Nrf9160 GPS long time to fix with healthy RF electronics

[bjornshnrf]

Helo Nordic support team,

I am supporting a client that has designed a product using the NRF9160 with GPS. The product has a small patch antenna mounted on the pcb board, a saw filter and a LNA upstream the GPS port of the NRF9160.

The product has been tested extensively in the open and they experience long time to fix - in some cases 10 to 15 minutes from cold start (and some time no fix at all). It was suspected that the antenna gain and match could explain long fix time due to lov SNR for the received GPS signal. The antenna match was investigated using a network analyser and found to be around -10dB RL for the GPS band.

We have compared the product respons with a Thingy:91 using a larger patch antenna with integrated LNA. This gave similar results with long time to fix (not tested as thorough as the product).

Next test was done in a lab version of an anechoic chamber called GTEM cell. It is not a perfect environment for antenna performance tests but gives a good indication on performance. Using this GTEM device with a signal source we were able to apply a -90dbm CW signal to the product (now running only with test software and battery power). The test software logged the SNR and RSSI using the AT AT%XRFTEST=2,1,-90 at command and after the test we extracted the data from the product to a log file using a J-link SEGGER with RTT Viewer. We did a similar test with the Thingy:91 with the external antenna and got similar results.

Our preliminary conclusion is that the product (and the Thingy:91) has good RF performance (around 14dB gain including LNA, SAW and antenna gain).

I tend to conclude that the long time to fix can be explained by software. 

There are no other use of the radio between GPS read, the LTE is not used.

Can the Nordic team comment on why do we experience long time to fix from cold condition even with apparently healthy RF electronics ?

Thanks,

Regards

Bjørn

[bjornshnrf]

Hi,

The client ran a test last week.

They made 15 consecutive tests with the GPS on a parking lot in front of the office. The products are powered and produce logs via a USB connector connected to a portable PC (this is slightly different compared to the tests made in Drammen).

The tests were executed in the following manner with two devices:

- Start both devices (giving them power)

- start logging

- Wait until both devices have fix.

- Power off

- Reiterate test

This was iterated 15 times with two in theory identical devices with the same SW setup (nrf SDK v1.4 base, using GPS driver in continues mode, no AGPS, modem fw 1.2.3)

These are the time to fix (in seconds, considering uptime of device) values we obtained:

Iteration    Device 1:        Device 2:
1                207                117
2                181                52
3                110                49
4                94                  39
5                92                  36
6                44                  53
7                145                51
8                43                  37
9                93                  47
10              51                  53
11              124                44
12              58                  51
13              70                  37
14              89                  32

Observations:

    - Large difference in time to fix between devices even though they are running at the same time (Device 1 took longer to get fix in all iterations except iteration 8 and iteration 10).

25022021.zip

 

[bjornshnrf]

Hi,

We also added some tests from a location in Drammen.

The test is on a Thingy:91 with external antenna and LNA. The Thingy is placed on the rooftop of the car and 15 cold start measurements were recorded (a cold start is a power on cycle when the unit is completely disconnected from any power supplies).

The next test is with the Thingly located on the dashboard of the car. This time we only took 5 measurements all from a cold start.

The last test was with the product located on the dashboard. 15 tests were recorded.

  thingy (roof) :  Product (dashboard)
1    33        52
2    37        46
3    21        32
4    21        52
5    33        43
6    37        41
7    38        44
8    39        48
9    37        42
10  36        42
11  29        48
12  43        46
13  38        52
14  36        42
15  38        35

      thingy (dashboard)
1    33
2    37
3    30
4    31
5    36

The log files from these measurement where created by connecting the JLink to the Thingy/Product. No USB ocnnection was used.

Thanks,

Regards

Bjørn

Thingy_27_2_Sundland.zip

[JONATHAN LL]

So the values that you gett from your test in Drammen looks to be completely normal\expected results, and the values fromt he parking lot test with the two custom HW then Device 2 seems to be within the limit of normal expected results. except for the first iteration.  Device 1 has a bit higher time in general but i asume that the two devices are identical ?  The ground plane of the Device might change when connecting the device to laptop with a usb cable,  then the behaviour of the GPS antenna might also change, that is somthing we have seen on the Thingy91. Are you able to test on Device 1 and 2 without a usb cable?

~20 seconds for GPS from cold start is as quick as it gets. 

Regards,
Jonathan

[bjornshnrf]

Hi Jonathan,

Thank you for your answer.

Could you comment the data fro the GTEM cell and comment on the SNR and RSSI ?

Thanks,

Regards

Bjørn 

[JONATHAN LL]

So the results indicate\shows that the better the CN0 value the quicker TTFF(Time to firt fix).Better antenna leads to better CN0 values witch leads to better TTFF. 

Device 2 seems to be much better, both in terms of TTFFs and CN0s. Is there a difference in the two devices ?

The values you got for the thingy is as good as it gets and everything that is in the 40s range is very good. 

Regards,
Jonathan