diff --git a/extras/tests/marine_sources/marine_sources.ino b/extras/tests/marine_sources/marine_sources.ino
new file mode 100644
index 0000000..f1fb048
--- /dev/null
+++ b/extras/tests/marine_sources/marine_sources.ino
@@ -0,0 +1,86 @@
+#include <Adafruit_GPS.h>
+
+Adafruit_GPS GPS;
+bool testsPassed = false;
+
+void setup() {
+  Serial.begin(115200);
+  // Wait briefly for the Serial Monitor on native USB boards.
+  while (!Serial && millis() < 5000) {
+    delay(10);
+  }
+  delay(250);
+  Serial.println(F("Adafruit GPS marine source regression"));
+
+  // Supported sentence types are independent of the source identifier.
+  // Exercise the new identifiers and every existing two-letter source.
+  const char sources[][3] = {
+    "HC", "TI", "SD", "AI", "II", "WI", "GP", "PG", "GL", "GA", "GN"
+  };
+  for (uint8_t i = 0; i < sizeof(sources) / sizeof(sources[0]); i++) {
+    char sentence[80] = "$GPGLL,4916.45,N,12311.12,W,225444,A";
+    sentence[1] = sources[i][0];
+    sentence[2] = sources[i][1];
+    if (!parseSentence(sentence, sources[i], "GLL")) {
+      return;
+    }
+  }
+
+  // PG must still be recognized before the shorter proprietary P prefix.
+  char legacy[24] = "$PGTOP,11,2";
+  if (!parseSentence(legacy, "PG", "TOP") || GPS.antenna != 2) {
+    return;
+  }
+  char proprietary[24] = "$PCD,11,1";
+  if (!parseSentence(proprietary, "P", "CD") || GPS.antenna != 2) {
+    return;
+  }
+
+  char unknown[80] = "$XXGLL,4916.45,N,12311.12,W,225444,A";
+  GPS.addChecksum(unknown);
+  if (GPS.parse(unknown)) {
+    Serial.println(F("FAIL: unknown source was accepted"));
+    return;
+  }
+  Serial.println(F("PASS: unknown source is rejected"));
+
+#ifdef NMEA_EXTENSIONS
+  // Typical magnetic heading, water temperature, and sounder sentences.
+  char compass[32] = "$HCHDM,238.5,M";
+  if (!parseSentence(compass, "HC", "HDM")) {
+    return;
+  }
+  char temperature[32] = "$TIMTW,18.5,C";
+  if (!parseSentence(temperature, "TI", "MTW")) {
+    return;
+  }
+  char depth[48] = "$SDDBT,32.8,f,10.0,M,5.5,F";
+  if (!parseSentence(depth, "SD", "DBT")) {
+    return;
+  }
+  Serial.println(F("PASS: marine extension sentences"));
+#endif
+  testsPassed = true;
+}
+
+void loop() {
+  if (testsPassed) {
+    Serial.println(F("PASS: marine source regression"));
+  } else {
+    Serial.println(F("FAIL: marine source regression"));
+  }
+  delay(2000);
+}
+
+bool parseSentence(char *sentence, const char *source, const char *type) {
+  GPS.addChecksum(sentence);
+  if (!GPS.parse(sentence) || strcmp(GPS.lastSource, source) ||
+      strcmp(GPS.lastSentence, type)) {
+    Serial.print(F("FAIL: source or sentence identity for "));
+    Serial.println(sentence);
+    return false;
+  }
+  Serial.print(F("PASS: "));
+  Serial.println(sentence);
+  return true;
+}
diff --git a/src/NMEA_parse.cpp b/src/NMEA_parse.cpp
index 59eb24b..ec7435d 100644
--- a/src/NMEA_parse.cpp
+++ b/src/NMEA_parse.cpp
@@ -23,7 +23,8 @@
 #include <Adafruit_GPS.h>
 
 const char PROGMEM Adafruit_GPS::sources[][4] = {"II", "WI", "GP", "PG", "GL",
-                                                 "GA", "GN", "P",  "ZZZ"};
+                                                 "GA", "GN", "HC", "TI", "SD",
+                                                 "AI", "P",  "ZZZ"};
 #ifdef NMEA_EXTENSIONS
 const char PROGMEM Adafruit_GPS::sentences_parsed[][4] = {
     "GGA", "GLL", "GSA", "RMC", "CD",  "DBT", "HDM", "HDT", "MDA", "MTW", "MWV",