UKHAS Wiki

UK High Altitude Society

User Tools

Site Tools


communication:protocol

Communications Protocol

This is the typical protocol used by UKHAS members to transmit GPS data balloon to ground on the unlicensed bands. Parsing is handled by habitat which can be configured to almost practically any format you can come up with, provided the string starts with $$CALLSIGN, end with a newline ('\n'), and contains printable ASCII characters. Fields are comma delimited.

 $$CALLSIGN,sentence_id,time,latitude,longitude,altitude,optional speed,optional bearing,optional internal temperature,*CHECKSUM\n 
  • The individual fields can be of variable length, the minimum and maximum can be set in the payloads XML file.
  • CALLSIGN simply identifies your balloon - doesn't have to be a real, licensed callsign.
  • sentence_id (sometimes known as count, sequence, incremental counter…) also helps people receiving many transmissions at once. This should go up for each new transmission. It can go up 1, 2, 3, or be the milliseconds since power on, or whatever you want. It just has to increase each time a new string is sent.
  • latitude and longitude can either be in decimal degrees (DD.dddd) or the NMEA format (DDMM.mmmm). If using decimal degrees take care that your conversion code does not break if it crosses the meridian. While the NMEA format is accepted you will still need to parse whether it its NSEW and use +/- appropriately.
  • The checksum is very strongly recommended. We support CRC16_CCITT, NMEA XOR and a couple of variants of Fletcher 16 checksums; however: the CRC16_CCITT is the most popular and is far better than xor, which is mainly supported for old payloads. There have actually been real xor collisions.
  • The newline at the end IS required. It does make the protocol much more easier to human-read, and dl-fldigi looks for that character to terminate the string. Although both will work, please use '\n', not '\r\n'.
  • Here are example strings:
XOR Checksum'd string:
$$A1,15254,15:36:34,52.145255,000.542061,00118,0000,03,3F4D3F2F,45*62
Example of a CRC16_CCITT'd string:
$$hadie,181,10:42:10,54.422829,-6.741293,27799.3,1:10*002A
Demonstration of the fact that the custom data can be anything:
$$icarus,12342,12:34:17,52.345645,-1.02342,10232,21.35,192.3,15.4,-22.34,-18.27,1232,Blah,Blah,Blah*0C

Usage example for the below two functions

#include <stdio.h>
#include <stdint.h>
#include <string.h>
 
char s[100];
 
void make_string(struct information i)
{
	char checksum[10];
 
	snprintf(s, sizeof(s), "$$MYPAYLOAD,%i,%s,%s", i->num, i->gps.lat, i->gps.lon);
 
	snprintf(checksum, sizeof(checksum), "*%04X\n", gps_CRC16_checksum(s));
	// or 	snprintf(checksum, sizeof(checksum), "*%02X\n", gps_xor_checksum(s));
 
	// It would be much more efficient to use the return value of snprintf here, rather than strlen
 
	if (strlen(s) > sizeof(s) - 4 - 1)
	{
		// Don't overflow the buffer. You should have made it bigger.
		return;
	}
 
	// Also copy checksum's terminating \0 (hence the +1).
	memcpy(s + strlen(s), checksum, strlen(checksum) + 1);
}

Useful code to calculate NMEA xor checksum

Please note: The use of xor checksum for payload telemetry is not advised, this code is useful for checking sentences sent by the GPS. The CCITT checksum below has much better performance and is preferred for payload telemetry

#include <stdio.h>
#include <stdint.h>
#include <string.h>
 
uint8_t gps_xor_checksum(char *string)
{
	size_t i;
	uint8_t XOR;
	uint8_t c;
 
	XOR = 0;
 
	// Calculate checksum ignoring the first two $s
	for (i = 2; i < strlen(string); i++)
	{
		c = string[i];
		XOR ^= c;
	}
 
	return XOR;
}

Useful code to calculate CRC16_CCITT checksum on the AVR

(Refer to this page for implementation on other microcontrollers)

#include <stdio.h>
#include <stdint.h>
#include <util/crc16.h>
#include <string.h>
 
uint16_t gps_CRC16_checksum (char *string)
{
	size_t i;
	uint16_t crc;
	uint8_t c;
 
	crc = 0xFFFF;
 
	// Calculate checksum ignoring the first two $s
	for (i = 2; i < strlen(string); i++)
	{
		c = string[i];
		crc = _crc_xmodem_update (crc, c);
	}
 
	return crc;
}

Getting Your Payload On the Tracker

You will need to fill out a flight document so the server can parse your telemetry strings and update the map.
Fill out the form here :Payload Document Generator the come and see us on IRC to get it added.

communication/protocol.txt · Last modified: 2013/02/01 17:43 by mbrejza