Navigation

Early Navigational Foundations

To aid in navigation and map making, a coordinate system was created using virtual lines of latitude and longitude that cross at 90 degree angles. Latitude is referenced to a circle circumscribed around the Earth called the equator, which is at what is called zero latitude. North of the equator, the latitude lines are parallel to the equator and are called "north latitude." The geographical North Pole is 90 degrees north latitude and the circle at that latitude has such a small radius it is virtually a point. The South Pole is 90 degrees south latitude. The angle of latitude is the angular difference between two lines: one drawn from the center of the Earth to the equator and one drawn from the center of the Earth to the latitude in question.

Longitude lines also run around the Earth but through the North and South Poles. The reference line, the zero meridian, runs through the Royal Observatory in Greenwich, just outside of London, England. Lines of longitude are designated east and west from the zero meridian. Time zones are also a function of longitude and the reference time zone from which all others are measured is called GMT, or Greenwich Mean Time. This brings up an important subject: the inseparable relationship between time and navigation.

Time Zones, Sundials, and Longitudinal Calculations

Nearly everyone is familiar with the sundial, which uses the shadow of its angled center piece, called the gnomon, to "tell" time. The sundial tells local time, based on its relationship to the Sun in any given place. As an example, at exactly "high noon" the gnomon produces no shadow as the Sun is precisely midway between sunrise and sunset. But high noon occurs at different times at different places on the Earth. This is why there are time zones.

There are generally 24 time zones corresponding roughly to the one-hour segments of a 24-hour Earth day. There are some odd time zones with half-hour and even smaller increments, but these are rare. The actual time of high noon does not jump in one-hour steps, of course, but changes gradually as one travels around the Earth. If the sundial is adjusted so the gnomon points to true north, the sundial will show true solar time. The difference between true solar time at some location and the true solar time at the zero meridian can be used to calculate longitude.

In order to use the sundial to determine longitude in relationship to the zero meridian, however, a traveler must have an accurate mechanical clock set to precise GMT before traveling. The English government offered a substantial reward in 1761 for the invention of an accurate clock that would operate on a ship for precisely this reason. While the latitude of a ship could be determined by measuring the position of the Sun at its highest point, without a point of reference to time, determining longitude without an accurate GMT reading required lunar observations and time-consuming, difficult mathematical computations. Trade and exploratory ships could travel more safely, accurately, and economically with the use of reliable time-keeping technology.

The requirements for navigation became much more stringent when humans began to travel by air. A ship traveling on open water is relatively slow, so finding a "fix" or position every few hours was sufficient. Even if fog or other bad weather prohibited taking fixes, the ship could slow down or stop until conditions improved. This is not possible with aircraft! Accurate position fixes must be available continuously. Clock and sundial technology could not perform this complex task!

From Radio Beacons to On-Board Computers

One of the first aircraft navigation systems, invented in the 1920s, used radio beacons. The aircraft could hop from one beacon to another on what were called airways. Position could be determined from these airways but this involved tedious procedures that were not only difficult but time-consuming, as well. The beacons were strategically located so that the airways passed directly over airports to simplify the navigation. Similar homing beacons were used for ships but only near shore due to the limited range of the beacon's radio signal.

Later, more sophisticated radio navigation systems for both air and sea actually measured the vessel's latitude and longitude, which was plotted on a navigation chart. This was acceptable for ships at sea but unfolding a large navigation chart and plotting a course in an aircraft cockpit was not particularly convenient. However, because it was the best option at the time, it was done.

What would have been ideal would be a computer that took the latitude and longitude information and automatically calculated steering information. Some ship navigators had access to such a computer, which worked with the first long-range radio navigation systems during World War II. These computers were huge mechanical monsters that were acceptable for a battleship but not suited for aircraft.

It was not until small digital computers became available that long-range navigation became commonplace in aircraft. The aircrew could enter the desired final or intermediate destinations, called "waypoints," into the computer, and the computer would calculate the steering information, which was displayed with an indicator. It was even possible to use the steering information in the form of electrical signals to control a ship or aircraft with an autopilot.

Long-Distance Navigation Systems

Since World War II, several improved long distance radio navigation systems have been developed. The first was LORAN, which stands for "long range navigation." Shortly after LORAN was Omega, which was followed by a much-improved LORAN called LORAN-C. Finally, in the late 1970s, the ultimate system was developed, the satellite-based Global Positioning System or GPS. GPS can provide navigation anywhere on Earth within less than one meter (about 3 feet) of error, which is superior to any previous navigation system.

The GPS navigation system consists of a "constellation" of 24 satellites in well-known orbits. A network of ground stations controls the orbits and functions of the satellites. Satellites transmit radio signals that are used to measure the distance from the user to each satellite. A computer solves the geometry problem and determines the user's position.

GPS depends on the very accurate atomic clocks located in the satellites and ground control stations. It is fascinating to realize that the secret to accurate navigation in 1761 was precise clocks, and the same remains true today.

In addition to a radio receiver, the GPS user equipment has a rather extensive computer. It is necessary to separate the signals from the satellites, which are all transmitted on the same frequency and sorted out by the computer. The computer knows which satellites are present and where they are in their orbits. It inserts a number of calibration factors and calculates the position of the user equipment in latitude, longitude, altitude, and precise time. Most GPS receivers used for aircraft have large databases, which include the locations of airports, radio navigation aids, airways, and so on.

GPS products for consumer use have become increasingly popular since the late 1990s. In addition to providing convenience and security to people driving in unfamiliar areas, GPS technology such as the General Motors "OnStar" navigational system, which connects drivers to assistance operators via GPS satellites, can help save lives by directing drivers to hospitals or police stations near where they are, should an emergency arise.

SEE ALSO AIRCRAFT TRAFFIC MANAGEMENT; GEOGRAPHICAL INFORMATION SYSTEMS; GLOBAL POSITIONING SYSTEMS; SATELLITE TECHNOLOGY.

Albert D. Helfrick

Bibliography

Clausing, Donald J. Aviator's Guide to Navigation. Blue Ridge Summit, PA: TAB Books, 1992.

Hotchkiss, Noel J. A Comprehensive Guide to Land Navigation with GPS. Herndon,

VA: Alexis, 1995.

Lewis, Ralph. By Dead Reckoning: Recollections of a Master Navigator. McLean, VA: Paladwr Press, 1994.

Sonnenberg, G. J. Radar and Electronic Navigation. Boston: Butterworths, 1988.