nRF 9160 P-GPS specific ephemerides

[Oette]

Hi,

I am working with a custom board that has an nRF9160 SiP (B0). I am using SDK v1.6.0 on a Windows 10 machine. Modem FW is v1.2.3.

Our device uses GPS periodically once per week. I would like to know if it is possible to download the ephemerides only for a specific timeframe to save battery life by reducing the amount of data transfered. The GPS-search occurs at the same time every week so one ephemerides prediction per week would be enough.

I would also like to ask about the importance of A-GPS in combination with P-GPS. It is not completely clear to me how they work together.

If I download the predictions for the next two weeks, how accurate are they going to be towards the end? Does the device need to download additional information and if it does, do you think it will still be an improvement power consumption-wise compared to just using A-GPS?

If I use P-GPS only and the predictions are not good enough, is it going to download the ephemerides directly from the satellites? Lastly if the predictions are not good, is some of the information used and thus the amount of downloaded data reduced?

Thanks in advance!

[Pete Skeggs]

Hello,

Oette said:

Our device uses GPS periodically once per week. I would like to know if it is possible to download the ephemerides only for a specific timeframe to save battery life by reducing the amount of data transfered. The GPS-search occurs at the same time every week so one ephemerides prediction per week would be enough.

Yes, you can specifically request a single prediction for some specific GPS day and GPS time of day within the coming two weeks. See:
https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/include/net/nrf_cloud_pgps.html#c.gps_pgps_request

Oette said:

I would also like to ask about the importance of A-GPS in combination with P-GPS. It is not completely clear to me how they work together.

When an application uses both A-GPS and P-GPS, it will have the following behaviors:

1. P-GPS data will be downloaded and stored to flash for the ephemerides for the configured number of predictions.

2. Whenever the application needs a GPS fix, and a cloud connection is active, a minimal set of A-GPS assistance data will be downloaded.  No ephemerides or almanacs will be downloaded, but at most rough location data, time data, ionosphere correction data, and integrity data.

3. When the application needs a GPS fix, and a cloud connection is not active (due to being out of range of any cell towers, or due to the application's need to conserve power), the P-GPS library will attempt to provide rough location and time data, if properly configured. The most recent known location will be provided if the application is built to provide this. The current time will be provided if the system time has been set by the date time library at some point since the last reset, or if the application utilizes an external clock / calendar chip to keep track of time.

These 3 components -- ephemerides, rough location, and accurate time -- are required for a fast fix; the others simply improve it.

Oette said:

If I download the predictions for the next two weeks, how accurate are they going to be towards the end?

Towards the end, the accuracy of the fix will degrade slightly, and the time to get a fix will increase a bit. It's hard to provide exact estimates.

Oette said:

Does the device need to download additional information and if it does, do you think it will still be an improvement power consumption-wise compared to just using A-GPS?

Whether it is an improvement power consumption-wise depends on many factors. If the system clock is accurate, and if the distance of the device compared to the last known fix is not larger than ~150 km, and the application implements last known location storage as shown in asset tracker with the CONFIG_PGPS_STORE_LOCATION option enabled, then no additional assistance will need to be downloaded.

In your case, since a fix is only needed rarely, you would probably get faster fixes and consume less power if you simply use A-GPS.  P-GPS is more beneficial when you need frequent fixes (many times in a week) during periods of no cloud connectivity.

Oette said:

If I use P-GPS only and the predictions are not good enough, is it going to download the ephemerides directly from the satellites?

It may do so for satellites for which predicted ephemerides are missing, which can occur towards the end of a two week period.

Oette said:

Lastly if the predictions are not good, is some of the information used and thus the amount of downloaded data reduced?

Thanks in advance!

See above.