Using the Global Positioning System (GPS, a process used to establish a position at any point on the globe) the
following two values can be determined anywhere on Earth (Figure 1):
1. One’s exact location (longitude, latitude and height co-ordinates) accurate to within a range of 20 m to approx. 1 mm.
2. The precise time (Universal Time Coordinated, UTC) accurate to within a range of 60ns to approx. 5ns.
Speed and direction of travel (course) can be derived from these co-ordinates as well as the time. The co-ordinates and time values are determined by 28 satellites orbiting the Earth.
GPS receivers are used for positioning, locating, navigating, surveying and determining the time and are employed both by private individuals (e.g. for leisure activities, such as trekking, balloon flights and cross-country
skiing etc.) and companies (surveying, determining the time, navigation, vehicle monitoring etc.).
GPS (the full description is: NAVigation System with Timing And Ranging Global Positioning System, NAVSTAR-
GPS) was developed by the U.S. Department of Defense (DoD) and can be used both by civilians and military
personnel.
The civil signal SPS (Standard Positioning Service) can be used freely by the general public, whilst the
military signal PPS (Precise Positioning Service) can only be used by authorised government agencies. The first
satellite was placed in orbit on 22nd
February 1978, and there are currently 28 operational satellites orbiting the
Earth at a height of 20,180 km on 6 different orbital planes. Their orbits are inclined at 55° to the equator,
ensuring that a least 4 satellites are in radio communication with any point on the planet. Each satellite orbits
the Earth in approximately 12 hours and has four atomic clocks on board.
During the development of the GPS system, particular emphasis was placed on the following three aspects:
1. It had to provide users with the capability of determining position, speed and time, whether in motion
or at rest.
2. It had to have a continuous, global, 3-dimensional positioning capability with a high degree of accuracy,
irrespective of the weather.
3. It had to offer potential for civilian use.
Generating GPS signal transit time
28 satellites inclined at 55° to the equator
orbit the Earth every 11 hours and 58
minutes at a height of 20,180 km on 6
different orbital planes (Figure 3).
Each one of these satellites has up to four
atomic clocks on board. Atomic clocks are
currently the most precise instruments
known, losing a maximum of one second
every 30,000 to 1,000,000 years. In order to
make them even more accurate, they are
regularly adjusted or synchronised from
various control points on Earth. Each satellite
transmits its exact position and its precise on
board clock time to Earth at a frequency of
1575.42 MHz. These signals are transmitted
at the speed of light (300,000 km/s) and
therefore require approx. 67.3 ms to reach a
position on the Earth’s surface located
directly below the satellite. The signals
require a further 3.33 us for each excess
kilometer of travel. If you wish to establish
your position on land (or at sea or in the air),
all you require is an accurate clock. By
comparing the arrival time of the satellite
signal with the on board clock time the
moment the signal was emitted, it is
possible to determine the transit time of that
signal.
The Global Positioning System (GPS) comprises three segments (Figure 8):
• The space segment (all functional satellites)
• The control segment (all ground stations involved in the monitoring of the system: master control station,
monitor stations, and ground control stations)
• The user segment (all civil and military GPS users)
