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Re: GPS Based Speedometers

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On 12/14/25 07:06, Daniel70 wrote:
> On 10/12/2025 12:14 am, knuttle wrote:
> 
> <Snip>
> 
>> GPS accuracy is 10 to 16 feet.   So the slower the vehicle is going 
>> the more imprecise that speed, as it is dependent on the accuracy of 
>> the starting position and the ending position. The time interval is 
>> also a factor in the accuracy of the speed by GPS. ie at 60MPH, the 
>> car travels 520 feet per minute, so 10 feet on the starting and ending 
>> positions has more affect than if the speed was at 30MPH or 1040 
>> feet.  These number could be higher depending on the interference in 
>> the area of the position.  It is best to put the numbers in a 
>> spreadsheet to understand the effect.
>>
>> The speed detecting radar is accurate to 1 to 2 miles per hour in 
>> ideal conditions.  In practice I suspect that if you stay within 10% 
>> of the speed, it is questionable if the radar detector can definitely 
>> say you are over the speed limit. (10% Based on actual experience with 
>> many electronic instruments)  Again there are many variables that 
>> affect the measurement of the speed detecting device.
> 
> And your GPS Speed is only indicating what you were doing, what, 0.5 
> Secs ago .... cause the signal has to go up to the Satellite and come 
> back down, doesn't it??

   With GPS it only has to come DOWN. If you
   can xmit UP then most govts in the world
   will want to have a long talk with you ....

   However the distance "down" depends on the exact
   location of the sats. Still isn't TOO bad, the
   sats are only about 12,500 miles high. Modern
   software can probably PREDICT your next position
   and speed based on the last few readings, that
   wouldn't be hard at all.

   OLD GPS ... one-second refresh intervals and
   dead zones. More modern GPS, one-tenth-second
   is common. Speed is very accurate, position is now
   typically around eight feet (can tell which traffic
   lane you're in) or better, altitude ... more iffy.

   And if your speed is going from legal to illegal
   and back in a tenth of a second then, wow ... the
   problem may not be GPS  :-)

   Some of the new military networks can surely
   out-perform ordinary GPS ... but YOU don't
   get to look at those.

   So, all in all, I'd say that GPS now out-performs
   the old transmission-linked speedos. Down-facing
   add-on doppler radar units exist and I've used
   them ... and CAN be even more accurate. Ag uses
   these a lot, bolt-ons for existing equipment
   so precision-ag can be employed. Likely newer
   GPS now comes on, or can be plugged-into, most
   any tractor/harvester/etc.

   https://store.agexpress.com/dickey-john-radar-iii/
   https://store.agexpress.com/dickey-john-radar-ii/

   Prices have gone WAY up since I used them ... GPS
   has pretty much taken over. I used the "II", RS-232
   (and maybe RS-485) interface.

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Re: GPS Based Speedometers Daniel70 <daniel47@nomail.afraid.org> - 2025-12-14 23:06 +1100
  Re: GPS Based Speedometers "Carlos E.R." <robin_listas@es.invalid> - 2025-12-14 13:43 +0100
  Re: GPS Based Speedometers rbowman <bowman@montana.com> - 2025-12-14 19:09 +0000
  Re: GPS Based Speedometers c186282 <c186282@nnada.net> - 2025-12-15 00:12 -0500
    Re: GPS Based Speedometers rbowman <bowman@montana.com> - 2025-12-15 19:55 +0000

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