GPS Primer

What is Navigation?

Since prehistoric times, people have been trying to figure out a reliable way to tell where they are and how to get to their destination—and back home again. Such knowledge often meant survival and economic power in society. Early cultures probably marked trails when they set out hunting for food. They later began making maps and, by the Classical Age of Greece, developed the use of latitude (your location on Earth measured north or south from the equator) and longitude (your location on Earth measured east or west of a designated prime meridian) as a way of locating places. Today the prime meridian used worldwide runs through the Greenwich Observatory in England.

Sextant

Early mariners followed the coast closely to keep from getting lost. When they learned to chart their course by following the stars, they could venture out into the open seas. The ancient Phoenicians used the North Star to journey from Egypt and Crete. According to Homer, the goddess Athena told Odysseus to "keep the Great Bear on his left” during his travels from Calypso’s Island. Unfortunately the stars are only visible at night—and only on clear nights. Sometimes lighthouses provided a light to guide mariners at night and warn them of nearby hazards.

The next major developments in navigation were the magnetic compass and the sextant. The needle of a compass always points to the magnetic North Pole, so it tells you your "heading,” or the direction you're going. Mariner's maps in the Age of Exploration often depicted the headings between key ports and were jealously guarded by their owners.

The sextant uses adjustable mirrors to measure the exact angle of the stars, moon, and sun above the horizon. From these angles and an "almanac" of the positions of the sun, moon and stars, you can determine your latitude in clear weather, day or night. Sailors, however, were still unable to determine their longitude. When you look at very old maps, you sometimes find that the latitudes of the coastlines are accurate, but the longitudes are off by hundreds of miles. This was such a serious problem that in the 17th century the British government formed a special Board of Longitude consisting of well-known scientists. This group offered 20,000 British pounds, equal today to about $32,000 to anybody who could find a way to determine a ship’s longitude within 30 nautical miles.

The offer paid off. The answer lay in knowing what time it is when you make your sextant measurements. For example, say your Greenwich almanac predicts that the sun is highest at noon. Your shipboard clock, synchronized to Greenwich time when you left port, says it's 2 p.m. when your sextant measures that event. Then you must be the equivalent of two hours west of Greenwich.

In 1761 a cabinetmaker named John Harrison developed a shipboard timepiece called a chronometer, which lost or gained only about one second a day—incredibly accurate for the time. For the next two centuries, sextants and chronometers were used in combination to provide latitudes and longitudes.

In the early 20th century several radio-based navigation systems were developed and used widely during World War II. Both allied and enemy ships and airplanes used ground-based radio-navigation systems as the technology advanced.

A few ground-based radio-navigation systems are still in use today. One drawback of using radio waves generated on the ground is that you have only two choices:

• a system that is very accurate but doesn’t cover a wide area; or
• a system that covers a wide area but is not very accurate

High-frequency radio waves (like cell phones) can provide accurate position location but can only be picked up in a small, localized area. Lower frequency radio waves (like FM radio) can cover a larger area, but are not a good yardstick to tell you exactly where you are.

Scientists, therefore, decided that the only way to provide accurate coverage for the entire world was to place high-frequency radio transmitters in space. A transmitter high above Earth would broadcast a high-frequency radio wave with a special coded signal that could cover a large area and still reach Earth far below at a useful power level. This is one of the main principles behind the GPS system. It brings together 2,000 years of advances in navigation by providing precisely located "lighthouses in space" that are all synchronized to a common time standard.

The GPS system can tell you your location anywhere on or above Earth to within about 20 to 30 feet. Even greater accuracy, usually within less than three feet, can be obtained with "differential corrections" calculated by a special GPS receiver at a known fixed location.