Patent GB-1415383-A (GB)
Title: Radio navigational aids
IPC: G01S1/54
Filed: 1973-02-14
Abstract: 1415383 Radio navigation RAYTHEON CO 14 Feb 1973 7323/73 Heading H4D In an instrument landing system a transmission frame is established during which an omnidirectionally radiated pulse is followed by a vertical fan beam scanning in azimuth in discrete steps and then by a beam omnidirectional in azimuth and scanning in elevation also in discrete steps. On the aircraft the receiver determines its azimuth by the period of time between the reception of the omnidirectional pulse and the radiation from the beam scanning in azimuth and similarly determines its elevation from the time between the reception of the omnidirectional pulse and the radiation from the beam scanning in elevation. Successive scan beams and elevation and azimuth are overlapping and to overcome any errors this may produce at the receiver, odd numbers of azimuth beams and elevation beams are transmitted in one frame and even numbered azimuth beams and elevation beams are transmitted in the next frame, whereby an interlaced format is produced. The frame sync. information and data information at a slightly different frequency are respectively transmitted between the omnidirectional pulse and the azimuth scanning beams and between the azimuth scanning beams and the elevation scanning beams. The aircraft receiver (Fig. 5) is provided with an angle clock 32 which by means of sync. control logic 41 receiving the various synchronizing signals in the ground transmission produces a series of angle indicative pulses in synchronism with the ground clock 28 (Fig. 1, not shown). A 10 megacycle oscillator 30 produces by means of by 10 multiplier 34 and offset oscillator 36 a heterodyne signal whereby the azimuth and elevation beam signals are changed to 35 megacycles and the sync. signals and data signals are changed to 40 megacycles. After IF amplification at 42 the azimuth and elevation signals are further mixed with a 30 megacycle signal to produce a 5 megacycle signal which on detection at 48 produces a set of azimuth responses and elevation responses, e.g. Fig. 4E and comprising a series of contiguous pulses one for each step of the beam and having summed energy contents which equalize about a weighted time centre 13 indicating the time of passage of a simulated continuously scanning beam. The frame sync. and data signals are separated out by a mixing process at 52 and detection at 60 whilst the slot sync. signals are separated out by detector 62. The signals of Fig. 4E the sync. and data signals and the angle indicating signals from clock 32 are all fed to a digital angle logic 50 feeding in turn a crossed pointer type of indicator 68. To enable a pilot to fly down a desired azimuth path and elevation path the angle logic circuit shown in detail in Fig. 7 incorporates means 80, 82 for selecting a desired azimuth and a desired elevation whereby after the actual elevation and azimuth of the aircraft has been determined said actual co-ordinates are subtracted from the selected co-ordinates and azimuth and elevation deviation signals are fed to said cross pointer or deviation indicator 88. The angle clock 32 is synchronized with the received sync. signals by means of the variable divider feeding signals to said angle clock and having a division factor of 4 or 5 controlled in a coarse and a fine manner in dependence on the detected relative synchronism of said clock and said received sync. signals. The weighted time centre line 13 is detected by means of a split frame gate (Fig. 6) having a positive to negative transistion point which ideally is aligned with said weighted time centre line 13 and gates through equal energy signals from before and after said time centre line.