Navigation and Seamanship

6.4.2. Working principle

The Department of Defense, USA, launched 24 satellites to go around the earth exclusively for their military purpose. Satellite have known orbits in term of position with regard to earth and they are continuously emitting radio signals. At a time, we can get signal from at least 4 satellites at any position on the earth. The micro computer available in GPS receiver makes the computation and gives the position in terms of latitude and longitude.

The GPS system consists of the following three segments,

1. Space segment

The original space segment consists of 24 satellites( 21 operational satellites with 3 active spares ) at the orbit height of around 11,000 nautical miles ( 20,200 km) . The satellites complete an orbit approximately once every 12 hours. S o there will be between five and eight satellites in view at any time, from any point on the Earth’s surface.

Each satellite is equipped with an atomic clock (made of Cesium and Rubidium) , which accurately keeps time to within three nanoseconds (that’s 0.000000003, or three-billionths of a second) . The signal travels from the satellite to the ground at the speed of light. The difference between the time when the signal is received and the time when it was sent, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To calculate its precise latitude, longitude, and altitude, the receiver measures the distance to four different satellites. In addition to the American GPS satellites, Russia established GLONASS system and GALLILEO system launched by European countries.

The GPS control segment consists of several ground stations located around the world with a master control station at Schriever Air Force Base in Colorado, USA. In addition, five unstaffed monitor stations are established at Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the Atlantic Ocean; and Colorado Springs, Colorado .

3. Receiving segment or user segment

The receiver of the GPS system is the user segment. The receivers are designed to fulfil the requirement of the different users. Now a days, the usage of GPS is in almost all the fields including fisheries. It receives the signal from the satellite, process it and display the required information including position in terms of latitude and longitude and speed of the vessel or aircraft that carries the receiver.

2. Control and monitoring ground segment