Re: GPS question
I think I know a couple of the sources of confusion here, might be right, might be wrong, 50/50 chance either way.<br /><br />First things first, Sloopy, the GPSMap-76S you have is a wonderful machine in that it has both WAAS and a barametric altimeter built into it. As the fellows here who are pilot know full well the way to calibrate a your altimeter is to know the current barimetric pressure, or, as is so common on private aircraft to simply set for the known altitude before takeoff and the pressure will take care of itself - but that is getting away from the point.<br /><br />I saw a mention above about how many reference points are needed to properly triangulate a position, and much of the discussion seemed to follow from that. It is the inherent inaccuracy in the argument. GPS doesn't triangulate anything.<br /><br />The notion of triangulation, as used here, comes as much as anything from the old radio direction finding location method. Simply put you used a very directional antenna to show you the direction from which several radio signals were being received. Using that method you'd operate on one of two ways. In one you would know where the transmitting stations were located, and commercial radio stations could be used for that purpose, and using your very directional antenna you found your relative bearing from several of them. You drew the lines on the map. If you had one station all you knew was that you were somewhere on a line, and you didn't even know which end of the line you were on. If you could receive a second station then you could plot intersecting lines, and that would give you a much better idea of where you were. If you were lucky enough to have three stations you could put down three lines and what you would get would be a little triangle on the map, and you could safely assume that you were located in the middle of that little triangle. Hence the term triangulation. In each case you were working to determine your positon on a line and it made no difference at all how far away you were from any of the three, or more, radio statons.<br /><br />GPS doesn't work that way at all. For one thing the transmitting stations are always moving relative to the receiving station. That alone means that there is absolutly no way to be able to triangulate anything. Nope, with GPS what is being measure is time delays from a know location at a fixed point in time. What you get when you visualize time delays is not nice straight lines, as you got with the radio signals, but instead you get spheres of a size dictated by the speed of the radio signal and its distance from the emitting source. If you have two of these spheres - which is to say you are receiving signals from two satellites - there will be one set of points in space where their intersecton will define circles. If you toss a third satellite into the mix you get something entirely different, you get two sets of points in space where an intersection of all three time delayed spheres can exist. it doesn't make a hoot in hell if those two points are on the earth's surface, above it, or below it. If you add the 4th bird to the picture you add accuracy, but not additional capability.<br /><br />So there are the basic differences. To triangulate, as we used to do with radio, you use direction and straight lines. A GPS, on the other hand, uses time delays instead of direction and uses spheres rather than lines. To compare the two beyond that is generally an exercise in futility.<br /><br />A GPS measures distance and speed by using its memory. It simply remembers where it is at an instant and then compares that to where it is at some other instant. Its very accurate internal clock is used in the comparison but the comparison is between two points, no matter where they are located - once again, on the earth's surface, below it, or above it, it makes no difference at all to the GPS. The GPS simply sees that it was at one point at some time in the past, it is now at some other point, and it took some time to get there (we're far to large to act like electrons in quantum mechanics). The calculation is easy from there, but the point is that its only the distance, not the relationship to the earth (which has no bearing at all on the calculation) and the time that make any difference. So elevation has absolutly no effect on the computation of speed. At least not for a GPS.<br /><br />If you were to go back to the Radio Direction finding example then elevation would indeed have an effect on speed, but even by the advent of LORAN-C as the primary means of location determination elevation lost any meaning on the calculation.<br /><br />Pretty straight forward actually.<br /><br />Thom
