",
+           (unsigned)dt.year,
+           (unsigned)dt.month,
+           (unsigned)dt.day,
+           (unsigned)dt.hour,
+           (unsigned)dt.minute,
+           (unsigned)dt.second);
+}
+
+static void formatUtcHuman(const DateTime& dt, char* out, size_t outLen) {
+  snprintf(out,
+           outLen,
+           "%04u-%02u-%02u %02u:%02u:%02u UTC",
+           (unsigned)dt.year,
+           (unsigned)dt.month,
+           (unsigned)dt.day,
+           (unsigned)dt.hour,
+           (unsigned)dt.minute,
+           (unsigned)dt.second);
+}
+
+static bool parseUInt2(const char* s, uint8_t& out) {
+  if (!s || !isdigit((unsigned char)s[0]) || !isdigit((unsigned char)s[1])) {
+    return false;
+  }
+  out = (uint8_t)((s[0] - '0') * 10 + (s[1] - '0'));
+  return true;
+}
+
+static bool parseNmeaCoordToDecimal(const char* raw, const char* hemi, bool isLat, double& outDeg) {
+  if (!raw || !hemi || raw[0] == '\0' || hemi[0] == '\0') {
+    return false;
+  }
+
+  // NMEA uses ddmm.mmmm (lat) and dddmm.mmmm (lon), with leading zeros preserved.
+  // Parse from string slices so longitudes like 071xx.xxxx do not collapse to 7xx.xxxx.
+  int degDigits = isLat ? 2 : 3;
+  size_t n = strlen(raw);
+  if (n <= (size_t)degDigits + 2) {
+    return false;
+  }
+
+  for (int i = 0; i < degDigits; ++i) {
+    if (!isdigit((unsigned char)raw[i])) {
+      return false;
+    }
+  }
+
+  char degBuf[4] = {0};
+  memcpy(degBuf, raw, degDigits);
+  int deg = atoi(degBuf);
+
+  const char* minPtr = raw + degDigits;
+  double minutes = atof(minPtr);
+  if (minutes < 0.0 || minutes >= 60.0) {
+    return false;
+  }
+
+  double dec = (double)deg + (minutes / 60.0);
+  char h = (char)toupper((unsigned char)hemi[0]);
+  if (h == 'S' || h == 'W') {
+    dec = -dec;
+  } else if (h != 'N' && h != 'E') {
+    return false;
+  }
+
+  outDeg = dec;
+  return true;
+}
+
+static void parseGga(char* fields[], int count) {
+  if (count <= 7) {
+    return;
+  }
+  const char* latRaw = (count > 2) ? fields[2] : nullptr;
+  const char* latHem = (count > 3) ? fields[3] : nullptr;
+  const char* lonRaw = (count > 4) ? fields[4] : nullptr;
+  const char* lonHem = (count > 5) ? fields[5] : nullptr;
+  int sats = atoi(fields[7]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsUsed = (uint8_t)sats;
+  }
+  if (count > 8 && fields[8] && fields[8][0] != '\0') {
+    g_gps.hdop = (float)atof(fields[8]);
+  }
+  if (count > 9 && fields[9] && fields[9][0] != '\0') {
+    g_gps.altitudeM = (float)atof(fields[9]);
+    g_gps.hasValidAltitude = true;
+  }
+
+  // Position fallback from GGA so we still log coordinates if RMC position is missing.
