For thousands of years, speed was limited to a walking pace and landmarks were used to find location. At sea, early navigators limited their voyages to coastal routes to avoid becoming lost. New methods for determining position arose as trade between distant ports increased. Polaris, the North Star, was used to determine north-south distance (latitude) in the northern hemisphere. But mariners also had to find latitude when sailing in the southern hemisphere, and they lacked a method for determining east-west position (longitude). The solution, celestial navigation, required accurate time. In the late 18th century this led to the development of the marine chronometer, an accurate sea-going timepiece. Beginning in the 19th century the U.S. Naval Observatory, the nation’s official timekeeper, provided accurate time for navigators from an array of chronometers.

Sextant
A sextant is used to make precise celestial observations of the Sun, stars, and planets. It measures height, in degrees, above the horizon, which is used with the exact time to calculate position. The sextant shown here was made in the first half of the 19th century. “Shooting the stars” remains a basic skill for the sea-going navigator.

Electronic Innovations
Electronic navigation introduced all-weather capability, ease of use, and eventually, increased accuracy. In the 1930’s radio beacons were used to provide bearings from airfields. During World War II radio navigation systems were developed, the best known being LORAN, or Long Range Aid to Navigation. Positions were determined by the timing of signals received from different LORAN transmitter stations. In the 1960s the Omega system provided worldwide electronic navigation coverage for the first time. These land-based electronic navigation systems were accurate to within several miles, equivalent to celestial navigation. In the mid-1960’s the U.S. Navy’s NAVigation SATellite System (NAVSAT), also known as TRANSIT, was developed to provide more accurate positions for ships and submarines.
TRANSIT Satellite
TRANSIT was the first operational satellite positioning system. Six satellites gave worldwide coverage every 90 minutes and provided positions that were accurate to within 200 meters (660 feet). Positions were obtained by measuring the Doppler shift of the satellite signal. TRANSIT was effective, but it was limited by low accuracy and lack of 24-hour availability. The TRANSIT system operated until 1996.

SI Photo #7B-32766
© Smithsonian Institution.

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