Full GPS versus a-GPS

[jhonan]

Apologies if this is a FAQ, feel free to lock the thread and link me to the right place.

 

I'm trying to understand the differences between a-GPS and full GPS. e.g. the type of GPS offered by a mobile phone versus the receiver in a handheld GPS device.

 

I know a-GPS comes in at many levels and guises. My experience with the a-GPS in my Samsung Jet is that it initially uses the nearest cell masts to attempt to triangulate my position to the nearest 500m while it gets a lock with the satellites. I also do a 'download' before using the GPS which downloads some data via 3G to get some basic positional info.

 

When it has achieved a lock with 4 or 5 satellites (which can take a few minutes) it then gives me my position to within a few metres.

 

I've used the Jet to find my first three caches. And I've found it very accurate when it's working with a satellite lock (e.g. usually taking me to within 6 feet of the cache). So is the a-GPS chip in my Jet actually using full GPS satellite positioning? Or is it still depending on the cell network? (I don't see why it would need the cell network if it has a GPS lock)

[+Rockin Roddy]

Can't answer to all of tht, but normally, the cell phone GPS is less accurate than an actual GPS unit. The handheld unit is also more rugged and usually waterproof unlikr the cell phone. Personally, I wouldn't depend on the cell phone for much and wouldn't use it for caching...but some do.

[jhonan]

The handheld unit is also more rugged and usually waterproof unlikr the cell phone.

Yeah, I hear what you're saying on the 'rugged' part. Cell phones just aren't designed to be dropped into lakes.

 

Still, I thought the phone GPS would be a nice way to get started with geocaching, and it hasn't let me down yet.

[+Rockin Roddy]

The handheld unit is also more rugged and usually waterproof unlikr the cell phone.

Yeah, I hear what you're saying on the 'rugged' part. Cell phones just aren't designed to be dropped into lakes.

 

Still, I thought the phone GPS would be a nice way to get started with geocaching, and it hasn't let me down yet.

 

Not knocking your approach, my friend!! And, if you do become hooked like we all did, you'll have the phone to fall back on while deciding what GPS to purchase!!

 

Does the use of the GPS eat into the minutes?

[jhonan]

Not knocking your approach, my friend!! And, if you do become hooked like we all did, you'll have the phone to fall back on while deciding what GPS to purchase!!

 

Does the use of the GPS eat into the minutes?

No, not at all. As soon as it has a satellite lock that's all it uses. I'm using Route 66 software on the phone so it has all the maps pre-loaded. What I do is enter the co-ordinates for the caches I want to find before leaving the house. Then when I'm on the road I select the cache I want to go to, and click 'Drive To'

 

This then gives me voice directions like an in-car nav system until I'm near the cache. Once I'm there, I take the phone with me and walk towards the cache. I can see it on the map marked as a star, while I'm the green dot getting closer to it. So it usually takes me to the rocks or trees where it's hidden. And from there I can search around.

 

Then when I find it, I use the phone to make a video or take some photos. At that point I can even upload the photos or videos to twitter/facebook/youtube/blogger from the phone. But I usually wait until I'm home before logging the visit or uploading videos etc.

 

If I had a wish-list, it'd be the ability to send selected cache GPS co-ords directly to the mapping software. But it's no big hassle typing them in really.

[NordicMan]

Yep you have a good understanding how A-GPS works in most smartphones

 

Nowadays a smartphone can be just as accurate as a standalone GPSr, they use the same-or-similar technology so why wouldn't they be as accurate? I've been fiddling around with a new Palm Pre and the a-GPS feature is awesome! I can be deep inside a building and the thing will still be able to determine my approximate location based on cell tower trinangulation. A "rugged standalone GPSr" would be useless in the same spot! Now the accuracy of cell tower triangulation is waayyyyy too rough to Geocache with, but at least it'll show your location to within a block or so, until you get a satellite fix.

 

The biggest handicap with Smartphones is their batttery life can sometimes be a limiting factor, and yes they are quite a bit more fragile than a rugged outdoor rated GPSr. But so long as you're careful with it, you should be fine.

 

Happy caching!

Edited October 7, 2009 by NordicMan

[jhonan]

The combination I'm thinking about now is;

 

- Samsung Jet for the in-car navigation, maps, and getting to the general location of the cache

- Buying a eTrex H for homing in on the final location

 

I went hunting for a cache tonight, and the Jet got me to within 50m. At that point, and because it was so insistent that I should be on a road, I found it difficult to home in on the last few metres.

 

What I really needed was something that would show me just a simple GPS co-ordinate or compass. Showing me as a moving green dot on a map is great, but only up until the final 50m or while I'm still on roads.

Edited October 7, 2009 by jhonan

[John E Cache]

How do you "triangulate" with cell towers? The only triangulate I am familiar with is getting two sight lines from two known points on a map to the unknown point. The lines cross on the map at the unknown point. I would be interested how cell phone triangulation is done. Do the towers provide a direction line to the cell phone? Apparently some info is provided for emergency services.

 

Also see: http://en.wikipedia.org/wiki/A-GPS

[NordicMan]

How do you "triangulate" with cell towers?

 

Not sure, but it's gotta be easier than triangulating with satellites that are 11,000 MILES away(!!!!)

[+DENelson83]

How do you "triangulate" with cell towers?

Not sure, but it's gotta be easier than triangulating with satellites that are 11,000 MILES away(!!!!)

Triangulation is done with azimuths. GPS positioning is done with trilateration, which uses distances. If you use at least three cell towers, you can get some idea of the accuracy of the triangulation.

[John E Cache]

How do you "triangulate" with cell towers?

Not sure, but it's gotta be easier than triangulating with satellites that are 11,000 MILES away(!!!!)

Triangulation is done with azimuths. GPS positioning is done with trilateration, which uses distances. If you use at least three cell towers, you can get some idea of the accuracy of the triangulation.

So the answer to my question is that the tower sends azimuth and tower location info to the cell phone, then?

[+twolpert]

So the answer to my question is that the tower sends azimuth and tower location info to the cell phone, then?

It's actually more complicated than that. And it depends on your network provider. If you're really interested, there's much more detailed information available here:

 

http://arstechnica.com/gadgets/news/2009/01/assisted-gps.ars