firmware: auto-detect GPS UART baud

firmware/xiao-esp32c6-sensor-node/README.md

@@ -9,7 +9,7 @@ This Arduino sketch reads a BNO086 IMU and MAX-M10S GPS and publishes versioned,
 | GPS TX to XIAO RX | D9 |
 | GPS RX from XIAO TX | D10 |
 
-Power both sensors from 3.3V with shared ground. Required Arduino libraries are Adafruit BNO08x, TinyGPSPlus, and ArduinoJson 6.x. Enable **USB CDC On Boot** for diagnostics. GPS uses hardware UART0 and RS485 uses hardware UART1.
+Power both sensors from 3.3V with shared ground. Required Arduino libraries are Adafruit BNO08x, TinyGPSPlus, and ArduinoJson 6.x. Enable **USB CDC On Boot** for diagnostics. GPS uses hardware UART0 and RS485 uses hardware UART1. Startup probes 9600, 38400, 115200, and 4800 baud and reports the first rate producing checksum-valid NMEA.
 
 At startup the firmware scans D7/D8 and accepts either valid BNO08x address, `0x4A` or `0x4B`. If neither appears in the scan, verify 3.3V/GND, swap-check SDA and SCL, and confirm the breakout's PS0/PS1 jumpers are configured for I2C.
 

firmware/xiao-esp32c6-sensor-node/gps.cpp

@@ -3,12 +3,48 @@
 #include "sensor_node_config.h"
 namespace { TinyGPSPlus parser; }
 
-void initGps(HardwareSerial& serial) { serial.begin(GPS_BAUD, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN); }
+namespace {
+uint32_t activeBaud = GPS_FALLBACK_BAUD;
+
+bool baudHasValidNmea(HardwareSerial& serial, uint32_t baud) {
+    TinyGPSPlus probe;
+    serial.end();
+    serial.begin(baud, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+    unsigned long started = millis();
+    while (millis() - started < GPS_BAUD_DETECT_MS) {
+        while (serial.available()) probe.encode(serial.read());
+        if (probe.passedChecksum() > 0) return true;
+        delay(1);
+    }
+    return false;
+}
+}
+
+void initGps(HardwareSerial& serial) {
+    const uint32_t candidates[] = {9600, 38400, 115200, 4800};
+    Serial.println("Detecting GPS UART baud...");
+    for (uint32_t baud : candidates) {
+        Serial.print("Trying GPS baud ");
+        Serial.println(baud);
+        if (!baudHasValidNmea(serial, baud)) continue;
+        activeBaud = baud;
+        Serial.print("Valid GPS NMEA detected at ");
+        Serial.println(activeBaud);
+        return;
+    }
+
+    activeBaud = GPS_FALLBACK_BAUD;
+    serial.end();
+    serial.begin(activeBaud, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+    Serial.println("No valid GPS NMEA detected; falling back to 9600 baud");
+}
+
 void updateGps(HardwareSerial& serial, GpsState& s) {
     while (serial.available()) { s.lastByteMs = millis(); parser.encode(serial.read()); }
     s.charactersProcessed = parser.charsProcessed();
     s.sentencesPassed = parser.passedChecksum();
     s.sentencesFailed = parser.failedChecksum();
+    s.baud = activeBaud;
     s.online = s.lastByteMs && millis() - s.lastByteMs <= SENSOR_STALE_MS;
     s.fix = parser.location.isValid() && parser.location.age() <= SENSOR_STALE_MS;
     s.satellites = parser.satellites.isValid() ? parser.satellites.value() : 0;

firmware/xiao-esp32c6-sensor-node/rs485_transport.cpp

@@ -19,6 +19,7 @@ void publishSensorStatus(HardwareSerial& serial, const ImuState& imu, const GpsS
     i["yaw_degrees"] = imu.yawDegrees; i["pitch_degrees"] = imu.pitchDegrees; i["roll_degrees"] = imu.rollDegrees;
     JsonObject g = doc.createNestedObject("gps");
     g["online"] = gps.online; g["fix"] = gps.fix; g["satellites"] = gps.satellites;
+    g["baud"] = gps.baud;
     JsonObject nmea = g.createNestedObject("nmea");
     nmea["characters"] = gps.charactersProcessed;
     nmea["sentences_passed"] = gps.sentencesPassed;

firmware/xiao-esp32c6-sensor-node/sensor_node_config.h

@@ -14,7 +14,8 @@
 #define BNO086_I2C_ADDRESS 0x4A
 #define BNO086_I2C_ADDRESS_ALTERNATE 0x4B
 #define BNO086_I2C_CLOCK_HZ 100000
-#define GPS_BAUD 9600
+#define GPS_FALLBACK_BAUD 9600
+#define GPS_BAUD_DETECT_MS 2500
 #define RS485_BAUD 115200
 #define IMU_REPORT_INTERVAL_US 20000
 #define TELEMETRY_INTERVAL_MS 200

firmware/xiao-esp32c6-sensor-node/sensor_state.h

@@ -19,5 +19,6 @@ struct GpsState {
     uint32_t charactersProcessed = 0;
     uint32_t sentencesPassed = 0;
     uint32_t sentencesFailed = 0;
+    uint32_t baud = 0;
     unsigned long lastByteMs = 0;
 };