Module 2 : Global Positioning System
Lecture 4 : Basic GPS
NAVSTAR GPS

GPS design objectives

The following broad objectives were outlined for the design of GPS system (http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap1/112.htm)

. Suitable for different platforms: aircraft, ship, land-based and space (missiles and satellites),
. Ability to handle a wide variety of dynamics,
. Real-time positioning, velocity and time determination capability to an appropriate accuracy,
. Single global geodetic datum (reference system) for defining position,
. Differential accuracy standards: highest accuracy to be restricted to a certain class of authorized users,
. Resistant to jamming (intentional and unintentional),
. Redundancy provisions to ensure the survivability of the system,
. Passive positioning system that does not require the transmission of signals from the user to the satellite(s),
. Ability to provide the service to an unlimited number of users and world-wide coverage
. Low cost, low power, therefore highly complex satellite segment,
. Total replacement of the Transit 1 satellite and other terrestrial navigation aid systems.
 
Background of GPS evolution
  • The US Air Force (USAF) conducted concept studies for a 3-dimensional navigation system called 621B which provided three dimensional (latitude, longitude, and altitude) navigation with continuous service. In April 1973, the Deputy Secretary of Defense designated the Air Force as the lead agency to consolidate the various satellite navigation concepts into a single comprehensive DoD system to be known as the Defense Navigation Satellite System (DNSS).
  • By September 1973, a compromise system was conceptualized which combined the best features of earlier Navy and Air Force programs and was called Navigation Satellite Timing and Ranging Global Positioning System or NAVSTAR GPS . The signal structure and frequencies were taken from the Air Force's 621B. Satellite orbits were based on those proposed for the Navy's Timation system, but higher in altitude, giving twelve-hour instead of eight-hour periods (Parkinson and Spilker, 1996).
  • Thus NAVSTAR can be expressed as a combination of two systems as:     NAVSTAR = USN TIMATION + USAF 621B
  • Popularly known as the Global Positioning System (GPS), the system was designed to take over from existing navigation systems.
  • NAVSTAR GPS is a satellite-based, radio based, radio-positioning and time positioning and time-transfer system, designed, financed, deployed and operated by the US DoD.
  • Developed by the US DoD, GPS is first and foremost a defense military system for providing worldwide coverage at all time and all places with a low end user cost and a navigation position accuracy of 10-20 m.
  • The first test signals from space were transmitted from the Navigation Test Satellite 2, launched in June 1977. The first NAVSTAR satellite was launched in 1978. The system was declared to have the full operational capability (FOC) in 1993.
  • Requiring a minimum of 4 satellites, three for 3D position and one to keep track of time information to minimize certain types of errors , GPS is the most advanced real time positioning technique based on transmitting a coded satellite message, which could be received by a ground portable receiving unit (receiver) consisting of antenna (radio receiving unit mounted on tripod), power supply source (battery), oscillator (usually quartz crystal), internal clock ,and mini computer.
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