radio KM4BA UHF DMR GPS APRS MotoTRBO

The Motorola DMR radios typically have integral GPS capability. This is directly supported by APRS, and is a very useful feature. 

There are multiple steps to enabling this functionality:

  • Attached an active antenna. These are available on ebay and amazon for under $10. Key is to have an SMA male connector and an "active" antenna. There seems to be a de facto standard in play. The one I bought for a car radio head unit worked perfectly. 
  • Register on Brandmeister which will enable you to utilize the 'Self Care' menu
  • In the Brandmeister Selfcare menu set the following:
    • GPS to Motorola
    • APRS Interval to 600 seconds
    • APRS Callsign to your callsign
    • APRS Text to something like "Alan's XPR-4550 Mobile" or similar

Note the Brandmeister Selfcare  entry will be applied to any radios using that DMR ID. So if you have a Moto radio and also another brand that uses a different GPS method ("Chinese" or similar) you will need to setup a different DMR ID for the 2nd radio. 

  • In the Motorola CPS set the following General settings
    • GNSS Enabled- This is the GPS, I don't think it can even be unchecked in the CPS
    • ARS Delay set to 30 minutes- If it's left at zero, when you power up the radio will try to register with the "controller" (repeater) 5-15 seconds afterwards. And then GPS sends ~20-30 seconds after that. Since when we power up, it's usually to talk, this can create channel busy. Setting to 30 minutes make the random ARS registration occur 5 seconds to 30 minutes later on a random basis. This will slow the start of your GPS reports, but make the radio much more usable. 
    • Persistant LRRP Request set to "Save"- my understanding is this will have the radio save the GPS info such that it can send when needed even if the current location is not available. 
  • Also in the Motorola CPS, set the following on Brandmeister channels you wish to allow APRS reports from:
    • ARS set to "on Site/System Change"
    • Compressed UDP Header set to "None" or "DMR" if that's an option for your radio. (It's not for the 4550). 
    • GNSS Revert set to "Selected"

After executing the above steps, you should see:

  • The little sat dish icon turn solid as it picks up a signal
  • The radio may transmit a few times the first time it registers on the network
  • I received some odd messages back from BM as texts when the radio first activated
  • The radio may transmit when you change channels as it appears to register once on the frequency. I sometimes see the dish icon go out, then back on when I change channels. 

From then out you should be able to see your status on https://aprs.fi Here is mine: https://aprs.fi/info/a/KM4BA 

  • The map and graph views are useful. Clicking on info takes you back to the main page for the station. 
  • Also note that there does appear to be some rate limiting applied in aprs.fi updates.

Notes:

  • GPS requires ARS, which then triggers the radio to register with the controller (repeater) upon channel change. This creates some traffic from the radio every time you change channels. This can be annoying, as it can cause "channel busy" messages when you try to reply to a call or similar. You have to let the radio finish before your call can be sent, which can take a few seconds. 
  • Having ARS delay per the above helps with power-up, but once active, if you change channels you will still see some ARS traffic. And thus possible see a "channel busy" if you try to transmit at the same time. 
  • Also note that some repeater systems (DMARC, K4USD, etc) do not allow/support text messaging. So ARS should not be enabled for those sites in most cases. (At least that's what they indicate)
  • You probably do not want to turn on GPS/ARS if you do not have an antenna or solid GPS signal path. It seems to generate quite a bit of radio keyup trying to do the reporting. Persistent LRRP set to save seems to help with this, but still I'd leave ARS/GPS off if you do not have an antenna attached. 

Observations:

  • Doing DMR based APRS GPS reporting with the Moto XPR-4550 uses the radios's reporting capability. From reading the moto docs, it appears the 'controller' (repeater) requests a position report from the radio, and can also tell the radio to report every X minutes, position delta, etc. That location request tells the radio when/how to report.

    I've not confirmed the exact request Brandmeister has the repeater / controller make, but it is configured and managed centrally. IE: You cannot tell the radio to update location every N minutes. I believe that is controlled by the Brandmeister entry and passed down to the controller/repeater. 

    A bit of insight on how MotoTRBO handles gps can be found here: 
    https://cwh050.blogspot.com/2012/10/mototrbo-gps-in-nutshell.html which is a MotoTRBO site run by a Moto support engineer.