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Fig. 15. Th© element* that form the electronics payload employed for the Project "Score." Being held at the right is the 10-ounce command receiver. Behlnd il is the electronics control unit. The large unit in the center is the FM

message transmitter. with łts power conrerter at the left.    Fig. 16. Being held łn the photograph abore is the scanning

Foreground unit is V4-lb. beacon or tracking transmitter.    device lor photoelectric celi sensory unit in the "Pioneer I."

Photoelectric Systems

Special typcsof optical sensory uniłs have been developed primarily for map-ping and crude surveillance of large areas. These units employ photocells with appropriate scanning devices and switches. Characteristically, they can distinguish only between light and dark areas; but they can map effectively in terms of black-and-white contrast— such as between sea and land regions. They can be used similarly to record the distribution of clouds over the sur-face of the earth or other planets, again in terms of black-and-white contrast. But in any application. photoelectric sensory units provide poor definition.

First of these optical sensory units was part of the electronics payload aboard the ‘‘Pioneer I.’’ but was not operative because the space probe failed to reach the vicinity of the moon. The sensory unit will be used on later space probes for the identical purpose of mapping areas of the moon and other planets.

Such a sensory unit consists essen-tially of a scanning device (Fig. 16> and two photocells and other elements sensitive to infrared illumination. The entire unit is contained in a barrel which turns as the space vehicle ro-tates in flight. There are two smali circular apertures on each side of the barrel. Each aperture is equipped with a smali mirror-type telescope and is armed with a hydraulic timer.

At an altitude above 100,000 miles, the earth offers too smali an image to acti\*ate the photocells but wlien the space vehicle is pointed properly on a pass near the moon or any other planet, the refiected sunlight from the planet is suflicient to enter both apertures and trigger the photocells simultaneously. These signal impulses are broadcast by the data transmitter.

By this method, a “strip" of the region is scanned, with the photocells registering impulses for all sunlight re-flecłions. Enough of these “strips,” placed alongside each other. provide a crude electronic “picture” of the dis-tant surface—devoid of much defini-tion, but with enough black-and-white contrast to differentiate between water and land masses.

Another type of optical sensory unit was used in “Vanguard II" satellite to determine the distribution of clouds around the earth. Essential elements of the unit are two photocells mounted behind circular. gridded Windows pro-jecting from opposite sides of the satellite. The photocells project opposite each other at an angle of 45 degrees from the spin axis of the satellite. so that one always sweeps the surface of the earth. After amplification, signal impulses from the photocells are fed directly to a magnet ic tape recorder containing a 75-foot erasable tape. The recorder operates only when the photocells are scanning the sunlit part of the earth—about 50 minutes out of every hour. The recorder is turned off during darkness by an automatic switch acti-vated by solar cells.

When intenogated by a ground station, the command receiver in the electronics payload triggers the rccord-er and the data transmitter and an entire 50 minutes of taped data is broadcast in a single 60-second "burst"

Fig. 17. Series bank of m«rcury batteries

used as power supply in "Pioneer III.” of data transmission. Then the payload is reset to record again as the satellite continues its orbit around the world.

Television Systems

Refined types of opt ical-viewing satellites for the futurę will utilize smali TV cameras as the sensory units of their electronics payload. In a very real sense. these are the sophisticated successors of the photocell devices de-scribed previously. A TV system is far morę desirable because of its higher definition characteristics.

Initial satellite to be launched will be in the shape of a shallow cylinder, rcsembling a “flying saucer” and spinning about ten times per minutę at an altitude of from 200 to 500 miles above the earth. Three TV cameras, of the RCA “Vidicon" type. are installed around the periphery of the cylinder.

One camera has a field of view of about 1000 miles with a resolution of about 2.5 square miles. The second camera, with a smaller field of view, resolves a square of about 0.5 mile. The third camera, with the smallest field of view, resohes a square of about 0.1 mile or slightly morę than 500 feet.

The TV camera with the widest field of view is aimed perpendicular to the spin axis of the satellite and the other two are aimed in the same direction buł along the spin axis.

Each camera produces signal impulses in accordance with the amount of sunlight refiected by the surface of the earth. Cloud areas rellect about 80 per-cent of the sunlight; land about 15 to 20 per-cent; water surfaces about 5 per-cent.

Each camera seans for a fraction of a second and Stores the data on a mag-netic tape during about two seconds. This results in three channels of data, one from each camera. Subsequently, on command from a ground station. the data is broadcast by a 3-channel mul-tiplex data transmitter.

The TV sensory units are controlled by an automatic switch activated by solar cells so that the cameras operate only during dayiight hours. An elec-iContinued on page 114»

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October. 1959