Originally Posted by syseng68k
While gps is primarily a system to calculate position, it can also be used to provide fairly accurate heading and ground speed data, in conjunction with appropriate software.
With processing accelerometers provide you with distance covered despite their primary data being acceleration. Similarly you can differentiate GPS location (and time) information to get velocity and a second differentiation gives acceleration.
Note that with GPS you are differencing two quantities that have significant error dimensions relative to the difference obtained. So the error band on velocity is large. The error band on acceleration is even larger.
I have a pair of GPS receivers a few feet to my right. They are for precision time keeping rather than navigation. But I note the wandering of their time signals relative to each other with stationary antennas. "A few feet" is the translation I get on errors at about 1ns per foot. (That is accurate within a three percent.) If a plane is proceeding at a ground speed of say 300mph that's about (mumble 88*6 feet per second or about 440'/second.. If you take a reading once a second you have a speed range of perhaps 430'/second or 293 MPH to 450'/second or about 307 MPH. That may be accurate enough if the plane is not in distress. If the plane's dynamics change on a second by second basis, as with turbulence, it might not tell you quite as much. At a tenth second you have 23% error bands without some smoothing rather than 2.3%.
I'm not sure what speed accuracy is needed for being happy within a cockpit. I suspect 2% is acceptable and 10% is not. GPS is perhaps marginal. Over a 10 second interval GPS velocity determination is, of course, much better than my bogey numbers.
Another detail to remember, if you are dealing with high dynamics or simply high speeds, is that GPS tells you where you were at the time of measurement not where you are at the time you read it. To the extent that the distance traveled during that interval matters this detail is important to keep in mind. Fortunately the various filters in the system "take up the slack" on this detail.
She knows more about GPS than she would like to bother with. You guys have, for more than a couple decades, been relying on some work I performed while at Rockwell International in Anaheim on the satellites as designed in the late 70s. At that time it was a pasttime of mine to "annoy" AF officers over their silly "dithering" pointing out differential techniques render it pretty much moot. It does provide a nice way to gain precision from the oscillators, though.