On 23 May 2011 14:25, RussellMc <apptechnz@gmail.com> wrote:
>> I don't think that would work very well, at least for normal affordable =
GPS
>> units. =A0The reported position can jump around quite a bit, especially =
when
>> you're moving. =A0Most GPS units have something like a 10 meter 50% erro=
r
>> radius.
>
> That sounds larger than I have observed in practice. Especially so for
> relatively time-adjacent readings.
> If you change satellite set you will very probably get a step change.
> Some years ago I took raw GPS data from driving around a network of
> local streets and plotted it using Excel. This allowed subsequent
> passes to & fro along the same road at similar and separated times to
> be displayed and differences calculated. =A0Roads were typical NZ urban
> residential roads (one lane each way plus kerbside parallel parking on
> both sides.
>
> A pass along a street, U turn at end and come back on opposite side,
> total trip under say 1 minute, gave there and back tracks of close to
> correct separation.
>
> Wait a few hours and repeat and the mean centreline of the =A0new track
> pair may be 20 to 40 FEET offset to one side.
>
> ie the second by second differential positional accuracy is "very
> good". The variability in side to side positional error compared to
> the essentially straight line being driven is small. From somewhat
> long ago memory it's perhaps a foot or so.
>
> Interest only: =A0 =A0In the very very early days of GPS when there were
> about zero commercial GPS receivers an academic published a paper in a
> scientific journal describing the receiver that they had designed and
> built. It was large and magnificently stationary but they published a
> scatter plot of the position that it reported. From memory it showed a
> locus about as wide as the building that they were in side to side
> across the satellite track - maybe a few 10's of feet, and several
> times larger along the track. =A0A footnote noted "now at ..." with the
> name of what is now one of the major GPS receiver makers. (Which one I
> don't recall). I will probably still have that paper here somewhere,
> but my finding it for you would probably cost you several days labour
> charges :-).
>
>
> =A0 =A0 Russell
>
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FWIW
GPS signals suffer from ionospheric variations which generally change
fairly slowly so a go-return plot is likely to be along the lines you
noted. Other problems in urban areas include multipath reception and
poor snr due to signal attenuation caused by buildings (or trees )
etc. There is also a difference between the accuracies obtained from
the various signals and channels available.

RTK systems use a differential system which corrects for the
ionospheric effects, generally at something like 1 second intervals
(by GRPS, or VLF, UHF radio) while survey grade DGPS systems use a
more direct differential correction I think. The survey grade systems
are now capable of accuracies better than 1cm (~10mm ??)
The equipment I'm currently working on is capable of an accuracy
somewhat better than 10cm with correction or about 30cm without. Quite
fun to play with.

RP

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