A-GPS Data Consumption

[Akash Patel]

Hello,

What the average data consumption for using A-GPS on the nRF9160 to download the Almanac and Ephemeris data? I would like to know how much data is billed through my carrier for performing this operation.

Regards,

Akash Patel

[Didrik Rokhaug]

Hi,

Exactly how much data is used depends on the protocol used, the data available, etc.

There was some numbers in the nRF91 Software roadmap from February:

(with SUPL, if I understood the powerpoint correctly):

- Cold start, All/most ephemeris/almanac data: 10s of kB

- Warm start, some ephemeris data: 1-10kB

- Hot start, no new data needed: 0B

Meanwhile, A-GPS through nRF Cloud was said to use ~4.5kB.

But you should be able to measure it yourself by using the %XCONNSTAT AT command.

Best regards,

Didrik

[Akash Patel]

Hi Didrik,

The nRF Cloud 4.5kB is for a cold start or warm start? What is it for the other?

Also, does that mean that nRF Cloud is not using SUPL? What is it using?

Regards,

Akash

[Didrik Rokhaug]

Akash Patel said:

Also, does that mean that nRF Cloud is not using SUPL? What is it using?

 No, it is not using SUPL. According to the software roadmap, it is using a "packed binary format", which is sent over the MQTT connection that has already been established.

 

Akash Patel said:

The nRF Cloud 4.5kB is for a cold start or warm start? What is it for the other?

 I asked the application team if they had any more information. This is the answer I got:

It looks like the total TCP payload is about 3.5 kB for A-GPS request and response, but this may vary a bit

For a cold start all assistance data is fetched, ie almanac, ephemeris, time, location estimate etc

For a warm start, it's assumed that only ephemeris is needed, which is about 2k for all satellites

[Didrik Rokhaug]

Another developer on the apps team did some cold start tests with an nRF9160 Feather which is running the A-GPS sample and using a hologram.io SIM. Those tests showed 16.91kB data used:

Note that these numbers include all the data sent between the device and the cloud, including the TLS handshakes which uses ~10kB data.

[Akash Patel]

Hi Didrik,

Looks like we are using this agps_write function to write the A-GPS data we have received to the modem before we switch to GPS mode. When we write the almanac/ephemeris data to the modem, is that get erased when we do a power cycle?

From my understanding the almanac data should be valid for a several weeks and the ephemeris data should be valid for up to 2 hours. So essentially, If I write this A-GPS data, use GPS for 5 minutes, power cycle, turn on GPS again, will I have to rewrite the A-GPS data or the GPS will be able to perform a hot start?

Regards,

Akash

[Didrik Rokhaug]

Akash Patel said:

If I write this A-GPS data, use GPS for 5 minutes, power cycle, turn on GPS again, will I have to rewrite the A-GPS data or the GPS will be able to perform a hot start?

 Yes, I believe that is the case.

However, if you turn the modem off gracefully with AT+CFUN=0, it should store the data to non-volatile memory so that you would not have to fetch it all again after a power cycle.

Although there is still information that has to be fetched each time, such as time, approximate location, etc.