Patent GB-885923-A (GB)
Title: An aircraft radio navigation system
IPC: G01S13/84
Filed: 1958-05-30
Abstract: 885,923. Air traffic supervision. TELEFUNKEN G.m.b.H. May 30, 1958 [June 5, 1957; Aug. 22, 1957], No. 17333/58. Class 118. [Also in Group XL (c)] An aircraft radio navigation system comprises (1) at the airport control station a transmitter 10 radiating on a continuous carrier frequency fo adapted to be modulated by selective calling frequencies s1, s 2  &c. and by a single distancemeasuring frequency r; (2) on the aircraft, means responsive to a selective calling frequency to establish radio-telephone communication with the airport, and a transmitter radiating on a carrier frequency f which is modulated by the received and demodulated distance-measuring signal; and (3) at the airport control station, means for measuring the phase displacement between the demodulated distance-measuring signal and the transmitted distance-measuring frequency. In the system described control station B transmitter 10 includes audio-frequency r generator 12 and rows 32, 33 of keys for additional modulation with selected calling frequencies s 1  . . . , t 1  . . . respectively. Each co-operating aircraft carries a receiver 24 tuned to frequency f o  and the demodulated signal at frequency r is modulated on to transmitter 25 having a carrier frequency fn individual to each aircraft. At the airport receivers such as 16, individual to each of the carriers fn, are provided and the demodulated tone r feeds an automatic phase shifter 14 controlled by a voltage derived from bridge network 13; the phase shifter is calibrated in distance, and the speed at which the aircraft approaches the airport along lane L is given by the rate of change of measured distance. The aircraft also carries a pulse radar relay comprising receiver 27, a transmitter 28 radiating at the radar frequency and means which delay the received radar pulse and re-transmits the delayed pulse followed by additional pulses; the radar relay is energized only when an appropriate call frequency tn is received on carrier fo. For radio-telephony between aircraft and airport, the aircraft may transmit on frequency fn and the airport on frequency fo. The aircraft may also transmit to the airport by radio-telegraphy its estimated time of arrival, ground speed, air speed, course and altitude. Each selective calling frequency may be replaced by several frequencies transmitted simultaneously or successively. In a modification, to economize in the number of frequencies fn, the aircraft transmitters 25 are switched on and off after calling by time-delay relays, e.g. 5 seconds on and 55 seconds off. Airport indicators, Figs. 1 and 2.-An indicator board, Fig. 1, for a traffic operator is divided into columns 1 ... 9, one for each aircraft, and into rows representing distance from the airport ; individual aircraft are represented by projected light-spots. The lower section of the board contains rows in which the frequencies fn are entered, an appropriate red light ensues when an aircraft wishes to transmit a message, the time of arrival AZ is entered as estimated from aircraft ground speed and range, and the course, altitude &c. data transmitted by the aircraft is entered. To show lateral aircraft separation a plan position indicator (P.P.I.), Fig. 2, of the air lane is provided and the same data as in the lower part of Fig. 1 is given to the radar operator, who has an additional row of keys for interrogating a selected aircraft. Locking means are provided to prevent simultaneous use of the radio-telephony channel by the operators. Additional radio navigation facilities.-The system may include a revolving radio beacon transmitter at the airport with corresponding apparatus in the aircraft to determine the deviation from a prescribed course and to report such deviation to the airport on frequency fn ;  this information is shown on an additional row on the indicator boards. Alternatively a radio direction finder may be provided at the airport, capable of being switched at will to any aircraft frequency fn. Ground-to-air instructions may be transmitted to the aircraft by television, the image being projected on to the pilot's windscreen; or signals may be transmitted to automatically bring the aircraft on to a prescribed course. For blind landing, ground-controlled approach data may be transmitted to the selected aircraft or may be used to provide radio-controlled aircraft guidance.