Where am I? How do I get to my destination? These questions are as old as the history of mankind.
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Identifying points of reference was easy on land; but it became a matter of
life and survival when man started to explore the oceans, where the only visible objects
were the Sun, the Moon and the stars. Naturally, they became the "points of
reference" and the era of celestial navigation began.
Celestial navigation was the first serious solution to the problem of finding one's position in unknown territories, where the Sun, the Moon and stars were used as points of reference. The relative position of stars and their geometrical arrangement look different from different locations on Earth. Therefore, by observing the configuration of stars one could estimate his position on Earth and the direction that he should take for his destination. The Great Bear and Small Bear constellations are two examples. The geometrical configurations of stars from the observer's point of view were more accurately determined later by measuring the relative angles between them. For better accuracy, special optical instruments were invented to measure the angles of view between stars. These measured angles were then used to determine the position of the observer with the aid of published pre-calculated charts that eased the tedious computation task.
The process of measuring the angles of the stars with optical instruments was time-consuming and inaccurate. It could not be used during the day or on cloudy nights. The measured angles had to be transferred to special charts and after tedious calculations, the derived position was good only to about several miles.
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The calculation process was the basic triangulation geometry, where the stars became the known points of reference and the measured angles between them and the navigator would solve for the triangles' components and determine the navigator's position. The triangulation would have been simpler if distances to the stars could have been measured also. In fact, they could have been used to solve for the triangle's components instead of angles, but such measurements were not possible. |
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In frustrated moments of trying to determine a position, many navigators must have dreamed, conceivably, of gadgets that would do such a task automatically and more accurately. There were probably people that pictured a device, or even worked on building one, that aligned itself with stars quickly, measured angles to these points of reference and computed its position automatically.
The idea of automatic computation of position through measurement of distances to points of reference became a reality only recently when radio signals were employed and the age of radio navigation began.
About the middle of this century, scientists discovered a way to measure distances using radio signals. The concept was to measure the time it took for special radio signals to travel from a transmitting station to a special device designed to receive them. Multiplying the signal travel time by the speed of the signal gives the distance between the transmitter and the receiver. The speed of radio signals is the same as the speed of light — about 300,000,000 meters per second (about 186,500 miles per second). Accurate measurement of signal travel time is important since one microsecond (one millionth of a second) of error in measuring the travel time is equal to 300 meters of error in distance. For precision positioning, therefore, the receiving radio should be able to measure the travel time much better than one millionth of a second, perhaps to one billionth of a second (one nano-second).
How could such a radio signal transmitter-receiver system be used to determine a person's location?
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Assume that a transmitting tower is installed at a known point, A, on the earth and we have a special radio that can receive signals from transmitter A and measure the distance to the transmitter. The exact location of point A is programmed in our special radio receiver. We are in some unknown location. We turn on the receiver and measure our distance to the transmitter as 12,325 meters. This does not tell us where we are, but it narrows our position to a point on a circle with the radius of 12,325 meters around the transmitter, as shown in the Figure 1.
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Next assume that a second transmitter tower is installed at another known point, B, on the earth. The same special receiver measures our distance to transmitter B as 9,792 meters. This tells us that we are somewhere on a circle with the radius of 9,792 around the transmitter B. Now we have two pieces of information: our distance to point A is 12,325 meters and our distance to point B is 9,792 meters. So we are on circle A and circle B at the same time. 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 transmitter C would identify exactly where we are. Now you can imagine how the system works: we turn on our special radio receiver; it quickly measures the distances to transmitters A, B, and C and will compute our location. Remember that the exact location of transmitters A, B, and C were previously programmed in our special radio receiver. 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. The range of a radio transmitter is about 500 kilometers.
Navigational systems that use such radio signals to measure distances to several transmitting towers located at known points are called radio navigation systems.
LORAN (LOng RAnge Navigation) is one such radio navigation system that became operational around 1950. Each LORAN chain consists of at least four transmitters and typically covers areas of about 500 miles. To provide LORAN coverage for larger areas, several LORAN chains are used. For example, two LORAN chains cover the West Coast of the United States.
