eHam

eHam Forums => APRS => Topic started by: LA9XNA on November 28, 2018, 06:25:09 PM



Title: What GPS sentence (NMEA) to use for APRS
Post by: LA9XNA on November 28, 2018, 06:25:09 PM
What is the NMEA sentence(S) to ya with APRS.
I'm making a APRS display with Raspberry Pi and  the PI 7"LCD monitor together with an UBLOX NEO-7M.
Currently only over internett but in the future I will also add transmitting capabilities.


Title: RE: What GPS sentence (NMEA) to use for APRS
Post by: VE7KNL on November 29, 2018, 05:26:34 AM
Is this what your looking for?
http://www.aprs.net/vm/DOS/PROTOCOL.HTM


Title: RE: What GPS sentence (NMEA) to use for APRS
Post by: LA9XNA on December 01, 2018, 11:30:37 PM
No it is what NMEA sentences the GPS must be programmet to work with aprs software.


Title: RE: What GPS sentence (NMEA) to use for APRS
Post by: W5UAA on December 02, 2018, 03:37:18 AM
https://www.u-blox.com/sites/default/files/products/documents/NEO-7_DataSheet_%28UBX-13003830%29.pdf

The UBLOX NEO-7M produces the following sentences:
GGA (3D location)
GLL (Lat/Long)
GSA (Satellite data)
GSV (Detailed Satellite data)
RMC (Position, velocity, time)
VTG (Velocity)
TXT (error messages)

Depending on the APRS software you're using (or programming), I would guess you could use any sentence with position data in it  (GGA, GLL, RMC) for an APRS application.

The challenge in using GPS sentences is (1st) using a GPS receiver that produces the sentences you want and (2) parsing the sentence(s) you want to use.

Accurate position is used in APRS.  You can use accurate time also.  And if you're mobile, accurate velocity can be used.

Along with the standard GPS sentences, some other GPS receiver manufacturers produces proprietary sentences.  And some GPS receivers allow you to send commands to it to customize the sentences it outputs and the baud rate it sends them out the serial port, like my Novus GPSDO.

(edit) and if you send it a command, the challenge is doing the XOR on the sentence you send it for the *hh field so it accepts it as a valid command.


Title: RE: What GPS sentence (NMEA) to use for APRS
Post by: LA9XNA on December 04, 2018, 04:13:14 AM
Im usinf Xastir on a Rasberry Pi with the Compass distribution.
The GPS is a Ublox Neo-7m with the sentences GGA, GLL & RMC.
I get the GPS to stream data to the RPI by using the command stty -F /dev/ttyS0 9600 and monitoring by the command cat </dev/ttyS0
Xastir is set up with 9600 bd and /dev/ttyS0 but i get "GPS data is older than 30 seconds" when checking the gps status.


Title: RE: What GPS sentence (NMEA) to use for APRS
Post by: W5UAA on December 04, 2018, 05:01:30 PM
I'm not sure what you're asking now, but I've played around with a couple of UBLOX GPS receivers along with Xastir (on a couple of distributions like Fedora and "Andy's ham radio Linux", but not on Compass.  A quick google of Compass indicates it's just another distribution off of the Debian/Ubuntu branch--I have a version 2 Pi, but it's loaded with Raspbian at the moment.)  My UBLOX's use USB for interfacing and setup for Xastir includes attaching to /dev/ttyACM0.  Since it's USB, baud rate doesn't matter, but it has to be set to something in Xastir.  (4800 8n1, 9600 8n1 and higher all work the same.)

In the past, I've had to change the group ownership on the port prior to starting up Xastir in order to get it to work.  (dialup group?  I think).  But the latest version of Xastir seems to have fixed this and I don't need to change group ownership any more.

If you see it on your first serial port using cat  (from root?), but Xastir isn't seeing it, all I can suggest is checking the permissions on that port. You may need to change the group to something Xastir is a member of.  (#chown or #chgrp)

Or maybe just update your version of Xastir to the latest version?


Title: RE: What GPS sentence (NMEA) to use for APRS
Post by: LA9XNA on December 04, 2018, 05:37:02 PM
I  managed to fix it.
The data form the GPS where all garbeled.
The fix is simpel just to ground the board and put in some decupeling capacitors on the 5V line.
After that the GPS came up in a few seconds.  :)

https://www.ebay.com/itm/Ublox-NEO-7M-000-MWC-APM2-6-GPS-Module-2-4G-Antenna-Replace-NEO-6M-GYGPSV3-NEO7-/182490600841?var=&hash=item2a7d499589 (https://www.ebay.com/itm/Ublox-NEO-7M-000-MWC-APM2-6-GPS-Module-2-4G-Antenna-Replace-NEO-6M-GYGPSV3-NEO7-/182490600841?var=&hash=item2a7d499589)