2.1.1 Introduction to GPS

WHAT IS GPS?

The NAVSTAR Global Positioning System (GPS) is a satellite-based positioning system, which by virtue of its special characteristics, is revolutionising the tasks of navigation and surveying:

Relatively high positioning accuracies, from the dekametre to the millimetre level.

Determination of velocity and time to an accuracy commensurate with position.

Available to users anywhere on the globe: in the air, on the ground, or at sea.

Relatively low cost system, with no user charges.

All-weather system, available 24 hours a day.

The position information is in three dimensions, that is, vertical as well as horizontal information is provided.

Who Developed GPS?

The Global Positioning System is, in the first instance, a military navigation system designed, financed, deployed and controlled by the U.S. Department of Defense.

Development work on GPS commenced in 1973 as a result of the merger of several R&D programs within the U.S. Department of Defense, namely the Navy's "TIMATION" project, and the Air Force's "621B" project. The first satellite was launched in 1978. (For a background to the development of the GPS program the reader is referred to PARKINSON, 1994, and PARKINSON et al, 1995.) The development and production program for the GPS is managed by the U.S. Air Force (USAF) Systems Command, Space Systems Division, Joint Program Office (JPO) at the Los Angeles Air Force Base, California. The JPO is manned by personnel from the USAF, U.S. Navy, U.S. Army, U.S. Marine Corps, U.S. Coast Guard, U.S. Defense Mapping Agency, NATO nations and Australia.

The aim of the JPO was to develop an all-weather, 24-hour, truly global navigation system to support the positioning requirements for the armed forces of the U.S. and its allies. As such a system was designed to replace the large variety of navigational systems already in use, a great import was placed on the system's utility, reliability and survivability. A number of stringent conditions therefore had to be met. In addition to those listed above, they included:

• suitable for all classes of platform: aircraft (jet to helicopter), ship, land (vehicle to handheld units) and space (missiles and satellites),
  
• able to handle a wide variety of dynamics,
  
• real-time positioning and velocity determination capability,
  
• the positioning results were to be available on a single global geodetic datum,
  
• highest accuracy to be restricted to a certain class of user,
  
• resistant to jamming,
  
• redundancy provisions to ensure the survivability of the system,
  
• low-cost, low power, and hence much of the complexity to be built into the satellite segment, and
  
• replace the ageing TRANSIT Doppler system and other navaids.

This led to a design concept based on:

• a system using measured ranges,
• the latest technology in clocks and microwave transmission technology,
• multiple satellites, and
• accuracy degradation that was graceful.

   

The total investment by the U.S. military in the GPS system to date is well over $10 BILLION (US)! However, although the primary goal of GPS is to provide land, air and marine positioning capabilities to the U.S. armed forces and its allies, GPS is freely available to all users. The number of civilian users is already far greater than the military users, and the applications of the positioning technology are growing rapidly. The civilian sector therefore represents an important user group that is increasingly lobbying in order to influence official GPS policy. The U.S. military however still operates several "levers" with which they control the performance of GPS. On the other hand, there is tremendous innovation occurring within the civilian sector, with the development of technology and procedures that are increasing making redundant many of the U.S. military procedures intended to restrict GPS performance.

Why a Satellite Positioning System?

Satellite Positioning Systems are global. The signals can be "seen" over a large area, and are not interfered with by terrain or geography to the same extent as conventional ground-based positioning systems.

GPS was designed to replace the TRANSIT Doppler satellite navigation system which has given good service to the navigation and geodetic community for over 20 years. The advantages of TRANSIT are essentially those of GPS as well. A microwave satellite-based system:

• Can transmit signals that can be "seen" over a far larger area than ground-based systems.
• Can transmit signals through cloud and rain.
• Can be used day or night, as long as the transmitting satellite is above the user's horizon.
• Recognises no national boundaries, and refers positions to a global datum uniquely defined for that system.
• The position information is three-dimensional.

However, in addition, the NAVSTAR Global Positioning System has some further advantages over other satellite-based positioning systems:

It is a one-way (listen only) system, in which the satellites transmit signals, but are unaware who is using the signal (no receiving function). The user (or listener) does not transmit a signal, and therefore:

• cannot be detected by the enemy (military context), and
• cannot be charged for using the system (civilian context).

As GPS is a multi-satellite system, there is always a number of satellites visible simultaneously anywhere on the globe, and at any time.

GPS can support a number of positioning and measurement modes in order to satisfy simultaneously a variety of users, from those only requiring navigation (dekametre) accuracies to those demanding very high (millimetre - centimetre) accuracies.

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© Chris Rizos, SNAP-UNSW, 1999