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Hand Held GPS Altimeters, how?

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Active member
Nov 26, 2001
Can anybody explain to me how a handheld GPS figures out altitude. Does it have some kind of baro altimeter feature or is it determined by the signal from satellites somehow?
Hand Helds

In addition, there are several watches on the market that offer altitude features. They are suprisingly accurate, considering their cost.

Didn't answer your question, but just wanted to add my .02:D
GPS determines position in THREE dimension via triangulation, signal timing, etc. --- exactly the same way as it determines horizontal position. Pretend the corners of your room are satellites. The triangulation from the those corners will be different distances from each satellite to a point on top of your desk than to a point directly underneath on the floor.
the watches mentioned could just be conventional altimeters fixed to 2992. after all, they just need the ambient pressure to get a reading.

not sure how the gps one's work...
>>>>"GPS determines position in THREE dimension via triangulation"

Actually, that is not correct. GPS determines a position by measuring the distances to the satellites. Determining a position by measured distances to refernce points (the satellites) is called "trilateration". Triangulation is determining a position by measuring the angles to reference points.

Here's how...

Each satellite broadcasts its signal with an embedded time code. Your receiver receives and processes this time code and compares it to its internal clock. The difference in the receiver clock to the broadcast clock gives you time traveled. Assuming a constant radio wave speed, the receiver figures distance from the satellite.

Imagine this distance from the satellite. It gives you a sphere the calculated distance from the satellite. Two satellites give you two spheres. The receiver knows where each satellite is because the satellites also broadcast position data for the entire satellite constellation. The two imaginary spheres intersect on a circle (plus or minus distance error caused by inaccurate satellite and receiver clocks, orbital errors, multi-path error, etc.). Add the distance from a third satellite and you derive two possible points (the receiver can generally discard one and assumes that you are somewhere near the surface of the earth). This gives you a three-dimensional solution (which is why we call it triangularization - three satellites triangulates your 3-d position). However, as noted, the orbit and GPS clock errors make this a fuzzy solution. This error adds a fourth variable to the position equation. A fourth satellite provides the information to fully resolve your position accurately.

Several good sites on GPS that address this in more detail.

Good luck.
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Where's the best place (cheapest place) to find a Garmin GPS 92? What's the lowest price you've seen these go for?

I basically have one good VOR and a com radio. Sure would be nice to have alittle something extra.

the gps 12 is the same exact model as

the gps 12 is the same exact 12 channel gps as the gps 92 without the aviation database. the 92=$500, the 12=$144. and you can input the aviation database yourself, up to 500 waypoints.

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