diff --git a/exercises/18_GPS_Field_QA/lib/field_qa/Config.h b/exercises/18_GPS_Field_QA/lib/field_qa/Config.h
index 3546029..04232f9 100644
--- a/exercises/18_GPS_Field_QA/lib/field_qa/Config.h
+++ b/exercises/18_GPS_Field_QA/lib/field_qa/Config.h
@@ -50,6 +50,10 @@
 #define LOG_AP_IP_OCTET 23
 #endif
 
+#ifndef GNSS_DIAG
+#define GNSS_DIAG 0
+#endif
+
 #define FIELD_QA_STR_INNER(x) #x
 #define FIELD_QA_STR(x) FIELD_QA_STR_INNER(x)
 
diff --git a/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.cpp b/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.cpp
index 2269bd4..4b37985 100644
--- a/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.cpp
+++ b/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.cpp
@@ -2,6 +2,7 @@
 
 #include <ctype.h>
 #include <math.h>
+#include <stdarg.h>
 #include <string.h>
 #include "Config.h"
 
@@ -57,6 +58,20 @@ static void copyTalker(const char* header, char* out) {
   out[2] = '\0';
 }
 
+static void gnssDiagf(const char* fmt, ...) {
+#if GNSS_DIAG
+  char buf[192];
+  va_list ap;
+  va_start(ap, fmt);
+  vsnprintf(buf, sizeof(buf), fmt, ap);
+  va_end(ap);
+  Serial.print("[GNSS] ");
+  Serial.println(buf);
+#else
+  (void)fmt;
+#endif
+}
+
 }  // namespace
 
 void GnssManager::begin() {
@@ -196,6 +211,13 @@ void GnssManager::parseGsa(char* fields[], int count) {
   if (count < 18) {
     return;
   }
+  const uint32_t now = millis();
+  if ((uint32_t)(now - m_lastGsaMs) > 1500U) {
+    m_usedPrnCount = 0;
+    memset(m_usedPrns, 0, sizeof(m_usedPrns));
+  }
+  m_lastGsaMs = now;
+  gnssDiagf("GSA %s", fields[0] ? fields[0] : "?");
   const int dim = atoi(fields[2]);
   m_state.fixDimension = dim;
   if (count > 15 && fields[15] && fields[15][0]) {
@@ -212,13 +234,10 @@ void GnssManager::parseGsa(char* fields[], int count) {
   }
 
   int satsUsed = 0;
-  m_usedPrnCount = 0;
   for (int i = 3; i <= 14 && i < count; ++i) {
     if (fields[i] && fields[i][0]) {
       ++satsUsed;
-      if (m_usedPrnCount < sizeof(m_usedPrns) / sizeof(m_usedPrns[0])) {
-        m_usedPrns[m_usedPrnCount++] = (uint8_t)atoi(fields[i]);
-      }
+      addUsedPrn((uint8_t)atoi(fields[i]));
     }
   }
   if (satsUsed > 0) {
@@ -247,6 +266,36 @@ void GnssManager::clearSatelliteView() {
   m_state.maxSnr = 0;
 }
 
+void GnssManager::clearTalkerSatellites(const char* talker) {
+  if (!talker || !talker[0]) {
+    return;
+  }
+
+  size_t write = 0;
+  size_t removed = 0;
+  for (size_t read = 0; read < m_satCount; ++read) {
+    SatelliteInfo sat = m_satellites[read];
+    if (sat.valid && strcmp(sat.talker, talker) == 0) {
+      ++removed;
+      continue;
+    }
+    if (write != read) {
+      m_satellites[write] = sat;
+    }
+    ++write;
+  }
+  for (size_t i = write; i < m_satCount; ++i) {
+    m_satellites[i] = SatelliteInfo{};
+  }
+  if (removed > 0) {
+    gnssDiagf("clear talker=%s removed=%u remaining=%u",
+              talker,
+              (unsigned)removed,
+              (unsigned)write);
+  }
+  m_satCount = write;
+}
+
 void GnssManager::finalizeSatelliteStats() {
   uint32_t snrSum = 0;
   uint32_t snrCount = 0;
@@ -282,6 +331,20 @@ void GnssManager::finalizeSatelliteStats() {
   m_state.meanSnr = snrCount > 0 ? ((float)snrSum / (float)snrCount) : -1.0f;
 }
 
