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
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