Each LORAN transmitter chain broadcasts radio signals on its own designated frequency. A LORAN receiver tunes in to the radio signals of the transmitters of the chain, measures distances to them automatically, and computes the position of the receiver. A LORAN receiver has the exact locations of all LORAN transmitter chains in its database. In a journey, one may pass through several LORAN chains. So, the navigator needs to know and tune in to the frequency of each LORAN chain he is passing through; in the same manner that one needs to change the frequency of an FM radio when leaving the coverage area of one FM station and entering into a coverage area of another.
The entire operational LORAN chains worldwide cover only a small portion of the earth. They are operated by local governments and are generally situated near coastal areas that have high traffic volume.
Although LORAN was a major breakthrough for navigation, it has the following shortcomings:
In general, the accuracy of LORAN is good to only 250 meters.
To overcome these limitations, satellite-based radio navigation systems were conceived in which improved radio transmitters were put aboard satellites orbiting the earth at high altitudes to give wider coverage. This is similar to the concept of a local TV station versus satellite TV. You can receive your land-based local TV station only in your city and within a distance of no more than 100 miles, while a satellite TV transmitter can cover an area as wide as the US (about 3000 miles). Signals from navigation satellites can cover large areas of the earth, and several satellites can cover the whole planet.
The theory behind the operation of the satellite navigation systems is similar to that of the land-based systems. In land-based navigation systems, the transmitting towers are the reference points located on the earth and the distance to them is measured by the receivers to compute the two-dimensional position (Latitude and Longitude or X and Y) by finding the intersection of several circles. In satellite-based systems, the satellites act as the reference points and the distance to them is measured to determine the three-dimensional position (Latitude, Longitude, and Altitude or X, Y, and Z) by finding the intersection of several spheres.
In land-based systems, the location of the transmitting towers are fixed, accurately known, and stored in the data base of the receivers. In satellite-based systems, the locations of the satellites are not fixed. They orbit the earth at high speeds. However, satellites have a mechanism of giving information about their location at any instant in time. The accuracy of the calculated location of the satellites, at the time at which distances to them are measured, affects the accuracy of calculated position of the receiver. In other words, the accuracy in computing our position depends on the accuracy in computing the location of our reference points.
In a positioning satellite system, satellite locations and their orbits are continuously monitored from several observation centers around the world by the organization responsible for keeping the orbit of the satellite within acceptable boundaries. This organization also predicts the orbit of the satellite for the next 24 hours based on the actual orbit information received by the observation posts for the previous 24 hours (similar to weather predictions). The predicted orbit information for the next 24 hours is relayed to each satellite by the control organization, so that it can be sent to receivers. Satellites broadcast their orbit information as part of their radio signal structure.
With satellite systems, we are once again "looking" up to the sky.
This time, however, we are "looking" at man-made objects instead of stars. And
unlike celestial navigation utilizing stars, man has now devised a scheme (using radio
signals and receivers) to measure distances to reference points.
One of the first satellite navigation systems was Transit. The experience gained from Transit and several other experimental systems led to the development of the current Global Positioning System (GPS) by the United States of America and GLObal Navigation Satellite Systems (GLONASS) by the Russian Federation. GPS and GLONASS are very similar, as we will discuss in the rest of this book.
In this chapter, we learned:
Stones, trees, and mountains were the early examples of "points of reference".
Celestial navigation was the first serious solution to the problem of finding one's position in unknown territories, where the Sun, the Moon and stars were used as points of reference.
Automatic computation of position, through measurement of distances to points of reference, became a reality only recently when radio signals were employed and the age of radio navigation began.
LORAN coverage is limited to about 5% of the surface of the earth where the chains are established. It is not a global system.
Identifying and remembering objects and landmarks as points of
reference were the techniques that the early man used to find his way through
jungles and deserts. Leaving stones, marking trees, referencing mountains were the early
navigational aids. Stones, trees and mountains were the early examples of "points of
reference", a concept that has evolved through times with the advent of (and the need
for) more sophisticated techniques, objects and instruments.
