4 ms·

GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) in an absolute sense, but in a relative sense they are much more accurate and would

GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) in an absolute sense, but in a relative sense they are much more accurate and would easily show circles being driven in a parking lot (it's just that the circle might be 10 metres away from the road). This is also used in surverying to do something called differential GPS with two receivers: one GPS receiver fixed in a known location, and another to measure GPS locations relative to the first GPS in the known location - absolute error is completely removed by this process (http://en.wikipedia.org/wiki/Differential_GPS http://en.wikipedia.org/wiki/Differential_GPS claims 10cm accuracy). If you think about it, two receivers separated by a little space is more or less equivalent to two receivers separated by a little time, so you'd expect a similar relative accuracy of 10cm.

lutusp 14y ago
Yes, all well understood. The problem comes up in cluttered terrain and an ordinary GPS unit that's in motion (no differential GPS option). In such a case, if the receiver encounters obstacles in its satellite line of sight, it may switch to more favorable satellites and recompute its position. As a result, the apparent position will jump around within the normal error bound (see below). I know, I've had the experience any number of times. > GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence): http://www.gps.gov/systems/gps/performance/accuracy/ http://www.gps.gov/systems/gps/performance/accuracy/
- Robin_Message 14y ago
  Good point about changing satelites. I guess an open parking lot would be fine, but who knows. I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.
  - lutusp 14y ago
    > I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.
bigiain 14y ago
I suspect the probem is not the gps positional accuracy, but rather the data loggers recording frequency - for 99.9% of what you'd want from a gps log on a vehicle, recording the position and speed every 10 or 20 seconds would be more than enough. To generate useful plots of a car driving in circles round a small parking lot would need a much higher recording rate. Even slow old-school gps chipsets output 1Hz position data, but logging all that would needlessly produce 10 or 20 ties more data than youd need for anything except showing up lying journalists. I'd put good money on Musk getting 1Hz or better gps logs of the _next_ bunch of test drivers...

lutusp 14y ago

Yes, all well understood. The problem comes up in cluttered terrain and an ordinary GPS unit that's in motion (no differential GPS option). In such a case, if the receiver encounters obstacles in its satellite line of sight, it may switch to more favorable satellites and recompute its position. As a result, the apparent position will jump around within the normal error bound (see below). I know, I've had the experience any number of times. > GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence): http://www.gps.gov/systems/gps/performance/accuracy/ http://www.gps.gov/systems/gps/performance/accuracy/

Robin_Message 14y ago
Good point about changing satelites. I guess an open parking lot would be fine, but who knows. I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.
- lutusp 14y ago
  > I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.

bigiain 14y ago

I suspect the probem is not the gps positional accuracy, but rather the data loggers recording frequency - for 99.9% of what you'd want from a gps log on a vehicle, recording the position and speed every 10 or 20 seconds would be more than enough. To generate useful plots of a car driving in circles round a small parking lot would need a much higher recording rate. Even slow old-school gps chipsets output 1Hz position data, but logging all that would needlessly produce 10 or 20 ties more data than youd need for anything except showing up lying journalists. I'd put good money on Musk getting 1Hz or better gps logs of the _next_ bunch of test drivers...