alkire@hplabsb.UUCP (06/06/85)
** In my ground school class the other night, we talked about radio navigation, specifically VOR navigation. I wanted to know more about the technical specifics about how VOR works; however, my instructor really only knew how to use it. Her description of how it worked seemed over-simplified and I think there would be better ways of accomplishing the same results. What I got from her was: there is first an omnidirectional signal which is broadcast and then a sweep of 360 degrees through 360 seperate directional antennas. The VOR receiver finds the one degree of sweep (a radial) with the highest signal level from the VOR transmitter. It determines the angle of that radial with respect to magnetic north by the phase relationship between the omnidirectional signal and the radial. The 360 seperate antennas doesn't seem like the most efficient way of sweeping 360 degrees. What I would like to find out is what sort of antenna arrangement VOR uses and what are the characteristics of the omnidirectional and radial signals i.e. What sort of modulation is used? AM? CW? FM? How often is the sweep? How long is the omnidirectional signal or is it continuous? What is the timing like between the omnidirectional signal and the radial? This doesn't have much to do with VOR navigation but it has my curiosity up and I would appreciate any input on how VOR works technically. Thank You, Bob Alkire @ hplabsb
bob@ulose.UUCP ( Bob Bismuth ) (06/08/85)
No, the VOR station does not have 360 seperate antenna elements. It has 5. There is one central element and 4 elements surrounding it, each of which radiates in a figure 8 pattern. They are all broacasting a signal modulated at 30 Hz. Elements across a diagonal are modulated with a phase difference of 90 degrees, and each pair of diagonals differs by a phase angle of 90 degrees. The central element broadcasts the reference modulation. The result is, if I understood it correctly, the effect of 2 signals whose phase relationship varies according to your position relative the the station, ie. what "radial" you are on. If you want to pursue an interest in how the electronics of aircraft work, there are several good books on the market. The one sitting on my bookshelf is "Aircraft Electricity and Electronics" by Bent/McKinley, published by McGraw Hill. It costs arround $30 and is intended as a testbook for those studying for A&P type qualifications. -- bob (decvax!ulose!bob) *** REPLACE THIS LINE WITH YOUR MESSAGE ***
dmmartindale@watcgl.UUCP (Dave Martindale) (06/10/85)
Some additional information: First, why expect 360 antennas? A VOR doesn't generate 360 radials, it generates a radio signal that changes continuously as the position of the receiving antenna relative to the ground station changes, and there are thus an infinite number of possible "radials" to fly. The actual position determination is done by comparing the phases of two 30Hz signals. The phase reference is provided by a 9960Hz subcarrier on the main signal that is frequency modulated at 30 Hz; this signal is broadcast by the omnidirctional centre antenna. The four other antennas are fed signals that are directly amplitude-modulated by a 30Hz signal whose phase varies 90 degrees from one antenna to the next. The phasing is so arranged that if an aircraft is directly (magnetic) north of the VOR, the phase of the AM modulation matches that of the FM reference signal extracted from the subcarrier. As the aircraft's angular position with respect to the VOR changes, the phase difference changes by the same amount. In the aircraft, one of the two received signals is shifted in phase by 0, 90, 180, and 270 degrees and then these four signals are fed to a phase-shift circuit in the VOR head that provides a continuous phase shift of from 0 to 360 degrees as the course selection knob is rotated. This shifted signal and the other received signal are then fed to phase-comparing circuits that drive the course deviation indicator and the TO/FROM indicator. Thus, the phase difference in the received signal depends on your position, and the amount of phase difference that causes the CDI needle to null is determined by the setting knob on the VOR head. The connections to the CDI galvanometers are arranged so that when the needle does deflect from centre, it does so in the direction that you must fly, and the gain is adjusted so that full-scale deflection occurs at the proper off-course angle. The TO/FROM indicator is really just a second CDI meter driven with one of the two signals shifted 90 degrees so its nulls are 90 degrees away from that of the main needle. Of course, the signal broadcast is also modulated with the Morse code identification for the VOR plus, sometimes, voice. The modulation depths of the various signals are controlled to allow them to coexist simultaneously.