Patent GB-584670-A (GB)
Title: Improvements in or relating to radio navigation systems
IPC: G01S1/08
Filed: 1943-10-01
Abstract: 584,670. Radio navigation systems; valve circuits ; pulse modulation. REEVES, A. H., and WILLIAMS, F. C. Oct. 1, 1943, No. 16165. [Class 40 (v)] A radio navigation system which enables a moving craft to follow a predetermined course comprises a transmitter which transmits to a receiver on the craft, signals which are modulated in accordance with the instantaneous distance therebetween. In the embodiment described for guiding a craft to a particular destination, a ground station A transmits a train of pulses in order to determine the instantaneous range of the craft and also transmits intercalated pulses, the phase of which, with respect to the first train is controlled in dependence upon the range of the craft to produce distinctive signals which define a navigational path. A second or B ground station also measures the range of the craft by one train of pulses, the phase of a second intercalated train being controlled to produce a distinctive signal when the craft reaches its destination at a predetermined distance from the B station. Both stations operate on the same radio frequency but different basic recurrence frequencies, to enable them to be differentiated by the craft. The A station has a basic pulse recurrence frequency of 133 cycles per second the phase of the intercalated train of pulses when the craft lies in the middle of the navigational path being such that the output of a filter tuned to 266 cycles per second and fed with pulses is at an intermediate level. If the craft lies to one side or other of its proper course, the phase of the intercalated pulses is advanced or retarded in a dot-dash rhythm in such a sense that the 266 cycles per second output on the craft is-modulated to produce predominantly dots or dashes depending on the direction of the range error of the craft. The basic recurrence frequency of the B station is 97 cycles per second and the phase of the intercalated pulses is such that a steady note of 194 cycles per second is received which is interrupted by the phase shifting of the second train of pulses to transmit signals to the craft. Information in the form of morse signals may be transmitted to the craft from the A or B station in place of the automatic navigation signals. The equipment at the A and B stations is identical and their functions are interchangeable. The control pulses for the operation of the apparatus are generated in the master control unit MCU, Fig. 13, which provides positive and negative-going square waves at the recurrence frequency for A or B station operation, positive calibration pulses corresponding to ranges of 1, 5 and 50 or 60 miles and a positive-going recurrence frequency square wave. with negative 5-mile calibration pulses superimposed. The positive square waves are used to control the main time base MTB which produces a sweep representing a range of about 350 miles on a main cathode-ray tube indicator CT1 and to remove the black-out on this tube during the time of sweep. The negative square wave may be selected by switch S1 to control the main time base to enable the interlaced transmitter pulse to be observed and is also used in conjunction with positive recurrence pulses to control the phase modulator DDPM and lock the fixed train of pulses produced by the transmitter 'TX and its modulator TXM, to one of the calibration pulses occurring near the commencement of the main time base. The positive square wave with negative 5-mile calibration pulses superimposed is used to control the single strobe unit SS which produces narrow positive and negative pulses about 10 miles wide which are movable in steps of 5 miles over the main time base sweep, the positive pulse being fed to the grid of CT to brighten up the selected portion of the trace, while the negative pulse is used to initiate a high-speed time base HSB which is displayed on a vernier cathode-ray tube CT2 and also to remove the black-out on this tube. Echo or re-transmitted signals from the craft obtained from the receiver REC are fed through a mixer and phase splitter unit MPS and a push-pull amplifier PPA2 to produce'deflection of the main time base on CT1, the signals being replaced by calibration pulses if desired. The signals are also converted to derived pulses of 4 microseconds width in the unit DS, mixed with calibration pulses in unit MPS, and fed through push-pull amplifier PPA1 to produce deflections on the vernier time base on CT2. For B station operation these derived signals may be replaced by the true signals. The positive single strobe pulse is also fed to the double strobe unit DS which produces a pair of pulses each 4 microseconds long, separated by one microsecond which can be adjusted to occur on any point on the highspeed time base sweep and in A station operation are fed as brilliance modulation to the grid of CT2, the gap between these two strobe pulses being adjusted to coincide