Did anyone manage to get SBAS reliable working on B2?

Playing with @BillM 's code. Despite forcing AT6558(R?) into the SBAS mode by sending

"PCAS15,4,FFFF"

to the chip, the error is still over 7 meters.
Is there a way to get the error below 2 meters as stated in most SBAS articles?

UPD:
After taking weighted by time between records average of recorded coordinates at a static point and comparing those average coordinates to the true coordinates, the error seems to be closer to 20 meters.

UPD2:
The command checksum is not included in the post. When the command is sent to the GPS chip the checksum is added.

I have tried to make the PCAS-commands a bit more usable in a library, but some seem to do nothing at all (e.g. setting Standby mode) while other work fine (update interval). Is there a way to dump the current config to check?

Does your command miss the checksum or did you omit that on purpose?

https://github.com/halemmerich/BangleApp­s/blob/pcas/modules/pcas.js

There is some helpful code at http://www.espruino.com/Bangle.js2+Techn­ical#gps that might help? It at least calculates the CRC

Thank for the reply. The post misses the checksum. The real code is:

function checksum(str) {
	var cs = 0;
  
	for (const c of str) {
		cs = cs ^ c.charCodeAt(0); //XOR
	}
  
	return cs.toString(16).toUpperCase().padStart(2­, '0');
}

function configureSBAS() {
   const thisTime = new Date(); 
   let timeString = thisTime.toISOString();
	
  // SBAS satellite (SBAS satellite No. 1-19, corresponding to PRN 120-138
  var cmd = "PCAS15,4,FFFF"; //turn on satellites 1-16 of SBAS
  //var cmd = "PCAS15,4,7FFFF"; //turn on satellites 1-19 of SBAS

  cmd = "$" + cmd + "*" + checksum(cmd);
  Serial1.println(cmd);
  writeLogFile(timeString + ": " + cmd + "\n");
  
  SBASon = true;
}

You are checksumming the command without the leading $, I think that needs to be included.

I thing line 19 of the presented code add the leading "$"

It does, but that happens after the checksum is calculated. The cmd going into checksum() does not yet contain it.

I'm not sure if "$" should be accounted in the checksum.
As shown here, NMEA checksum calculation does not include "$" and "*"

Lets assume the following NMEA sentence:

$GPGLL,5300.97914,N,00259.98174,E,125926­,A*28

In this sentence the checksum is the character representation of the
hexadecimal value 28. The string that the checksum is calculated over
is

GPGLL,5300.97914,N,00259.98174,E,125926,­A

To calculate the checksum you parse all characters between $ and *
from the NMEA sentence into a new string.

I have tried it with PCAS02, the command to change the update rate and without the $ included in the checksum it changes nothing while including it works.

print(require("pcas").checksum("$GPGLL,5­300.97914,N,00259.98174,E,125926,A")); creates a checksum of 28 which matches your example. Doing the same without $ calculates 6F as checksum.

require("pcas").checksum is as follows:

function checksum(cmd) {
  var cs = 0;
  for (var i = 1; i < cmd.length; i++)
    cs = cs ^ cmd.charCodeAt(i);
  return cmd + "*" + cs.toString(16).toUpperCase().padStart(2­, '0');
}

Looks like @BillM implemented the checksum function another way. His version computes 28 without "$" sign. Attached screenshot

1 Attachment

Oh wow, the confusion was on my end 😑.
My function ignores the first character and so it needs the $ given to it to directly ignore it...