NRF9160 - NO GPS Fix, Good C/N0 with 12 satellites

[GabeA]

I have read through the other posts related to GPS seeing satellites but without lock but they all seem to come down to bad signal quality. We have a device that is not getting lock (> 12 hours).

We are using a custom PCBA but an external GPS antenna with built in amplifier. Our on-board circuit is very simple, just a pull up inductor and a DC coupling capacitor.

Modem firmware is 1.3.4

We are not using A-GPS but assume that A-GPS would greatly help this problem. Some of our devices do not have cellular reception though.

Signal Quality data for this device is good and C/N0 can be seen here.

This morning:
"tracked": 12,
"fixed": 0,
"cn0": 

"0": 34.9,
"1": 26.7,
"2": 49.5,
"3": 44.7,
"4": 31.0,
"5": 46.4,
"6": 42.2,
"7": 44.0,
"8": 32.2,
"9": 26.3,
"10": 26.7,
"11": 24.5

Last night:
"tracked": 11,
"fixed": 0,
"cn0": 

"0": 47.5,
"1": 46.6,
"2": 40.1,
"3": 47.3,
"4": 43.1,
"5": 38.9,
"6": 41.0,
"7": 46.0,
"8": 42.0,
"9": 42.5,
"10": 49.6,
"11": 49.0

[Charlie]

Hi Gabe,

I suggest you generate a private case to request a HW review first.

At the same time, you can do a side-by-side TTFF measurement test with nRF9160DK using the same external antenna. 

The result and log printouts will help to understand what happens.

You can refer to the following blog about TTFF measurement. Please try under the open sky outdoor if possible.

(+) Up to 4x battery life: The benefits of assisted GPS in asset tracking - Blogs - Nordic Blog - Nordic DevZone (nordicsemi.com)

Best regards,

Charlie

[GabeA]

Charlie , Thank you for your response, I have been able to gain more insight into the problem and would like your feedback. 

Our devices are running in psm in a connected state with GPS enabled. Normally these devices work just fine and the GPS is able to get enough time between transmissions to get a lock and continue to keep a good signal over time. 

If I run a unit that is struggling and then switch to LTE off (i.e. CFUN=20) the device get's GPS lock in under 30 seconds and everything works as expected so I think we can determine that this is not a hardware issue.

It seems that the issue occurs only on some towers that may not be allowing PSM to be enabled properly. What would you recommend in this case for allowing the modem enough down time to get GPS lock regularly? We are running a geofence so need accurate continuous GPS readings.

There is another related post from 4 years ago:
https://devzone.nordicsemi.com/f/nordic-q-a/51756/gps-without-psm-using-external-antenna-on-thingy-91

How are we supposed to use GPS reliably without PSM? I can put the modem into CFUN=20 for 4 minutes of every 5 minutes but I think the carriers would not like this? If switching between CFUN=20 and CFUN=21 on that 5 minute interval is alright then we can move forward that way.

Thanks,

Gabe

[Charlie]

Hi Gabe,

Thanks for sharing such information. Let me comment on them one by one.

GabeA said:

Our devices are running in psm in a connected state with GPS enabled. Normally these devices work just fine and the GPS is able to get enough time between transmissions to get a lock and continue to keep a good signal over time. 

GabeA said:

If I run a unit that is struggling and then switch to LTE off (i.e. CFUN=20) the device get's GPS lock in under 30 seconds and everything works as expected so I think we can determine that this is not a hardware issue.

Yes, you are correct. The hardware should have no problem if GPS works in PSM, LTE connection is deactivated and RRC Idle mode as you observed, which is also clarified in the nRF9160 specification GNSS chapter.

I would say it is a limitation of nRF9160 instead of an issue since its GNSS operation is designed to be time multiplexed with LTE modem on chip considering interference, radio performance and so on.

GabeA said:

It seems that the issue occurs only on some towers that may not be allowing PSM to be enabled properly.

As far as I know, PSM feature is not decided by a single tower but by the network operator. If the local network operator enabled this feature on their network, then your device can use this feature as long as it uses SIM card in its home network. If your device uses a roaming network, it depends on the roaming agreement between these two operators. It is common that the PSM feature is disabled for roaming devices.

GabeA said:

What would you recommend in this case for allowing the modem enough down time to get GPS lock regularly? We are running a geofence so need accurate continuous GPS readings.

GabeA said:

How are we supposed to use GPS reliably without PSM? I can put the modem into CFUN=20 for 4 minutes of every 5 minutes but I think the carriers would not like this? If switching between CFUN=20 and CFUN=21 on that 5 minute interval is alright then we can move forward that way.

This is related to your application design. It is OK to deactivate and activate the LTE connection periodically to get a GPS fix if the power consumption is not the concern.

Regarding your concern about carriers don't like this, there is actually a related restriction about activation/deactivation requests by nRF9160 MFW. The GSMA specified Radio Policy Manager (RPM) feature for blocking the ATTACH command (TS.34_8.2.4_REQ_010, page 43 in this pdf) specifies 30 activation/deactivation requests per hour. nRF9160 MFW just follow this restriction. You plan to do this 12 times per hour should have no problem.

It is worth mentioning that we recently released MFW v1.3.6 for nRF9160. It mainly improved the GNSS performance. You can refer to the release note on its downloading page. nRF9160 DK - Downloads - nordicsemi.com

Best regards,

Charlie