nRF9151 no GPS fix

[Urmaz]

Hi,

My custom boards with nRF9151 don't get gps fix. I'm using the same GPS antenna and LNA as DK. Soft is from DevAcademi example, but tried also GGSN sample and some other samples, but never got fix. Same soft with nRF9160 board works ok (devicetree overlay is different). UART, SPI, I2C, sensors, flash, pMIC, all ok.

When GPS cycle starts, then always during 2minutes and 10sec nRF9151 finds only 0...3 satellites, all with cn0: 0. On 2:10 satellite count jumps to 12, all with good cn0. It lasts 6...7sek, then drops again to 0...3 sat, but this time cn0 is different from 0 (seems real). It repeats with every GPS cycle, never gets fix and always 2:10 from start and lasts 6...8sek. Current monitored with PPK2 seems normal. 

LNA is connected to COEX0, but tested it also with always power on - same result.

Tried modem FW 2.0.0, 2.0.2, SDK 2.7, 2.8, 2.9 - no diffrence.

Tried different LTE PS modes and values - same GPS pattern. 

Tried to set GPS priority over LTE - same.

Tried with two boards (same batch) - same pattern.

NRF9151-LACA-R7 bought from Mouser, details installed to PCB in factory.

Br 

Urmas

...

[00:02:13.645,202] <inf> gw2_2: Searching...

[00:02:13.649,810] <inf> gw2_2: Number of current satellites: 0

[00:02:14.569,488] <inf> gw2_2: Searching...

[00:02:14.574,066] <inf> gw2_2: Number of current satellites: 0

[00:02:15.573,944] <inf> gw2_2: Searching...

[00:02:15.578,521] <inf> gw2_2: Number of current satellites: 0

[00:02:16.574,371] <inf> gw2_2: Searching...

[00:02:16.578,979] <inf> gw2_2: sv: 4, cn0: 207, signal: 1

[00:02:16.584,838] <inf> gw2_2: sv: 11, cn0: 210, signal: 1

[00:02:16.590,759] <inf> gw2_2: sv: 16, cn0: 217, signal: 1

[00:02:16.596,710] <inf> gw2_2: sv: 3, cn0: 288, signal: 1

[00:02:16.602,569] <inf> gw2_2: sv: 31, cn0: 213, signal: 1

[00:02:16.608,489] <inf> gw2_2: sv: 1, cn0: 213, signal: 1

[00:02:16.614,349] <inf> gw2_2: sv: 24, cn0: 207, signal: 1

[00:02:16.620,269] <inf> gw2_2: sv: 30, cn0: 211, signal: 1

[00:02:16.626,220] <inf> gw2_2: sv: 23, cn0: 207, signal: 1

[00:02:16.632,171] <inf> gw2_2: sv: 7, cn0: 215, signal: 1

[00:02:16.638,000] <inf> gw2_2: sv: 199, cn0: 214, signal: 3

[00:02:16.644,042] <inf> gw2_2: sv: 25, cn0: 220, signal: 1

[00:02:16.649,963] <inf> gw2_2: Number of current satellites: 12

[00:02:17.574,401] <inf> gw2_2: Searching...

[00:02:17.579,010] <inf> gw2_2: sv: 4, cn0: 239, signal: 1

[00:02:17.584,869] <inf> gw2_2: sv: 11, cn0: 242, signal: 1

[00:02:17.590,820] <inf> gw2_2: sv: 16, cn0: 241, signal: 1

[00:02:17.596,740] <inf> gw2_2: sv: 3, cn0: 252, signal: 1

[00:02:17.602,600] <inf> gw2_2: sv: 31, cn0: 239, signal: 1

[00:02:17.608,551] <inf> gw2_2: sv: 1, cn0: 243, signal: 1

[00:02:17.614,379] <inf> gw2_2: sv: 24, cn0: 244, signal: 1

[00:02:17.620,330] <inf> gw2_2: sv: 30, cn0: 243, signal: 1

[00:02:17.626,281] <inf> gw2_2: sv: 23, cn0: 241, signal: 1

[00:02:17.632,202] <inf> gw2_2: sv: 7, cn0: 242, signal: 1

[00:02:17.638,061] <inf> gw2_2: sv: 199, cn0: 242, signal: 3

[00:02:17.644,104] <inf> gw2_2: sv: 25, cn0: 244, signal: 1

[00:02:17.650,024] <inf> gw2_2: Number of current satellites: 12

[00:02:18.573,486] <inf> gw2_2: Searching...

...

