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From: Roger Merriman <roger@sarlet.com>
Newsgroups: rec.bicycles.tech
Subject: Re: Difference between GPS and measured distace.
Date: 3 Nov 2024 20:37:47 GMT
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cyclintom <cyclintom@yahoo.com> wrote:
> On Fri Aug 23 20:59:03 2024 Roger Merriman  wrote:
>> cyclintom <cyclintom@yahoo.com> wrote:
>>> On Sun Jul 28 21:19:08 2024 Roger Merriman  wrote:
>>>> AMuzi <am@yellowjersey.org> wrote:
>>>>> On 7/28/2024 2:04 PM, Tom Kunich wrote:
>>>>>> On Sun Jul 28 17:57:01 2024 Roger Merriman  wrote:
>>>>>>> Tom Kunich <cyclintom@yahoo.com> wrote:
>>>>>>>> T have been doing a route past the airport and back that has pretty large
>>>>>>>> variations in distance on the Garmin but mostly 25 miles. Yesterday I
>>>>>>>> installed a wheel sensor on the bike and added it to the sensor group.
>>>>>>>> The 25 mile route came in at 27 miles. Now upon occasion it showed that
>>>>>>>> distance on straight GPS but it appears that civilian GPS is pretty
>>>>>>>> easy to interfere with.
>>>>>>>> 
>>>>>>> You?ve done something wrong to be getting that degree of inaccuracy. If
>>>>>>> it?s GPS drift is possible to map that.
>>>>>>> 
>>>>>>> I use GPS only, but even so very minimal deviations certainly not into
>>>>>>> miles or kilometres!
>>>>>>> 
>>>>>>> Roger Merriman
>>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> This only occurs around the airport and not up in the hills. So I think
>>>>>> that it is due to radio interference.
>>>>> 
>>>>> or perhaps:
>>>>> https://spectrum.ieee.org/faa-files-reveal-a-surprising-threat-to-airline-safety-the-us-militarys-gps-tests
>>>>> 
>>>>> https://www.gps.gov/spectrum/
>>>>> 
>>>>> https://arstechnica.com/information-technology/2022/10/cause-is-unknown-for-mysterious-gps-outage-that-rerouted-texas-air-traffic/
>>>>> 
>>>>> Nice phrase in that one, "experiencing GPS anomalies".  Just 
>>>>> like you!
>>>>> 
>>>>> 
>>>> Seems unlikely considering for Tom?s cause more likely frankly set up.
>>>> 
>>>> I ride past or through Heathrow on the commute! It?s within 0.06 miles let
>>>> alone the military stuff just to the north. Which is generally same sort of
>>>> tolerance one gets riding with others ie the difference, between folks
>>>> recorded distance.
>>>> 
>>>> And Tom?s device almost certainly would be multi band ie uses either the
>>>> Russian or European alternative to get more points and thus more accurate
>>>> though I believe most devices are fairly good and frankly for roads it?s
>>>> kinda a non issue, used to get some drift in the woods and so on but it?s
>>>> at least from a user experience spot on.
>>>> 
>>>> Note that for MTB trails may run parallel and within a few meters but
>>>> modern devices don?t wander and can even use foresight to tell you which
>>>> trail your pointed at, which is often not clear ie multiple trails all
>>>> heading off from a clearing in the woods!
>>>> 
>>>> Roger Merriman
>>>> 
>>> 
>>> 
>>> 
>>> 
>>> The US invented the GPS system and I don't know if the Garmin can pick up
>>> the Russian or European satellites. In any case I don't think that it is
>>> interference with the GPS signals. I think that it is the ILS (instrument
>>> landing system) simply overpoweroing the very low signal strength of the
>>> GPS. Though it's been a long time, I do not believe that they ever
>>> replaced this system which can land the plane entirely without the help
>>> of the pilot. Modern commercial aircraft are entirely too large for the
>>> pilot to make perfect judgments on landing and I have yet to see tires
>>> blow out from misguessing altitude.
>>> 
>> I have exactly the same Garmin as you the Edge 830 and my commute
>> occasionally I go essentially though London Heathrow which is degree of
>> magnitude larger, and also surrounding by well London.
>> 
>> This isn?t a technical issue, but a Tom issue for what ever reason.
>> 
>> Roger Merriman
>> 
>> 
> 
> 
> 
> 
> Roger, I don't understand your meaning. Can you be plainer with your
> comments? I worked on aircraft for a little less than 4 years in the Air
> Force and then worked in Commercial aircraft for 6 more years. I traveled
> all over northern California to virtually every commercial airport during
> the Vietnam Airlift and I expect I have a MUCH better idea of aircraft than you.
> 
> DME and Tacan are in the same approximate frequencies as GPS and are very
> much stronger in and adjacent to airports. So it would be no surprise if
> there were interference.
> 

And yes one of my routes is though Heathrow ie one of the busiest airports
in the world with the same unit that you have, ie Edge 830 that shows the
same margin of error as any other route or even other places, ie not enough
to worry about.

And certainly shows no correlation with proximity to airports.

Roger Merriman