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From: George Neuner <gneuner2@comcast.net>
Newsgroups: comp.arch
Subject: Re: Radians Or Degrees?
Date: Fri, 15 Mar 2024 11:40:57 -0400
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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