Need info on AGPS

[Marcus Bauer]

On Wed, 2007-01-24 at 23:09 +0100, Marc Verwerft wrote:
> All,
> 
> I've been reading on this lists for some time now and saw that there
> are quite intelligent people here ;-)
> I am not very up to date in my knowledge of GPS. Can somebody answer
> me a couple of quick questions? 
>      1. Has anybody here ever used AGPS? I'd like to hear your
>         experiences.

Everybody who has used a modern GPS has used AGPS. It is usually called
warm-start or hot-start. AGPS is purely a marketing term. To calculate
the position a GPS chip needs:

      * almanac = coarse position of satellites
      * ephemeris = precise positon of satellites

The almanac is broadcast in a loop of 12.5 minutes and valid for at
least six weeks. The ephemeris is broadcast in a loop of 30 seconds and
valid for ~2 hours.

Time is mostly irrelevant, as modern chips synchronise within a second
with the satellites.

The receiver chipsets store this data in flash and load it from there
onto the chip in order to _assist_ the hot or warm start.

AGPS now means to load the almanac and the ephemeris from elsewhere,
i.e. via a network. For example for free from the american government:
http://www.navcen.uscg.gov/gps/precise/default.htm

_A_GPS is a nice convenience yet the success and proper functioning of
TomTom and Navigon PDAs shows that you don't need that at all. 

>      1. The chip in the Neo1973 is a Global Locate AGPS. Anybody know
>         what type? Hammerhead maybe?

Should be Hammerhead.

>         I understand the concept of assisted GPS. But does the phone
>         have it's own antenna/receiver so that it can work without
>         'assistance'? That's not really clear to me. 

See above, the important part is the GPS and not the assistance. Antenna
is thus compulsory.

>         Has anybody info on the whereabouts of assistance servers,
>         especially in Belgium and Europe?

Can be anywhere on the net. Alternatively a service from the cellphone
operators.

>         Using the assistance servers will probably mean that I will
>         have to pay for that service. Any idea of the costs?

They use the low cost of their chip as selling point. Their website
implies that this is a service that comes with the chip. I'd call it not
very clever if they are going to charge you - it would change their
image from lowcost to money grabber and the reverse engineering of their
binary protocol would happen even faster.

Last not least: Global Locate boasts itself to get a first fix in 8 sec
without AGPS. The importance of AGPS depends whether the part of their
website you are reading is targeted at cell phone operators, or not.

Should be a nice chip nevertheless.

Marcus