Re: Galileo

On Wed, 28 Dec 2005 20:05:44 +0000 (UTC), The Cardinal
<thegrot@xxxxxxxxxxxx> wrote:

>Paul Cooper wrote:
>> On Wed, 28 Dec 2005 11:25:15 -0000, "Nigel" <jassira53@xxxxxxxxxxx>
>> wrote:
>>
>>
>>>"Gary" <plumperNOSPAM@xxxxxxx> wrote in message
>>>news:y_Srf.74824$2k.52399@xxxxxxxxxxx
>>>
>>>>Ronald Raygun wrote:
>
>>>>>Keep two receivers, one in the bows and one at the stern. If both
>>>>>have sub-metre accuracy, it will be possible for a clever box to
>>>>>work out your heading even when the boat is not moving.
>
>>>
>>>My understanding of GPS is that there is an element of error / inaccuracy
>>>due to atmospheric conditions which would affect both GPS's equally so for
>>>the purpose of heading could be ignored, and element of inaccuracy due the
>>>limitations of the GPS system and the quality of the receivers which would
>>>affect each GPS differently. Therefore if you were heading east and one GPS
>>>had a northerly error of 0.5m and the other a southerly error 0.5m, my
>>>calculations for a 10m boat show a "compass" error of over 5 degrees. Even
>>>0.1m gives an error over 1 degree
>>>
>>
>>
>> Absolute, stand-alone accuracy is as you state. However, the
>> relationship between two receivers can be measured extremely
>> precisely, to millimeter accuracy.
>>
>> Paul
>
>Interestingly we tried an experiment this summer. We compared the
>position of a handheld Garmin and a handheld Magellan against the
>installed Garmin. They were all set to the same datum and none of them
>agreed with the others. As I recall there was a difference of about 100
>yards between the two hand helds and a difference of about 50 yards
>between the two Garmins.
>
>I doubt that you can buy a pinpoint accurate GPS for a couple of hundred
>pounds and I doubt that the error between the suggested GPS at the bow
>and stern would be as little as 1 meter.
>
>db

As you surmise, the accuracy I report is not possible with consumer
grade kit! This is survey quality equipment, with the capacity of
comparing the phase of the carrier signal vetween different receivers.
This is extremely accurate - we routinely use it in Antarctica to
determine the position of survey markers with sub-centimetre absolute
positions.

Your errors do seem too large, though. They should agree to within
10-15 metres at worst, and your results suggest that there is some
other confounding factor. Were you in a marina or at sea when you
carried out the experiment? If the former, then I suspect you were
being badly affected by multi-path reception - that is, the receivers
being affected to different extents by signals being "reflected by
rigging and masts of surrounding boats. This is a well known problem
for urban areas, and I guess a mrina would be much the same. Different
receivers respond differently in these circumstances, and even three
identical receivers might give different results because even small
changes in position could cahnge the path-lengths. Another possible
cause of the discrepancy is local RF noise - GPS is more susceptible
to this than most people realize.

Paul
.

Relevant Pages

  • Re: Galileo
    ... inaccuracy due to atmospheric conditions which would affect both GPS's equally so for the purpose of heading could be ignored, and element of inaccuracy due the limitations of the GPS system and the quality of the receivers which would affect each GPS differently. ... We compared the position of a handheld Garmin and a handheld Magellan against the installed Garmin. ...
    (uk.rec.sailing)
  • Re: Galileo
    ... due to atmospheric conditions which would affect both GPS's equally so for the purpose of heading could be ignored, and element of inaccuracy due the limitations of the GPS system and the quality of the receivers which would affect each GPS differently. ... Therefore if you were heading east and one GPS had a northerly error of 0.5m and the other a southerly error 0.5m, my calculations for a 10m boat show a "compass" error of over 5 degrees. ...
    (uk.rec.sailing)
  • Re: Galileo
    ... >>> Keep two receivers, one in the bows and one at the stern. ... >>> work out your heading even when the boat is not moving. ... >> It's already available with GPS. ... stand-alone accuracy is as you state. ...
    (uk.rec.sailing)
  • Re: Galileo
    ... >> Keep two receivers, one in the bows and one at the stern. ... >> work out your heading even when the boat is not moving. ... My understanding of GPS is that there is an element of error / inaccuracy ...
    (uk.rec.sailing)
  • Re: L2C and other signals
    ... I am wondering whether the new gps signals will replace the oldies. ... what's the critical number of l2c transmiting satellites for seeing ... After 2020, legacy dual frequency receivers may still work, but the DoD would no longer assure that Ppower levels and the navigation message would remain the same. ... Just the United States CORS (Continually Operating Reference Stations) network alone comprises more than 1,000 receivers. ...
    (sci.geo.satellite-nav)
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