GPS

The Stone AgeIn 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 may have four or more transmitters in order to have better coverage. However, the range of a radio transmitter is about 500 kilometers and the system is only two dimensional, it can not calculate or attitude or elevation above an object.

GPS (Global Positioning System)Left the Stones and now into Satellites

GPS is similar to star based navigation. Except now the stars are man made and emit radio signals instead of light. GPS is composed of 24 satellites in 6 different planes located 11,000 miles above the earth that act as fixed reference points. By measuring the travel time of a signal transmitted from a satellites, a receiver can calculate its distance from that satellite. When receiving the signals from at least 4 satellites, a receiver can determine latitude, longitude, altitude, and time.

There are five master ground stations located at Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs that continually track and correct the satellites for variations in position and time and errors.

Components of the SatelliteEach satellite is equipped with solar panels. These panels capture energy from the sun which provides power for the satellite throughout its life.

Internal components include such items as atomic clocks and radio transmitters. Each satellite contains four atomic clocks. These clocks are accurate to a billionth of a second or a nanosecond.

External components include a variety of antennas. The signals generated by the radio transmitter are sent to GPS receivers via the L-band antennas. Each of the 24 satellites transmits its own unique code in the signal. Since GPS was developed by the Department of National Defense, each satellite emits two separate signals, one for military purposes and one for civilian use.

How to determine the distance to a satellite How does the handheld receiver know when the signal left the satellite?

To better explain this lets use a simplistic analogy. Suppose there is a satellite orbiting 10 000km above you in space and you are on the ground with a hand held receiver. At the same time, the receiver and the satellite begin playing a song (The inky dinky spider). Than standing at the receiver, you would hear two versions out of sync, one from the receiver and one from the satellite (delayed). If you wanted to see just how delayed the satellite’s version was, you could start delaying the receiver’s version until they fell into perfect sync. That delay time is equal to the travel time of the satellites version. So we just multiply that time times the speed of light and you ve got the distance to the satellite. The song played is a complex digital binary (on/off) wave known as the Pseudo-Random Code.

Picture from http://www.casecorp.com/agricultural/afs/howgpsworks.htmlA graphical version of the Pseudo-Random Code (a series of on/off poises). Note the timing difference due to travel time of the sate