Geoscience Reference
In-Depth Information
The D code, known as the navigation message, at 50 bits per second, carries the
position of satellites, status information, etc.
GPS Positioning Services
GPS offers two positioning services: the Precise Positioning Service (PPS) and the
Standard Positioning Service (SPS).
PPS. Authorized users of the PPS, including the US military, certain US
government agencies, and civil users specifically approved by the US government,
need cryptographic equipment and special receivers. The positioning accuracy of
PPS is several meters and the time accuracy reaches 40 ns.
SPS. For common civilian users, the US Government provides the SPS to take
control of positioning accuracy. Users worldwide use the SPS without charge or
restrictions. In the initial stages of GPS system implementation, SPS accuracy was
intentionally degraded by the US Department of Defense by the use of so-called
selective availability (SA). Under the effect of SA, the positioning accuracy of SPS
is degraded to approximately 100 m and the time service accuracy is about 340 ns.
In May 2000, the USA announced it was discontinuing the use of SA. At present,
SPS provides a positioning accuracy of approximately 10 m and a time service
accuracy of about 100 ns.
GPS Coordinate System and Time System
World Geodetic System 1984 (WGS84)
For a worldwide unified geodetic coordinate system, the US Defense Mapping
Agency (DMA) has provided WGS60 since the 1960s and later developed the
improved WGS66 and WGS72. WGS84, currently used by GPS, is a more accurate
global geodetic coordinate system.
The coordinate origin of WGS84 is at the Earth's center of mass. Its Z-axis is the
direction of the Conventional Terrestrial Pole (CTP), as defined by BIH1984.0. The
X-axis points to the intersection of the zero meridian plane defined by the
BIH1984.0 and the plane of the CTP's equator. The Y-axis constitutes a right-
handed coordinate system.
The coordinates of GPS single point positioning and the baseline vector in
relative positioning solution belong to the WGS84 geodetic coordinate system on
which the GPS satellite ephemeris is based. However, practical measurement
results often belong to a national or local coordinate system. In real applications,
one needs to solve the transformation parameters in order to transform coordinates.
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