Patent GB-580752-A (GB)
Title: Improvements in or relating to radio altimeters
IPC: G01S13/34
Filed: 1944-03-20
Abstract: 580,752. Radiolocation. STANDARD TELEPHONES & CABLES, Ltd., and WESTON, J. D. March 20, 1944, No. 5207. [Class 40 (v)] [Also in Group XXXVI] An electrical frequency measuring device applicable to a frequency modulated radio altimeter comprises a ratiometer having two operating coils, one of which is fed with a current of magnitude depending upon the frequency to be measured while the other is fed with a reference current derived from a different source. In a known frequency modulated altimeter, Fig. 1 (not shown), the beat frequency is measured by applying it to a frequency counter circuit which produces a direct current proportional to the beat frequency, which can be measured on a D.C. indicating instrument calibrated in altitude. The counter circuit generally comprises a valve, which conducts on alternate half-cycles of the applied frequency, followed by a differentiating condenser and full-wave rectifier producing the direct current proportional to the applied frequency. The counter-circuit may also produce a gain controlling voltage to be fed back to preceding amplifiers. Variation in the anode voltage of the switch valve and temperature variations in the constants of the counter-circuit causes errors in readings and these may be overcome according to the invention by employing a ratiometer R, the windings of which are fed from similar counters. The application of the two counter arrangement to an altimeter is shown in Fig. 4 in which a transmitter T radiates waves frequency modulated in saw-tooth manner at a frequency fm by the modulator M. Direct and reflected waves beat in a detector D to produce the beat frequency fd which is amplified at A. The frequencies fd and fm are fed to the counters C1 and C2 respectively, the output currents Id, Im, of which feed the coils W1, W2 of the ratiometer R, and as the beat frequency fd is directly proportional to the modulating frequency fm and the maximum frequency variation F, the reading of the ratiometer R is not affected by variations in fm and in the potential source V, and is also substantially independent of variations in the counter-circuits produced by temperature changes. Errors in indications due to variations in F with temperature may be compensated by causing the ratio of the characteristics of the counter to vary in a suitable manner by the use of attenuating networks with temperature dependent resistors. As the altitude indication with the arrangement of Fig. 4 is not affected by variation of fm, the modulating frequency may be varied in accordance with altitude in order to reduce the required receiver band width. This variation may be obtained by appying the control voltage derived from counter C1 for amplifier A, in an appropriate manner over lead X1 to modulator M. In a simplified arrangement, Fig. 2 (not shown), counter C2 is omitted and the winding W2 is fed with current derived from the H.T. source V so that only variations in V are cancelled out. A preferred arrangement of the two countercircuits is shown in Fig. 5. Circuit C fed with the frequency f or fd comprises an amplifier V3 followed by the switching valve V1, in the anode circuit of which is the condenser K1, bridge rectifier Q1 and winding W1 of the ratiometer R. Counter-circuit C2 fed with frequency f2 or fm comprises the switching valve V2, condenser K2, bridge rectifier Q2 and winding W2 of the ratiometer. The windings W1 and W2 are shunted by smoothing condensers K3 and K4 and a variable resistor r8 enables the current through winding W2 to be adjusted to set the readings of the meter. Valves V1 and V2 are blocked and unblocked for substantially the whole of alternate half periods of the applied waves, the condensers K1 and K2 being charged from the positive source V through resistors r3, r4 and discharged through the valves. Both charge and. discharge currents now in the same direction through the ratiometer windings due to the rectifier bridges so that the currents through the windings are proportional to the frequencies of the applied waves. Switches S1a, S1b which are ganged are provided to interchange valves V1 and V2 to check that large variations in the valves are not introducing errors. Switch S2 is similarly provided to vary the amplitude of the waves fed to valve VI by short-circuiting the bias resistor r11, to verify that the operation of the counter-circuit is substantially independent of the input amplitude. To supply the feed-back voltage for controlling the amplifier A or to control the modulation frequency fm, an auxiliary counter-circuit C3 may be employed, comprising the condenser K9 and the bridge formed of rectifiers Q3, Q5 and resistors r17, r19. A balanced smoothing circuit comprising resistors r13, r15 and condensers K13, K15 produces a balanced voltage at terminals 5 and 6 which is proportional to the applied frequency f1 or fd. The modulating frequency fm may be conveniently varied if it is generated by means of a relaxation oscillator, the control voltage from terminals 5 and 6 being employed to vary the plate circuit impedance of a valve forming part of the oscillator circuit. If the control effect is made sufficiently large, the current Id may be made nearly constant while the reference current Im varies inversely with altitude. With this arrangement, temporary loss of the incoming signal would cause the modulating frequency fm to assume its maximum value so that the beat fd might be outside the pass band of the receiver when signals reappear. This may be prevented by giving the control circuit a long time constant. A loss of signal will also occur if the aircraft rises beyond the range of the altimeter and then descends and may be prevented by arranging an auxiliary valve to so bias the modulator in the absence of signals that the minimum value of fm is produced. Alternatively, the receiver may be arranged to have some response over a narrow band corresponding to the beat frequency produced at maximum altitude with maximum value of fm, then the apparatus would always come into operation on descent if the control circuit had a large enough time constant. As an alternative to the continuous automatic variation of the modulation frequency, fm may be varied in several relatively large steps producing a corresponding number of separate ranges for the equipment. The scale factor of the ratiometer can be simultaneously changed to give increased precision of reading by maintaining the reference current Im proportional to the product of the modulation frequency and scale factor by adjustment of resistance r8. Preferably, however, the modulator M is arranged to provide a constant frequency fm to the counter C2 while a frequency fm/n is employed to modulate the transmitter; n having a different value for each range. This may be done by deriving the frequency fm from an electromagnetic generator driven by a motor, a rotary condenser performing the transmitter modulation being driven from the same shaft through adjustable ratio gearing.