I got involved in a project to locate the actual boundary monuments
which delimit our town. The USGS 7.5' topographic maps appeared to be
in error at some locations by several hundred feet. About 30 monument
locations were involved.
We are in rural NH which is mostly wooded, which results in a lot of
satellite signal attenuation, i.e., fewer receivable satellites.
I did a first effort using a Garmin eTrex 20 (~ $200) which received
both the US GPS satellites and the Russian GLONASS satellites. Receiving
both is important in our high attenuation environment because it
significantly increases the number of receivable satellites.
Those measurements supported my suspicions about topo errors, but had
error bounds in the tens of meters - not accurate enough for my purpose.
Next I used a Bad Elf Surveyor (~ $600 + Mac iPad) to record 30 minutes
of data into RINEX files which were then sent to CSRS-PPP for
post-processing. This somewhat reduced the error bounds, but they were
still ~10 meters wide, even for some 60 minute recordings.
Finally I used an iGage iGS3 receiver (~ $2400) to record US GPS and
GLONASS satellites for at least 30 minutes each (up to 1 hour under
heavy foliage) into RINEX files sent for post-processing to CSRS-PPP.
This approach finally realized the ±1 meter with 95% probability I needed.
A graph of the error ellipses for the 30 monuments as predicted by
CSRS-PPP post-processing, shown on a 1 foot grid, can be seen here:
<http://www.Mason-NH.org/Specials/Mon_Ellipses_Final_1_ft_grid.jpg>
Note that almost all are within ± 2 feet.
GPS satellites broadcast on two frequencies, L1 and L5. A key difference
is that the hand-held Garmin units and the Bad Elf Surveyor only use the
L1 frequency whereas the iGS3 is a dual frequency receiver (L1 and L5).
One large source of errors is the variable signal propagation delays in
the ionosphere, which have predictable differences between the L1 and L5
frequencies. Recording both signals allows a better estimation of, and
correction for, the ionospheric delays.
A word about post-processing.
I use the Canadian Geodetic Survey's CSRS-PPP processing because they
accept data from both US GPS and Russian GLONASS satellites whereas the
US Geodetic Survey's OPUS only accepts US GPS satellite data. In our
heavily wooded environment the ability to use both constellations of
satellites provides a crucial boost in performance.
Post-processing services continuously record L1/L5 signals from hundreds
of fixed sites. This allows them to accurately model the time-changing
errors in GPS signals, primarily ionospheric delays but also errors in
the satellite orbits and their clocks.
When RINEX data is submitted, the service can look at its
contemporaneous data from fixed receivers to model the errors at the
time and location of the RINEX recording and thereby apply corrections.
CSRS-PPP offers 3 levels of correction: ultra-rapid (90 minutes), rapid
(24 hour) and final (2 weeks). Which is applied depends on how long you
wait between recording the RINEX file and submitting it for post-processing.
The ultra-rapid will use the 90 minutes of data preceding the recording;
the rapid will use 24 hours of data, and the final will use 2 weeks of
data in calculating the corrections. More data gives better models and
corrections. In my experience the differences between 24 hour and 2 week
corrections are minor compared to my ±1 m target accuracy.
A fixed + rover approach is the gold standard for accuracy. This
technique uses 2 receivers - one fixed for the duration and the other
"roving" around to the various locations to be measured. As long as the
two stations are fairly close they will be affected identically by the
same GPS errors which can then be cancelled out. The cancelling can be
done in real-time if the fixed station broadcasts its data to the rover,
or it can be done by post-processing. Of course using two receivers
doubles (or more) the equipment cost.--
Garth Fletcher
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