Just curious. I am new to the Nuvi200W. Does it tell you the altitude you are at?
Louise in FLA
My 200 does. if you touch the car it will show you the current location coordinates and also the elevation, although I don't think the latter is all that accurate (I think there has been some discussion on that here in the past).
Guttermouth is correct. The elevation is no very accurate on the 200. But it gives you a general idea.
Altitude in GPS is complicated -- it gets into the geoid model, and the standard model used (wgs84) for consumer products is a good approximation. More specialized models are available for surveying and the like.
(the geoid model specifies the parameters for the ellipsoidal surface used to model the Earth. Unlike latitude and longitude, altitude is the height above something -- and the ellipsoid determines that something, based on latitude and longitude)
You can also go to favorites and click "My Current Location" if you have fat fingers. Altitude accuracy improves some with WAAS enabled but eats your battery faster.
From what I remember GPS altitude accuracy is specified at 1.5 x the horizontal accuracy. I can't remember any details on how many satellites and their geometry is assumed for that value. In general "widely spaced" satellites sufficiently above the horizon for decent reception are best.
Also, at least 4 satellites are required to calculate an altitude (3D fix) while only 3 are required for position (2D fix), which assumes an altitude, generally the last one available.
For the math buffs...why are 3 sats required for 2D and 4 sats for 3D...the receiver also has to calculate the value of time. (It's getting different values from each satellite due to their different distances and relative velocities)
For the math buffs...why are 3 sats required for 2D and 4 sats for 3D...
Two satellites define a full circle of possibilities (the intersection of two spheres). Three satellites give you two possibilities (the intersection of the previously mentioned circle and another sphere). A fourth satellite is necessary to tell you (in 3D) which altitude is the correct one.
Every calculation of the distance from a satellite is based on the time it takes for the signal to arrive. For instance, only two spheres around satellites define an intersecting circle where distances provide the time difference in signals observed from the two satellites.
There's a pretty good explanation of how a 2D position could be calculated with only 2 satellites IF the GPS receiver had a precise value for time such as an atomic clock... It starts about half way down the page.
... IF the GPS receiver had a precise value for time such as an atomic clock...
Jeez, Garmin can't even get it's software to work properly. Now you want them to put little atomic reactors in the GPS. That's just asking for trouble!
Yes, and we want to use cesium which is highly toxic too! So if the atomic clock doesn't explode and get you with the radiation, the poison cloud from the cesium will definitely finish the job.
If you're gonna do it - do it right!
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