Systems that provide a variety of navigational parameters through the application of the electronic sciences. Typical parameters are distance and bearing from a vehicle to a known point, and the present position of the vehicle in a particular coordinate system. From the knowledge of present position, the course and distance to a destination can be calculated. Most systems are based on the use of electromagnetic (radio) waves, and those described here are used by aircraft, ships, land vehicles, and space vehicles.
The use of radio waves has been found to be attractive because of their known, and nearly constant, velocity of propagation, namely, the speed of light (c), which is about 1.86 × 105 mi/s (3 × 108 m/s). Thus, if the time (t) of travel of the radio signal between two points is accurately measured, the distance or range (d) between the points can be accurately determined from the equation d = ct. Ships also use systems based on underwater sound waves, with due allowance for the much slower speed of sound in water. Also, electromagnetic waves in the visible (optical) or near-visible spectrum can be used in a similar manner for distance measurement. See also Electromagnetic radiation; Light; Sonar; Underwater navigation; Underwater sound.
From an electronic viewpoint, all systems can be classified as either cooperative or self-contained. From a navigational viewpoint, systems are frequently classified as positioning or dead-reckoning systems. Most positioning systems are cooperative systems, while most dead-reckoning systems are self contained. Many modern systems combine the data from cooperative and self-contained sensors to obtain a more accurate solution. Such systems have been called multisensor, integrated, or hybrid systems. See also Dead reckoning.
Cooperative systems
The two general categories of cooperative radio systems are point-source systems and multiple-source systems. Point-source systems typically determine position in a relative coordinate system by measuring the distance and bearing to a transmitting source at a known location. See also Rho-theta system.
Perhaps the earliest form of a point-source radio system is the direction finder, in which the signal from a single transmitter source is received at two known points or elements. The direction from the vehicle to the source is determined by measuring the differential distance (or phase) of the signals received at the two points or elements. For operational convenience (size), it is desirable to have the two receiving points close together and to use common circuitry at both measuring points. A loop antenna fulfills both of these requirements. The loop is rotated until the currents in the two vertical arms of the loop are equal in amplitude and phase so that the output of the receiver is zero. The transmitter source is then located 90° from the plane of the loop. In simple loops, there is a 180° ambiguity, but this is typically resolved by temporarily connecting an omnidirectional antenna to the receiver input. See also Direction-finding equipment.
The angular measurement errors, particularly on aircraft and ships, can be quite large (3° or more). However, direction finders are still used, particularly for backup navigation and emergency homing.
Another class of point-source angle-measuring systems is based on the use of rotating or scanning antenna beams at the transmitter source and reception (or reflection) of the transmitted signal by the user vehicle receiver. For example, if a ground transmitter generates a rotating cardioid amplitude pattern at a fixed rate plus an omnidirectional reference signal, the user receiver can measure the relative phase difference between these two signals and thereby determine the bearing to the transmitter source. The very high frequency (VHF) omnidirectional range (VOR), used worldwide for short-distance aircraft navigation, is based on this principle. See also VOR (VHF omnidirectional range).
Another point-source technique for bearing measurement is based on using a radiated dual-lobe structure, with the carrier in each lobe being modulated at different frequency tones and phases. The user receiver can then detect when the vehicle is operating at the intersection of the two beams. The instrument landing system (ILS), which is used worldwide for aircraft approach and landing, is based on that principle. See also Instrument landing system (ILS).
Theoretically, there are two possible types of point-source systems for distance determination. One is based on the two-way (round-trip) ranging principle. The interrogator, which may be located on the navigating vehicle, transmits a signal, typically a pulse or pair of pulses, at a known time, which is stored in the equipment. The signal is received at a transponder and, after a fixed, known delay, is retransmitted toward the interrogator. When received by the interrogator's receiver, it becomes an input to the ranging circuit. This circuit measures the time difference between the original transmission time and the time of reception (less the known fixed delay), which (when multiplied by the speed of light) is a direct measure of the two-way distance. Such a system is the basis of the distance-measuring equipment (DME), used for short-range navigation worldwide, and also of the Air-Traffic Control Radar Beacon System (ATCRBS), used for air-traffic control surveillance on a global basis. See also Air-traffic control; Distance-measuring equipment.
