ABX-3 / MBX-3 Reference Manual
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1. Introduction
This chapter provides a brief overview of GPS, differential GPS beacon
technology, and a description of the ABX-3 and MBX-3 receivers,
antennas, and accessories.
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 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.
Summary of Contents for ABX-3
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