Path: ...!weretis.net!feeder6.news.weretis.net!i2pn.org!i2pn2.org!.POSTED!not-for-mail From: George Neuner Newsgroups: comp.arch Subject: Re: Radians Or Degrees? Date: Fri, 15 Mar 2024 11:40:57 -0400 Organization: i2pn2 (i2pn.org) Message-ID: <8mq8vi9ci3pdrejn240hqcfppj7g87d5e4@4ax.com> References: <20240222015920.00000260@yahoo.com> <20240222233838.0000572f@yahoo.com> <20240223140209.00007157@yahoo.com> <80b12fb00e6c32b534bb6cb5ce2d4c19@www.novabbs.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Info: i2pn2.org; logging-data="2086414"; mail-complaints-to="usenet@i2pn2.org"; posting-account="h5eMH71iFfocGZucc+SnA0y5I+72/ecoTCcIjMd3Uww"; User-Agent: ForteAgent/8.00.32.1272 X-Spam-Checker-Version: SpamAssassin 4.0.0 Bytes: 3817 Lines: 58 On Fri, 15 Mar 2024 12:00:45 +0100, Terje Mathisen 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