A second possible technique for point-source distance determination is one-way signal-delay (time-of-arrival) measurement between a transmitter source at a known location and a user receiver. In this case, a successful measurement is possible only if the transmitter oscillator (clock) and the user receiver oscillator (clock) are precisely time synchronized. Since this synchronization is frequently impossible at reasonable equipment cost, no practical point-source distance measurement system based on such synchronization has been developed, but several modern multisensor systems (discussed below) have modes that employ this principle.
Multiple-source radio systems employ multiple transmitter sources, and user vehicle equipment typically consisting of a receiver or a receiver-transmitter. The four major categories of such systems (with some implementations using combinations of these) are hyperbolic systems, pseudoranging systems, one-way synchronous ranging (direct-ranging) systems, and two-way (round-trip) ranging systems.
Hyperbolic systems were the first to be developed, but they are still in widespread use. Three or more transmitter sources transmit time-synchronized continuous-wave or pulsed signals. The user receiver measures time differences of signals from pairs of stations. Loci of constant time differences, or (equivalently) constant differences in distance, from two stations form hyperbolic lines of position (LOPs). The point where two lines of position cross is the position of the user vehicle. The Loran C system is based on this hyperbolic principle. See also Hyperbolic navigation system; Loran.
Pseudoranging has some similarity in configuration to hyperbolic systems, but does not use time differences. Several transmitter sources (for example, space vehicles or terrestrial stations), whose positions are made known to the user, transmit highly time-synchronized signals based on an established system time. With the periodic times of transmission epochs of the signals from the sources provided or known to the user, the user measures the time of arrival (TOA) of each signal with respect to the user's own clock time, which normally has some offset from system time. This range measurement is called pseudorange, since it differs from the true range as a result of the offset of the user's clock relative to the system (transmitter) time. From successive or simultaneous time-of-arrival (pseudorange) measurements from four (or more) sources, the user receiver then calculates the three-dimensional position coordinates and its own clock offset (from system time). This is accomplished by solving four simultaneous quadratic equations (usually the computations are linearized), involving the three known position coordinates of the sources and the four unknowns, namely, the three user position coordinates and the user's clock offset. Thus, such a system accurately determines not only the user's three-dimensional position but also system time, which is easily related to Universal Time Coordinated (UTC). The pseudoranging concept is the basis of operation of two major satellite navigation systems for worldwide use, the U.S. Global Positioning System (GPS) and the Russian Global Navigation Satellite System. See also Satellite navigation systems.
The implementation of one-way synchronous ranging between one of the transmitter sources and the user receiver was discussed above. In order for a true one-way range measurement to be made, the clock of the user receiver must first be synchronized to that of the transmitter source. In some systems, this is accomplished by an independent means, for example, in the JTIDS-RelNav system and in the Position Location Reporting System (PLRS). The concept is also used in one mode of certain hyperbolic systems, notably Loran C. Two implementations are possible, the rho-rho method or the multiple-rho method. The rho-rho method requires only two source transmitters, but also requires a highly stable user receiver oscillator (clock) and precise knowledge of the time of transmission from the source stations. The multiple-rho method requires at least three stations. Using three lines of position permits clock oscillator self-calibration, much as the previously discussed pseudoranging concept, and therefore leads to a much less stringent clock oscillator requirement.
Two-way (round-trip) ranging involves multiple two-way distance measurements of the type discussed above. To obtain a completely unambiguous horizontal position fix, three such two-way ranges are required; however, two are usually sufficient. The major disadvantage of this concept is that the user requires a transmitter as well as a receiver. A typical application of this concept is DME-DME or multiple-DME operation for civil aviation navigation.
Self-contained systems
These systems can be classified as radiating or nonradiating. Radiating systems may be subject to jamming and to homing by radiation-seeking missiles, although the effects of jamming are typically small. Radiating systems include the radar altimeter, airborne mapping radar, map matching, and Doppler radar. See also Airborne radar; Altimeter; Doppler radar; Electronic warfare; Homing; Jamming.
Nonradiating systems, such as inertial, offer essentially complete protection against jamming. Inertial navigation systems, consisting of accelerometers, gyroscopes, and computers, continuously determine position, velocity, acceleration, direction, and vehicle attitude (pitch and roll), based on Newton's second law. See also Accelerometer; Gyroscope; Inertial guidance system.
