Link between microG location services and system location settings (A-GPS and SUPL)

Apps & Core Settings
1

Hi!
I wonder how the microG location services are linked to the system location settings, especially A-GPS and SUPL.

I don’t fully understand the mechanisms here. Please correct me if I am wrong.

I understand the mechanism the following way:

  1. If I activate Assisted GPS under system location settings the location service selected in microG (e.g. BeaconDB) is used for providing the rough location.
  2. The phone sends the ID of the radio cell that is currently used to a location service (e.g. BeaconDB) and receives back its rough location (which is the cell tower’s location).
  3. The rough location is then sent to an A-GPS server which sends back the GPS almanac and ephemerides that the GPS needs for a quick GPS fix.
  4. The communication with the A-GPS server is done either by the OS via User Plane Protocol (SUPL) or by the baseband chip (within the firmware running on the radio processor) via Control Plane Protocol.
  5. The communication with the A-GPS server (Google) via SUPL can be disabled in iodéOS under location settings.
  6. I may never know whether the baseband chip of my phone communicates (as well) via Control Plane Protocol and which (personal) information it sends to the A-GPS server (Google).

What I don’t understand:

  1. Why is a location service (e.g. BeaconDB) necessary? Can the A-GPS server not handle the cell IDs directly? Or did I misunderstand the mechanisms of location service and A-GPS server?
  2. How are the switches under system location settings “Use assisted GPS” and “Force disable Secure User Plane Location (SUPL)” interrelated?
  3. The SUPL switch states “Always disable SUPL assisted location support regardless of carrier configuration”. What is the “carrier configuration”?

Is it possible to redirect the SUPL requests to the SUPL proxy of GrapheneOS with the iodé app or in any other way?

supl.google.com to supl.grapheneos.org
or
supl.google.com to 146.59.144.170

By the way, does anybody know if the Fairphone 5 uses A-GPS via baseband?

Any help is much appreciated.

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