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ALAN MAP 600
User’s Manual
The Global Positioning System (GPS) network
13. The Global Positioning System (GPS) network
GPS is a world-wide radio navigation system formed by a group of 24 satellites (21
operating and 3 spares) and their associated ground stations. GPS uses these satel-
lites, appropriately called NAVSTAR (Navigation Satellite Timing and Ranging), to
calculate ground positions. The basis of GPS operation is the use of triangulation
from the satellites. To triangulate, a GPS receiver measures distance using the tra-
vel time of radio signals. However, to measure travel time, GPS needs very accura-
te timing, plus it needs to know exactly where the satellites are in space. To solve
this problem, each of the 24 satellites is inserted into an orbit high enough (12,000
miles) to preclude interference from other objects, both man-made and natural, and
to insure overlapping coverage on the ground so that a GPS receiver can always
receive from at least four of them at any given time. In addition, compensation is
inserted for any delay the signal experiences as it travels through the atmosphere to
the receiver.
The GPS network was originally conceived by the U.S. Department of Defense
(DOD) to aid navigation.
13.1 How the system works
With the satellites operating at 12,000 miles above the earth’s surface, they are
arranged in strategic positions and orbit around the earth at a speed of 17,000
miles-per-hour, thereby completing an earth orbit every 12 hours. Each is powered
by solar energy; if that fails, they are equipped with on-board backup batteries to
maintain operational GPS integrity, and with small rocket boosters to keep them
flying along the correct path.
13.2 Satellite Frequency and control signals
Each satellite transmits a low-power radio signal in the UHF frequency range; the
frequencies used are designated as L1, L2, etc. GPS receivers, such as the PMR-
GPS unit, listen on the L1 frequency of 1575.42 MHz. This signal, since it is line-of-
sight, will reach the ground receiver unless it is obstructed by solid objects, such as
buildings and mountains.
The L1 signal is accompanied by a pair of pseudo-random signals (referred to as a
pseudo-random code) which is unique to each satellite. These codes are identified
by the GPS receiver and allow for the calculation of the travel time from the satelli-
te to the ground. If this travel time is multiplied by the speed of light, the result is the
satellite range (distance from satellite to receiver). The navigation information provi-
ded by each satellite consists of orbital and clock data, plus delay information based
on an ionospheric model. Signal timing is provided by highly accurate atomic
clocks.
13.3 Ground control
There are five GPS ground control stations - Hawaii, Ascension Island, Diego Garcia,
Kwajalein and Colorado Springs - that control the satellites by checking their ope-
rational disposition and exact position in space. Four of these stations are unman-
ned, and the fifth -- Colorado Springs – is the Master station. The four unmanned
stations constantly receive data and send it to the Master station. The Master sta-
tion then provides corrections for satellite Ephemeris constants and clock offsets
and, in conjunction with two other antenna sites, uplinks these information to the
satellites.
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