gpsd and AGPS

Ken Yale kyale at
Fri Sep 28 16:39:43 CEST 2007


Good questons.   See comments below.


-----Original Message-----
From: Alexey Feldgendler [mailto:alexey at]
Sent: Fri 9/28/2007 12:57 AM
To: List for OpenMoko community discussion
Subject: Re: gpsd and AGPS
On Mon, 03 Sep 2007 21:24:45 +0200, Ken Yale <kyale at> wrote:

> Normally, ephemeris satellite data is downloaded at 50 bps from the  
> satellites, but only when the signal strength is above about -142 dBm.   
> Live ephemeris data is good for about 2 hours.  With a data connection  
> (I use a USB TCP/IP bridge to a PC, and then to the network), we can  
> download 7 days of ephemeris in 3 or 4 seconds, independent of GPS  
> signal conditions.

Wasn't the last sentence supposed to say "almanac"? As I understand, the  
ephemeris data is short-lived and doesn't make sense to cache ahead of  
time. The almanac, on the other hand, is valid for some days once acquired.

[Ken]  Almanac and Ephemeris.  The Broadcom LTO data service has been
purchased by FIC for use with the Hammerhead chipset inside the GTA01.
LTO is a 7-day prediction of the ephemeris.

> - database of cellID --> initial position look up.  I understand network  
> operators cherish and protect this database.

Sure, the cell operators won't gladly share this data with anybody, but  
there's still something that could be done: the phone could learn the  
CellID <-> area association and use it later (if we are registered at some  
cell we've already been at, we can't be miles away from where we were last  
time at this cell).

[Ken]  Exactly!  This would be a good feature for an Open SUPL server.
The Broadcom SUPL server has this feature also.

[Ken[ Your second point:  presumed proximity based on most recent
location is hard-coded into the GTA01 GPS already.  However, the GPS
must derate the accuracy of the position as a function of time.  Most
GPS receivers have this feature already.  One problem is when you've flown
across an ocean, and a 1-or-2 day old (or even 8 hour old) position
would actually be a negative assistance.

[Ken]  This could be a feature to be added to the GLLIN by FIC:  detect
this large position change.  Some ideas:
- "flight mode"  - tap the city (or airport code) your flying to.
- "discard position"  - force the GPS to discard the GPS position cache
- "reset gps" - actually, this feaure is already there:  gllin -recover on the
    command line has a "Reset GPS" button in the OMGUI.

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