====== Background ======
GPS was the first Global Navigation Satellite System (GNSS) and the term GPS is often used as a synonym for GNSS.
Older software often only checks for NMEA 0183 sentences starting with "$GP" (= GPS only) instead of looking for all GNSS systems.
Newer GNSS receivers often use multiple GNSS systems and therefore "$GN" is used ("$GNRMC...") if it recives a combination of GPS, Galileo and GLONASS signals.
It would be better to us instead of "$GPRMC" either "$G.RMC" ("." = anychar) for using GPS, Galileo and/or GLONASS (GNSS) or even "$..RMC" which means any source.
NMEA 0183 sentence "$XXYYY..." explanation:
"$" = Start
"XX" = Source device (https://de.wikipedia.org/wiki/NMEA_0183#Ger%C3%A4te-IDs)
BD = Beidu
GA = Galileo
GL = GLONASS
GP = GPS
GN = GNSS position fix from more than one constellation (eg. GPS + GLONASS)
"YYY" = Record
RMC = Recommended Minimum Sentence C (used in most cases)
GGA = Global Positioning System Fix Data
GNS = GNSS fixed data
GLL = Geographic Position - Latitude/Longitude
...
Source: https://de.wikipedia.org/wiki/NMEA_0183
====== SDK Script gps-udp-client-GNSStoGPS.are ======
/* DESC: This script sends the local GPS NMEA stream to a remote UDP server.
* DESC: It checks for the correct CR/LF end of line characters and convert messages $GN to $GP
* Copyright (C) 2013-2019) NetModule AG, Switzerland
*/
/* default values */
local_ip = "127.0.0.1";
local_port = 2947; /* would be better to take it from the configuaration */
gpsd = -1;
void usage() {
printf("usage: gps-udp-client.are \n");
exit(1);
}
int gpsd_restart ()
{
nb_syslog("Restarting GPS daemon");
nb_config_set("gpsd.0.status=0");
do {
sleep(3);
} while ( nb_config_done() != 0)
nb_config_set("gpsd.0.status=1");
do {
sleep(5);
} while ( nb_config_done() != 0)
return 0;
}
int gpsd_connect ()
{
nb_syslog("Connecting to GPS daemon");
gpsd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (gpsd < 0) {
nb_syslog("ERROR: Unable to open socket");
exit(-1);
}
for (attempt = 0; attempt <= 15; attempt++) {
sleep(15);
if (connect(gpsd, local_ip, local_port) < 0) {
nb_syslog("Could not connect to daemon");
if (attempt == 12) {
nb_syslog("Unable to connect, restarting daemon");
gpsd_restart();
}
} else {
break;
}
}
nb_syslog("Connected to daemon, requesting NMEA");
send(gpsd, "R=1\n");
nb_syslog("Processing NMEA");
return gpsd;
}
string checksum(string s)
{
pos = strchr(s, "*");
if (is_void(pos))
return s; /* no checksum to correct */
cs = 0;
chars = explode(s);
for (i = 1; i < pos; i++) { /* skip $ and *XX */
c = ord(chars[i]);
cs ^= c;
}
r = sprintf("%s%02X%s", substr(s, 0, pos + 1), cs, substr(s, pos + 3));
return r;
}
/* main() */
if (argc < 2) {
usage();
}
SERVER = trim((string) argv[1]);
PORT = (int) argv[2];
/* open UDP server socket */
server = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (server < 0) {
nb_syslog("Unable to open socket");
exit(-1);
}
/* process NMEA stream */
nb_syslog("Processing NMEA data");
while(1) {
/* connect to gpsd */
if(gpsd < 0) {
gpsd = gpsd_connect(gpsd);
if(gpsd < 0) {
nb_syslog("ERROR: Unable to connect to daemon");
sleep(5);
continue;
}
}
/* wait for client socket data */
rv = select(gpsd, 3);
if(rv == -1) {
nb_syslog("ERROR: select failed, re-connecting");
close(gpsd);
gpsd = -1;
continue;
} else if(rv == 0) {
/* nothing received */
continue;
}
data = recv(gpsd);
len = strlen(data);
/* Check for correct framing: CR+LF */
if(len > 2) {
if(strrchr(data, "\r") != len-2 || strrchr(data, "\n") != len-1) {
nb_syslog("ERROR: NMEA framing wrong");
close(gpsd);
gpsd = -1;
gpsd_restart();
continue;
}
}
if(len == 0) {
nb_syslog("ERROR: no data, re-connecting");
close(gpsd);
gpsd = -1;
continue;
}
/* Convert message $GN to $GP for backward compatibility */
if(strstr(data, "$GN") == 0) {;
a_arr = explode(data);
a_arr[2] = "P";
data = checksum(implode(a_arr));
}
/* skip all messages different from GPGGA
* if(strstr(data,"$GPGGA")==())
* continue;
*/
sent = sendto(server, data, SERVER, PORT);
if(sent != len) {
nb_syslog("ERROR: Unable to send %d bytes to %s:%d", len, SERVER, PORT);
}
}
if (gpsd > -1) {
close(gpsd);
}
if (server > -1) {
close(server);
}
exit(-1);