CSI Wireless Seres, Reference Manual

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Seres Reference Manual
1
Introduction
This chapter provides a brief overview of GPS, differential GPS, Space
Based Augmentation Systems (SBAS) such as WAAS / EGNOS / MSAS, in
addition to a description of the Seres positioning system.
1.1 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.
1.1.1 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).