[00:02:22.574,523] <inf> gw2_2: sv: 4, cn0: 246, signal: 1

[00:02:22.580,383] <inf> gw2_2: sv: 6, cn0: 255, signal: 1

[00:02:22.586,242] <inf> gw2_2: sv: 16, cn0: 243, signal: 1

[00:02:22.592,163] <inf> gw2_2: sv: 31, cn0: 244, signal: 1

[00:02:22.598,114] <inf> gw2_2: sv: 1, cn0: 246, signal: 1

[00:02:22.603,973] <inf> gw2_2: sv: 24, cn0: 246, signal: 1

[00:02:22.609,893] <inf> gw2_2: sv: 30, cn0: 245, signal: 1

[00:02:22.615,844] <inf> gw2_2: sv: 23, cn0: 247, signal: 1

[00:02:22.621,765] <inf> gw2_2: sv: 7, cn0: 249, signal: 1

[00:02:22.627,624] <inf> gw2_2: sv: 199, cn0: 243, signal: 3

[00:02:22.633,636] <inf> gw2_2: sv: 25, cn0: 242, signal: 1

[00:02:22.639,587] <inf> gw2_2: Number of current satellites: 11

[00:02:23.568,969] <inf> gw2_2: Searching...

[00:02:23.573,577] <inf> gw2_2: sv: 6, cn0: 249, signal: 1

[00:02:23.579,437] <inf> gw2_2: Number of current satellites: 1

[00:02:24.568,847] <inf> gw2_2: Searching...

[00:02:24.573,455] <inf> gw2_2: sv: 6, cn0: 249, signal: 1

[00:02:24.579,315] <inf> gw2_2: Number of current satellites: 1

[00:02:25.568,420] <inf> gw2_2: Searching...

[00:02:25.573,028] <inf> gw2_2: Number of current satellites: 0

[JONATHAN LL]

Hi,

Have you had the custom board reviewed by us? 

What antenna are you using? 

Regards,
Jonathan

[Urmaz]

Not reviewed.
Antenna is DSGP.1575.18.4.A.02.

U.

[JONATHAN LL]

Comments to the schematic:
From the parts that you have shared it looks like a few of our recommended components are missing for the nRF9151. 

For the VDD you should have the extra components for filtering of the power rail. It is needed for the device to preform as expected an laving them out will impact RF and GNSS performance. 

For the nPM1300 we also recommend to have a few extra components on the VOUT1 for filtering power noise. 

See the nRF9151DK or the Thingy91X as reference: https://nsscprodmedia.blob.core.windows.net/prod/software-and-other-downloads/dev-kits/thingy91-x/hardware-files/pca20065-thingy91-x-1_0_0.zip 


In both logs there's a jump in the number of acquired/tracked satellites after 2 minutes, which you also described above. This is caused by GNSS switching the acquisition mode. Those ~12 satellites which are shown for less than 10 seconds are not real satellites, but just noise.


Regards,
Jonathan

[Urmaz]

Sorry, as you asked for part of schematic, I missed part of it. 

[Urmaz]

I can share full schematic and PCB drawings in private conversation. Is it normal, that nrf9151 shows "false satellites" for 10 seconds? Why it don't aquire those more time? As I have read, first fix sometimes needs 15 minutes to get. I haven't seen this kind of behavior with Thingy91 or 9160.

[Urmaz]

I got it working. Thank you for your help!

Issue was with GPS antenna tuning circuitry. On schematics are L and C, that are NC, but on PCB were real LC, for wrong frequency. As modem saw 12 sat, I didn't dig in right place. It would be useful if the documentation mentioned somewhere which cn0 values ​​indicate good reception and that the modem can also show ghost(/noise) satellites.

[JONATHAN LL]

Urmaz said:

I got it working. Thank you for your help!

Great!

Urmaz said:

if the documentation mentioned somewhere which cn0 values ​​indicate good reception and that the modem can also show ghost(/noise) satellites.

Will bring it up internally and and maybe we are able to update some docs or if we can get that info through some other way. Thanks for the feedback. 

But C/N0 (cn0) values it is generally considered that you should have 30dB-Hz or higher to get tracking. As you can see you get around 20 to 25 and that will not be sufficient. 

Urmaz said:

As I have read, first fix sometimes needs 15 minutes to get.

Where in the docs did you see 15 minutes? 

Cold start can result in a slow time to fix, but should be 30.5sec or better, hot fix should yield 1.3 sec in best cases on average. 
See docs : https://docs.nordicsemi.com/bundle/ps_nrf9151/page/gps.html#ariaid-title2 

Urmaz said:

I can share full schematic and PCB drawings in private conversation

We do full reviews, just create a private ticket and we will review schematic and layout for future projects. 

Regards,
Jonathan