Throughout time people have developed a variety of ways to figure out their position on earth and to navigate from one place to another. Early mariners relied on angular measurements to celestial bodies like sun and stars to calculate their location. The 1920s witnessed the introduction of more advanced technique-radio navigation-based at first on radios that allowed navigators to locate the direction of shore-based transmitters when in range. Later development of artificial satellites made possible the transmission of more precise, line of sight radio navigation signals and sparked a new era in navigation technology. Satellites are first used in position finding in a simple but reliable 2D Navy system called Transit. This laid the groundwork for a system that would later revolutionize navigation for ever-the Global Positioning System.
The Global Positioning System (GPS) is a satellite based navigation system. The concept of GPS was introduced by the United States Department of Defense (DoD). It is in the year 1994 that the GPS was completely developed. The GPS is developed to provide continuous, highly precise positions, velocity and time information to the land, sea, air and space based users. The intent of system is to use a combination of ground stations, orbiting satellites and special receivers to provide navigation capabilities to virtually everyone, at any time, anywhere in the world, regardless of weather conditions.
THE GPS SEGMENTS
The Space Segment
The space segments, also known as satellite segment, consist of 24 operational satellites revolving around earth in 6 orbital planes approximately 600 GPS satellites are not geosynchronous. First satellite was launched in the year 1978. The satellites take approximately 12 hours to orbit Earth. These satellites revolve the earth in a circular pattern with an inclined orbit. Out of the 24 satellites 21 are working satellites and the remaining 3 satellites will be in standby. In the event of a satellite failure, one of the spare space vehicles can be moved in to its place using modern propulsion and guidance system. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers. At a time 5 to 8 satellites can be viewed by the user, there by ensuring worldwide coverage. The information from three satellites is needed to calculate a navigational unit’s horizontal location on Earth’s surface (2D-Reporting), but information from four satellites enables a receiver to determine its altitude (3D-Reporting). Each satellite contains a Cesium Atomic Clock and all these clocks will be synchronized and are accurate within a few nanoseconds.
The Control Segment
The GPS control segment (CS), called the Operational Control System (OCS), includes all the fixed locations ground-based monitor stations located throughout the world, a Master Control Station (MCS) and the up-link transmitters. The monitor stations are simply GPS receivers that track the satellites as they pass overhead and accumulate ranging and ephemeris data from them. This information is relayed to the Master Control Station. These ground stations around the world are responsible for monitoring the flight paths of the GPS satellites and synchronizing the satellite’s onboard atomic clocks. This information is relayed to MCS where it is processed and compares the actual satellite position with the GPS computed position. The MCS receives data from the monitor stations in real time 24hrs a day, and uses that information to determine if any satellite are experiencing clock or ephemeris change and to detect malfunctions. Corrections are done and then it is uploaded to the satellites twice per day by the uplink antennae.
The User Segment
The GPS user segment consists of all the GPS receivers and the user community. Initially the GPS service was available for military purpose only. But in 1980 the Government of United States made the GPS service available to civilians also. The Fig 2.3 shows a GPS receiver. GPS receivers convert signals received from space vehicles into position, velocity and time estimates .The GPS navigation set contains antennae, receiver, data processor and a display unit. The satellite signals are further processed by data processor of the navigation set to demodulate the data and then decode it to get the user’s 3D position coordinates. The GPS receivers are used for navigation, positioning, aviation, shipping, geology and other purposes.
Consider the case of a lightning followed by a thunder. A few seconds after seeing the lightning we hear the thunder. If we know the time taken for the sound waves to travel from the lightning place to the listener, we can calculate the distance between the listener and the lightning place. Similar principle is used in the working of GPS. The GPS system works by determining how long it takes a radio signal transmitted from a satellite to reach a land-based receiver and then, using that time to calculate the distance between the satellite and the Earth station receiver. Radio waves travel at approximately the speed of light, 3×108 m/s. if a receiver can determine exactly when a satellite began sending a radio signal and exactly when the signal was received, then it can determine the propagation time. From propagation time, the receiver can determine the distance between it and the satellite using the mathematical relationship
d = v × t Where d = distance between satellite and receiver (meters) v = velocity (3 x 108 m/s)
t = propagation time (seconds)
Time is the most important factor in the working of GPS. Time synchronization between the GPS receiver and the on-board clocks is very important. Then only the Ranging calculations can be done accurately. The satellite transmitter and the Earth station receiver produce identical synchronizing (pseudorandom) codes at exactly the same time. This time will be accurate up to a few nanoseconds. Each satellite continuously transmits its precise synchronizing code. After a synchronizing code is acquired, the receiver compares the received code with its own locally produced code to determine propagation time. The time difference multiplied by the velocity of radio signal gives the distance to satellite.
