RE: nRF9160 GPS Cold Fix Guidance

Didrik Rokhaug — Thu, 19 Mar 2020 14:25:43 GMT

I asked our GPS expert for input, and here is what he answered:

Cold start can roughly be divided into satellite search and ephemeris download.

With a good antenna and open sky, the satellite search could very well be done in 15 s. In more challenging conditions, the satellite search may take considerably longer. This is not a big problem, GPS just interrupts its activities when LTE uses the radio, and continues where it left off when it gets the radio back. In these conditions, satellite search just takes 15 seconds longer per interrupt.

Ephemeris download is more tricky. Nominally, there is 18 seconds of continuous reception needed to download the full data. Ideally, GPS should be provided at least 36 seconds to guarantee full download of ephemeris.

If needed, Ephemeris can be split into 3 x 6 seconds of download, whereby the 15 s LTE , 15 s of GPS is enough. However, in this case the LTE could overlap with the time instant when Ephemeris is available from the satellites. If now the cycle is a multiple of 30 seconds, it could take a very long time to actually wait for the timing to change so that Ephemeris availability coincides with GPS having the RF (LTE eDRX is actually a multiple of 10.24 seconds, while Ephemeris is available in an exactly 30 seconds cycle, so there is indeed a timing difference, albeit very small).

So the recommendation is: If possible, use a long enough interval between LTE transmissions, at least 1 minute, but the longer the better. Secondly, if possible, do not use an eDRX period that is (close) to a multiple of 30 seconds, e.g. 50 or 70 seconds would be better than 60 seconds.

Almanac suffers from same issues as Ephemeris, however Almanac is not crucial for cold fix.

In other words, you should be fine, though you do risk having to wait for a very long time if you have bad luck with the timing. You could avoid a lot of these problems by using A-GPS. That will let you download the almanac and ephemerides over LTE, and provide it to the modem.

RE: nRF9160 GPS Cold Fix Guidance

GNotman — Wed, 18 Mar 2020 11:22:34 GMT

Hi,

I guess my question is really, how might these interruptions affect the performance of achieving the first fix?

How much of an impact might these interruptions have on the device's ability to download the Almanac and Ephemeris data?

Due to the interruptions, some parts of that data will be missing. For the Ephemeris that should be too much of a problem as the transmission period is 30s. The Almanac data however, is transmitted with a 750s period.

RE: nRF9160 GPS Cold Fix Guidance

Didrik Rokhaug — Wed, 18 Mar 2020 11:00:47 GMT

Hi.

If the GPS gets interrupted, it should continue where it left off the next time it starts.

So you should eventually be able to get a fix.

Best regards,

Didrik