I think I understand how A-GPS works: the phone has some chips that measure signal strength from satellites and/or cell towers, and then forwards that information to the tower, where the location information is calculated. So the phone can't know its information without help.

But, because you can look at your location information in debug mode, I assume there's a way to get the tower to send your location back to you. (I get all 0s, presumably because there is no support for E-911 in my location?)

So my question is: does anyone here know if it's possible to use a different, independent server to resolve the location information? (say, one I run on my home machine?) Or is there a way to get the GPS-satellite-location information (I assume it's predictable) and try to do the calculation myself on the 7135?

I see a lot of potential benefits to having location information available to me, and controlled by me. Of course the carriers could care less about my desires though, so I'm thinking that being proactive is the only way I'll get anything like what *I* want out of this A-GPS thing.

I assumed that Assisted GPS worked just like regular GPS (position calculated inside the phone) but when satellite coverage became spotty or dissappeared the phone would then get updated positioning from the base stations using normal triangulation techniques coupled with the last known position.

This would imply that A-GPS would be more accurate during those times that regular gps was having trouble getting an accurate fix (or any fix at all).

Also to save battery power, A-GPS may have other tricks. It could let the satellite reciever go into sleep mode whenever the phone determined it wasn't moving based upon a stable signal from base station(s) and wake up the receiver when movement was detected via regular triangulation methods.

Of course that's just my speculation. I would be interested to hear how it really works from someone who actually knows.

__________________
-Data Rox

I haven't worked with A-GPS in a while, but I'm reasonably familiar with it.

As Data Rox suspects, there are multiple A-GPS modes. The mode determines how the location is computed, which gives a strong indication of how closely the computed location will match the actual location.

Based on what I've seen, I'm pretty sure that the 7135 can use A-GPS mode 3. Mode 3 means that when the 7135 can be "seen" by three cooperating cell towers the location can be determined to within 300 meters at worst case. The 7135 may be able to use mode 5, which would allow location to be determined to within 65 meters at worst case.

The biggest problem is getting three cooperating towers. Without the carrier being involved, I can't see how you are going to make that happen.

I don't think that the 7135 can use modes 8 or higher, which would allow self-location based purely on satelite signals and wouldn't need any help from the carrier.

-PatP

Thanks for the info, Pat .. that's very helpful. For some reason, clear explanations of A-GPS are kinda hard to come by. :-/

I'm sure you're right about the 7135 not being able to use mode 8+, i.e. doing location resolution on its own. I'm curious if my previous understanding of the 7135's capabilities does correspond to one of the defined A-GPS modes, though; that is, that the 7135 does measure signal strength (or delay? or something else?) from the satellites, but then passes that on to the carrier for resolution.

What I've seen here and there is that 1. A-GPS is intended as a solution for devices that don't have the CPU to do the calculations needed to resolve location on their own, and 2. using signal from GPS satellites provides better resolution, but doesn't work indoors, so resolving using signal from towers can act as a substitute. Is that correct, or a misunderstanding?

If a reply would involve typing in a bunch of info that's out there somewhere (but that I haven't been able to find), feel free also to point me at a URL or two, if it's more convenient for you.

Yeah, that definitely seems like the case .. well, that rules out doing it on my own. Maybe once VZW roles out the service, though, I can still find a way to query the location information on my own..

Thanks again for the info.

Pat, I recall that the FCC 911 mandate was that phones had to be able to transmit the location within 50 FEET 95% of the time. That's just from memory so I could be wrong.

The 7135 uses a Qualcomm chip that has A-GPS built in. I believe Qualcomm bought the technology called Snaptrack. Here is a link to more Snaptrack info:

http://www.snaptrack.com/technology/whitepapers.jsp

They claim the following:

"SnapTrack's multimode solution works indoors and out, in all environments, flexibly selecting between multiple GPS navigation modes and conventional GPS, depending on the situation."

Maybe they are using the term "conventional GPS" very loosely but to me that implies it's capable of working without assistance from cellular towers. But maybe only devices with powerful and dedicated GPS processors can access this mode?

__________________
-Data Rox

You can get basic A-GPS information relevant to the Kyocera 7135 at the Qualcom site, and more general information from the GIS Developement site. A good collection of links to all kinds "location oriented" sites is the Wireless Developer Network links page. It isn't A-GPS specific, but it gives you a "one stop shop" for the currently popular location technologies.

The E911 spec did call for 50 foot resolution for 90% of the attempts at one point in time, and more importantly it called for meeting that requirement without using handset specific features (so a carrier could not force all of its customers to buy new phones in order to comply with the requirement)! I believe that none of the major carriers were able to comply, and the FCC could not produce a test system that could consistantly meet that requirement. That has lead to a lot of interesting debate, but it has shed much more heat than light on the subject! Note that the specs I gave were "worst case" for a successful computation... The compuation can always fail to resolve a location, and the best case could at least theoretically be under one meter for any mode.

Somewhere I've got a spec that shows all eleven A-GPS modes that I know about, with a brief definition of how they work and what accuracy they provide. I'll see if I can dig that up, but that won't be a "quick and easy" so it might have to wait until the weekend.

-PatP

Cool, thanks very much ..

Sure, if you get a chance -- if not, no biggie, you've already been very helpful.