iSXblue/SXBlue II GNSS Series Technical Reference Manual
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Appendix C - Introduction to GPS 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 (Inter 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.
In order to maintain a strategic advantage, the US DoD used to artificially degrade the performance of the
SPS so that the positioning accuracy was limited to 100 meters 95% of the time. This intentional
degradation was called Selective Availability (SA). The effect of SA has been turned to zero since mid-
2000, and has been officially ‘turned off’ since then (2007).