If the Earth station receiver knows the location of the single satellite and the distance the satellite is from the receiver, it knows that it must be located somewhere on an imaginary sphere centered on the satellite with a radius equal to the distance the satellite is from the receiver. If the receiver knows the location of the two satellites and their distances from the receiver, it can narrow its location to somewhere on the circle formed where the two spheres intersect. If the location and distance to a third satellite is known, a receiver can pinpoint its location to one of the two possible locations in space. If the location and distance from a fourth satellite is known, the altitude or the 3D position of the Earth station can also be determined.
LEVELS OF SERVICES
Standard Positioning Service (Sps).
It is the positioning and timing service that is made available to all GPS users (Military, Private and Commercial) on a continuous, worldwide basis. It provides a horizontal accuracy of 100m, a vertical accuracy of 156m and a 3D accuracy of 185m. SPS will be provided on GPS L1 frequency (1575.42 MHz).
Precise Positioning Service (Pps)
PPS is a highly accurate military positioning, velocity and timing service which will be available on a continuous, worldwide basis to users authorized by the Department of Defense. PPS will be provided on GPS L2 frequency (1227.60 MHz). Both L1 and L2 frequencies are used for high precision works. . It has a horizontal accuracy of 20m, vertical accuracy of 27m and a 3D accuracy of 35m. Cryptographic equipments are used to prevent the unauthorized use of these PPS. This is more precise than the SPS.
APPLICATIONS OF GPS
The GPS can be used to find the exact position of a person or a vehicle etc very easily. Whatever may be the whether conditions we can easily locate a person or a vehicle having the GPS receiver. The Fig 5.1 shows the positioning of a receiver in a smoky weather condition. Precise location data for any point on planet is possible using this GPS. This system is used to locate persons and vehicles when they are lost.
For the Navigation purposes GPS receivers are used in cars, aeroplanes, ships and even space vehicles. The Fig 5.2 shows a GPS system used in cars. The current location of the vehicle and the road maps will be displayed on an LCD screen. Thus the shortest path to the destination can be selected. This is used in Aviation purposes to know the exact location of the plane and its distance from the different ports. Automatic pilot system is based on this technology. This is also used by sailors and other cruise ships to know their position in sea. The space vehicles (SV) also use this GPS to know their current locations.
The path through which a person or a vehicle moves can be traced easily using this GPS. So it is used in Shipping and Aviation purposes to know the path of the vehicle. Its tracking facility is used in industrial applications to know the processes through which the product moves. The velocity of the vehicle can also be calculated.
GPS is used in military for setting the targets and guiding the missiles. During the war period, military uses this technology to know the positions of their forces and their movements in the war region. Aeroplanes like F-16, B2 Bomber, KC-135 Arial refuelers make use of this technology. Missiles like Toma-Hawk are guided using GPS to destroy the targets. When a country uses the GPS guided missiles against an enemy, it is possible for the enemy country to locate some of the points in its trajectory by making use of its own GPS. Using these points the actual trajectory can be manipulated and location from which the missile was fired could be predicted. There is a good chance that this location would be a military base. Fig 5.4 shows GPS guided missile.
The GPS technology is used to help the people in case of emergencies. When an emergency call is made by a person, the call is automatically forwarded to a public-safety answering point (PSAP), also called an emergency call center. When the call is answered, the call center operator is provided with automatic location information (ALI), pinpointing the exact position of the call. The PSAP will give information to the nearby rescue operation team and thus makes the rescue operations fast.
Many synchronization systems use GPS as a source of accurate time; hence one of the commonest applications of this use is that of GPS as a reference clock for time code generators. For instance, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately. For geographically dispersed stations, the time synchronization is done using the GPS.
Though originally designed to help US Forces around the world to locate targets and move quickly, it is now being used across the world, from mountaineers climbing up Mount Everest to sail boats journeying in to oceans. Its functions have been extended to over positioning, tracking, mapping etc.GPS’s future seems secure. Its biggest push now is the Federal Communications Commission (FCC) enhanced 911 mandates. The new cell phones will be enabled with GPS. GPS receivers in future will be able to give accuracy up to 5mm. there is still room for improvement for GPS, as it does so, we will find it being used more and more in our daily lives, to the point where it would be hard to perform many travel and industry tasks without it.