GB626610A - Improvements in or relating to frequency modulation radio altimeters - Google Patents

Abstract

626,610. Radiolocation. SHARPE, B. A. May 3, 1945, No. 11307. [Class 40 (vii)] In a frequency modulation radio altimeter of the type providing an electrical output which is a function both of the height to be measured and the modulation frequency, the indicator comprises a pointer driven by servomotor means for comparing the said electrical output with a further output which is proportional to the instantaneous modulation frequency, so that the indication provided is independent of the modulation frequency. A frequencymodulated signal generated at TX is radiated from the aerial TA and the echo signals received at RA are mixed with the signals then being transmitted, the beat frequency signal being applied to a frequency counting circuit FC1 which generates a voltage Va proportional to the beat frequency. A variable condenser in the transmitter TX is driven continuously by the motor MM to provide the frequency modulation. A voltage ripple produced in the field winding F and consequently related to the modulation frequency, is applied through bandpass filter BPF and limiter L2 to the frequency counter FC2 which provides a voltage Vb proportional to the modulation frequency. A fraction of the potential Vb is tapped off by the potentiometer POT and applied in opposition to the potential Va to the grid of a valve, the anode circuit of which contains a change-over relay RE arranged to control a reversible motor SM which accordingly drives the potentiometer arm to maintain the voltage applied to the grid of the valve constant. A pointer P attached to the potentiometer arm indicates the altitude. The indication, which is a function of the ratio of Va to Vb is consequently independent of the modulation frequency as both potentials are directly proportional thereto. Moreover by supplying at least both the limiter circuits from a common H.T. supply, Va and Vb may be made to vary proportionately should the H.T. supply alter, whereby the meter reading may be rendered independent of the supply voltage. Since the indicator may be made to provide substantial torque, the system is suitable for providing remote indications.