WHAT IS GPS?

Global Positioning System or GPS is a global navigation system using satellites. A constellation of at least twenty-four satellites at medium earth orbit send microwave signals to the GPS receiver to pinpoint its location, direction and calculate the speed. Its official name is Navstar GPS. It was developed by the US Department of Defense and is now being managed by the US Air Force's 50 th Space Wing. The maintenance of the GPS system, including research and development and satellite repairs and replacements costs approximately 750 million USD annually, but the service is free for civilian use.

The GPS Navigation System

How does the system work? Different forms of receivers determine their locations by making precise measurements between three or more GPS satellites in orbit. The distance between the receiver and the satellite is calculated by means of measuring the time it takes for a signal to be transmitted and received again. Trilateration is the method used by the receiver to determine its relative location. This requires at least three satellites and uses the geometry of triangles, similar to triangulation.

Parts of the GPS Navigation System

There are three main parts of the GPS system: the space segment, the control segment, and the user segment. The space segment consists of the GPS satellites in orbit called space vehicles. These are distributed equally in six circular orbital planes having approximately 55 degrees inclination relative to the Earth's equator, and have a 60 degree angles starting from the equator that separates each plane. Right now there are 30 working space vehicles in orbit. These additional broadcasting satellites increase the precision of the system by providing redundant measurements for error checking. The satellites cruise in orbit at around 20,200 kilometers above sea level and positioned in such a way that any point on Earth would have six satellites in its range.

The second division of the GPS system is the control segment composed of the tracking system operated by the US Department of Defense. US Air Force stations monitors the flight paths of the space vehicles and then the data is sent to Air Force Space Command's master control center at Colorado Springs where the Schriver Air Force Base is located. The 2d Space Operations Squadron of the US Air Force that manages Schriver Air Force Base updates the location and clocks of the space vehicles regularly from data gathered from space weather information, ground monitoring stations and other important data centers.

The user segment is mainly the GPS receiver. These have antennas set at frequencies used by the space vehicles, processors, and a clock that is very stable. Crystal oscillators are often used for their stability. An interface for the user can sometimes be included in the unit, or there can be interfaces via computer monitors for vehicle tracking devices. It all depends on the format of the receivers. Data that are transmitted to PCs use various protocols such as NMEA 0183, NMEA 2000, GPSD, u-blox, SiRF and MTK.