Navigation
When ships began venturing out of the sight of land, independent navigation became a necessity. Since ancient times, navigators learned to use the stars and planets to locate themselves. Hand-held tools were aimed at various planetary bodies to measure their altitude, or the angle made with the horizon. The altitude, combined with other information including the day of the year, was used to determine the latitude (Fairbridge, 1966). Below are two examples of early navigation devices, the astrolabe and the cross-staff.
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| Astrolabe, a ring marked in degrees for measuring celestial altitudes, in use. A rotatable alidade carried sighting pinnules. If held at eye level, a star could be sighted through the pinnules, and the mariner would read the star s altitude from the point where the alidade crossed the scale. For a sun sight, the astrolabe was allowed to hang freely and the alidade was adjusted so that a ray of sunlight passed through the hole in the upper vane and fell precisely on the hole in the lower vane. (Ifland, 1998) Courtesy of Krieger Publishing. |
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The cross-staff gave rise to the phrase 'shooting the stars’. In the process of sighting the chosen star, the cross-staff was held up to the user's eye with one hand, with the vertical piece, the transom, which slid along the staff, grasped in the other hand so that the person looks like an archer taking aim at the sun. The transom slides along the staff so the star can be sighted over the upper edge of the transom while the horizon is aligned with the bottom edge (Ifland, 1998).
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| Using the Cross-staff. Courtesy of The Mariner's Museum, Newport News, VA. |
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The sextant replaced the astrolabe in the mid - 1750’s. The principle is to measure the angle between the direct ray of the fixed horizon and the doubly reflected ray from a celestial body into coincidence. The celestial body is reflected in two opposing mirrors. The yields greater precision for several reasons, including the removal of motion artifacts due to the moving ship (after Bowditch, 2002).
Much later, electromagnetic energy, commonly in the form of radio waves, was used to develop more robust and accurate systems. Those systems could be operated independent of the visibility of the stars, a frequent limitation on the older celestial navigational methods. With evolution of the radio-based systems, multiple orientations became available, so that triangulation could be used to more precisely determine a geographic location. Radio Acoustic Ranging (RAR), developed in 1923, incorporated the principle of triangulation to yield ranges at frequent intervals. Sound energy from small explosives deployed from a ship was recorded on the ship and on several hydrophones at known locations; the ship's position was computed from time delays. (Sverdrup, Johnson, and Fleming, p. 341)
Radar (RAdio Detection And Ranging), developed in the Second World War, uses high-frequency electromagnetic energy emitted from a station to compute distance and direction from the time associated with the reflected energy returning from an object. The radio waves can be output at great strengths, yet be detected at very low strengths, then amplified for use, making RADAR versatile. (Brown, 1994)
Loran (LOng RAnge Navigation) also developed during WW1I, uses the time differential between low frequency radio transmitters - a stationary master and two slave stations to compute a geographic fix. Interference and energy propagation loss limit the effectiveness of LORAN. (Spradley, 1985)
In the 1960's satellites were launched and placed in orbit specifically for use in navigation. Early systems used the Doppler shift arising from the frequency change between the broadcast signal relative to the received signal due to the movement of the satellite. By monitoring this frequency shift over a short time interval, the receiver can determine its location relative to the satellite. These measurements combined with an exact description of the satellite's orbit can fix a particular position. (Wells, 1986)
Modern systems are more direct. The Global Positioning System (GPS) consists of 24 to 27 satellites. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. By knowing its distance from three or more satellites, the receiver can calculate its position by solving a set of equations, based on the principal of triangulation. Various corrections and techniques can be applied to increase accuracies to 2 to 3 centimeters (Wells, 1986).
This brief history is not intended to be a complete one. It is an overview from which readers may further explore the topic in libraries or on the web, using the topics presented above as starting points.
References
Bowditch, Nathaniel, 2002, The American Practical Navigator, Chapter 16: Instruments for celestial navigation, Paradise Cay Publications, 896 pp.
Brown, R.H., 1994, Robert Watson-Watt, the Father of RADAR, Engineering Science and educational Journal, IEE, v.3, no. 1.
Fairbridge, R.W.,1966, The Encyclopedia of Oceanography, Encyclopedia of Earth Sciences Series, vol. 1, Reinhold Publishing Corp., New York, p.535.
Ifland, P., 1998, Taking the Stars: Celestial Navigation from Argonauts to Astronauts, Melbourne: Krieger Publishing, 240 pp.
Krieger Publishing: www.krieger-publishing.com
Spradley, L.H., 1985, Surveying and Navigation for geophysical exploration, Boston: IHRDC, 289pp.
Sverdrup, H.U., M. W. Johnson, and R.H. Fleming, 1942, The Oceans: Their physics, chemistry, and general biology, Englewood Cliffs: Prentice-Hall, 1087 pp.
The Mariner’s Museum, Newport News, VA: http://www.mariner.org/
Wells, David, 1986, Guide to GPS positioning, Canadian GPS Associates, Fredericton: University of New Brunswick Graphic Services.
