Re: Radians Or Degrees?

MitchAlsup1 wrote:

Terje Mathisen wrote:

This is, except for today being a 64-bit world as opposed to 30 years earlier, exactly the same reasoning Garmin's programmers used to decide that all their lat/long calculations would use 2^32 as the full circle.

 

With a signed 32-bit int you get a resolution of 40e6m / 2^32 = 0.0093m or 9.3 mm, which they considered was more than good enough back in the days of SA and its ~100 RMS noise, and even after Clinton got rid of that (May 2 2000 or 2001?), sub-cm GPS is very rarely available.

 The drift rate of the oscillators in the satellites is such it will neve be.
The clock drift is reset every orbit.

Sub-meter (typically 2 cm at 10-20 Hz) GPS requires a nearby (static) base station which can measure the individual pseudo-range errors from each sat and send that to the measuring device (the rover) over a separate channel.
If you record all the pseudorange values, then you can do the same with post-processing, this can be useful for DIY surveying.

 Also note: hackers are now using ground based GPS transmitters to alter
where your GPS calculates where you think you are. This is most annoying
around airports when planes use GPS to auto guide the planes to runways.

The potential for this attack is one of the reasons the military signal is encrypted, so that it cannot be spoofed.

 

Doing the same with 64-bit means that you get a resolution of 2.17e-12 m which is 2.17 picometers or 0.0217 Ã…, so significantly smaller than a single H atom which is about 1 Ã… in size.

 And yet, driving by Edwards AFB sometimes my car's GPS shows my 50m off the
interstate quality road, and sometimes not.

50 m is a bit high, but still fairly typical for what commercial receivers can do in the vicinity of refelcting surfaces like cliffs or buildings.
Modern multi-system receivers are actually getting much better at detecting and mitigating such problems. Starting with GPS+Glonass+Galileo having worldwide coverage, then adding in the Chinese and Japanese sats means that as long as you don't worry to much about pwer usage, you can do quite well. My personal target is sub 3m when under a wet forest canopy, that is good enough for orienteering map field survey work.
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"