nrf9160, AGPS works longer than GPS

[Stas Jis]

We use nRF9160 on our own board.
The firmware "Serial lte modem" is installed on the chip with a connection to an external MCU (#define MQTT_MESSAGE_BUFFER_LEN 8192)
We tested the environment with periodic switching from AGPS to GPS.
And in tests, we constantly see that the operating time with AGPS is twice as long as with just using GPS.

Average waiting time when using AGPS: 1 minute + 10-15 seconds;
Average waiting time when using GPS: 30-40 seconds.

What could be the reason for such behavior? It was expected that the operating time with AGPS would be significantly faster than with GPS.

The command sequence for AGPS:
AT+CEREG=5
AT+CFUN=1
AT#XNRFCLOUD=1
AT#XAGPS=1,1

The command sequence for GPS:
AT+CEREG=5
AT+CFUN=31
AT#XGPS=1,1

The tests were carried out statically, without movement.

[Stas Jis]

Dear Håkon Alseth,
The links you provided are very useful for reference, but they do not give me a solution on how to convert LTE to IDLE mode, to speed up getting coordinates via AGPS.
Is there an AT command that I can use to manually put LTE into Idle mode?

[Håkon Alseth]

Hi,

 

Stas Jis said:

The links you provided are very useful for reference, but they do not give me a solution on how to convert LTE to IDLE mode, to speed up getting coordinates via AGPS.

This happens based on what the network sets, and after a given period of idle from your application (ie. not any more data to send).

Stas Jis said:

Is there an AT command that I can use to manually put LTE into Idle mode?

If you check the CSCON notification, you shall be notified on when the link is idle.

 

Kind regards,

Håkon

[Stas Jis]

Dear Håkon Alseth,
If I check CSCON, the status notification is "0" (Idle) 15-18 seconds after connecting to the nRF cloud and executing command AT#XAGPS=1,1. This is unacceptably long for a low-power energy device.

[Håkon Alseth]

Hi,

 

This is because it connects, over IP, to nrf cloud to download the GPS almanac, time-of-week, ephemerides etc.

You are effective downloading a binary blob and pushing this to the GNSS (ie. GPS), and you must wait until the LTE Link is idle before starting it. This is what takes more time compared to just starting the GPS.

 

If you already have refreshed the AGPS, you can just start the GPS with AT#XGPS instead.

 

Kind regards,

Håkon

[Roman1982]

Hello 

we perform the initial tests with SDK 1.7.1 and observe totally different behavior .

the AGPS including the connection and download files taken about 15-20 sec until GPS lock.

from your reply i understand that there is no reason to use AGPS? 

[Håkon Alseth]

Hi,

 

You are counting from the wrong timestamp. You must compare the two options when the GPS is actually running.

When using AGPS, you will first download GPS data from the internet, then you'll have to wait until the GPS can run (ie. LTE Link is idle) before you will get valid pvt/nmea frames.

 

If your conclusion is that you do not need AGPS, that is an application specific decision, but we do offer this to avoid a cold start scenario for the GPS. 

 

Kind regards,

Håkon