326 Radio Engineering for Wireless Communication and Sensor Applications

The European Union has a definite plan to deploy a new satellite navigation system called Galileo starting in 2005. The Galileo system should be ready for full operation in 2008 when all 30 satellites are orbiting at the altitude of 24,000 km.

12.5.3 Navigation Systems in Aviation

In addition to hyperbolic and satellite navigation systems, many other radionavigation systems are in use in aviation.

VHF Omnidirectional Range (VOR) is based on omnidirectional beacons operating in the range of 108 to 118 MHz. The carrier is amplitude or frequency modulated at 30 Hz so that the phase of modulation depends on the azimuth angle. Two subcarriers 9,960 Hz apart from the carrier are frequency modulated at 30 Hz and are angle-independent reference signals. The receiver on an aircraft measures the phase difference of the 30-Hz signals and thus reveals the direction of the beacon.

Distance Measuring Equipment (DME) operates in the range of 962 to 1,213 MHz and is usually located with a VOR beacon. Its frequency can be ‘‘paired’’ with VOR or ILS. The distance between an interrogator on an aircraft and a transponder at a ground station is obtained from the time it takes for the signal to propagate from the aircraft to the ground station and back, as shown in Figure 12.12. The interrogator sends a pair of pulses. The transponder delays its response by 50 m s and changes the frequency by 63 MHz.

The Instrument Landing System (ILS) and Microwave Landing System

(MLS) are landing systems that give guidance for airplanes approaching a runway. ILS was introduced in the 1940s. It consists of three radio systems, as indicated in Figure 12.13: localizer, glide slope, and marker signals. The localizer signal (108–112 MHz) provides lateral guidance. The right side of the antenna pattern, as seen by an approaching aircraft, is modulated at

Figure 12.12 Operating principle of DME.

		Applications		327

Figure 12.13 ILS localizer, glide slope, and marker beams.

150 Hz, and the left side at 90 Hz. On the correct track, the 90 and 150 Hz signals are of equal intensity. The glide slope signal (329–335 MHz) provides vertical guidance. The upper part of the pattern is modulated at 90 Hz and the lower part at 150 Hz. The intensities of the modulating signals are equal in the optimum glide angle, which is typically 2.5° to 3°. Marker beacons at 75 MHz provide information on the distance from the runway.

MLS is a precision landing system that will replace ILS. MLS allows different glide angles and curved approach paths, and has many other advantages over ILS. Navigation is based on five signals: (1) the scanning azimuth signal, ±40° or ±60°; (2) the scanning elevation signal, maximum scan 0.9° to 30°; (3) the back-azimuth signal for missed approaches; (4) precision DME (DME/P); and (5) data signals. With the exception of DME/P, all MLS signals are transmitted on a single frequency through time-sharing. The operating range, 5,031 to 5,091 MHz, contains 200 channels. Figure 12.14 shows how the azimuth angle of an approaching plane is measured. The narrow beam produced by a phased antenna array sweeps at a fixed scan rate, and the receiver on the plane measures the time interval between sweeps,

Figure 12.14 Azimuth signal of MLS.