GPS is the most sophisticated system of pinpointing a position on earth so far developed by man. This chapter discusses in more detail the development of GPS and an explanation of how the technology works, so you will better understand what is happening when you use the system in the outdoors. A very short history of navigation is given to show how GPS navigation is so much more advanced than anything the world has ever seen. Some of the technical terms of the system are explained. See also the Glossary, which starts on page 108.

Early Navigation

In ancient times, most travellers ascertained their location by sight. Navigation was achieved by detailed observation of landscape features, some rudimentary observation of the stars and by detailed knowledge of a relatively small territory passed down from generation to generation. The compass was an important discovery because it oriented the traveller, but it alone could not fix a person's position. The astrolabe, the quadrant and the sextant opened new vistas in travel. They enabled navigators to easily determine their latitude. Longitude calculations required astronomical tables that detailed the exact positions of the planets at exact times. Before the invention of accurate chronometers in 1735, few adventurers had the skills to determine both latitude and longitude. The 15th century voyager, Amerigo Vespucci, taught himself how to measure longitude and made an important longitude calculation.

Vespucci performed his measurement on the Brazilian coast. He thought he was in the Indies because that is what Columbus had reported. Vespucci carried a book, called an almanac, which listed the exact times and positions of various planets. His book was made in Italy, so the times of the celestial events were based on the time as measured in Ferrara, Italy. At midnight on August 23, 1499, the moon would cross Mars in Ferrara. Vespucci watched the crossing in Brazil. He noted that the conjunction occurred 6.5 hours after it was seen in Ferrara. Using the difference in time and Ptolemy's value of the circumference of the earth, he was able to calculate his distance, or longitude, from Ferrara. The results convinced him that he was not in the Indies and that Columbus had discovered a new continent. Vespucci was the first person to know the truth about Columbus' discovery and only because he could determine his exact position.

As romantic as it seems, celestial navigation takes a lot of practice. It is accurate to about one mile, but no readings can be taken in bad weather. All of these disadvantages were eliminated by radio.

Radio Navigation

The use of radio signals to determine position was a significant advance in navigation. Equipment for radio navigation appeared in 1912. It was not very accurate, but it sufficed until World War Two brought pulse radar and the ability to measure short time differences between transmitted and received radio waves.

Determining position using radio signals requires measuring the time difference between arriving signals that come from known locations. An example involves two radio towers. Radio signals are sent from both towers at exactly the same time and travel the same speed. The receiver in the middle detects which signal arrives first and the amount of time until the arrival of the second signal. In this case, the west signal arrived before the east signal. If the operator knows the exact locations of both radio towers, the speed of the radio waves and the time difference between the two signals, he can calculate a one-dimensional location. He knows where he is on a line between the two towers.

Information of a position in one dimension is not good enough. If three radio towers are used, a two-dimensional fix can be made. An additional radio tower broadcasts in the example of this. Once again, the signals are sent at exactly the same time. The receiver records which signal arrives first and the time differences between the other signals. Using tower locations, signal speeds and time differences, it calculates a two-dimensional position in latitude and longitude. The Global Positioning System (GPS) works on the very same principles. The radio towers are replaced by satellites that orbit 20,200 km (12,500 miles) above the earth. The GPS system is even better than what has already been described because it fixes position in three-dimensions: latitude, longitude and altitude.

The GPS System

The Global Positioning System was conceived in 1960 and development started under the auspices of the U.S. Air Force. In 1974 the other branches of U.S. military service joined the effort and renamed it Navstar, but the name, GPS, persisted. The system was declared fully operational only recently, in 1995. It cost $10 billion to develop. Twenty-four satellites circle the globe every 12 hours to provide global coverage.

Tests performed in 1972 showed that the worse case accuracy of the system was 15 m (49'). The best case accuracy was 1 m (3.3'). A concern arose that enemies of the United States would use the system against them, so two tiers of accuracy were implemented for authorized (military) and unauthorized (civilian) users. The military has GPS receivers that are more accurate than civilian receivers. They are accurate to 1 m (3.3'). The accuracy of civilian receivers varies between 15 and 100 m (49' - 327') because of Selective Availability.

Each satellite broadcasts a signal that contains: Precision (P) codes, Coarse Acquisition (CA) codes and status information. Like other radio navigation systems, all of the satellites send their radio signal at exactly the same time, so the user can measure differences in arrival. The time of the satellite system is called GPS Time. The time is passed on to the user's receiver. Two receivers any place in the world will have the same time to within milliseconds. GPS time is highly accurate because each satellite has precise atomic clocks on board.

The receiver also has to know the satellites' locations, so a list of the satellites' positions (known as an ephemeris or almanac) is transmitted from each satellite to the receiver. The first time a receiver starts up, it takes about 15 minutes to get a fix because it first loads the satellites' almanacs. Ground control sites track the satellites and keeps their almanacs accurate.

Each satellite has unique P and CA codes, so that the receiver can distinguish between the satellites. The P codes are more complex than the CA codes, and only military users can recognize them because their receivers hold the value of the P codes in memory for comparison to the arriving signals. The time differences between P codes can be measured more accurately than between the CA codes. It is these more precise measurements that make military receivers more accurate than civilian ones.

Civilian receivers measure time differences between the arrival of CA codes. As stated before, civilian receivers would enjoy accuracy around 15 m (49') if ground control did not introduce error by interfering with the signal timing. The whole concept of radio navigation depends on simultaneous transmission of the radio signals. If the signals are not all sent at exactly the same time, the receiver cannot accurately calculate position. Ground control interferes by telling some of the satellites to send their CA signals either slightly earlier or later than the other satellites. Deliberate interference is the source of Selective Availability. Civilians do not know the amount of error, only that it is changed randomly between 15 and 100 m (49' - 327'). Military receivers are not affected.

There is another source of error that affects single frequency civilian receivers: ionospheric interference. When a radio signal travels through the free electrons in the ionosphere, it experiences a certain amount of delay. Signals of different frequencies are delayed differently. In order to detect the amount of delay caused by free electrons, the GPS satellites send the P code on two radio waves of different frequency, called L1 and L2. One signal is delayed more than the other as they pass through the ionosphere. Expensive receivers track both frequencies and measure the difference in arrival between L1 and L2, calculate the amount of delay caused by free electrons and make corrections for the effect of the ionosphere. Civilian receivers cannot correct for ionospheric interference because the CA codes are broadcast only on the L1 frequency.

There exist specialized receivers, known as codeless receivers, that are super accurate. They use the P code, but not directly. The receivers do not know the P code values like the military receivers know it, so they gain their accuracy by using special signal processing techniques. They listen to and process the P code for a number of days. After doing many computations, they can provide position fixes that are accurate to 10 mm (0.39"). This is great for surveying.

Operator using signals from two radio towers to calculate a one-dimensional location.