GB1207565A - Radio altimeters - Google Patents

Abstract

1,207,565. Radio altimeter. STANDARD TELEPHONES & CABLES Ltd. 2 July, 1969 [11 July, 1968], No. 33046/68. Heading H4D. A radio altimeter, Fig. 2, comprises a transmitter 3 frequency modulated by a triangular waveform, Fig. la, and transmitting its output to: (a) a first receiver 10, both directly (path 4c) and via aerials 5, 9 and the ground return path, to produce therefrom a height/frequency dependant beat signal VF10, Fig. 1c; and (b) a second receiver 7, both directly (path 4b) and via a standard delay line 6, to produce therefrom a reference beat frequency signal VF7, Fig. 1b. A clock pulse generator 1 produces clock pulses, Fig. 1, and also pulses t o , defining the turn-over minimum of the modulation triangular waveform and t 1 , defining the start of the usable linear portion of said waveform. The wavelength of the reference beat frequency signal VF7 is determined in counter 8 by counting the number of clock pulses n rc in the first N R half cycles of said signal after t 1 , beginning at t 2 and ending at t 4 . Beat frequency signal VF10 is fed to counter 11 which is also fed with the clock pulses and pulses defining t 2 and t 4 . Counter 11 determines the number of half cycles No of VF10 from the commencement of the first crossover point after t 2 at time t 3 , to time t 5 of the first crossover point after t 4 , and also the number of clock pulses ng c occurring during period t 3 to t 5 . Signals representing the t 3 , t 4 , t 5 , N R and No are sent to computer 12, wherein the height is computed using the formula: N R and n rc should be equal to predetermined values whereby: Height = K . N G /n gc Values of N R and n rc are monitored at 15 and a detected error causes an alarm and/or shut down of the height information output. If N R and n rc can be assumed stable, outputs 8b and 8c of counter 8 can be omitted. If N G is large, then n gc #n rc , whereas (Fig. 2A, not shown): Height = K<SP>1</SP>N G .