Please note that the very first fix after device start-up is a rather different measurement than any of the following consecutive fixes.

[Roman Veselskyi]

Hello,

We’re testing the nRF9160 GNSS performance on our custom PCB and have noticed that, even with A-GPS, the first fix after powering on the device takes significantly longer than subsequent fixes. This is a bit confusing because we’re using the GNSS sample for testing, and it should clear the GPS data during TTFF testing. While reading an article, we came across the following statement:

"Please note that the very first fix after device start-up is a rather different measurement than any of the following consecutive fixes."

Could someone please explain why this first fix behaves differently and is also related to AGPS? 

[Edvin]

Hello,

I believe this would be as expected. Even though you have A-GPS, you still need to download the Almanac from the network. So you need to connect to the cellular network, download the Almanac, and then use this to get a fix (which would be quicker than doing a cold start of the GPS). But when you have your first fix, the GPS data is already present on the nRF91, and it is more accurate than the Almanac data that you would download (I think). So 1: You don't need to download the data, and 2: it is more accurate. Therefore the consecutive fixes will be quicker.

Please note that we are short staffed due to public Holiday in Norway. Please expect some delay in our answers for the next week. We will be almost full staffed from Tuesday next week.

Best regards,

Edvin

[Roman Veselskyi]

Hello,

Thank you for your response, but with all due respect, your explanation unclears for me. Using phrases like “I believe” or “I think” can create concern that even the chip manufacturer's support team is unsure about how the device works.

Have you checked the GNSS sample in TTFF (Time to First Fix) testing mode? The usual approach is for the firmware to delete all GPS data for each test, then search for satellites from a cold start. You can enable AGPS or run it without AGPS (assisted or unassisted GNSS).

When we delete the GPS data, we expect each fix to require roughly the same amount of time. However, we see the first fix taking longer. This raises the question: is the sample not actually deleting all GNSS data, or is there something else at play?

This issue is critical for our device because we request GPS location infrequently—perhaps only once a month. If the first fix, even with AGPS enabled, does not benefit from faster acquisition (because additional satellite data is still required), then AGPS offers no real advantage. We only need that one fix, so faster subsequent fixes provide no benefit.

[Edvin]

Hello Roman

Roman Veselskyi said:

Using phrases like “I believe” or “I think” can create concern that even the chip manufacturer's support team is unsure about how the device works.

I understand. Sorry for the confusion. We are very short staffed due to public holiday here in Norway, and I though an answer would be better than no answer. I have barely worked with the nRF91-series before, and due to the low staffing, I didn't have time to read up or test. So this is all based on some things that I learned a long time ago, but haven't looked at since. 

I will hand this ticket over to someone with more nRF91 experience, but please be aware that due to the public holiday in Norway, they will not return until mid-next week. I am sorry for the inconvenience.

Best regards,

Edvin

[Edvin]

Hello,

I discussed this with a colleague now, and even though you delete the GPS data, parts of this is still stored in RAM. You need to reset the nRF after deleting the GPS data to make the TTFF consistent. 

It is mentioned in the ncs\nrf\samples\cellular\gnss sample's readme that you should:

"1. Connect your nRF91 Series DK to the PC using a USB cable and power on or reset your nRF91 Series DK."

If I understand your claim correctly, you are saying that it takes longer the first time you get the fix than the consecutive ones (after deleting the GPS data). Try deleting it, and then reset the application, and you should see that the time is more consistent.

Best regards,

Edvin