SX BLUE GPS iSXblue II+ GNSSTM, Руководство пользователя

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SXblue Series User Manual 86
Appendix B - Introduction to GNSS
and DGPS Services
This chapter provides a brief overview of GPS, differential GPS, Space Based
Augmentation Systems (SBAS) such as WAAS / EGNOS / MSAS / GAGAN.
GPS
The United States Department of Defense (DoD) operates a reliable, 24 hour a
day, all weather Global Positioning System (GPS).
Navstar, the original name given to this geographic positioning and navigation tool,
includes an official constellation of 24 satellites (plus active spares) orbiting the
Earth at an altitude of approximately 22,000 km.
How it Works
These satellites transmit coded information to GPS users at UHF (1.575 GHz)
frequencies that allows user equipment to calculate a range to each satellite. GPS
is essentially a timing system - ranges are calculated by timing how long it takes
for the coded GPS signal to reach the user’s GPS antenna.
To calculate a geographic position, the GPS receiver uses a complex algorithm
incorporating satellite coordinates and ranges to each satellite. Reception of any
four or more of these signals allows a GPS receiver to compute 3D coordinates.
Tracking of only three satellites reduces the position fix to 2D coordinates
(horizontal with fixed vertical).
The GPS receiver calculates its position with respect to the phase center of the
GPS antenna. The latitude, longitude, and altitude of the antenna are referenced
according to the current version of the World Geodetic System 1984 ellipsoid
(WGS-84). At the time of print, the current WGS-84 version is G1150 and follows
the ITRF 2000 datum (International Terrestrial Reference Frame)
GPS Services
The positioning accuracy offered by GPS varies depending upon the type of
service and equipment available. For security reasons, two GPS services exist:
the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS).
The SPS uses a code modulated onto the signal for measurements and is referred
to as the Coarse Acquisition code (C/A code). The US Department of Defense
(DoD) reserves the PPS for use by its personnel and authorized partners. The
PPS uses a different code than the SPS, referred to as the Precise Code (P-code)
and contains more resolution than the C/A code. The DoD provides the SPS free
of charge, worldwide, to all civilian users.