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Aviation History
1951
1951 - 1447.PDF
112 FLIGHT, ?j July 1951 On the left the Grasshopper is seen ready for operation; the telescopic mast is about 20ft high. The centre view shows the device after it has landed, but before the leg-release device has functioned. On the extreme right is an interior view, with the clock mechanism in the right foreground. Air-Launched Weather Station Ingeniously Designed Equipment for U.S. Navy A SELF-CONTAINED automatic weather station, whichtransmits weather data by radio, has recently beendeveloped by Percival D. Lowell and William Hakkarinen of the U.S. National Bureau of Standards for the U.S. Navy Bureau of Ships. Named the Grasshopper, the device can be • parachuted from aircraft on to inaccessible territory. It will auto- matically set itself up and periodically make and transmit weather observations. It may also be used as a radio marker beacon. Designed in the shape of a bomb, and packing its own parachute, the Grasshopper is loaded on a bomb carrier. When the unit is released, the parachute is automatically opened by a static line; simultaneously, an electric clock, which controls subsequent operations of the station, is turned on. The impact of landing sets of! a small explosive charge which disengages the parachute and prevents the station from being pulled along the ground. Either immediately or after a pre-set dormancy period, another explosive charge causes the station to rise to an upright operating position. This is done through an arrangement of six legs to which springs are attached; the explosive charge operates a release, permitting the springs to pull the legs into position. A third explosive charge extends a telescopic vertical antenna to a height of some 20ft. The station is then ready for automatic transmission at intervals predetermined by the built-in timing mechanism. The Grasshopper could be adapted to transmit various kinds of information, but in the standard design only temperature, pressure, and humidity data are reported. Separate mechanisms responsive to changes in these atmospheric conditions each cause an associated resistor to vary. At predetermined intervals the timing mechanism turns on the radio transmitter and connects one resistor after another to a critical point in the transmitter circuit. The transmitter is designed so that the emitted radio signal pulses on and off at a rate proportional to the value of the resistor so connected. The station is calibrated before use by subjecting it to known temperatures, pressures, and humidities and measuring the resulting pulse rates. At the receiving station the transmitter pulse rate can then be read as temperature or humidity, depending on the phase of the predetermined clockwork cycle. The radio transmitter proper consists of a crystal oscillator followed by a radio-frequency amplifier stage. A relay in the plate circuit of a separate relaxation oscillator turns the crystal oscillator on and off at a rate proportional to the value of whatever resistor is temporarily inserted (by the clock mechanism) into the relaxationoscillator circuit. When the station is to be used as a beacon, the radio transmitter and its control mechanisms may, of course, besimplified. The .clock, in addition to inserting the several weather-responsive resistors into the circuit in a predetermined sequence, connects two other resistors at appropriate intervals. These area reference resistor and an identification resistor, both of constant value. The pulse rate produced by the fixed reference resistor isobserved during initial tests of the transmitter. Any subsequent deviation in the reference resistor pulse rate warns the receivingstation that a correction fa'tor must be applied to pulse rates of the weather-responsive resistors. Such deviation could arise fromtransmitter damage or aging. The identification resistor is of a value selected to produce a pulse rate characteristic of theparticular station; this enables the receiving station operator to identify the station. A special technique is used to ensuie maximum accuracy of thetransmitted data despite possible deformation of the weather- responsive mechanisms due to landing impact; a buzzer vibrateseach weather-responsive device for a short time before its associated resistor is inserted in the relaxation oscillator circuit.This forced vibration counteracts friction, which may have been increased by landing impact deformation, and thus aids in theattainment of a true equilibrium condition. The developmental model had an output of the order of 5 watts.Operating on a frequency in the neighbourhood of 5 megacycles, it performed reliably over land at ranges of more than 100 miles.The dry batteries used provided power for transmission of weather reports at three-hour intervals for more than 15 days. RUSSIAN GLIDERS IN GERMANY A CORRESPONDENT reports that a few days ago, in the veryearly morning, three Li-2 (Russian-built Dakota) glider tugs crossed over Zehlendorf, south-west of Berlin in the Americansector. They were flying at about 1,500ft and one of these machines was towing a large glider, the span of which was slightlymore than that of the Dakotas, though the fuselage was shorter. The same correspondent believes that Russian paratroops aretraining at Stendal, near the frontier of the British Zone.
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