fixed missing function

main.c

@@ -233,6 +233,22 @@ void send_rtty_packet() {
   send_cun++;
 }
 
+uint16_t gps_CRC16_checksum(char *string) {
+  uint16_t crc = 0xffff;
+  char i;
+  while (*(string) != 0) {
+    crc = crc ^ (*(string++) << 8);
+    for (i = 0; i < 8; i++) {
+      if (crc & 0x8000)
+        crc = (uint16_t) ((crc << 1) ^ 0x1021);
+      else
+        crc <<= 1;
+    }
+  }
+  return crc;
+}
+
+
 #ifdef  DEBUG
 void assert_failed(uint8_t* file, uint32_t line)
 {