GPS Altitude Error !?!?!?

**ShawJohn** · 10-12-2004, 11:56 AM

Hmmm, GPS triangulated in a horizontal axis?!??!

Triangulation infers a triangle. And its actually a series of trangles in a 3d space.
As I understand GPS, it works by working out the time the signal takes to get from satellite to GPS Receiver for each satelite. This gives a distance to each satellite, therefore by using the known position of each satellite it works out a position on earth using simple trig. calculations.

The same calculation that allow it to work out lat. and lon. will also give it altitude surely?

How can they be horizontlly acurate to 15m yet altitude out by 170ft (80m)

Unless like you say its deliberate to stop people building there own missiles etc... ;-)
Anyway it doesn't matter that much really, I'd just like to be able to trust all the info that I'm given from my hardware. Maybe Im asking too much ;-)

John

**kiltjim** · 10-12-2004, 01:07 PM

Well, at least thats wht I've been led to believe. To figure lat. and lon. you need at bare minimum 2 satellites to lock. This would give you a possible location of 4 different places in space. A reasonable guess could be made and put you on 2 different places on the globe. With a 3rd satellite you could accurately get location, but not altitude. We all know this.

What I'm thinking, using the same theory that I've been given, I would think that with 3 satellites telling you that: "you may be here, but we're not going to be as accurate as you would like" the accuracy would be so-so. But when you start to rely on 4 different satellites all telling you the same thing, but not with any real accuracy, I would think that altitude (the only real reason you need 4) may be the one to suffer. The others may say that you are driving off the road, but when you think of 4 spheres moving around (to simulate inaccuracy) altitude data may move significantly.

**Change** · 10-12-2004, 02:14 PM

http://gpsinformation.net/main/altitude.htm

http://members.cox.net/sd_run_maps/gpsprimer.htm

**carpcnoobie** · 10-12-2004, 04:55 PM

Originally Posted by Change

http://gpsinformation.net/main/altitude.htm

http://members.cox.net/sd_run_maps/gpsprimer.htm

Thanks for the links. Interesting reads.

**VanMan69** · 10-12-2004, 04:59 PM

Originally Posted by ukchris

Makes you wonder about missile guidance though, hmm, maybe they use altimeters too, or maybe the satellite imaging used for targetting is measured from the same satellites and therefore helps them avoid missile bath-time.

I'm going to assume the military uses something more sophisticated than a $60 GPS mouse to guide their cruise missiles.

**aguila** · 10-13-2004, 03:18 PM

I live below sealevel, but my GPS does not seem to believe this (it shows 60 meters + !!

**Marsupial** · 10-13-2004, 03:33 PM

you live under sea level, so you have a subputer?

lol

**Change** · 10-13-2004, 06:37 PM

Originally Posted by VanMan69

I'm going to assume the military uses something more sophisticated than a $60 GPS mouse to guide their cruise missiles.

A bit

Actually, civilian GPS receivers are required to stop tracking if they detect some particular set of speed and altitude conditions.

**chenko** · 10-13-2004, 07:11 PM

The "Sea Level" IIRC, is taken from a nail in a dock wall on the south coast of the UK somewhere, well that is what OS uses.

I have a BU-303 and its altitude isnt far off for my location.

Originally Posted by VanMan69

I'm going to assume the military uses something more sophisticated than a $60 GPS mouse to guide their cruise missiles.

That and they have the <1m accuracy which we don't

we could get that accuracy too without much hassle, but civvies are not allowed

**TruckinMP3** · 10-13-2004, 07:55 PM

Originally Posted by kiltjim

I'm not an expert, so I couldn't tell you if the sea level in specific areas differs around the globe. I always thought sea was pretty much at the same level globally except for the tides.

I would think it would be the GPS. Older GPS units had error built in, so that you couldn't build a rocket with pin point accuracy. If you look at the raw stream of GPS data, you will see that it jumps around alot. VisualGPS shows you this. While monitoring mine over a day or so, it jumped upto 500'. Download it, and just have your GPS sit on you desk for a day or so. That should tell you if its the GPS, or the sea, or whatever.

Some famous harbor on the east coast of Canada (I think) has a 20 Plus foot difference between high and low tide. Most of the coastal plains arround the US are a few feet at most. (this from the guy in Colorado = land locked

)