A P P E N D I X
General Information
6.1
GPS Basics
The Global Positioning System (GPS) is a space-based radio-
navigation system. It consists of 24 satellites which orbit the earth
at an altitude of approximately 11,000 miles and ground stations.
GPS provides users with accurate information on position, velocity
and time. This is available anywhere in the world and in most
weather conditions.
GPS was initiated by The United States Department of Defense in
1973 to reduce the proliferation of navigation aids. By creating a
robust system that overcame the limitations of many previously
existing navigation systems, GPS became attractive to a broader
spectrum of users. GPS has been successful in classic applications
such as aviation and marine navigation since it was first developed.
Since then, it has become useful for a wide range of people because
its capabilities are accessible using small, affordable equipment.
What was once available only in very expensive equipment, the
Cobra Electronics Corporation brings to you with our high quality line
of affordable handheld GPS receivers.
How GPS Works
GPS uses a global network of 24 satellites that transmit radio signals
to earth from high earth orbit. Thankfully for users, The United States
has offered free use of the system to the international community.
Many international user groups have accepted GPS as the core for
their international civil satellite navigation system.
GPS service provides users with extremely accurate positioning
information anywhere on or near the surface of the earth. To
accomplish this, each of the 24 satellites emits signals to receivers
below. GPS determines location by computing the difference
between the time that a signal is sent and the time it is received.
GPS satellites carry atomic clocks that provide extremely accurate
time information. The time information is placed in the codes
broadcast by the satellite so that a receiver can continuously
determine the time the signal was broadcast. The signal contains
data that your GPS receiver uses to compute the locations of the
satellites and to make other adjustments needed for accurate
positioning. Your GPS receiver uses the time difference between the
time of signal reception and the broadcast time to compute the
distance, from the receiver to the satellite. With information about
the distance to four satellites and the location of each satellite when
the signal was sent, your receiver can compute its own three-
dimensional position: latitude, longitude and altitude. People think of
GPS as having three segments: control, space and user.
Control Segment
The Control Segment of GPS consists of the Master Control Station,
Monitor Stations and Ground Antennas.
The one master control station, located at Falcon Air Force Base in
Colorado Springs, Colorado, USA is responsible for overall
management of the remote monitoring and transmission sites. As
the center for support operations, it calculates any position or clock
errors for each individual satellite, based on information received
from the monitor stations. Then it “orders” the appropriate ground
antennas to relay the corrective information back to that satellite.
Five monitor stations are located at Falcon Air Force Base in
Colorado, Hawaii, Ascension Island in the Atlantic Ocean, Diego
Garcia Atoll in the Indian Ocean and Kwajalein Island in the South
Pacific Ocean. Each of the monitor stations checks the exact altitude,
position, speed and overall health of the orbiting satellites. The
control segment uses measurements collected by the monitor stations
to predict each satellite’s orbit and clock. The prediction data is
up-linked, or transmitted, to the satellites for transmission back to
the users. The control segment also ensures that the GPS satellite
orbits and clocks remain as designed. A station can track up to 11
satellites at a time. Each station performs this “check-up” twice a
day, as the satellites complete their journeys around the earth.
GPS General Information
