GPS - From Stones to Satellites
Uploaded by picco20 on Feb 09, 2002
The Stone Age
In the early days of man, navigation was composed remembering objects as fixed points of reference. Leaving a trial of stones, marking trees and referencing mountains are examples of primitive navigational aids. The principles of this kind of navigation has evolved and is even present in today?s sophisticated navigational aids.
The Star Age (Trigonometry)
Identifying points of reference on land was easy. However, man started to explore the oceans where the only points of reference were the sun, the moon and the stars. Naturally they became points of reference and celestial navigation began. The position of stars and their geometrical arrangement looks different at different positions on the earth. Therefore, by observing the configuration of stars, one could estimate his position. For better accuracy special optical instruments were invented to measure the angles between stars. The data was then transferred to special charts where a position could be calculated. However, measuring angles with stars was limited to clear nights and only accurate to several miles.
Radio Age (Distance = Velocity X Time)
Around the middle of the century, scientists found a way to measure distances using radio signals. The basic concept is simple and works by using the relationship between distance, speed (around the speed of light for radio signals), and time. Accurate measurement of the signals time is important since on microsecond (one millionth of a second) equals 300m.
Below is how a radio receiver/transmitter (LORAN system) calculates ones position
The exact location of point A is programmed into a special radio receiver. The receiver when turned on measures the distance from the transmitter as 1000m. However, this does not tell us where we are but rather narrows our location to any point on a 1000m circle around the tower as shown in Figure 1.
Next assume that a second radio tower B is programmed into the receiver. The receiver when turned on measures the distance to tower B at 750m. Now we have two pieces of information: our distance to point A is1000m and our distance to point B is750m. So we are on circle A and circle B at the same time. Therefore we must be at the intersection of the two circles, one of the two points P or Q shown in Figure 2.
Measuring our distance to a third radio tower C would identify exactly where we are. Transmitters A, B, and C together are called a transmitter "chain". A chain...