Patent GB-564505-A (GB)
Title: Improvements in or relating to radio altimeter
IPC: G01S13/36
Filed: 1941-03-06
Abstract: 564,505. Radio echo apparatus; phase-difference measurements. FORMAN, J., and PYE, Ltd. March 6, 1941, No. 3045. [Class 37] A radio range finder or altimeter, particularly for use at short distances or low altitudes; comprises an ultra-high frequency transmitter which is modulated with sinusoidal oscillations of constant frequency and a receiver for receiving the transmitted signals after reflection, wherein the received modulated oscillations are limited or converted to produce sinusoidal oscillations of the modulating frequency and of constant amplitude, and wherein these oscillations are mixed with oscillations of the same amplitude derived from the modulation applied to the transmitter after one of these series of oscillations has been reversed in phase, the resultant peak voltage being measured to give an indication of the phase difference and thus of the distance or altitude. In the altimeter shown in Fig. 1, an oscillator 1 generating sinusoidal voltages modulates an ultra-high frequency transmitter 2. Modulated waves are transmitted from an antenna 3, reflected from the earth's surface, received on an antenna 4, and passed to a receiver 5. The output of this receiver, which is a sinusoidal voltage of the same frequency as that generated by the oscillator 1, but of different amplitude and phase, is fed either to a limiter or to an oscillator 6 generating sinusoidal voltages of the modulating frequency. This oscillator is then synchronized by the output of the receiver 5 so that its oscillations are in phase with those from the receiver The output from the limiter or oscillator 6 is fed to an amplifier 7 where it is amplified to equal the voltage of the oscillator 1, the bias of the amplifier 7 being automatically controlled through a rectifier 10 by the output of the oscillator 1. The amplifier 7 contains the requisite number of stages to cause a phase shift of 180 degrees. The outputs from the amplifier 7 and the oscillator 1 are fed to a mixer 8 and thence to a peak diode voltmeter 9, which reads the altitude directly. In a modification the output from the mixer 8 is fed through a peak diode rectifier to one side of a discriminator circuit, the other side of which is fed through a second peak diode rectifier from the oscillator 1 with a sinusoidal voltage equal in frequency, phase, and amplitude to that fed to the mixer. The discriminator circuit is shown in Fig. 3. Two identical valves 1, 2 have identical resistors 5, 6 in their grid circuits and identical resistors 9, 10 in their anode circuits. Bias is applied by a battery 7 and anode potential by a battery 8. A micro-ammeter 11 is connected across the anodes of the valves 1, 2, a battery 13 and variable resistor 12 being provided to balance out inequalities between the valves. The input 3 to one valve is fed from the rectified voltage of the oscillator 1 while the input 4 to the other valve is fed from the rectified output of the mixer 8. In another arrangement, Fig. 4, a high frequency oscillator 1 modulates an ultrahigh frequency transmitter 2 and synchronizes one grid of a double balanced valve 7 via the condenser 9 after passing through a 180 degrees phase shifter 24. Radio waves emitted from the antenna 3 are reflected from the earth 4, received by the antenna 5, and detected by the receiver 6. The output of this, containing the component derived from the oscillator 1, synchronizes the other grid of the valve 7 via a condenser 8. The valve 7 with associated circuits 14, 15, 16, 17 is made to oscillate in each of its halves at the frequency of the oscillator 1 by causing it to oscillate at approximately that frequency by means of tuned circuits 10, 11 and 12, 13 and then synchronizing the exact frequency of the oscillator 1. The balanced output of the valve 7 is applied via pick-up coils 18, 19 to the mixer 20, the output of which is rectified by the peak rectifier 21 and fed to the discriminator 22 giving direct readings of altitude on the meter 23. In order to extend the scale of the instrument for lower altitudes, the slope of the voltmeter may be made to vary with the output of the altimeter so that amplification is greater for small inputs. The output of the mixer 1, Fig. 5, is fed to the grid of a variable-mu valve 4 across a grid resistor 6 and also to a peak rectifier 2. The output from this is fed in the correct sense across the biassing resistor 3 of the valve 4, so that the bias alters according to the output of the altimeter and the slope of the valve varies accordingly and extends the scale of the meter 5 at low altitudes. With the circuit shown in Fig. 4 the deflection is a maximum for minimum altitude, and the scale may be extended by amplifying the output of the discriminator in a non-linear manner. To effect this the amplifying valve may be positively biassed so as to draw grid current for larger inputs, thereby increasing the anode current at low altitudes while allowing normal amplification above a certain height. Alternatively, the amplifier may be operated on the curved portion of its characteristic.