Patent GB-1050006-A (GB)
IPC: G01S13/14
Abstract: 1,050,006. Radar. BOLKOW G.m.b.H. Oct. 8, 1964 [Oct. 9, 1963], No. 41066/64. Heading H4D. In an aircraft altimeter device for providing continuous instrument readings of height use is made of a ground observation radar carried by the aircraft. The radar receiver output and the pulse trigger line are connected to separate inputs of the device, and a first circuit arrangement provides for the suppression of signals due to directly received pulses. A second circuit arrangement checks that ground echoes are present and then sets into operation a third circuit arrangement which allows current to flow through a measuring instrument during the intervals between the transmission of pulses and the reception of the first ground echoes. The average current is proportional to height, and the instrument is calibrated to read the latter. As described, two multivibrators each comprising two transistors, T 1 , T 2  and T 3 , T 4  respectively, Fig. 1, are connected in series. The negative video output of the radar receiver is supplied via terminal 3 to the base of transistor T 1  through a summation resistor R 3  and the negative pulse trigger is supplied via terminal 4 to (i) the base of transistor T 2  and (ii) through a reversing stage, comprising transistor T 5 , and a summation resistor R 4  to the base of transistor T 1 , to cancel the direct video signal due to the transmitted pulse. The output from the collector of transistor T 4  is taken through a measuring instrument 5, in series with a variable resistor R 14 . Initially, transistors T 1  and T 4  are cut off and transistors T 2  and T 3  are conductive. The arrival of the first ground echo reverses the states of transistors T 1  and T 2  and in consequence, the collector of transistor T 2  falls in potential and the differentiating circuit C 9 , R 9  causes a short negative pulse to pass through condenser C 10  to the base of transistor T 3 . This has no effect as transistor T 3  is already conductive, the base being negatively biased through resistor R 10 . When the next pulse trigger reaches the base of transistor T 2  that transistor becomes conductive and transistor T 1  is cut off. A positive pulse is passed on via condenser C 10  to the base of transistor T 3  causing that transistor to be cut off and transistor T 4  to become conductive, so that current passes through the instrument 5. The next first ground echo causes transistor T 1  to become conductive and transistor T 2  to be cut off, a negative pulse being passed to the base of transistor T 3  so that it becomes conductive and transistor T 4  is cut off, the current through the instrument 5 ceasing. The sequence continues with current flowing through the instrument 5 in the intervals between transmission of radar pulses and reception of first ground echoes. The average current reading is thus proportional to the height of the aircraft above the ground. A pulse shaping circuit IV may be connected to the trigger input in order to adjust its length to that of the direct video impulse. A range switch 6 may be provided which bridges a resistor R 15  across the measuring instrument 5 and its series variable resistor R 14  and parallels a capacitance C 12  across the capacitance C 11  of the multivibrator comprising transistors T 3 , T 4 . Voltage stabilized power may be provided by means of a dropping resistor R 16  and Zener diodes Z 1 , Z 2 . The components of the device may be incorporated in the housing of a somewhat elongated conventional direct current indicating instrument, Fig. 3 (not shown). The output of the device may also be conveyed to other devices, such as a fire-control computer. A method of calibration is described using a rectangular pulse generator. Differentiated negative pulses are fed to terminal 4 and calibration is effected by variation of the resistor R 14 .