Patent GB-624558-A (GB)
Title: Improvements in or relating to radio navigational systems
IPC: G01S13/70
Filed: 1945-04-19
Abstract: 624,558. Radio navigation ; thermionic valve circuits. JONES, F. E., ROBERTS, W. L., HARRISON, F., UTTLEY, A. M., and WILLIAMS, F. C. April 19, 1945, No. 9899. [Classes 40 (vi) and 40 (vii)] In a radio navigation system of the type in which signals are transmitted from fixed stations to a moving craft ; the indications produced at the craft are dependent upon the instantaneous distance of the craft from the desired course, and also the rate of change of that distance. The invention is applied to a system similar to that described in Specification 584,670, in which signals from one ground station are modulated in accordance with instantaneous distance of the craft so that the craft is caused to follow a path at a constant radius from the ground station passing over a desired point while a second ground station spaced from the first observes the range of the craft and transmits suitable signals to indicate when the craft has reached a desired point. The additional feature of the present invention, whereby the " error " signal transmitted to the craft is made dependent also upon the rate of change of the distance off course, reduces " weaving " of the craft in endeavouring to follow the course and the craft actually follows a curve which merges exponentially with the correct track. The system embodying the present invention, unlike that described in Specification 584,670 employs width modulation of the pulse transmissions for the conveyance of the navigational information. The ground station defining the required track transmits on a pulse recurrence frequency of 133 c.p.s., and the radio transmissions employed are of a wavelength of approximately 10 cms., the mean pulse width employed being 3 Á secs. At the craft the width. modulated pulses are converted to amplitude modulation and give rise to an audible note. Signalling is by variation of the intensity of the audible notes in a dot-dash rhythm in dependence on the deviation of the craft from the course so that the course correcting signals are heard in craft as dots or dashes of tone, the on-course condition corresponding to a steady note. In the system described, which is concerned with the track defining ground station, the operation of the station, Fig. 1, is controlled by a calibration unit CAL which is fed with the output from 93.117 kc/s. crystal oscillator TCCO and provides +ve 1 and 5 mile calibration pulses, + ve recurrence frequency pulses fed to the marker unit MAR and +ve recurrence frequency pulses fed to the transmitter modulator MOD which are delayed so that the transmitted pulses are visible on the main C.R.T. display. The pulse transmitter and T.R. unit T.R. feed a common aerial system A, returned signals from the craft being fed to the receiver REC. The unit MAR generates 25 mile calibration pulses which are fed to a mixer unit MIX together with 1 and 5 mile pulses and +ve signals from the receiver. The display comprises two cathode-ray tube indicators, one, the long time base indicator, Fig. 5, A and B, which is generated by unit LTB triggered by a -ve pulse recurrence frequency pulse from unit MAR and another on which a selected portion of the range displayed on the long time base is presented to a magnified scale. On the magnified display the time base corresponds to a range interval of 15 or less miles and is delayed with respect to the commencement of the long time base by a selected number of 5 and 1 mile steps, Fig. 5, C and E. This delay is controlled by part of unit MTB which is triggered by - ve pulse recurrence frequency pulses from unit MAR and determines the delay by counting a selected number of - ve 5 and 1 mile pulses fed also from unit MAR before triggering the time base circuit of the indicator. The interval selected for display by unit MTB is shown on the long time base by a brightening pulse corresponding to the position of the magnified time base which is fed from unit MTB to unit LTB. A unit WS fed with a negative trigger pulse from the magnified time base unit and negative signals from unit MIX generates a " walking strobe " brilliance pulse which appears on the magnified display and may be caused to occur at any position on the time base scan. The walking strobe pulse can be caused to lock to the leading edge of any returned signal and to follow any movement of such a signal corresponding to speeds of the craft producing it up to 600 m.p.h. when the signal falls within an "active region " on the magnified display. The walking strobe unit also provides a blackout pulse which may be accurately positioned on the magnified time base at a range corresponding to that of the target, i.e. the radius of the desired track from the station. The walking strobe unit generates a range voltage proportional to the difference in range between the walking strobe pulse when locked to a signal and the target-marker pulse, i.e. the error in range of the craft, and also a velocity voltage proportional to the rate of change of the range error. The range and velocity voltages are fed to a pulse width control unit PWCU which feeds voltages to the modulator MOD to produce width modulation of the transmitted pulses according to the dot-dash tone variation, if any, corresponding to that deviation and rate of change of deviation. Certain parts of the complete system of Fig. 1, namely part of the magnified time base concerned with the adjustment of the delay of that trace, the walking strobe unit and the pulse width control unit are described in detail with reference to Figs. 2 (not shown), 3 and 6. Magnifted time base, Fig. 2 (not shown).