EM7455 Loosing GPS Fix


Apologies for the long post.

I’m having issues with EM7455 loosing GPS fix (with seemingly no reason) and I’m out of ideas at this point. Hoping someone might be able to help troubleshoot this further.

We’re working on project that involves around 60 commercial vehicles being tracked with GPS and collecting data from other sensors along with GPS coordinates. Here is the physical setup:

  • EM7455 modules are installed in small industrial PCs that are running Windows 10 IoT
  • Modules have 9999999_9904609_SWI9X30C_02.38.00.00_00_GENERIC_002.082_000 and 9999999_9904780_SWI9X30C_02.33.03.00_00_VERIZON_002.079_001 firmware images installed with the Verizon image being the active one.
  • We’re using Sierra Wireless 3-in-1 Shark Fin Antennas (part #6001119). The antennas are mounted on top of the vehicle’s cab which is the highest point and should be unobstructed.
  • The EM7455 modules are configured to expose the NMEA serial port
  • We’re using Verizon LTE to send data to our backend server.

On startup we send the following AT command to start the GPS tracking:

  • AT!ENTERCND=“A710”
  • AT+WANT=1
  • AT!GPSTRACK=1,255,20,1000,1

After that the application reads and parses the NMEA sentences and keeps track of current coordinates.

The issue we’re seeing is that sometimes a unit will lose its GPS fix for seemingly no reason. Sometimes it will recover the GPS fix and sometimes it will stay down until the vehicle runs the next day (the PC is restarted). On some days there will be no issues at all.

Usually that happens when the vehicles are in residential neighborhoods where there are no tall buildings and generally no major obstructions.

Recently we made a change to the software where every 30 seconds the following commands are sent to the modem to reset the GPS tracking:

  • AT+WANT=0
  • AT+WANT=1
  • AT!GPSTRACK=1,255,20,1000,1

This seems to help somewhat but there are some vehicles that are still losing GPS tracking and we prefer to avoid this if possible.

I should also mention that the units are shutdown when the vehicles are done working for the day and started backup the next day (hardware is reset).

I’m attaching a partial NMEA log from one of the vehicles that is having the issue: NmeaDiagnostic_00.zip (31.8 KB)

Any help is appreciated.

did you compare the AT!GPSSATINFO? in OK case and NOK case?

Does the AT!GPSCOLDSTART help?

I will try to find an unit that is having the issue and run the AT!GPSSATINFO? command in both cases as well as try the AT!GPSCOLDSTART command. However, I really like to avoid AT!GPSCOLDSTART since it take about 1.5 minutes to obtain a fix.

Is there a way to determine if this is a antenna related issue? The odd thing about the NMEA log I’ve uploaded it that when the unit lost GPS fix, there were several NMEA messages that did not report any visible satellites which usually happens if the antenna cable is disconnected.

You can compare the at!gpssatinfo of two devices in same location to know if there is antenna issue

@jyijyi, can you please help me clarify something? Is the code that doe the GPS computations part of the regular firmware files that can be loaded into the module or is it permanently loaded into the module? I’m asking because I was thinking of switching some of the devices to a previous Verizon firmware to see if there will be a difference or even switching some of the devices to a different carrier.

I think it is inside the FW code