ardupilot/libraries/APO/examples/MavlinkTest/MavlinkTest.pde

/*
  AnalogReadSerial
 Reads an analog input on pin 0, prints the result to the serial monitor 
 
 This example code is in the public domain.
 */

#include <GCS_MAVLink.h>

int packetDrops = 0;

void handleMessage(mavlink_message_t * msg) {
  Serial.print(", received mavlink message: ");
  Serial.print(msg->msgid,DEC);
}

void setup() {
  Serial.begin(57600);
  Serial3.begin(57600);
  mavlink_comm_0_port = &Serial3;
  packetDrops = 0;
}

void loop() {
  mavlink_msg_heartbeat_send(MAVLINK_COMM_0,mavlink_system.type,MAV_AUTOPILOT_ARDUPILOTMEGA);
  Serial.print("heartbeat sent");
    
  // receive new packets
  mavlink_message_t msg;
  mavlink_status_t status;

  Serial.print(", bytes available: "); Serial.print(comm_get_available(MAVLINK_COMM_0));
  while(comm_get_available(MAVLINK_COMM_0)) {
      uint8_t c = comm_receive_ch(MAVLINK_COMM_0);

      // Try to get a new message
      if(mavlink_parse_char(MAVLINK_COMM_0, c, &msg, &status)) handleMessage(&msg);
  }

  // Update packet drops counter
  packetDrops += status.packet_rx_drop_count;
  
  Serial.print(", dropped packets: ");
  Serial.println(packetDrops);
  delay(1000);
}