NRF9160: GPS Tracking satellites, but can't get fix.

[mlang3440]

Hi, we have a custom board using the nrf9160, running modem firmware 1.1, and using the nrf9160_gps driver.

Sometimes we see messages like this:

Tracking: 8 Using: 0 Unhealthy: 0

Where the GPS is tracking satellites but is not using any, and never gets a fix. Eventually we time-out and stop the GPS. But this behavior continues until the next device is restarted.

Is this a bug with the modem firmware? Is there any fix for this.

Thanks

[Øyvind]

Hello, 

What GPS antenna do you have on your custom board? Can you provide any schematics? Are you outside or inside when testing?

Kind regards,
Øyvind

[phipi]

Hi Øyvind, we're using a yageo patch antenna (ANT1818B00AT1575S), and I've attached the relevant section of the design here.

[Øyvind]

Hello, 
I've been discussing your schematic with one of our HW engineers, and there is one potential issue: when the LTE radio is operating, the RF signal can intermodulate in the GPS LNA input stage and create unwanted harmonics. You should be safe either using an LNA with an integrated filter or by adding a filter on the input of the LNA.

Your design deviates from the reference design from the LNA vendor. You can also test with an external antenna. And please, test outside in open space.

[Øyvind]

Are you able to get a modem trace while it is failing to get a fix? This should help our R&D team see what is failing.

[mlang3440]

I'm working on getting a modem trace.. Do you need to see the Log output as well?

[Øyvind]

Yes, please. 

Where are you conducting your tests? Where is the GPS receiver when you are tracking the satellites?

[Hucky]

Is there any news on this issue? I do see the same issue with tracking several SVs but not using any of them on my Thingy 91 occasionally happening too.

[RicardoPereira]

Hi Hucky, 

Did you find the solution ? I'am having the same issue 

[Jan Tore Guggedal]

Hei Ricardo (and others having the same issues of seeing many satellites tracked, but no fix),

There's a PR that can help us debug this issue a bit without capturing modem traces: https://github.com/nrfconnect/sdk-nrf/pull/2385

This PR introduces NMEA-only output to the GPS sample (found in samples/nrf9160/gps/), which might help us narrow down what's going on.
Until the PR is merged into NCS, you can experiment with it by using git and pull in the changes like this (assuming your NCS git remote is named "ncs", otherwise change it to your remote's name):

git fetch ncs refs/pull/2385/head
git checkout FETCH_HEAD (add " - b <branch_name>" if you want it on a branch)
west update

Now you have a new option in the GPS sample Kconfig. You can enable it by setting CONFIG_GPS_SAMPLE_NMEA_ONLY=y (note that it's default "n" /disabled  at the bottom line of the sample's prj.conf). After setting the option, you can build and flash using the tool you prefer.
I'm attaching hex files for nRF9160 DK and Thingy:91:
gps_sample_nmea_nrf91dk.hex
gps_sample_nmea_thingy91.hex

In the log output, you'll see NMEA strings only.

You can share this log with us, and will see if we can see what's going on. Note that you can create a private ticket and share the files there if you prefer.

In some cases, we'll also need modem traces to know exactly what's going on.

Best regards,

Jan Tore