+  double lat = 0.0;
+  double lon = 0.0;
+  if (parseNmeaCoordToDecimal(latRaw, latHem, true, lat) &&
+      parseNmeaCoordToDecimal(lonRaw, lonHem, false, lon)) {
+    g_gps.latitudeDeg = lat;
+    g_gps.longitudeDeg = lon;
+    g_gps.hasValidPosition = true;
+  }
+}
+
+static void parseGsv(char* fields[], int count) {
+  if (count <= 3) {
+    return;
+  }
+  int sats = atoi(fields[3]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsInView = (uint8_t)sats;
+  }
+}
+
+static void parseRmc(char* fields[], int count) {
+  if (count <= 9) {
+    return;
+  }
+
+  const char* utc = fields[1];
+  const char* status = fields[2];
+  const char* latRaw = fields[3];
+  const char* latHem = fields[4];
+  const char* lonRaw = fields[5];
+  const char* lonHem = fields[6];
+  const char* date = fields[9];
+
+  if (!status || status[0] != 'A') {
+    return;
+  }
+
+  if (!utc || strlen(utc) < 6 || !date || strlen(date) < 6) {
+    return;
+  }
+
+  uint8_t hh = 0, mm = 0, ss = 0;
+  uint8_t dd = 0, mo = 0, yy = 0;
+  if (!parseUInt2(utc + 0, hh) || !parseUInt2(utc + 2, mm) || !parseUInt2(utc + 4, ss)) {
+    return;
+  }
+  if (!parseUInt2(date + 0, dd) || !parseUInt2(date + 2, mo) || !parseUInt2(date + 4, yy)) {
+    return;
+  }
+
+  g_gps.utc.hour = hh;
+  g_gps.utc.minute = mm;
+  g_gps.utc.second = ss;
+  g_gps.utc.day = dd;
+  g_gps.utc.month = mo;
+  g_gps.utc.year = (uint16_t)(2000U + yy);
+  g_gps.hasValidUtc = true;
+  g_gps.lastUtcMs = millis();
+
+  double lat = 0.0;
+  double lon = 0.0;
+  if (parseNmeaCoordToDecimal(latRaw, latHem, true, lat) &&
+      parseNmeaCoordToDecimal(lonRaw, lonHem, false, lon)) {
+    g_gps.latitudeDeg = lat;
+    g_gps.longitudeDeg = lon;
+    g_gps.hasValidPosition = true;
+  }
+}
+
+static void processNmeaLine(char* line) {
+  if (!line || line[0] != '$') {
+    return;
+  }
+
+  g_gps.sawAnySentence = true;
+
+  char* star = strchr(line, '*');
+  if (star) {
+    *star = '\0';
+  }
+
+  char* fields[24] = {0};
+  int count = 0;
+  char* saveptr = nullptr;
+  char* tok = strtok_r(line, ",", &saveptr);
+  while (tok && count < 24) {
+    fields[count++] = tok;
+    tok = strtok_r(nullptr, ",", &saveptr);
+  }
+
+  if (count <= 0 || !fields[0]) {
+    return;
+  }
+
+  const char* header = fields[0];
+  size_t n = strlen(header);
+  if (n < 6) {
+    return;
+  }
+
+  const char* type = header + (n - 3);
+  if (strcmp(type, "GGA") == 0) {
+    parseGga(fields, count);
+  } else if (strcmp(type, "GSV") == 0) {
+    parseGsv(fields, count);
+  } else if (strcmp(type, "RMC") == 0) {
+    parseRmc(fields, count);
+  }
+}
+
+static void pollGpsSerial() {
+  while (g_gpsSerial.available() > 0) {
+    char c = (char)g_gpsSerial.read();
+    if (c == '\r') {
+      continue;
+    }
+    if (c == '\n') {
+      if (g_gpsLineLen > 0) {
+        g_gpsLine[g_gpsLineLen] = '\0';
+        processNmeaLine(g_gpsLine);
+        g_gpsLineLen = 0;
+      }
+      continue;
+    }
+
+    if (g_gpsLineLen + 1 < sizeof(g_gpsLine)) {
+      g_gpsLine[g_gpsLineLen++] = c;
+    } else {
+      g_gpsLineLen = 0;
+    }
+  }
+}
+
+static bool collectGpsTraffic(uint32_t windowMs) {
+  uint32_t start = millis();
+  bool sawBytes = false;
+  while ((uint32_t)(millis() - start) < windowMs) {
+    if (g_gpsSerial.available() > 0) {
+      sawBytes = true;
+    }
+    pollGpsSerial();
+    g_sd.update();
+    delay(2);
+  }
+  return sawBytes || g_gps.sawAnySentence;
+}
+
+static void initialGpsProbe() {
+  logf("GPS startup probe at %u baud", (unsigned)GPS_BAUD);
+  (void)collectGpsTraffic(kGpsStartupProbeMs);
+  logf("GPS probe complete: nmea=%s sats_used=%u sats_view=%u utc=%s",
+       g_gps.sawAnySentence ? "yes" : "no",
+       (unsigned)g_gps.satsUsed,
+       (unsigned)g_gps.satsInView,
+       g_gps.hasValidUtc ? "yes" : "no");
+}
+
+static IRAM_ATTR void onPpsEdge() {
+  g_ppsEdgeCount++;
+}
+
+static uint8_t bestSatelliteCount() {
+  return (g_gps.satsUsed > g_gps.satsInView) ? g_gps.satsUsed : g_gps.satsInView;
+}
+
+static bool ensureGpsLogPathReady() {
+  if (!g_sd.isMounted()) {
+    g_gpsPathReady = false;
+    return false;
+  }
+
+  if (g_gpsPathReady) {
+    return true;
+  }
+
+  if (!g_sd.ensureDirRecursive("/gps")) {
+    logf("Could not create /gps directory");
+    return false;
+  }
+
+  // Touch the log file so a clean SD card is prepared before first discipline event.
