The Global Positioning System The global positioning system (GPS) is a satellite-based navigation system, consisting of a network of 24 orbiting satellites travelling on six different orbital paths. These satellites, referred to as NAVSTAR satellites, are constantly moving, making two complete orbits around earth per day. The first GPS satellite was launched in February, 1978. Satellites now weight approximately 1 tonne, and are about 5 metres across with extended solar panels. GPS needs at least 24 satellites to provide full coverage of every point on the earth, all the time.
GPS satellites consist of 3 segments: space, control and user. The space segment consists of the 24 satellites in six orbital planes(four satellites in each plain). Each satellite repeats the same 12-hour orbit, but appears four minutes earlier each day. Each satellite orbits at 55 degrees in respect to the equatorial plane with a 12-hour period to orbit the earth.
The segment we are familiar with is the user segment. The user segment is what receives GPS signals, determines the distance between a satellite and a receiver and solves the navigation equations, all in order to obtain the coordinates of a specific place. The space segment consists of 31 satellites but there is an availability of at least 24 satellites that are approximately 6 000-12 000 miles above the earth. These satellites are arranged in this way because it ensures that at least four satellites are in view from literally any point on earth. This ensures the accurate and efficient operation of the global positioning system.
“Global Positioning System: Theory and Applications – vol. I and II”, American Institute of Aeronautics and Astronautics, Washington, 1996.  http://edu-observatory.org/gps/gps_accuracy.html, visited on 1/12/10.  http://www. elsevierdirect.com/companions/ 9780126836301/ appendices/ Chapter_2_ECI_ Cartesian_Coordinates_to_Kepler_Orbit_Elements_Conversion.pdf  http://www.squidoo.com/GPS-coordinate-systems-datums  Iyiade Adeniyi, “Ionospheric Error in GPS Applications” (www.ursi.org/proceedings /procGA05/pdf/GP1.
GPS Segments To understand the working of GPS, it is necessary to know the three Segments of GPS . 1. Space Segment- It consists of 24 satellites orbiting 12,000 nautical miles above the earth. 2. Control Segment- It consists of 5 ground stations around the globe that manage the operational health of the satellites by transmitting orbital corrections and clock updates.
GPS uses twenty-six satellites and ground tracking stations around the world to compute distances using time. This is done by finding the difference between the time a signal is sent and the time it is received. The satellites have atomic clocks so the time is extremely accurate. The receivers position is determined by using three satellites, this is called triangulation.(kAPLAN,eLLIOTT. UNDERSTANDING GPS:PRINCPLES AND APPLICATIONS) GPS is made up of three segments: space, control
There are about 31 active satellites used for GPS communication which are rotating around the earth’s axis. Their orbit is inclined 55 degrees with respect to the equator. Satellites are placed at 20,000 kilometers from the earth’s surface and their orbits are designed in such a way that each satellite rotates two orbits per day. Orbits are planned so that at least 6 satellites in view, from most area on the earth . C. Trilateration Trilateration is a simple mathematical approach used to estimate the vehicle location by calculating the distance from the satellites in view.
By the middle of 1961, RCA had a contract with NASA to build, a 4000 mile high, medium-orbit, active communications satellite called RELAY, AT&T was working on its own medium-orbit satellite called TELSTAR, and Hughes Aircraft Company had an exclusive contract to build a 24-hour orbit, 20,000 mile high satellite, called SYNCOM. By 1964, two TELSTARs, two RELAYs, and two SYNCOMs had operated successfully in space. The transponder technology used by AT&T in the TELSTAR I satellite is current technology in use today (Whalen, n.d.). On April 6, 1965, a new company called COMSAT launched its first satellite, EARLY BIRD, from Cape Canaveral beginning Global satellite communications. The EARLY BIRD satellite provided almost 10 times the capacity of submarine telephone cables for almost 1/10th the price.
From then, SpaceQuest followed its characteristic rapid development process to produce three operational spacecraft – AprizeSat 5 is retained as a software test- bed and flight spare. AS3 and AS4 were launched into a sun-synchronous, 10:30 orbit aboard a Dnepr rocket (converted SS-18 Russian ICBM) by ISC Kosmotras. Automated commissioning of the spacecraft was accomplished in the first two orbits, even before contacting the ground station for the first time. MISSION The AprizeSat spacecraft have a dual-purpose mission: AIS and M2M. Both missions can be performed in tandem, without interfering with the other.
Global-navigation Satellite Systems (GNSS) provide navigational data to airborne transportations and various other applications. Mainly two satellite constellations are in use today for navigational purposes. They are global positioning system (GPS) which is provided by the United States and global navigation satellite systems (GLONASS) which is provided by the Russian federation . GPS and GLONASS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km and 19,100 km respectively. These satellites primarily transmit the timing and a data message consisting of its orbital parameters which are received by the GNSS receivers to calculate the range from the satellite which will further provide a relative three dimensional position and time from the satellites.