MASA Avionics Telemetry System

Long-Range 2.4GHz LoRa Communication for High-Altitude Rockets

Project Overview

This project, as part of the Michigan Aeronautical Science Association (MASA), focuses on developing a robust, long-range telemetry system for high-altitude rockets. The primary goal is to ensure reliable, real-time data transmission from the rocket to a ground station, even at altitudes up to 75,000 feet.

The system utilizes LoRa (Long Range) communication in the 2.4GHz band, leveraging Chirp Spread Spectrum (CSS) modulation to achieve a resilient communication link. This is critical for post-flight analysis, especially in scenarios where the primary black box data recorder may be unrecoverable. This project addresses a key failure point from past launches by creating a redundant, live telemetry backup.

Avionics Hardware Placeholder

Technical Approach

LoRa Communication & Antennas

The core of the system is built around the SX1280 LoRa transceiver module. On the rocket, a patch antenna is integrated into one of the tailfins to provide a compact and aerodynamic solution for transmitting data. This placement helps maintain a clear signal path, especially during the rocket's descent phase.

LoRa modulation provides excellent resilience to interference and requires low power, making it ideal for a rocket's constrained environment. Our target is a 40Kbps telemetry data rate, which is a balance between data throughput and link reliability over long distances.

Dual-Antenna Ground Station

To maximize signal reception, the ground station employs a dual-antenna setup. This includes a high-gain directional Yagi antenna that must be aimed at the rocket and a semi-omni-directional dome antenna. This diversity helps mitigate signal loss due to rocket orientation changes during flight. An antenna splitter combines the signals before they are processed.

Link Budget Analysis

A critical part of the design process is the RF link budget analysis. By accounting for transmitter power (12.5 dBm), antenna gains (8 dBi ground, 6 dBi rocket), free-space path loss at 20km, and other system losses, we calculated a received power of -108.9 dBm. This is well within the SX1280's receive sensitivity of -132 dBm, providing a healthy link margin to ensure reliable communication.

Data Analysis & Results

Initial testing of the transmitter and receiver test boards has yielded promising data. The following plots show the Received Signal Strength Indicator (RSSI) and Signal-to-Noise Ratio (SNR) over time during a hotfire test, demonstrating a stable link against rocket plume.

RSSI Over Time

RSSI Plume Data

RSSI remains consistently around -75 dBm, indicating a strong signal.

SNR Over Time

SNR Plume Data

SNR is stable above 10 dB, indicating a clear signal with low noise.

Firmware Snippets

Transmitter Code

TransmitterCodeLoRa.txt

void loop() {
  // Prepare message and buffer
  const char* message = "Hello to MASA!";
  size_t msgLen = strlen(message);
  uint8_t buffer[128] = {0};

  // Prepend packet counter to the message
  memcpy(buffer, &packetCounter, sizeof(packetCounter));
  memcpy(buffer + sizeof(packetCounter), message, msgLen);

  size_t totalLen = sizeof(packetCounter) + msgLen;

  Serial.print("[SX1280] Transmitting packet #");
  Serial.print(packetCounter);
  
  // Transmit data using RadioLib
  int state = radio.transmit(buffer, totalLen);

  if (state == RADIOLIB_ERR_NONE) {
    Serial.println("Success!");
    packetCounter++; 
  } else {
    Serial.print("Transmission failed, code ");
    Serial.println(state);
  }

  delay(1000);  
}

Receiver Code

ReceiverCodeLoRa.txt

void handleReceivedPacket() {
    uint8_t buffer[128] = {0}; 
    int state = radio.readData(buffer, sizeof(buffer));
    
    float rssi = radio.getRSSI();
    float snr = radio.getSNR();

    if (state == RADIOLIB_ERR_NONE) { 
        uint32_t packetCounter;
        memcpy(&packetCounter, buffer, sizeof(packetCounter));

        metrics.totalPackets++;

        if (metrics.totalPackets == 1) {
            metrics.expectedPacketCounter = packetCounter + 1;
        } else {
            if (packetCounter != (metrics.expectedPacketCounter - 1)) {
                // Logic to detect and count lost packets
                uint32_t lost = packetCounter - metrics.expectedPacketCounter;
                metrics.lostPackets += lost;
            }
            metrics.expectedPacketCounter = packetCounter + 1;
        }
        
        // Update signal metrics (RSSI, SNR, Power)
        updateSignalMetrics(rssi, snr);
    }
    
    radio.startReceive(); // Re-enter receive mode
}
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