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.

[Håkon Alseth]

You only need to load the AGPS data once, to ensure that you do not start the GPS in a cold start (which takes min. 40 seconds to obtain fix).

Subsequent fixes does not need to use AT#XAGPS.

 

There are some scenarios where you usually want to always fetch AGPS, but this is normally when obtaining a location fix in a higher interval (>4 hours).

Roman1982 said:

2. stream continues location one at 1 min

This can potentially be problematic in some networks, where you're not able to enter PSM (or other idle modes) this quickly. All networks has a required paging period, and that can potentially block the GPS from running.

 

Kind regards,

Håkon

[Roman1982]

what the sequence ? 

can you please advice the sequence for example : 

once in 4 hours we need perform an assistance table load .

for doing this we need to use L 

AT+CEREG=5
AT+CFUN=1
AT#XNRFCLOUD=1
AT#XAGPS=1,1

and check the status of update with the next command:

XXXXXXXX 

in case the tables are update we suppose to use : 

AT+CEREG=5
AT+CFUN=31
AT#XGPS=1,1 

in order to have a fast lock 

please advice the sequence 

[Håkon Alseth]

Hi,

 

Roman1982 said:

AT+CEREG=5
AT+CFUN=1
AT#XNRFCLOUD=1
AT#XAGPS=1,1

This sequence will fetch AGPS from nrfcloud, and then start the GPS.

Roman1982 said:

in case the tables are update we suppose to use : 

AT+CEREG=5
AT+CFUN=31
AT#XGPS=1,1 

This will activate GPS (without changing the LTE permissions), and start the GPS directly (ie. no AGPS fetching from nRFCloud).

 

Note that you are continuously running the GPS now, as per the documentation:

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/2.0.0/nrf/applications/serial_lte_modem/doc/GNSS_AT_commands.html#run-gnss

 

Kind rergards,

Håkon

[Roman1982]

what the period for tables refresh ?

what the AT command that assistant tables update is complete?

(in order not to wait a lot of time)

does ordinary GPS operated always with assistant table (AT+CFUN=31 .AT#XGPS=1,1)

or we need to configure it that it starts with hot start

[Håkon Alseth]

Hi,

 

Ephemerides are valid in 4 hours (+/- 2 hours), and the almanac is valid for up to 90 days. 

The GPS will start from a warm/hot start if the interval is <4 hours since last successful fix.

 

If the above is in order, it will provide a fix within a shorter time period, provided that you are able to track satellites successfully.

Roman1982 said:

or we need to configure it that it starts with hot start

No configuration needed, other than just starting the GPS.

Roman1982 said:

what the period for tables refresh ?

what the AT command that assistant tables update is complete?

When the GPS is running, it will continuously fetch the latest updates from the satellites. This happens automatically.

 

Kind regards,

Håkon