The system calculates the position of the receiver by Triangulation method, which is similar to manner in which a seismologist locates position of an earthquake epicentre using arrival time of seismic waves. When distance from one satellite is measured, the position gives us radius of a sphere. Similar measurement from another satellite gives another sphere intersecting the first in an overlapping region of a circle. The third measurement gives another sphere intersecting the circle at two points.
Now one of the two points can be eliminated by either a fourth measurement, or due to unreasonable location or speed. Thus we have an accurate position of the GPS receiver. This position can be determined by just two measurements at sea level due to a well defined datum, and software assisted elimination of unreasonable datum.
Now with known speed of signal and extremely accurate time keeping, the distance from the satellites can be calculated accurately. With more than four satellites normally visible at any given time, position calculated becomes more accurate with each additional satellite.
Satellite Configuration Geometry causes the position information to be suspect due to position of satellites themselves, if they are bunched together the position accuracy will suffer negatively and if they are more or less well spread out in space, the accuracy is much better. This aspect is called Dilution of Position (DOP).
Limitations and Future of GPS
There are some inherent limitations of the GPS as given below:
GPS Signal Reception. Accurate functioning of a GPS receiver requires at least four GPS satellites. These signals cannot penetrate water or soil. Hence, GPS cannot be used for underwater marine navigation or underground surveys like mines or tunnels. Similarly, in areas of skyscrapers or high surrounding terrain, the number of visible satellites or signal strength can fall below acceptable strength hampering GPS functioning.
GPS Signal Integrity Monitoring. Since the datum for all calculations is GPS satellite, wrong satellite positions or wrong range measurements from satellites can be potentially disastrous. Thus GPS Integrity Channel (GIC) and Receiver Autonomous Integrity Monitoring (RAIM) are two schemes to accurately monitor integrity of signal information and warn the receiver whenever a conflict situation exists.
GPS Signal Accuracy. A GPS receiver normally measures time taken for a signal to arrive at the receiver and converts it into range information. This measurement can be corrupted in many ways called collectively as User Equivalent Range error (UERE). These are corrected by directly deducting the known error from the calculations or reduce these errors by changing the way we make measurements. Uncorrected errors creep into the calculations causing different kinds of inaccuracies or errors affecting performance of the receiver.
The future looks good with intense competition among world players to have their own GPS, mainly due to security aspirations. Russia, Europe and Japan are currently in advanced stage of system development/testing and many other regional countries are thinking about their own GPS. This would also lead to better accuracy and world wide acceptance of GPS for commercial and city use. The advantages would be better prices, more redundancy especially in the field of aviation and extensive market penetration due to hand held ground based GPS. The