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've never used any kind of GPS. How does a pilot use a GPS designed for hiking when he/she is in an airplane? If the pilot wants to know how close he is to a VOR (or an airport) then he has to enter that VOR's exact location himself, right? This sounds like a lot of data entering - or can aviation data bases be easily downloaded from the internet into one of these cheaper units? I assume that altitude and magnetic bearing info is read right off the face of the GPS?
I've heard of other pilots using "non-aviation" GPS units while flying. This seems like an inexpensive way to improve my positional awareness. I do understand that none of the handhelds are leagle substitutes for TSO'd equipment.
If you look in the AFD you can find the lat/long for VORs and airports. Just enter the lat/long into the GPS and save it as a waypoint. The Garmin 12 also has a nifty way of enter intersections. You can start with a VOR and define an intersection based on the intersections relative position to your vor waypoint. Before your flight you can enter all the points you'll need. Or if you get lost you can enter the lat/long of an airport/vor near you and navigate to it that way. Not as slick as the Garmin 430 I use now, but you'll save $9851.
I bought my GPS 92 on an Ebay auction for $160 in new condition. Just do a search for aviation gps. The GPS 92 has everything you could want: updateable Jeppesen database downloadable from the internet, map view with controlled airspaces, SUA, nearest airport, etc. SUA alarm which will alert you 10 minutes before entering the airspace, airport information including runway lengths and frequencies. Everything you could want in a GPS which fits in a flight bag! I love mine. They show up regularly on Ebay and can be gotten very reasonably.
I'd like to upgrade from my Magellan. What's the best all around unit for flying as well as driving (for $1000 or less)? I'd like all the capabilities such as a database so I don't have to enter everything manually, external antenna, easy-to-see/read screen, external power adaptor, etc. I saw some favorable comments on another post about the Garmin 195 and III Pilot. Any other opinions? Thanks!
Snoopy, the eTrek has essentially all the same features as the 12 but weighs ~40% less. The display is about 30% smaller for the eTrek but is 4-level gray versus b&w for the 12. The eTrek has higher screen resolution (128 x 64 v. 100 x 64). The eTrek has ~50% more tracklog memory. The eTrek uses batteries at about half the rate of the 12.
Go to the Garmin site and make your own comparison (link below).
Wiggums - The eTrek will also do the projected waypoint.
Mine stays in my flight bag as a backup. Yes, it's a pain to enter the points, but I just have a couple dozen airports. I use it to check students on dual cross countries and other times I want to double check my location versus the instruments already in the plane.
Better still, use this guy's website to extract a subset of waypoints from the FAA database into something you can then use a utility like Waypoint+ or G7toWin to upload the waypoints to your GPS unit.
the 12 displays 5 different nav screens, the exact same ones the gps92 aviation unit displays, check it out yourself and u will see. the etrex is a poor excuse of a gps, the 12 is alot more useful to a pilot.
here are links to download a free manual to both the gps 12 and the etrex and you will see how much better the 12 is for 20 dollars more than the etrex. http://www.garmin.com/manuals/gps12.pdf http://www.garmin.com/manuals/etrex.pdf