This guide dives deeply into how an ESP32-based system collects GPS data using TinyGPS++, stores it locally, and transmits it over a cellular GPRS connection using a SIM800 module. We’ll cover each step with explanations, practical details, and insights.

System Architecture Overview

✅ Hardware components:

  • ESP32 microcontroller
  • SIM800L (or SIM800C) GPRS modem
  • NEO-6M or similar GPS module
  • Power supply and antennas

✅ Main functionalities:

  • Read real-time GPS coordinates and speed
  • Log data locally using LittleFS
  • Batch-send collected data every 10 seconds over HTTP POST
  • Handle communication either over WiFi or SIM800, depending on mode

In this article, we focus specifically on GPS data acquisition and SIM800 transmission.

Step 1: Reading GPS Data with TinyGPS++

The GPS module sends NMEA sentences (standardized ASCII messages) over serial. The TinyGPS++ library parses these into usable data:

while (gpsSerial.available()) {
    gps.encode(gpsSerial.read());
}

if (gps.location.isUpdated()) {
    float latitude = gps.location.lat();
    float longitude = gps.location.lng();
    float speedKmph = gps.speed.kmph();
    String timeUTC = getFormattedTime();

    Serial.printf("Lat: %.6f, Lng: %.6f, Speed: %.2f km/h, Time: %s\n",
                  latitude, longitude, speedKmph, timeUTC.c_str());
}

✅ Important notes:

  • Always check isUpdated() to avoid stale data.
  • Use .lat() and .lng() for high-precision coordinates.
  • Retrieve time only if gps.time.isValid().

Step 2: Storing Data Locally with LittleFS

Collected points are stored in a text file (/gpslog.txt) on the ESP32’s flash memory:

String record = String(latitude, 6) + "," + String(longitude, 6) + "," +
                String(speedKmph, 2) + "," + timeUTC;

appendToFile("/gpslog.txt", record);

✅ Why store locally?

  • If the connection is down, you avoid data loss.
  • You can batch-send multiple points in one POST request, reducing overhead.

Step 3: Preparing JSON for Transmission

Before sending, the code reads the log file and builds a valid JSON array:

[
  {
    "deviceId": "SCM-AA0001",
    "latitude": 41.123456,
    "longitude": 29.987654,
    "speed": 12.34,
    "timeStamp": "2025-05-09T12:34:56Z"
  },
  ...
]

✅ Batching advantage:

  • Reduce SIM800 data session usage.
  • Ensure multiple data points are preserved.

Step 4: Sending Data over SIM800 GPRS

Using AT commands, the ESP32 communicates with SIM800 via serial (sim800Serial):

sendATCommand("AT+HTTPINIT", 1000);
sendATCommand("AT+HTTPPARA=\"CID\",1", 1000);
sendATCommand("AT+HTTPPARA=\"URL\",\"http://<IP>:<PORT>/api/TelemetryData\"", 1000);
sendATCommand("AT+HTTPPARA=\"CONTENT\",\"application/json\"", 1000);
sendATCommand("AT+HTTPDATA=" + String(jsonArray.length()) + ",10000", 1000);

sim800Serial.print(jsonArray);
delay(2000);

sendATCommand("AT+HTTPACTION=1", 10000);  // POST
sendATCommand("AT+HTTPREAD", 3000);
sendATCommand("AT+HTTPTERM", 1000);

✅ Key points:

  • Always initialize HTTP (HTTPINIT) before sending.
  • Use HTTPPARA to set headers and target URL.
  • Set CID to 1, which refers to the active GPRS context (SAPBR).
  • Use HTTPDATA to indicate payload length and prepare the modem.

✅ GPRS prerequisites:

  • Ensure APN is set (AT+SAPBR=3,1,"APN","internet").
  • Ensure GPRS session is activated (AT+SAPBR=1,1).

Example AT Command Flow (SIM800)

1️⃣ Check module: AT → OK
2️⃣ Open bearer: AT+SAPBR=1,1
3️⃣ Initialize HTTP: AT+HTTPINIT
4️⃣ Set URL: AT+HTTPPARA="URL",...
5️⃣ Load data: AT+HTTPDATA
6️⃣ Execute POST: AT+HTTPACTION=1
7️⃣ Read result: AT+HTTPREAD
8️⃣ Terminate HTTP: AT+HTTPTERM

If anything fails, you can reset the connection with:

restartSIM800Connection();

Debugging Tips

✅ Enable serial prints to monitor AT command responses.
✅ Check signal strength with AT+CSQ.
✅ Use AT+CGATT? to ensure GPRS is attached.
✅ Retry failed actions or implement a reconnection loop.

Final Thoughts

By combining ESP32, TinyGPS++, and SIM800, you build a resilient, flexible GPS tracker capable of working in both WiFi and mobile network environments. Understanding the AT command flow is crucial for reliable data transmission.