Re: Radians Or Degrees?

On Fri, 15 Mar 2024 12:00:45 +0100, Terje Mathisen
<terje.mathisen@tmsw.no> wrote:

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.

Also the AFB operates differential GPS for its runways. An OTS unit
might be confused by close proximity to the ground transmitter.

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