The navigation systems which are protected by regulation were developments of the 1940's, and -- except for interference! -- work pretty well. They are analog technology, using phase and amplitudes of audio frequency tones to determine position and/or deviation from course.
For landing, the ILS system transmits amplitude modulated signals between 108 and 112 MHz (if I remember correctly -- it s been a long time) with a 90 Hz modulated beam to one side of the runway, and a 150 Hz modulated beam on the other. Where the two tones are demodulated at equal strength is (with a properly aligned and functioning system) along the runway center line. The receiver has filters to discriminate between the tones, but interference can displace the indicated position to one side or the other, with quite possibly fatal results. The Glide Slope system operates similarly at around 330 MHz, with beams aligned upwards at an angle such that equally demodulated tones indicate a correct rate of descent. VOR is also an analog system. VOR operates (if I remember rightly) between 112 and 116 Mhz. Basically, it is a scanning directional array, which sweeps its azimuthal pattern maximum around 30 times each second. At the same time it transmits a subcarrier of (approximately) 10 KHz, which is frequency modulated with a reference 30 Hz, such that when the beam is North, the phase of the tones demodulated in an aircraft North of the VOR station will agree with each other. The difference in phase between the received signal modulated as a result of the patterns' rotation, and the subcarrier's modulation, indicates direction to the aircraft from the VOR station. Again, this is easily interfered with. A substantial area around each VOR must be restricted for construction, as even multipath from re-radiated signals will cause the indicated bearing from the station to be off. All of these systems have detectors to indicate the absence of a tone, and a warning flag pops up in its display to show an invalid signal (or none). However, the flag is only an indicator of total absence of tone. If an interfering signal presents a tone, the receiver will not distinguish between it and a valid signal, and the flag will not be displayed. Consequently, the safety of these systems depends on how well their signals are received. Cortland ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
