Appendix f— gps primer, Background, Gps position determining concept – BendixKing KLN 89B - Pilots Guide User Manual
APPENDIX F— GPS PRIMER
The Global Positioning System (GPS) is a satellite-based navigation
system that was originally conceived and implemented by the United
States Department of Defense (DoD). The system is, however, avail-
able to all civilian users free of charge. GPS provides extremely
precise position, velocity, and time information.
The satellites are not geosynchronous, as is the case with many
weather and television satellites. That is, each satellite is not above a
fixed spot on the Earth all the time, but rather is continuously moving
across the sky. In fact, each satellite completely orbits the Earth two
times per day.
The Department of Defense imposes an intentional accuracy degra-
dation of the GPS system. This degradation is known as Selective
Availability (SA). When SA is active, only U.S. military users have
access to full GPS accuracy. For civilian users, position accuracy is
degraded to no worse than 100 meters. At the time of this writing,
Selective Availability is on (and therefore accuracy is degraded) near-
ly 100% of the time.
GPS POSITION DETERMINING CONCEPT
The technique used to determine position is fundamentally very sim-
ple. The complicated part is accounting for and correcting all the
possible errors in the position.
The GPS receiver is able to determine the time it takes a radio signal
to travel from the satellite to the GPS antenna. Since this radio signal
travels at the speed of light (approximately 186,000 statute miles per
second), the time delay can very easily be used to determine the
receiver’s distance from a given satellite. If measurements are taken
from four satellites (or three satellites and an input from an aircraft
altimeter), the receiver can identify its position very precisely.
KLN 89/KLN 89B Pilot’s Guide