Hardware:AGPS
From Openmoko
Neo1973's A(ssisted) GPS
More info on GPS in general and the OpenMoko chip in particular.
Q: 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.
Q: The chip in the Neo1973 is a Global Locate AGPS. Anybody know what type? Hammerhead maybe?
A 1: It is probably the same chipset as in ipaq hw6[59]15: http://wiki.xda-developers.com/index.php?pagename=SableGPS A 2: Should be Hammerhead.
Q: I understand the concept of assisted GPS. But does the phone have it's own antenna/receiver so that it can work without 'assistance'?
A: See above, the important part is the GPS and not the assistance. Antenna is thus compulsory.
Q: Has anybody info on the whereabouts of assistance servers, especially in Belgium and Europe?
A: Can be anywhere on the net. Alternatively a service from the cellphone operators.
Q: Using the assistance servers will probably mean that I will have to pay for that service. Any idea of the costs?
A: 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 but 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.
Q: Is there any "A-GPS standard" whatsoever?
A: no. It's a broad term for many variants of GPS
A: As far as I understand it: yes. I will ask our GPS engineer to comment on those questions.
Q: 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)?
A: GPRS. so its up to you whether you want that extra traffic (and cost, unless you're flat) or not.
Q: if the answer to above is GPRS: is it possible to estimate in advance how much additional traffic (in Kbytes/day of full operation)?
A: I am sure it's a quite fixed amount of
Q: Are there any known estimations on the overall (A)GPS performance on the Neo (esp. fix time)
A: There are some numbers in the data sheet of our GPS chipset vendor, but that data sheet is closed. I will ask them to give us some document/numbers that we can publicize.
Q: Coming to the Neo1973. In order to save costs, can the "Assisted" function in A-GPS be disabled through software API?
A: yes, it can be disabled through preferences.
Q: Is it possible to tell whether A-GPS is actually in use or not?
A: yes. either you have enabled it in preferences or nor ;)
Q: Is it possible to tell/know which is the A-GPS server currently in use?
A: yes.
Q: Can choosing to use GPS (even with A-GPS disabled) enable others to track me? Is there a mode where this is not the case?
A: ?
