====== 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 <nowiki>"$GP"</nowiki> (= GPS only) instead of looking for  all GNSS systems.

Newer GNSS receivers often use multiple GNSS systems and therefore <nowiki>"$GN"</nowiki> is used (<nowiki>"$GNRMC..."</nowiki>) if it recives a combination of GPS, Galileo and GLONASS signals.

It would be better to us instead of <nowiki>"$GPRMC"</nowiki> either <nowiki>"$G.RMC"</nowiki> ("." = anychar) for using GPS, Galileo and/or GLONASS (GNSS) or even <nowiki>"$..RMC"</nowiki> which means any source.

<code>
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
</code>

====== SDK Script gps2serial-GNSStoGPS.are ======
<code c gps2serial-gnsstogps.are>
/* DESC: This script sends the local GPS NMEA stream to the serial interface.
* DESC: It checks for the correct CR/LF end of line characters and convert messages $GN to $GP 
* Copyright (C) 2013-2021) NetModule AG, Switzerland
*/
 /* Options to configure by user*/
SER_SPEED = (int) argv[2];   /* Serial Baud Rate 4800, 9600, 19200, 38400, 57600, 115200 */
SER_DATABIT = 8;   /*  number of data bits (5, 6, 7, 8) */
SER_STOPBIT = 1;   /*  number of stop bits (1, 2) */   
SER_PARITY = 0;      /* parity (0=no parity, 1=odd parity, 2=even parity) */
SER_FLOW = 0;      /*  flow control (0=none, 1=xon/xoff, 2=hardware) */
DEV = "SERIAL1";   /*  name of the Serial Interface */

/* default values */
local_ip = "127.0.0.1";
local_port = (int) nb_config_get("gpsd.0.port");
 
gpsd = -1;
      
// start serial connection
int start_serial() 
{
   if (nb_config_get("serial.0.status") != "2") {
      nb_syslog("serial not assinged to SDK\n");
      exit(2);
   }
   // printf("set serial attributes\n");
   if ((rc = nb_serial_setattr(DEV, SER_SPEED, SER_DATABIT, SER_STOPBIT, SER_PARITY, SER_FLOW)) != 0) {
      nb_syslog("ERROR: unable to set serial attributes (rc %d) to %s\n", rc, DEV);
      return -1;
   }
   // nb_syslog("Open serial device %s\n", DEV);
   if ((fd = nb_serial_open(DEV)) < 0) {
      nb_syslog("ERROR: can't open serial port %s\n", DEV);
      return -1;
   }
   return fd;
}
// restart GPS deamon
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;
}

// start TCP Socket for GPS deamon loop 
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();
}
 
 
/* starting serial */
nb_syslog("starting serial");
if ((serial_fd = start_serial()) < 0 ) {
   nb_syslog("Could not start serial, exiting\n");
   exit(4);
}

 
/* 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 = write(serial_fd, data, len);
   nb_syslog("sending Data via serial interface");
   if (write(serial_fd, data, len) != len){
       nb_syslog("Unable to send message via Serial, data %s, len: %i\n", data, len);
  }
}
 
close(gpsd);
close(serial_fd);
exit(6);
</code>