Vector Sensor Reference Manual 125
Appendix C - Introduction to GPS and
SBAS
This chapter provides a brief overview of GPS, differential GPS, Space Based Augmentation
Systems (SBAS) such as WAAS / EGNOS / MSAS, and beacon differential services.
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 a
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 World Geodetic
System 1984 ellipsoid (WGS-84).
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
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