Global Positioning System Reference
In-Depth Information
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The deployment of GPS satellites came to a sudden halt due to the tragic January
28, 1986
Challenger
accident. Several years passed until the Delta II launch vehicle
was modified to carry GPS satellites. However, the theoretical developments contin-
ued at full speed. They were certainly facilitated by the publication of Remondi's
(1984) dissertation, the very successful First International Symposium on Precise
Positioning with the Global Positioning System (Goad, 1985), and a specialty con-
ference on GPS held by the American Society of Civil Engineers in Nashville in 1988.
Kinematic GPS was used for decimeter positioning of airplanes relative to re-
ceivers on the ground (Mader, 1986; Krabill and Martin, 1987). The goal of these tests
was to reduce the need for traditional and expensive ground control in photogram-
metry. These early successes not only made it clear that precise airplane positioning
would play a major role in photogrammetry, but they also highlighted the interest in
positioning other remote sensing devices in airplanes and spacecraft.
Lichten and Border (1987) report repeatability of 2-5 parts in 10
8
in all three com-
ponents for static baselines. Note that 1 part in 10
8
corresponds to 1 mm in 100 km.
Such highly accurate solutions require satellite positions of about 1 m and better. Be-
cause such accurate orbits were not yet available at the time, researchers were forced
to estimate improved GPS orbits simultaneously with baseline estimation. The need
for a precise orbital service became apparent. Other limitations, such as the uncer-
tainty in the tropospheric delay over long baselines, also became apparent and created
an interest in exploring water vapor radiometers to measure the wet part of the tro-
posphere along the path of the satellite transmissions. The geophysical community
requires high baseline accuracy for obvious reasons; e.g., slow-moving crustal mo-
tions can be detected earlier with more accurate baseline observations. However, the
GPS positioning capability of a few parts in 10
8
was also noticed by surveyors for
its potential to change well-established methods of spatial referencing and geodetic
network design.
Perhaps the year 1989 could be labeled the year when “modern GPS” position-
ing began in earnest. This was the year when the first production satellite, Block
II, was launched. Seeber and Wübbena (1989) discussed a kinematic technique that
used carrier phases and resolved the ambiguity “on-the-way.” This technique is to-
day usually called “on-the-fly” (OTF) ambiguity resolution (fixing), meaning there
is no static initialization required to resolve the ambiguities. The technique works
for postprocessing and real-time applications. OTF is one of the modern techniques
that applies to both navigation and surveying. The navigation community began in
1989 to take advantage of relative positioning, in order to eliminate errors common
to co-observing receivers, and to make attempts to extend the distance in relative
positioning. Brown (1989) referred to it as extended differential GPS, but it is more
frequently referred to as wide area differential GPS (WADGPS). Many efforts were
made to standardize real-time differential GPS procedures, resulting in several pub-
lications by the Radio Technical Commission for Maritime Services. The U.S. Coast
Guard established the GPS Information Center (GPSIC) to serve nonmilitary user
needs for GPS information.
The introduction of the geoid model GEOID90 in reference to the NAD83 datum
represented a major advancement for combining GPS (ellipsoidal) and orthometric
height differences. The most recent version is GEOID99.
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