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Aviation History
1976
1976 - 3002.PDF
FLIGHT International, 18 December 1976 pixels or picture elements). Just as in a newspaper picture, the detail is provided by the relative brightness of the matrix of dots. Since the brightness of each dot was deter mined by six binary digits, or bits, it took 8hr to compose each of the 21 frames on Earth at a transmission rate of 8-3 bits/sec. Maximum transmission rate depends on a number of factors, chiefly spacecraft transmitter power, receiver sensitivity, distance of the spacecraft and the performance of the data storage and processing equipment on Earth. To improve the resolution of images and increase the number of pictures, Mariners 6 and 7 each carried two television cameras, with focal lengths of 52mm and 508mm, taking pictures comprising 704 lines of 935 pixels/line. Resolution was 0-3km and 3km for wide and narrow-angle cameras respectively, and data was trans mitted at 16,200 bits/sec before the spacecraft reached the planet. During fly-by so much information was coming in that it mostly had to be stored aboard digital and analogue tape recorders. About 36 pictures could be stored in this way, for playback at the considerably slower rate of 270 bits/sec, after the craft had passed Mars. Better quality pictures Despite the better picture quality from these two Mariners, performance was still below the level required for Mariner 9, the first spacecraft to go into orbit about Mars. But again the system used on the Mariner 6 and 7 fly-bys formed the basis for new equipment, and the Mariner 9 cameras had a 700-line format with a total of 582,400 pixels per picture. At the same transmission rate as its predecessor—16,200 bits/sec—pictures were recon stituted at the rate of one every 5min, and more than 7,000 frames were received during its operational life in orbit. The next objective, a fly-by of Venus and Mercury by Mariner 10, focused attention again on the limitations of data acquisition in the brief duration of each planetary encounter. With two 1,500m focal-length cameras Mariner 10 married the experience of rapid-scan techniques developed for Mariners 6 and 7 to the high data trans mission rates built up through development of the Mariner 9 communication system. Every 42sec Mariner 10 returned high-resolution views direct to Earth, as they were taken, while it was near Venus and Mercury. During these periods the rate was 117,600 bits/sec. Each picture comprised 582,400 pixels, the same as that employed for Mariner 9. This was the first time that the data transmission rate was sufficient to permit pictures to be sent back as they were taken, so that Mariner 10 could be said to have carried the first true television camera to the planets. (Mariner 9 could do this, but only on a sampling basis and not during fly-by.) On earlier craft pictures could be taken much faster than they could be transmitted so they had 1785 Though Nasa's Mariner 2 made a successful reconnaissance of Venus in 1962, Mariner 4 (pictured here) made a bigger public impact because it sent back the first close-up photographs from another planet. It passed Mars on July 14, 1965, at a distance of 6,100 miles, taking 21 pictures which revealed a cratered surface resembling the Moon's to be stored on tape recorders and sent back at a rate determined by the telemetry equipment. But direct trans mission of pictures from the refined Viking cameras, which provided 1,056 lines of 1,182 pixels each, would have necessitated a much improved rate compared with that achieved by Mariner 10. With 1,248,000 pixels per frame and 7 bits/pixel, each picture contained 8-7 million bits of information so that "real-time" transmission was out of the question. Because of this, and the relatively relaxed picture-taking schedule—the pictures and other scientific information did not all have to be secured in the few hours when the craft was passing the planet—the Viking orbiters store pictures on tape and return them to Earth at a rate of 16,000 bits/sec, synthesis of each picture taking 9min. Unlike the global-mapping task assigned to Mariner 9, the Viking orbiter carries two 475m focal-length cameras Russia in 1962 made three attempts to launch a spacecraft towards Mars, but only one of them, Mars I, managed to leave Earth-parking orbit and begin its journey. Signals ceased after about four months, however, and the probe maintained silence as it flew past Mars in June 1963 at a range of 120,000 miles
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