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.

[phipi]

Some more info about this:

It seems that on reset the GPS fix works every time but eventually gets into this state where it's tracking multiple SVs but not using any. If we reset when it's in that state it will get a fix on boot, but then return to the same fail mode on the next fix which happens several (4~ 6) hours later?

Is it possible that this is related to the modem clock? On boot we get the time from an NTP server (time.nrc.ca) which is the only item that we can identify as different from the fix that happens several hours after boot.

Is there anything we need to do to keep the modem time updated?

[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.

[Pema]

Just dropping in on this question... I was under the impression that the LTE and GPS couldn't be operated at the same time, you can't use them simultaneously they have to be switched in between. So how can the LTE generate intermodulation to the LNA then?

Peter