+void GnssManager::addUsedPrn(uint8_t prn) {
+  if (prn == 0) {
+    return;
+  }
+  for (size_t i = 0; i < m_usedPrnCount; ++i) {
+    if (m_usedPrns[i] == prn) {
+      return;
+    }
+  }
+  if (m_usedPrnCount < sizeof(m_usedPrns) / sizeof(m_usedPrns[0])) {
+    m_usedPrns[m_usedPrnCount++] = prn;
+  }
+}
+
 void GnssManager::parseGsv(char* fields[], int count) {
   if (count < 4) {
     return;
@@ -290,7 +353,14 @@ void GnssManager::parseGsv(char* fields[], int count) {
   const int msgNum = atoi(fields[2]);
   const int satsInView = atoi(fields[3]);
   if (msgNum == 1) {
-    clearSatelliteView();
+    char talker[3];
+    copyTalker(fields[0], talker);
+    gnssDiagf("GSV %s %d/%d sats_in_view=%d", talker, msgNum, totalMsgs, satsInView);
+    clearTalkerSatellites(talker);
+  } else {
+    char talker[3];
+    copyTalker(fields[0], talker);
+    gnssDiagf("GSV %s %d/%d sats_in_view=%d", talker, msgNum, totalMsgs, satsInView);
   }
   if (satsInView >= 0) {
     m_state.satsInView = satsInView;
@@ -393,6 +463,9 @@ void GnssManager::processNmeaLine(char* line) {
   }
   m_sawSentence = true;
   m_state.sawSentence = true;
+  if (strstr(line, "GSV") || strstr(line, "GSA")) {
+    gnssDiagf("raw %s", line);
+  }
   char* star = strchr(line, '*');
   if (star) {
     *star = '\0';
diff --git a/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.h b/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.h
index 8b3d204..0c5a94f 100644
--- a/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.h
+++ b/exercises/18_GPS_Field_QA/lib/field_qa/GnssManager.h
@@ -30,7 +30,9 @@ class GnssManager {
   static bool parseUInt2(const char* s, uint8_t& out);
   static double parseNmeaCoord(const char* value, const char* hemi);
   void clearSatelliteView();
+  void clearTalkerSatellites(const char* talker);
   void finalizeSatelliteStats();
+  void addUsedPrn(uint8_t prn);
   bool prnUsedInSolution(uint8_t prn) const;
 
   HardwareSerial m_serial{1};
@@ -43,9 +45,10 @@ class GnssManager {
 
   GnssSample m_state;
   SatelliteInfo m_satellites[kMaxSatellites];
-  uint8_t m_usedPrns[16] = {0};
+  uint8_t m_usedPrns[kMaxSatellites] = {0};
   size_t m_usedPrnCount = 0;
   size_t m_satCount = 0;
+  uint32_t m_lastGsaMs = 0;
   uint32_t m_lastGsvMs = 0;
   uint32_t m_lastFixMs = 0;
   uint32_t m_bootMs = 0;
diff --git a/exercises/18_GPS_Field_QA/src/main.cpp b/exercises/18_GPS_Field_QA/src/main.cpp
index 5c7b179..15a2c20 100644
--- a/exercises/18_GPS_Field_QA/src/main.cpp
+++ b/exercises/18_GPS_Field_QA/src/main.cpp
@@ -83,6 +83,42 @@ void IRAM_ATTR onPpsEdge() {
 
 String htmlEscape(const String& in);
 
+void logGnssTalkerCounts(uint32_t sampleSeq, const SatelliteInfo* sats, size_t satCount) {
+#if GNSS_DIAG
+  size_t gp = 0, ga = 0, gl = 0, gb = 0, gi = 0, gq = 0, gs = 0, gn = 0, other = 0;
+  for (size_t i = 0; i < satCount; ++i) {
+    if (!sats[i].valid) {
+      continue;
+    }
+    if (strcmp(sats[i].talker, "GP") == 0) ++gp;
+    else if (strcmp(sats[i].talker, "GA") == 0) ++ga;
+    else if (strcmp(sats[i].talker, "GL") == 0) ++gl;
+    else if (strcmp(sats[i].talker, "GB") == 0 || strcmp(sats[i].talker, "BD") == 0) ++gb;
+    else if (strcmp(sats[i].talker, "GI") == 0) ++gi;
+    else if (strcmp(sats[i].talker, "GQ") == 0) ++gq;
+    else if (strcmp(sats[i].talker, "GS") == 0) ++gs;
+    else if (strcmp(sats[i].talker, "GN") == 0) ++gn;
+    else ++other;
+  }
+  Serial.printf("[GNSS] flush sample=%lu satCount=%u GP=%u GA=%u GL=%u GB=%u GI=%u GQ=%u GS=%u GN=%u other=%u\n",
+                (unsigned long)sampleSeq,
+                (unsigned)satCount,
+                (unsigned)gp,
+                (unsigned)ga,
+                (unsigned)gl,
+                (unsigned)gb,
+                (unsigned)gi,
+                (unsigned)gq,
+                (unsigned)gs,
+                (unsigned)gn,
+                (unsigned)other);
+#else
+  (void)sampleSeq;
+  (void)sats;
+  (void)satCount;
+#endif
+}
+
 void recordEvent(const char* fmt, ...) {
   char msg[72];
   va_list args;
@@ -439,6 +475,7 @@ void sampleAndMaybeLog() {
     const uint32_t msSinceRunStart = millis() - g_runStartMs;
     SatelliteInfo sats[kMaxSatellites];
     const size_t satCount = g_gnss.copySatellites(sats, kMaxSatellites);
+    logGnssTalkerCounts(sampleSeq, sats, satCount);
     if (!g_storage.appendSampleCsv(sample, sampleSeq, msSinceRunStart, g_runId, g_bootTimestampUtc)) {
       stopLoggingForStorageFailure("append_sample");
     } else if (satCount > 0 &&