Axolotl wrote:I'm interested in how accurate these GPS things seem to be now. If you can rely on the speed indication over a short distance, they must be extremely accurate. How do they get round the military-imposed error in positioning? Or has that been removed now?
SA, or Selective Availablity was removed sveral years ago due to the great number of companies that boradcast a correction service. They compared the GPS location with their known location and broadcast the resulting error to subscribers.
Clinton announced it would stop and Bush enacted it, I think.
In any case, that never had much effect on speed readings, as the error was not a "+/- 100 metres" type error but a steadily but slowly changing error. If your plotted your postion, then you'd see yourself apparently moving in a rough circle (allowing for the receiver's error of typically a few metres). The error changed slowly enough that your change in position from one second to the next (and hence speed) was relatively accurate. You can certainly rely on the figure to within a few tenths of mph.
The barrier to accuracy is how accurate the receiver can compare the time signals from the satellites using some pretty severe maths. Better receivers have allowed accuracy to improve but the next barrier will be more accurate clocks in the satellites themselves.
Military receivers are still more accurate than civilian equipment as they have access to a different signal from the satellites. Accuracies of a few centimetres are possible.
I bought one years ago, as a walking aid, and it was great in the open, but as soon as the path went into the woods, or it rained, no signal.
It has to see at least four satellites, so inevitably you'll have problems under cover. And most people don't realise that rain can halve the signal strength.
What have they done to improve that?
Not a lot they can do about cover, really. Most terminals now can track up to a dozen satellites which means they have a better chance of giving you a reading.
Receiver sensitivity has improved so rain is less of an issue.