How GPS WorksThe impact of Global Positioning Systems (GPS) technology on modern navigation techniques has been very profound. It has also induced fundamental changes in surveying and mapping methods. Creative innovation in GPS technology and features, as well as significant reductions in the average price of GPS equipment in the last few years has flowed on to diverse uses for GPS in cadastral mapping and engineering, GIS surveys and environmental studies. For precision mapping, GPS technology is deployed in spacecraft, aircraft, and land-based vehicles. It provides precise navigation and tracking systems to airplanes, ships at sea, taxi fleets and other land-based transportation systems, emergency vehicles, and even golf carts. Hikers, campers and people on the move use GPS receivers to get an accurate fix on their location at a given moment. Almost every day, new applications spring up for GPS technology. The GPS Satellite NetworkThe GPS navigation system is based on an orbiting network of 24 one-ton satellites launched by the United States Department of Defence, starting in 1978 and completed in 1994. Although initially intended for use solely by the military, the U.S. government decided to allow civilian use of the system. The satellites were launched into orbital paths 12,000 miles above the Earth, and each unit orbits the planet completely two times a day. Each satellite transmits precise time signals back to Earth, using atomic clocks for the purpose.
The nature of the signalThe satellites transmit signals (at only 50 watts of power) in two radio frequencies. UHF-band signals at 1575.42 MHz (called the L1 frequency) are used for civilian GPS; the other frequency (L2) is reserved for military and government uses. Satellite transmissions continue 24 hours daily, whatever the weather conditions or wherever in the world. The pattern in which the 24 satellites are arranged makes it possible for any earth-based GPS receiver to lock onto the signals of at least 4 satellites at any time. There are three components of the signal: * A pseudorandom code identifies the particular satellite originating the transmission. * Ephemeris data gives the orbital position of the satellite and other orbital information. * Almanac data contains various data; the most important portions are the current date and time, which are useful for calculating position. The signals move along lines of sight. They pass easily through glass or plastic but cannot penetrate solid objects like mountains or buildings. Signals may be degraded, which affects GPS accuracy. Some factors that cause signal degradation are: * Atmospheric distortion. The signal is slowed down by the ionosphere and troposphere. * Multi-path error. The signal may bounce off dense objects, e.g. high-rise buildings, rocks, etc., resulting in a time delay before the GPS receiver gets it. The longer travel time causes signal error. * Receiver clock accuracy. The GPS receiver clock may not be as precise as the atomic clocks used in satellites, causing slight mismatches in timing. * Orbital position error. The reported location of a particular satellite may be inaccurate. This is sometimes called ephemeris error. * Satellite reference. Accuracy is immensely improved when a GPS can reference or lock on to more satellites. The line of sight may be blocked by buildings, earth surface features, radio interference, or thick foliage. Disrupted signal reception results in poor (or no) position readings. [Note: GPS receivers usually cannot be used indoors or underground.] * Relative satellite positions. Satellite geometry is most favourable when they are positioned at wide angles relative to the other satellites. There is poor geometry when they are tightly grouped or positioned along a line.
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