Patent GB-842430-A (GB)
Title: Radio navigation system
IPC: G01S1/02
Filed: 1958-08-15
Abstract: 842,430. Radio navigation. STANDARD TELEPHONES & CABLES Ltd. Aug. 15,1958 [Aug. 16, 1957], No. 26296/58. Class 40(7). Relates to a TACAN radio beacon with a directivity pattern 3, Fig. 2, comprising a major lobe 7 and eight equally spaced minor lobes 8 and rotating at 15 c/s. in which the transmitted signal comprises (a) semi-random pulse pairs with a fixed 12Á-sec spacing between the pulses of a pair, Fig. 7A, the mean repetition rate of the pulse pairs being 2,700 c/s. (b) a 15 c/s. main reference signal comprising a group of 12 pulse pairs (12Á-sec spacing between the pulses of a pair and 30Á-sec spacing between corresponding pulses of adjacent pairs, Fig. 7B) transmitted each time the major lobe 7, Fig. 2, is directed in a datum direction and (c) a 135 c/s. auxiliary reference signal comprising a group of 12 pulses (12Á-sec spacing between adjacent pulses, Fig. 7C) transmitted each time a minor lobe 8, Fig. 2 is directed in the datum direction such that at a mobile receiver (righthand portion of Fig. 1) the envelope of the received pulse signals, Fig. 3 (not shown), comprises a 15 c/s. component and a 135 c/s. component which, when compared in phase comparators 42 and 44 with main and auxiliary reference signals derived from the received main and auxiliary pulse reference signals, give coarse and fine bearing indications. As shown the 15 c/s. and 135 c/s. bearing-dependent signals are selected by filters 36, 37 and the output from the receiver 34 is also applied to a decoder 38 producing a single pulse from each 12Á-sec pulse pair, the output from the decoder 38 being applied to ringing circuits 39, 40 tuned to 33 kc./s. and 80 kc./s. respectively to give the main and auxiliary reference signals. In such a system it has been proposed to identify the beacon transmission by periodically switching off the semi-random pulse signal and transmitting in its place 2,700 c/s. recurrent pulse pairs with a regular spacing between adjacent pairs, the switching being effected in a coded sequence characteristic of the beacon identity; at the receiver the corresponding pulse identity signal is selected by a ringing circuit 41. With such an arrangement it has been found that the phase of the bearing-dependent output signals from the filters 36, 37 is shifted during transmission of the identity signals as described with reference to Figs. 4 and 5 (not shown) and according to the present invention such phase shift is reduced by transmitting the identity signal in the form shown in Fig. 7D comprising groups of pulses recurring regularly at 1350 c/s, i.e. half the mean recurrence frequency of the semi-random pulses, each group comprising two pairs of pulses, the spacing between the pulses of a pair being 12Á-sec and the spacing between the two pairs being 100Á-sec. As shown in Fig. 1 the semi-random pulses, the main reference pulses and the auxiliary reference pulses are initiated respectively by a pulse generator 19 (average recurrence frequency 2,700 c/s.), pulse generator 22 (output 12 pulses, 30Á-sec spacing) and pulse generator 22 (output 6 pulses, 24Á-sec spacing), the outputs from the pulse generators being applied in parallel to the RF stage 21 through a pulse coder 20 which converts each input pulse into a 12Á-sec pulse pair. The main and auxiliary reference pulse generators 22, 23 are triggered by timing pulses generated respectively in fixed pick-up coils 28, 26 co-operating with soft iron slugs attached to the rotating aerial 2, and the timing pulses are also applied to block the semi-random pulse generator 19 during the periods of operation of generators 22 and 23. The identity pulses are initiated by applying the output from a 1350 c/s. oscillator 11 to trigger a 100Á-sec monostable multivibrator 12 and generating a pulse at the leading and trailing edges of the output therefrom, the resultant output pulses being applied to the coder 20 in place of the semi-random pulses from generator 19 by means of the identity coding switch 19a. The oscillator 11 is synchronized by the timing pulses from the aerial pick-up coil 26 such that the main and auxiliary reference signal pulse groups, Fig. 7B and C, occur during the vacant periods between the groups of identity pulses.