ETL 1110-1-183
1 Apr 98
g. Process with a minimum elevation mask of 15
relative to the precision of the measurements used to
degrees. A 15-degree elevation mask will reduce noise
determine the position. Large VDOP values represent
poor satellite geometry that will generally produce weak
embedded in low elevation satellite data and also
positioning solutions, although VDOP is primarily
minimize potential multipath effects from nearby objects
intended as a quality indicator for GPS code solutions.
surrounding the antenna. For obstructions low on the
horizon, a 20-degree elevation mask may be used during
c. Use fixed tripods poles. Fixed height tripods
baseline processing.
provide a consistent station occupation method that can
h. Process GPS data with Precise Ephemeris. The
reduce the likelihood of antenna height measurement
blunders.
broadcast ephemeris is the prediction of where the
satellites will be, but the precise ephemeris is the actual
d. Use dual frequency receivers. Dual frequency
true orbit of the satellites. Use of a precise ephemeris will
receivers can correct GPS measurements for ionosphere
reduce the error between predicted and actual satellite
based range errors. This will extend the feasible baseline
orbit and increase the accuracy of the survey. The precise
length and resolve integer ambiguities reliably within 20
ephemeris is available approximately 7 days after a
km. Dual frequency receiver should be used on all
survey through the U.S. Coast Guard Navigation Center
baselines longer than 10 km.
home page (www.navcen.uscg.mil) or the National
Geodetic Survey home page (www.ngs.noaa.gov).
e. Use identical geodetic quality antennas with
ground plane. Different makes and models of GPS
i. Use only ionosphere free fixed baseline
solutions for baselines greater than 10 k. Ionosphere
antennas can have different phase centers. Mixing of
different type of antennas can cause errors in the vertical
free solutions indicate the use of dual frequency receivers
component up to 100 mm. Only if the processing
and processing can model and eliminate errors due to
software can account for the phase center difference in the
signal delay in the ionosphere. Fixed baseline solutions
GPS antennas should mixing of antenna types occur. The
indicate a statistically accurate integer ambiguity was
ground plane on the antenna will reduce the amount of
established from the GPS data. A normal, (not
ground reflecting multipath.
ionosphere free) fixed baseline is sufficient for baselines
less than 10 km.
f. Occupy points a minimum of twice with different
satellite constellations and on different days. The
j. Use relative geoid height values. Application of
purpose is to ensure different atmospheric conditions
the geoid model to both reference and remote stations will
(different days) and significantly different satellite
produce two absolute geoid heights. The relative geoid
geometry (different times) for the two occupations. For
height value is determined from the difference between
example, if the first day observation were made between
the absolute geoid model height values taken at both ends
8:00 am to 8:30 am, the second 30-minute observation
of a given baseline. Relative geoid heights, when added
would be made on the next day anytime between 11:30
to measure ellipsoid height differences, produced the best
am and 5:30 pm. If the second observation is not made
vertical accuracy based on the ground truth test results.
for a couple of days or even a week, be sure to
k. For further information on using DGPS for
compensate for the daily 4-minute change in the GPS
satellite constellation is accounted for. It has been shown
elevations, see Zilkoski (1997).
that the average ellipsoid height of repeat observations is
closer to the truth, with a few exceptions, than the
ellipsoid height of a single observation.
A-5
Previous Page
