Where am I? For the 19th Century mariners, a sextant measured the angle of the Sun, stars and planets above the horizon. Coupled with an accurate timepiece, their approximate location was determined.
Today’s new constellation of artificial celestial objects in Earth-orbit sends signals to Global Positioning System receivers for precise latitude, longitude and elevation anywhere on our planet. However, “shooting the stars” with a sextant remains a basic skill for all sea-going navigators.
Twenty-four orbiting satellites and ground equipment help military and civilians chart their course over land, sea and through the air. The signals are available to anyone with a modest investment in a small GPS receiver.
Civil aircraft fly from one ground beacon (waypoint) to another, zigzagging to their final destination. Equipped with advanced GPS, a private pilot flys a direct route and lands even in poor visibility, a savings of time, fuel, and increased safety.
In the 1930s, radio beacons from airfields provided navigational bearings. During World War II, LORAN (Long Range Aid to Navigation) was useful in calculating location by timing the radio signals. By the 1960s, the accuracy of land-based electronic navigation was comparable to celestial navigation, within several miles. Later that decade, the US Navy’s six Transit satellites measured the Doppler Shift of the separate signals; these narrowing the window to about 660 feet. Measurements were only possible every twelve hours when satellites were in range. Transit operated until 1996.
The Department of Defense in 1973 combined the incompatible US Navy and Air Force navigation systems into a group of satellites each carrying atomic clocks. It was the first block called the Navstar Global Positioning System.
The latest evolution, Block II, introduced ground stations, coupled to the atomic clock, that controlled all the satellites for less money. The former Soviet Union’s similar navigation system, GLONASS is in operation still.
The launch of the current Block II began in 1989 and the two dozen satellites became fully operational in 1995 at a total cost of $12,000M. The military’s Precision Lightweight GPS Receiver (PLGR), nicknamed by the infantry as “Plugger,” is similar to civilian receivers, only more accurate.
After the Soviets shot down the Korean commercial airliner, Flight 007 in 1983, President Reagan directed that GPS signals would be available to everyone world-wide at no charge. Commercial utilization of GPS expanded rapidly.
The media attention upon the Persian Gulf War of 1991 unveiled the GPS secret weapon to public awareness. A critical shortage of GPS receivers to the military coalition was filled by civilian GPS units. For highest accuracy, the US Department of Defense temporarily suspended its scrambling of the satellite signals during those crucial months.
In May of this year, “Selective Availability,” intentional degrading of the timing signal for non-military use, was eased from 20 meters (66 feet) to 10 meters. However, the undisrupted satellite signals to a superior receiver is reportedly a coordinate accurate to a quarter of an inch.
Civilian GPS receivers outnumber those for the military by 1000%. Backpackers maintain their bearings with handheld units; pedestrians carry their GPS-equipped Personal Data Assistants; truck and automobile drivers see maps of the road ahead continually updated on dashboard displays or a laptop computer.
Global Positioning receivers do not transmit. The antenna must be primarily outdoors, distanced from tall trees and buildings. A clear reception of the signals from three satellites is necessary to calculate a position in two dimensions; four or more are needed in three dimensions.
Scientific field studies are more accurate, using Differential GPS, a method of correcting the arbitrary variance errors: Cartographers correctly reposition the location of small isolated Pacific Islands. Surveyors shoot accurate plat boundaries in mountainous terrain. Ecologists map changes in forest canopies. Biologists track animals wearing radio collars, and document specimen locations. Geologists measure the expansion of volcanoes and land movement along earthquake faults. And ranchers plot the perimeters of noxious weeds.
A repeat of maritime accidents of the past are less likely with the aid of GPS. Vessels are efficient routed through fog in heavy traffic lanes and crowded ports. With GPS, air/sea rescuers locate survivors, and police/fire departments dispatch their resources more quickly and efficiently. Railroads monitor their freight movement in real-time.
Recently, newly discovered waterfalls in Yellowstone National Park were data
collected using GPS. Locations of power poles and new subdivision roads and
utility hookup are easily archived. You can even set your watch with a GPS
receiver; it receives the time of day accurate to 150 billionth of a second.