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?
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.
>>>>"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.
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.
I have a $129. garmin gps 12, been using it for 4 years on everything from cubs to jets. it is always accurate within 200 feet of msl altitude with no barometric setting.
cheap and nice to have 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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