- A circuit of the type described in Specification 582,758 is triggered by the -ve recurrence frequency pulses and counts an adjustable number of 5 mile calibration pulses, the back edge of the resultant long pulse being differentiated to produce a pulse, delayed by a selected multiple of 5 miles from the recurrence frequency pulse, which is fed to trigger the WS unit and also fed to trigger a similar dividing circuit fed with the 1 mile calibration pulse. The output of the second counter circuit after differentiation provides the pulse to initiate the magnified time base at an instant corresponding to a range of m x 5 miles + n x 1 mile, m and n being selected integers, after the commencement of the long time base which is itself initiated by the recurrence frequency pulse. Walking strobe unit, Fig. 3.-The negative pulse delayed by m x 5 miles from the MTB unit is fed via terminal T4 and differentiating circuit C1, R8, to diode V2 which is biassed to remove any 5 mile calibration pulses which may be present in the applied wave. The negative pulse produced cuts off valve V1, and the resulting +ve pulse is fed to a variable delay circuit comprising valves V3, V4 of the type described in Specification 582,758. The delay set by potential divider R119 determines the interval between the 5 mile delayed pulse and the commencement of the " active region " of the MTB, Fig. 5E. The screen-waveform from V3 is differentiated by C8, R17, R19, and the - ve peaks fed to V5 to produce +ve going output pulses which are fed to the targetmarker delay circuit comprising valves V6 and V7 and also the walking strobe circuit. Valves V6, V7 operate as described in Specification 582,758 to produce a screen wave form of width approximately equivalent to 6 miles, the back edge being adjustable by Œ 1 mile under the control of potentiometer R34. This wave is fed to the normally non-conducting valve V8 and due to the short-circuited ¢Á sec. delay network in its anode load this valve produces ve and +ve 1 Á sec. pulses, corresponding to the leading and trailing edges of the applied wave, which are fed to valve V9, the +ve pulse corresponds to the adjustable back edge and produces a - ve output from valve V9 which is fed via terminal T5 to produce the target blackout pulse on the magnified time base. The delayed output pulses from V5 are also fed through C11 and potentiometer R23 to the suppressor grid of V15. This valve forms part of a delay circuit operating as described in Specification 582,758 to produce a waveform depending in length upon the cathode potential of diode V16, which is fed to the control grid of the normally biassedoff valve V17. The anode load of V17 includes a short-circuited delay network having a ¢ Á sec. delay, the resultant anode waveform thus consisting of a 1 Á sec. negative going pulse at the beginning and 1 Á sec. positive going pulse at the end of the input waveform. These pulses are fed to V12 which is normally biassed to cut-off so that the positive going peak only is effective and the resultant delayed output from T9 is used to brighten the trace on the MTB to indicate the position of the walking strobe. Adjustment is made by R34 so that when S4 is on contact 3 and 165 volts applied to V16 cathode the delay introduced is such as to make the target marker and the walking strobe coincide. If switch S2 is on contact 2 the control grid of V14 (the " range " valve) is connected to the junction of R64, R65 and its potential is dependent on that of S1 and the variable contact on potentiometer R67. This latter is adjusted so that when there is 165 volts on S (i.e. position 3) there is no current flow into or out of C26 and thus the walking strobe is steady at the target range and does not move. For other voltages on S1 the condenser C26 will either charge or discharge and the strobe move along the trace. If S2 is set in position 3, the voltage on the anode of V14 then adjusts itself to the potential of the arm of S2, which may be controlled by potentiometer R75 which potentiometer can thus be used to manually place the strobe pulse into coincidence with an echo signal. When the echo signal and' strobe coincide S2 is moved to contact 2 and with S1 on contact 2 the walking strobe will move with echo. Negative echo signals are fed in at T6 and cut-off V10 to pro