Patent GB-1109710-A (GB)
Title: Self-calibrating, self-testing radio altimeter
IPC: G01S7/40
Filed: 1966-12-08
Abstract: 1,109,710. Radio altimeter semi-conductor circuits. BENDIX CORPORATION. 8 Dec., 1966 [26 Jan., 1966]. No. 54888/66. Headings H3T and H4D. The Specification describes an F.M.-C.W. radar altimeter comprising a self-calibrating circuit and means whereby all parts of the altimeter are continuously checked and an alarm given if any part should be found in error. The output of VHF oscillator 10, Fig. 1, is frequency modulated by a variable-slope sawtooth waveform produced by integrating a variableamplitude square waveform generator 28, in generator 12 A high-frequency ripple frequency modulation is added by generator 31 to overcome step errors. Part of the thus frequency-modulated wave is tapped at 21 from its feed to the transmitter antenna, and is fed to a balanced mixer 23, both directly, and via a delay line 22. Line 22 has a delay corresponding to a known altitude, and thus the frequency of mixer output, as determined by frequency counter 25, should give rise to a voltage at one input of comparator 26, equal to a reference voltage applied to the other input thereof. Any difference detected by the comparator produces a signal which, after amplification of 27, is used to vary the amplitude of the square waves from generator 28. The operation of frequency counter 25 and comparator 26, is checked by means of a duplicate counter and comparator 33 and 34 fed with the mixer output and with the reference voltage. Any appreciable output from comparator 34 causes a flag signal to be produced. All the flag signals described are marked by the cessation of a normally present signal. To ensure that sufficient signal is being obtained from the mixer 23 a filter 32 is tuned to 6 kc/s., the expected difference frequencies, and feeds a threshold detector 36. Lack of sufficient signal produces a flag signal. The echo signal is received at 15 and mixed at 17 with a tapped-off portion of the transmitted wave to produce a difference frequency proportional to altitude. The difference frequency signal is fed to a low-pass filter 38 having an attenuation characteristic with a sloping shoulder which varies in frequency according to the amplitude of the signal passing therethrough as detected at 42, such that a high-amplitude signal causes the shoulder to reduce in frequency until the signal is attenuated to a predetermined level. Likewise if the signal should drop in amplitude, then the shoulder will rise in frequency and decrease the attenuation thereof until said predetermined level is reached. Difference frequency signals of approximately constant amplitude are thus produced by filter 38, and any decrease in signal with increase of altitude is counterbalanced by an equalization network 39. Leakage of the transmitted wave direct to the receiver causes a low-frequency difference frequency, and this is removed by high-pass filter 40. The detector 42, as well as providing a control signal for filter 38 produces a warning signal when the signal amplitude falls below a predetermined value. This fall may be due to a system fault, or to actual loss of echo signal and in the latter case no warning is wanted. Thus a system fault is detected by means of a leakage amplifier and detector 43, 46, detecting the small amount of leakage signal passing filter 40. Any output indicates that the system is not at fault, and the warning signal is inhibited. The frequency of the difference signal is determined by two identical frequency counters 44, 45 and output drivers 51, 52. Driver 52 feeds the aircraft autopilot and driver 51 feeds an indicator 53. The indication comprises a comparator, feed with the driver output and with the output of a servo-controlled potentiometer 55. A difference output from the comparator 56 energizes a motor 54, which moves an altitude indicator 58 and the contact of potentiometer 55. The output of comparator 56 should thus be around zero, and this is detected at 63, any other valve causing a flag signal to appear. The outputs of drivers 51 and 52 should be equal, and this equality is detected by a comparator 61, departure from equality producing a flag signal. A flat signal causes the addition of a signal at 48 to both driver outputs whereby the pointer of indicator 58 is driven off scale and the autopilot is disconnected from driver 52. Since a flag signal is indicated by cessation of a normally continuous signal, all said normally continuous signals are fed to a NAND gate, Figs. 2 and 5 (not shown), whereby when one of said signals ceases, indicating a flag signal, the NAND gate produces a warning output. The variable bandwidth filter 38, together with network 39 and signal level detector 42 are shown in greater detail in Fig. 3. If the signal increases at the input of detector 42, transistor 123 increases conduction and causes a decrease in the base voltage of transistor 127. This in turn causes a decrease in the base voltages of transistors 115 and 115<SP>1</SP> and an increase in the voltages at the cathode ends of diodes 108/111 and 108<SP>1</SP>/111<SP>1</SP>. The difference frequency signal from mixer 17 appears at the constant bias voltage emitter of transistor 101. The increasing voltage of the cathode end of the diodes shuts them off in turn thus removing resistors 105, 104, 103 in turn from parallel connection with resistor 102. The time constant of the CR circuit comprising capacitor 107 and the connected resistors 102/105 is therefore increased. This produces a decrease in the corner frequency of the filter and corresponding increase in the attenuation of the difference frequency signal.