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

To have a rewrite of these value we created a sdk script for legacy reasons.

The Script should be used as follows:

• Trigger: sdk-startup
• Argument: Server Port (default should be 2947)

Please requierments:

Please set the GNSS Server to a alternative port. e.g. 22947. (https://simulator.netmodule.com/gnss.php.html) The SDK Skript will handle the default gpsd Port 2947 after that.

gps-tcp-server-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 = (int) nb_config_get("gpsd.0.port");
server_port = (int)trim((string) argv[1]);
 
gpsd = -1;
MAX_CONNECTIONS=10;
 
void usage() {
	printf("usage: gps-udp-client.are <server> <port>\n");
	exit(1);
}
 
int gpsd_restart ()
{
	nb_syslog("Restarting GPS daemon");
 
	nb_config_set("gpsd.0.status=0");
	sleep(3);
	nb_config_set("gpsd.0.status=1");
	sleep(5);
 
	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 <= 5; attempt++) {
		sleep(3);
 
		if (connect(gpsd, local_ip, local_port) < 0) {
			nb_syslog("Could not connect to daemon");
			if (attempt == 3) {
				nb_syslog("Unable to connect, restarting daemon");
				gpsd_restart();
			}
		} else {
			break;
		}
	}
	nb_syslog("Connected to daemon, requesting NMEA");
	if (nb_config_get("gpsd.0.cmode") == "0") 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();
}
 
 
/* open TCP server socket */
server = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (server < 0) {
	nb_syslog("Unable to open tcp socket");
	exit(-1);
}
 
 
ret = bind(server, server_port, "");
if (ret == -1) {
    printf("Unable to bind port %d\n", port);
    close(sock);
    exit(1);
}
 
ret = listen(server, MAX_CONNECTIONS);
if (ret == -1) {
    printf("Unable to listen\n");
    close(sock);
    exit(1);
 
}
 
 
while (!quit) {
    client = accept(server);
    if (client < 0) {
        nb_syslog("accept failed\n");
        sleep(1);
        continue;
    }  else {
		nb_syslog("new client connection: %i",client);
	    clients=mkarray(client);
	} 
    clientConnection=true;
    nb_syslog("New client connected\n");
 
 
	// Sending NMEA Loop
 
	/* process NMEA stream */
	nb_syslog("Processing NMEA data");
	while(clientConnection) {
		/* 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(array_merge(clients,mkarray(gpsd,server)), 1);
 
		if(rv == -1) {
			nb_syslog("ERROR: select failed, re-connecting");
			close(gpsd);
			gpsd = -1;
			continue;
		} else if(rv == 0) {
			/* nothing received */
			continue;
		} else if(rv == server) {
			newclient = accept(server);
			if (newclient < 0) {
			nb_syslog("accept failed\n");
			continue;
			} else {
				nb_syslog("new client connection: %i",newclient);
				clients=array_merge(clients,mkarray(newclient));
 
		    }
		}
 
 
 
 
		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;
		*/ 
	    for (i=0;i<length(clients);i++) {
		sent = send(clients[i], data );
		if(sent < 0 ) {
			nb_syslog("ERROR: Unable to send, removing client (%i)",clients[i]); 
            clients=array_compact(array_unset(clients,i));
		} 
		} 
	}
 
 
 
}
 
if (gpsd > -1)	{
	close(gpsd);
}
if (server > -1) {
	close(server);
}
exit(-1);