Patent GB-722396-A (GB)
Title: Frequency-modulated radar system for developing a signal representative of velocity
IPC: G01S13/60
Filed: 1952-12-02
Abstract: 722,396. Frequencymodulated radar. HAZELTINE CORPORATION. Dec.2,1952[Dec. 26, 1951], No. 30561/52. Class 40 (7). In a radio altimeter of the frequency-modulated type in which the height of an aircraft above ground is indicated by a signal derived from the instantaneous frequency difference between the transmitted and received frequency-modulated waves the vertical component of the velocity of the aircraft is indicated by the rate of change of phase of the derived signal between the predetermined reference times. As shown in Fig. 1, the direct output (curve C, Fig. 2) of a frequencymodulated transmitter 13 (produced by a modulating signal of triangular waveform derived by integrating in 17 the output of a multivibrator 16) is combined, in receiver 12, with the same signal (D, Fig. 2) received by aerial 15 after reflection at the ground and the combined (heterodyned) signal (E, Fig. 2) is then converted into a corresponding pulse train (F, Fig. 2) in amplifier and limiter 18. The rate of occurrence of these pulses is proportional to the height of the aircraft and by deriving a unidirectional potential the magnitude of which is representative of this rate (by means of pulse counter circuit 19) the height may be displayed on a D.C. type altitude indicator 20. To determine the vertical component of the velocity of the aircraft the rate of change of phase of the pulse train F, Fig. 2, between two predetermined reference times is derived by supplying the latter train to the control grid 39 of a valve 36 to the suppressor grid 37 of which the pulse trains J, Fig. 2 (derived by suitably shaping and then differentiating in the elements 30 to 35 the output of multivibrator 16) is also supplied the arrangement being such that valve 36 conducts when a pulse J coincides with a positively going one of the pulses of train F (Fig. 2). When the height of the aircraft is such that valve 36 conducts a series of output pulses K (Fig. 2) is obtained, but if this height is altered by a distance equal to one quarter of the wavelength of transmitter 13 a negatively going one of the pulses F will coincide with a pulse J so that no output signal is developed by valve 36. Accordingly the group repetition frequency of the pulses K (shown as pulses L on a reduced time scale) is a measure of the vertical component of the velocity of the aircraft and by means of detector 44 and amplifier and limiter 45 a rectangular wave N (Fig. 2) having a frequency equal to the group repetition frequency of pulses K (or L) is produced and employed to generate a unidirectional potential (by means of pulsecounter circuit 46), the magnitude of which is proportional to this rate and which is employed to operate a velocity indicator 55. The output signals of the counter circuits 19 and 46 are also supplied to a computer 29 in which a signal representative of the deviation of the aircraft from a desired flight path is developed and this signal may be indicated on a suitadle meter or may be applied to an automatic pilot to control the flight fo the aircraft. Specification 656,094 is referred to.