R: Need info on AGPS

[Marcus Bauer]

On Thu, 2007-01-25 at 14:38 +0100, Michele Manzato wrote:

> 1. Is there any "A-GPS standard" whatsoever?

The Open Mobile Alliance (OMA) has defined a couple of protocols how to
provide assistance data on the mobile network layer.

This is mostly important in emergency situations (911/112 calls) to get
a position fix in seconds. 

It is a future requirement in the US and companies like Global Locate
are providing the infrastructure for that. Servers with 99.999% uptime,
carrier grade equipment etc.pp. 

This is costly but nothing you need for your daily GPS use.

> 2. I have heard elsewhere (Wikipedia)

The wikipedia article is mostly a repetition of the marketing
information of said companies. Kind of 'the emperor in new clothes',
making everybody believe that this is important.

As all the TomTom gadgets prove: it isn't. GPS works pretty well without
the A.

>  that in A-GPS the computation effort
> is shared between the device and the A-GPS Server.

That was one of the original ideas of UMTS that the network computes
your position. However it has proven impractical and the advance in
technology of the GPS chips has made this approach mostly obsolete. Not
very much to the liking of the network operators, who would prefer if
you pay them for your position.

>  According to a previous
> post, the device just downloads the ephemeris table so there isn't any
> actual "computation sharing", but rather a download of a pre-computed table
> download. Correct?

Absolutely.

> 3. A-GPS involves additional data traffic and thus (potential) additional
> costs. Does it use a normal GSM/GPRS IP-based data transfer? does it use
> some out-of-band GSM/GPRS control messages? or does it get data from
> broadcasts in the local cell (e.g. GSM cell-broadcast)?

The phone is being charged via USB. It should be possible to download
the data whenever it is plugged in. The ephemeris can be pre-computed
with decent accuracy for ten days and you will most likely charge your
phone more often.

If you need the assistance data on the road via GSM/GPRS, the ephemeris
is 90 Bytes/satellite, thus 900 Bytes for ten satellites above your
head. No big deal.

> 4. if the answer to above is GPRS: is it possible to estimate in advance how
> much additional traffic (in Kbytes/day of full operation)?

You only need that for the *fast* (and slightly more accurate) first
fix. A modern receiver gets a first fix on average in 38 secs without
assistance. 

> 5. Are there any known estimations on the overall (A)GPS performance on the
> Neo (esp. fix time)

Global Locate claims a TTFF (time to first fix) under all circumstances
in less than 8 secs. In real life it performs similar to recent SiRF
chips. Partially that depends on the quality of the radio too but
accuracy down to 2m under good conditions are possible.

> 6. Coming to the Neo1973. In order to save costs, can the "Assisted"
> function in A-GPS be disabled through software API?

If not it would be the dream of the mobile operators: a phone that sucks
out money of your pockets once switched on.

> 7. Is it possible to tell whether A-GPS is actually in use or not?

It will most likely be a pull option. Unless you request the assistance
it will just work like any other GPS. Believe me, it is mostly marketing
blurb. Simply go into the next electronics shop and look if any of the
TomToms or Navigons have AGPS. They don't. Rumours go that they are
pretty usable nevertheless.

> 8. Is it possible to tell/know which is the A-GPS server currently in use?

Should be configurable. The Globale Locate guys will get lots of flak
and ruin their reputation if they get silly ideas with the closed gpsd.
SiRF will happily provide chips without that closed nonsense - they do
already.

Sean has promised to provide a truly open mobile platform. Expect future
versions to have either an open chip or Global Locate publishing the
binary protocol specs of their chip. In case Sean doesn't keep his word,
I'm sure some other company will step up. OpenMoko is not tied to FIC.

> I realise that 1..6 of the above are indeed general questions about A-GPS
> technology, not really specific to the Neo or to OpenMoko. Sorry it I sound
> a bit off-topic, but I did not find them answered plainly anywhere.
> Reference links would be great.

Serach google for "oma supl" to find out how the mobile operators would
like to help you to get your position.

A really good one about the functioning of gps is:
http://www.zyfer.com/briefings/gps/gps%20basics%2010-04.pdf


Marcus