with the range of the navigational path which is to be defined. Unequal overlap of the derived signal pulses and double strobe pulses produces a D.C. output from unit DS which is fed to unit DDPM to control the sign and amplitude of the dot-dash phase modulation of the second train of transmitted pulses so that a receiver on the craft gives an output of dots or dashes of tone when off course to one side or other. In B station operation a series of black-out calibration pulses of adjustable interval are fed to the grid of CT2, under the control of a switch S2, from an automatic timing device ATD-which is controlled by the positive single strobe pulse. When the equipment is used for controlling bombing, the ground controller observing the passage of the signals from the aircraft over the black-out calibration pulses on the vernier cathode-ray tube, operates switches on the unit ATD which cause it to control the modulator, interrupting the steady tone first as a warning signal and finally as a bombing signal at the appropriate instant to allow for the time of bomb fall and the speed of the aircraft. Master Control Unit. (Fig. 2). This comprises a crystal controlled 93.12 kilocycles per second oscillator VI followed by four stages of frequency division by 5, 12, 8, 2 or 5, 10, 7, 2 as desired, to produce output at 97 or 133 cycles per second for operation of the B or A station. The output of VI is squared by V2 and differentiated by condenser C7 and resistance R12, the negative peaks triggering the first divider comprising double diode V3 and pentode V4, of the type described in Specification 582,758. The output from the cathode of V4 at 18.626 kilocycles per second is fed to a second divider V7, V8, V9 which produces an output at the cathode of V8 divided by 10 or 12 depending on the position of S3, which is further divided in a similar stage V10, V11, V12 by 7 or 8 according to the setting of S4, which is ganged with S3. The square wave output from the cathode of V11 at 194 or 266 cycles per second is differentiated by condenser C23 and resistance R61 and negative peaks. selected by V13 are used to trigger a multi-vibrator V14 which divides by two, positive and negative-going square waves being obtained from the cathodes of V15, V16. Calibration pulses corresponding to 1, 5 and 50 or 60 mile intervals are obtained by applying the differentiated output from V2, the screen of V 17, the grid of which is fed from the screen of V4, and V8, to the grid of V5. Positive calibration pulses are obtained from the cathode follower V6. Valves V16 and V17 have a common anode load and from the anode of V17 is obtained the recurrence frequency square wave with negative 5-mile calibration pulses superposed. Mixer and phase-splitter unit. (Fig. 3). Positive signals from the receiver .are phase reversed in V7 and fed through.cathode follower V8 to the double strobe unit where positive signals are derived and fed back -to the grid of V1 which has a common anode load with the valve V2, the grid of which is fed with calibration pulses. The combined output is fed to a phase-splitting valve V3, push-pull output being obtained from the cathodes of V3, V4 and fed to the deflection plates of the vernier cathode-ray tube. Positive-going signals are also fed to a phase-splitting valve V5, push-pull output being obtained from the cathodes of V5, V6 and fed to the main cathode-ray tube. A switch S5 enables the signals to be replaced by calibration pulses when required. Switch S6, not shown, is provided to enable signals instead of the derived pulses to be fed to the vernier cathode-ray tube when operating as a B station. Circuit generating single strobe pulses. (Fig. 4). This pulse which is variable between 50-150 microseconds in duration, is generated by feeding the positive recurrence frequency square wave on which is superposed negative 5-mile calibration pulses to the grid of V1, which with V2 forms a multi-vibrator circuit in which V2 is normally conducting. The square wave triggers the multi-vibrator, V1 becoming conducting and a negative pulse approximately equal to the H.T. voltage applied to V1 is fed to the grid of V2. Condenser C3 then charges approximately linearly through resistance R10 and the multi-vibrator restores after a time approximately proportional to the H.T. voltage of V1, i.e. the setting of potential divider P. The actual reversion of the circuit to its stable condition is locked to the nearest 5-mile calibration pulse fed to the grid of V1 so that the negative-going edge of the wave-form at the. anode of V2 can be moved over the duration of the main time base in 5-mile steps under the control of potential divider P. The anode voltage of V2 is differentiated by the condenser C1, and resistance R1 and the negativegoing edge produces a positive pulse at the anode of V4, the width of which is controlled by potential divider P1. The phase-splitting valve V5 and amplifier V6 provide positive and negative strobe pulse outputs from their cathodes. Double strobe p