Global Positioning System
The Global Positioning System (GPS) is the only fully functional Global Navigation
Satellite System (GNSS). Utilizing a constellation of at least 24 Medium Earth Orbit
satellites that transmit precise microwave signals, the system enables a GPS
receiver to determine its location, speed, direction, and time.
Developed by the United States Department of Defense, GPS is officially named
NAVSTAR GPS The satellite constellation is managed by the United States Air
Force 50th Space Wing. The cost of maintaining the system is approximately
US$750 million per year, including the replacement of aging satellites, and research
The current GPS consists of three major segments. These are the space segment,
comprising orbiting GPS satellites (SS), a control segment, where the flight paths of
the satellites are tracked by earth stations (CS), and a user segment, the user's GPS
A visual example of the GPS constellation in motion with the Earth rotating. Notice how
the number of satellites in view from a given point on the Earth's surface, in this
example at 45°N, changes with time.
Orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or 10,900
nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV
makes two complete orbits each sidereal day.
Each satellite transmits its navigation message with at least two distinct spread
spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to
the public, and the Precise (P) code, which is usually encrypted and reserved for
Augmentation methods of improving accuracy rely on external information being
integrated into the calculation process. Examples of augmentation systems include
the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems
and Assisted GPS.
Many civilian applications benefit from GPS signals, using one or more of three basic
components of the GPS: absolute location, relative movement, and time transfer.
The ability to determine the receiver's absolute location allows GPS receivers to perform
as a surveying tool or as an aid to navigation. The capacity to determine relative
movement enables a receiver to calculate local velocity and orientation, useful in
vessels or observations of the Earth. Being able to synchronize clocks to exacting
standards enables time transfer, which is critical in large communication and
How the position is established:
The positional fix is calculated by means of trilateration (distance – distance
intersection) The receiver is able to determine the signal propagation time, by using the
time stamp that is transmitted, to establish the distance.
Since the intersection of all the distances will not meet exactly, due to various factors,
including the atmospheric conditions, the receiver clock is adjusted until the smallest
error is obtained.
Once the receiver has established its position, the fix will be dynamic, and as the
receiver is moved, it will track its position. This information can then be extrapolated into
direction, distance and speed. The position can also be superimposed on a digital map,
which provides an almost foolproof navigational tool. The commercial aircraft industry is
working towards automated landing, assisted by GPS positioning. In the surveying
profession, use is made of differential GPS, were a control receiver is mounted at a
fixed, known position, and the “mobile” unit is setup at the unknown site. The position of
the base receiver in continuously calculated, compared to its known location, and the
correction is either transmitted to the rover, or used in the “post processing” of the
position This procedure can yield relative accuracies, over long distances (100km), of 1