Electronic navigation systems employing artificial satellites as radio signal sources and position references. Satellites have the advantage that their signals have line-of-sight propagation to almost an entire hemisphere of the Earth. These systems have become the preeminent radio navigation systems and are gradually replacing ground-based systems such as Loran and Omega. Two main types of satellite navigation systems have been developed, Doppler systems and differential-time-of-arrival systems. See also Electronic navigation systems; Loran; Satellite (spacecraft).
The first operational systems were Doppler systems. The Doppler technique uses the Doppler shift of signals received from a low-orbit satellite. The frequency of the signal received from a moving source is shifted by an amount proportional to its velocity toward or away from the receiver. As the satellite passes the receiver, the Doppler shift will decrease from a positive to a negative value. The distance from the satellite track determines the magnitude of the shift. The crossover from a positive to negative shift will occur at the point of closest approach. The Earth's rotation during the satellite pass changes the shape of the curve in a way that indicates which side of the satellite track the receiver is on. These satellites orbit at relatively low altitude, about 900 km (550 mi), in order to produce a large Doppler shift. See also Doppler effect.
Doppler systems have inherent limitations. They provide only two-dimensional position fixes. Altitude must be determined by some other means. They provide limited accuracy and no velocity information. Because of their low altitude and limited coverage area, they provide at most a few position fixes a day. As a result, these systems have been phased out.
In differential-time-of-arrival systems, the receiver measures the difference of the time of arrival of signals transmitted simultaneously from several satellites. Two such systems have been put into operation. The Global Positioning System (GPS) was developed by the United States. The former Soviet Union developed a similar system called Glonass, which is now operated by Russia and operates with some minor differences, similarly to GPS.
The Global Positioning System provides worldwide coverage with four satellites in each of six planes for a total of 24 satellites. The orbits are nearly circular and have an inclination of 55°. The ascending nodes of the six planes are equally spaced around the Equator. An altitude of 20,200 km (12,500 mi) gives the orbits a subsynchronous period such that they repeat their ground track every second orbit. The satellites are three-axis-stabilized by a combination of reaction wheels and magnetic torque applied against the Earth's magnetic field. The satellite is rotated so that the Earth panel with the antennas always faces the Earth and the solar panels are at right angles to the Sun.
Each satellite broadcasts a ranging signal which is measured by all satellites in view. The satellites then broadcast a data message which contains the satellite ephemeris and all the measurements which it received. In this way, each member of a pair of ranging satellites has access to the pseudorange measurements in both directions (from the first satellite to the second, and vice versa). Each intersatellite range measurement is a combination of the geometric distance between the satellites and the difference of their clock biases. The two pseudoranges can be combined to yield an independent derived clock measurement and a derived true range measurement. The actual implementation of this requires additional terms to compensate for the satellite's motion and to account for the general-relativistic offset between the two satellites.
In a satellite navigation receiver, the antenna converts the radio signal from the satellite to electric current which can be filtered, amplified, and processed by the receiver. The phase center of the antenna is the position which the receiver locates. A preamplifier filters and amplifies the radio-frequency signal. A mixer shifts the L-band frequency to a lower intermediate frequency (IF) by, in effect, subtracting the frequency of a local oscillator from the incoming signal while preserving the signal modulation. Most newer receiver designs convert the intermediate-frequency signal to digital samples at this point. See also Analog-to-digital converter; Mixer.
In addition to providing navigation service to aircraft, vehicles, and watercraft, many other uses have become practical because of the accuracy, global coverage, and low cost of satellite navigation receivers. Small, inexpensive, handheld receivers are available for use in hiker, boating, and other recreational activities. In other applications, the receiver is combined with a communication system or a geographic database. Surveying has been revolutionized by a very accurate type of differential system. A surveying system can take advantage of the fact that it is not moving for the duration of its measurements. This yields very precise position estimates relative to a local reference point. The tectonic motion of the Earth's crust around fault lines has been measured with an accuracy of 1 cm (0.4 in.) or less. See also Surveying.
In automobile systems, the navigation receiver is combined with a display and a digitized map which is stored on a CD-ROM. Vehicle tracking systems use satellite navigation to determine a vehicle's position, which is then periodically reported to a central location by using a cellular telephone or some other communications system. Variations of this concept have been employed to track livestock, weather balloons, wildlife migration, and many other things. The ability to easily record the exact location at any point makes satellite navigation receivers ideal for mapping environmental data such as soil conditions, forest growth patterns, and pollution. Such data are used to monitor environmental damage. In aircraft navigation systems, satellite navigation is often combined with an inertial navigation system. The inertial system improves short-term accuracy, particularly during a maneuver. The satellite navigation maintains accuracy over the long term. See also Inertial guidance system.
