INTRODUCTION - GPS Satellite Surveying - page 8

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mospheric monitoring system (which could provide high vertical-resolution profiles
of atmospheric temperature across the globe).
The surveying community promptly responded to the opportunities and challenges
that came with GPS. The American Congress on Surveying and Mapping (ACSM)
tasked an ad hoc committee in 1993 to study the accuracy standards to be used in
the era of GPS. The committee addressed questions concerning relative and absolute
accuracy standards. The National Geodetic Survey (NGS) enlisted the advice of
experts regarding the shape and content of the geodetic reference frame; these efforts
eventually resulted in the continuously operating reference stations (CORS). Orange
County (California) established 2000 plus stations to support geographic information
systems (GIS) and cadastral activities. There are many other examples.
Zumberge et al. (1998a,b) report single-point positioning at the couple of centime-
ters level for static receivers and at the subdecimeter level for moving receivers. This
technique became available at the Jet Propulsion Laboratory (JPL) around 1995. The
technique that requires dual-frequency observations, a precise ephemeris, and precise
clock corrections is referred to as precise point positioning (PPP). These remarkable
results were achieved with postprocessed ephemerides at a time when selective avail-
ability (SA) was still active. Since 1998 JPL has offered automated data processing
and analysis for PPP on the Internet (Zumberge, 1998). Users submit the observa-
tion file over the Internet and retrieve the results via FTP soon thereafter. Since 1999
JPL has operated an Internet-based dual-frequency global differential GPS system
(IGDG). This system determines satellite orbits, satellite clock corrections, and earth
orientation parameters in real-time and makes corrections available via the Internet
for real time positioning. A website at JPL demonstrates real-time kinematic posi-
tioning at the subdecimeter of a receiver located at JPL's facilities in Pasadena.
Finally, during 1998 and 1999, major decisions were announced regarding the
modernization of GPS. In 2000, SA was set to zero as per Presidential Directive.
When active, SA entails an intentional falsification of the satellite clock (SA-dither)
and of the broadcast satellite ephemeris (SA-epsilon); when active it is effectively an
intentional denial to civilian users of the full capability of GPS.

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1.2 GEODETIC ASPECTS

The three-dimensional (3D) geodetic model is definitely the preferred model for ad-
justing three-dimensional GPS vector observations and combining them with classi-
cal terrestrial observations such as slant distance, horizontal angle, azimuth, vertical
angle, and, with some restrictions, leveled height differences. The three-dimensional
model is applicable with equal ease to the following: small surveys the size of a par-
cel or smaller, large surveys covering whole regions and nations, three-dimensional
surveys for measuring and monitoring engineering structures, and the “pseudo three-
dimensional” surveys typical of classical geodetic networks or in “plane surveying.”
Application of simple concepts from the theory of adjustments, such as “weighted
parameters” and “significance of parameters,” make it possible to use the three-
dimensional model in all of these applications in a uniform manner. Perhaps the most