An introduction to the Global Positioning System
Lucas M. Hjelle
Links Point, Inc.
One Selleck Street
Suite 330
Norwalk CT
06855
Abstract
The Global Positioning System is a satellite-based navigation system developed by the
United States Department of Defense (DOD) for military and government use, but the
information it provides is available free for civilian and commercial uses. GPS is a
broadcasting system in which satellites transmit information toward Earth. GPS
receivers take the transmitted information and use a form of triangulation to calculate the
user’s exact location. The basic premise of the technology is that the GPS receiver
compares signal transmission time with the signal reception time, and then uses the time
difference and the propagation speed to deduce the distance from each of the visible
satellites. GPS offers a wide range of applications and uses, and can provide GITA
professionals with a wide range of valuable data.
Three segments of the global positioning system
Space Segment
The Space Segment consists of a minimum of 24
satellites (sometimes referred to as space vehicles, or
SVs) orbiting 12,600 miles above the earth. Because
each satellite lasts about ten years, there are often
extra satellites already in orbit to replace the failing
older satellites (right now there are 26). Each satellite
travels at about 7,000 miles per hour, enabling them to
orbit the earth in just under twelve hours; the altitude
and orbital inclination are such that each satellite repeats the same ground track in each
twelve-hour orbit. The satellites are arranged in six orbital planes, spaced equally at 60
degrees apart, and each inclined at about fifty-five degrees with respect to the equatorial
Source: National Telecommunications and
Information Administration
plane. This spacing is intended to ensure that the required four satellites are viewable at
any given time from any spot on Earth, however there are often eight and up to twelve
satellites visible. Each satellite weighs approximately 2,000 pounds, is approximately 17
feet across, and uses solar panels to power its electronics and transmit the GPS signal.
It’s worth noting that at 50 watts or less, the GPS signal is at approximately the same
level as the background noise of the universe by the time it reaches Earth.
Control Segment
The Control Segment consists of a
network of monitor stations located
around the world used to track the
“health” of all of the satellites, as well as
one master control facility located at
Schriever Air Force Base in Colorado
Springs. The orbital models for each
satellite describes the degree to which
each SV is on its proper flight path; the
monitor stations measure certain signals
from the satellites, determine to what degree each satellite is off course, and compute
new orbital data and clock corrections. The monitor stations then send the new orbital
information (known as ephemeris data) and the clock corrections to the master control
station, which then relays the information to the satellites.
User Segment
The User Segment consists of the GPS receivers in the hands of the community of GPS
users. GPS receivers convert satellite signals into position and time estimates, and often
use this information to calculate other information such as velocity and heading. GPS
receivers make positioning, navigation, and time dissemination possible. This
information is then used for recreational, educational, commercial, research, and many
other applications. The receivers in the user segment are often used for aviation,
marine, and terrestrial vehicular and non-vehicular navigation.