+  File f = SD.open("/gps/discipline_rtc.log", FILE_APPEND);
+  if (!f) {
+    logf("Could not open /gps/discipline_rtc.log for append");
+    return false;
+  }
+  f.close();
+
+  g_gpsPathReady = true;
+  return true;
+}
+
+static bool appendDisciplineLog(const DateTime& gpsUtc,
+                                bool havePriorRtc,
+                                int64_t rtcMinusGpsSeconds,
+                                uint8_t sats,
+                                uint32_t utcAgeMs,
+                                uint32_t ppsEdges,
+                                char* outTs,
+                                size_t outTsLen) {
+  if (!ensureGpsLogPathReady()) {
+    logf("SD not mounted, skipping append to gps/discipline_rtc.log");
+    return false;
+  }
+
+  File f = SD.open("/gps/discipline_rtc.log", FILE_APPEND);
+  if (!f) {
+    logf("Could not open /gps/discipline_rtc.log for append");
+    return false;
+  }
+
+  char ts[32];
+  formatUtcCompact(gpsUtc, ts, sizeof(ts));
+  if (outTs && outTsLen > 0) {
+    snprintf(outTs, outTsLen, "%s", ts);
+  }
+
+  char drift[40];
+  if (havePriorRtc) {
+    snprintf(drift, sizeof(drift), "%+lld s", (long long)rtcMinusGpsSeconds);
+  } else {
+    snprintf(drift, sizeof(drift), "RTC_unset");
+  }
+
+  char pos[64];
+  if (g_gps.hasValidPosition) {
+    snprintf(pos, sizeof(pos), "lat=%.6f lon=%.6f", g_gps.latitudeDeg, g_gps.longitudeDeg);
+  } else {
+    snprintf(pos, sizeof(pos), "lat=NA lon=NA");
+  }
+
+  char hdop[16];
+  if (g_gps.hdop > 0.0f) {
+    snprintf(hdop, sizeof(hdop), "%.1f", g_gps.hdop);
+  } else {
+    snprintf(hdop, sizeof(hdop), "NA");
+  }
+
+  char alt[16];
+  if (g_gps.hasValidAltitude) {
+    snprintf(alt, sizeof(alt), "%.1f", g_gps.altitudeM);
+  } else {
+    snprintf(alt, sizeof(alt), "NA");
+  }
+
+  char line[320];
+  snprintf(line,
+           sizeof(line),
+           "%s\t set RTC to GPS using 1PPS pulse-per-second discipline\t"
+           "rtc-gps drift=%s; sats=%u; %s; alt_m=%s; hdop=%s; utc_age_ms=%lu; pps_edges=%lu; "
+           "fw_epoch=%lu; fw_build_utc=%s",
+           ts,
+           drift,
+           (unsigned)sats,
+           pos,
+           alt,
+           hdop,
+           (unsigned long)utcAgeMs,
+           (unsigned long)ppsEdges,
+           (unsigned long)FW_BUILD_EPOCH,
+           FW_BUILD_UTC);
+
+  size_t wrote = f.println(line);
+  f.close();
+  if (wrote == 0) {
+    logf("Append write failed: /gps/discipline_rtc.log");
+    return false;
+  }
+  return true;
+}
+
+static bool gpsUtcIsFresh() {
+  if (!g_gps.hasValidUtc) {
+    return false;
+  }
+  return (uint32_t)(millis() - g_gps.lastUtcMs) <= 2000;
+}
+
+static bool waitForNextPps(uint32_t timeoutMs) {
+  uint32_t startCount = g_ppsEdgeCount;
+  uint32_t startMs = millis();
+  while ((uint32_t)(millis() - startMs) < timeoutMs) {
+    pollGpsSerial();
+    g_sd.update();
+    if (g_ppsEdgeCount != startCount) {
+      return true;
+    }
+    delay(2);
+  }
+  return false;
+}
+
+static void waitWithUpdates(uint32_t delayMs) {
+  uint32_t start = millis();
+  while ((uint32_t)(millis() - start) < delayMs) {
+    pollGpsSerial();
+    g_sd.update();
+    delay(10);
+  }
+}
+
+static void showNoTimeAndDelay() {
+  uint8_t sats = bestSatelliteCount();
+  char l3[24];
+  snprintf(l3, sizeof(l3), "Satellites: %u", (unsigned)sats);
+  oledShowLines("GPS time unavailable", "RTC NOT disciplined", l3, "Retry in 30 seconds");
+  logf("GPS UTC unavailable. satellites=%u. Waiting 30 seconds.", (unsigned)sats);
+  waitWithUpdates(kNoTimeDelayMs);
+}
+
+static bool disciplineRtcToGps() {
+  if (!gpsUtcIsFresh()) {
+    showNoTimeAndDelay();
+    return false;
+  }
+
+  DateTime priorRtc{};
