Basics of Global Positioning System
Origin & History:
The Global Positioning System is originally a part of Navigation System with Timing and Range (NAVSTAR) was developed by US Department of Defense. The concept of GPS first developed in the late 1960’s but the first satellite was lunched in the year 1978 and the last one in the series of 24 earth orbiting satellite was placed into orbit in June of 1993. Its main purpose was only for military purpose including 24 hours navigation capabilities for navy and air forces. Since then it has become an important part in various civilian application, industries, and for recreational activities.
These 24 satellites are circulating earth above a distance of 20, 200 km in 6 orbits (4 in each) with an inclination of 55 degrees to the equatorial plane. All these satellites are controlled by the GPS Master Control Station (MCS) situated in Colorado. There are four real time tracker antennas in the ground and five passive- tracking stations to monitor all kinds of GPS activities. GPS receivers use triangulation technique in which the area is divided into a series of triangles for accurate 3D measurements of distances.
Navigation Signals - Satellites provide two signals with different frequencies, the L1 caries the navigation codes and the L2 is used to measure the ionosphere delays of signals. L1 frequency (1575.42 MHz) is used for civilian purpose. The satellites also transmit two kinds of clock code, the Coarse-acquisition (C/A) code contains the L1 frequency and the Precise code (P) usually used by the military purposes. Each satellite has its unique C/A code that is the identification of satellites. Signals generally contain three different kinds of data – pseudorandom code, ephemeris data and almanac data.
- Pseudorandom code contains a Pseudo Random Numbers (PRN) that identifies the satellite that transmits information.
- Ephemeris data tells about the orbital information for that satellite.
- Almanac data provides information about the status of the satellite with particular date and time that is essential to determine position.
Measuring Position: To tackle the problem of interfering signals, the P code is encrypted into the Y-code in the help of anti-spoofing (AS) mode that later decrypted and coordinates are calculated according to the World Geodetic System. Calculating position of a unit requires exact time and hence the satellites are equipped with atomic clocks and the receiving unit with the help of a crystal oscillating clock update time.
The receiver identifies each satellite signals and calculates the delayed time by producing an identical code sequence of he C/A. Taking these two sequences, the receiver can calculate a rough distance to each satellite. Later, the satellite orbital positions are taken into account to calculate its exact position. The location and distance of satellite indicates that the object is situated on the earth in an imaginary sphere. So now the intersection point of measurement by different satellites reveals the real position of the object.