+  bool lowV = false;
+  bool havePriorRtc = rtcRead(priorRtc, lowV);
+  if (havePriorRtc && (lowV || !isValidDateTime(priorRtc))) {
+    havePriorRtc = false;
+  }
+
+  DateTime gpsSnap = g_gps.utc;
+  if (!waitForNextPps(kPpsWaitTimeoutMs)) {
+    oledShowLines("GPS 1PPS missing", "RTC NOT disciplined", "Retry in 30 seconds");
+    logf("No 1PPS edge observed within timeout. Waiting 30 seconds.");
+    waitWithUpdates(kNoTimeDelayMs);
+    return false;
+  }
+
+  DateTime target = gpsSnap;
+  addOneSecond(target);
+
+  if (!rtcWrite(target)) {
+    oledShowLines("RTC write failed", "Could not set from GPS");
+    logf("RTC write failed");
+    return false;
+  }
+
+  int64_t driftSec = 0;
+  if (havePriorRtc) {
+    driftSec = toEpochSeconds(priorRtc) - toEpochSeconds(target);
+  }
+
+  uint8_t sats = bestSatelliteCount();
+  uint32_t utcAgeMs = (uint32_t)(millis() - g_gps.lastUtcMs);
+  uint32_t ppsEdges = g_ppsEdgeCount;
+  char tsCompact[32];
+  bool logOk = appendDisciplineLog(target,
+                                   havePriorRtc,
+                                   driftSec,
+                                   sats,
+                                   utcAgeMs,
+                                   ppsEdges,
+                                   tsCompact,
+                                   sizeof(tsCompact));
+
+  char utcLine[36];
+  char driftLine[36];
+  char logLine[36];
+  formatUtcHuman(target, utcLine, sizeof(utcLine));
+  if (havePriorRtc) {
+    snprintf(driftLine, sizeof(driftLine), "rtc-gps drift: %+lld s", (long long)driftSec);
+  } else {
+    snprintf(driftLine, sizeof(driftLine), "rtc-gps drift: RTC_unset");
+  }
+  snprintf(logLine, sizeof(logLine), "Log:%s sats:%u", logOk ? "ok" : "fail", (unsigned)sats);
+
+  oledShowLines("RTC disciplined to GPS", utcLine, driftLine, logLine, tsCompact);
+
+  logf("RTC disciplined to GPS with 1PPS. %s drift=%+llds lowV=%s",
+       utcLine,
+       (long long)driftSec,
+       lowV ? "yes" : "no");
+  return true;
+}
+
+void setup() {
+  Serial.begin(115200);
+  delay(kSerialDelayMs);
+
+  Serial.println("\r\n==================================================");
+  Serial.println("Exercise 11: Set RTC to GPS with 1PPS discipline");
+  Serial.println("==================================================");
+
+  if (!tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial)) {
+    logf("PMU init failed");
+  }
+
+  Wire.begin(OLED_SDA, OLED_SCL);
+  g_oled.setI2CAddress(OLED_ADDR << 1);
+  g_oled.begin();
+  oledShowLines("Exercise 11", "RTC <- GPS (1PPS)", "Booting...");
+
+  SdWatcherConfig sdCfg{};
+  if (!g_sd.begin(sdCfg, nullptr)) {
+    logf("SD startup manager begin() failed");
+  }
+  (void)ensureGpsLogPathReady();
+
+#ifdef GPS_WAKEUP_PIN
+  pinMode(GPS_WAKEUP_PIN, INPUT);
+#endif
+#ifdef GPS_1PPS_PIN
+  pinMode(GPS_1PPS_PIN, INPUT);
+  attachInterrupt(digitalPinToInterrupt(GPS_1PPS_PIN), onPpsEdge, RISING);
+#endif
+
+  g_gpsSerial.setRxBufferSize(1024);
+  g_gpsSerial.begin(GPS_BAUD, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+  logf("GPS UART started: RX=%d TX=%d baud=%u", GPS_RX_PIN, GPS_TX_PIN, (unsigned)GPS_BAUD);
+
+  oledShowLines("GPS startup probe", "Checking UTC + 1PPS");
+  initialGpsProbe();
+
+  g_nextDisciplineMs = millis();
+}
+
+void loop() {
+  pollGpsSerial();
+  g_sd.update();
+
+  if (g_sd.consumeMountedEvent()) {
+    g_gpsPathReady = false;
+    (void)ensureGpsLogPathReady();
+  }
+  if (g_sd.consumeRemovedEvent()) {
+    g_gpsPathReady = false;
+  }
+
+  uint32_t now = millis();
+  if ((int32_t)(now - g_nextDisciplineMs) >= 0) {
+    bool ok = disciplineRtcToGps();
+    g_nextDisciplineMs = now + (ok ? kLoopMsDiscipline : kNoTimeDelayMs);
+  }
+
+  delay(5);
+}