Radar And Mariner 10, Best possible mapping, pre-Messenger |
Radar And Mariner 10, Best possible mapping, pre-Messenger |
Apr 28 2005, 06:21 PM
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
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May 22 2006, 08:45 AM
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Senior Member Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 |
Mariner 10 and Voyager had very similar camera systems. Selenium Sulfide vidicon tubes, who's design went back to Mariner 69 and maybe some Mariner Mars 64 heritage.
Mariner 4 had a horrendous light leak -- PAST the shutter mechanism -- and impossibly limited data storage. Mariners 6 and 7 tried to solve the data storage problem with a combined analog/digital dual recorder system that *SORT OF* worked but was compromized by tape-crud contamination buildup on the analog recorder heads. Mariner 9 upgraded to all digital, 9 bit (!) data and could stuff about 33 pics on a tape recorder load, doing 2 orbits a day and dumping 1 tape load just after a periapsis and the second just before the second periapsis (only one 210 foot DSN dish at the time). Unfortunately, the Mariner 69 and 71 data were badly corrupted by residual images. Approach images by Mariner 9 show multiple ghost imagtes of Mars in subsequent images in the sequence, slowly fading with each following shot. Mariner 10 (and Viking orbiters, and Voyagers) used brilliant (literally) brute force engineering to eliminate the residual image problem. After each frame, they turned lightbulbs on INSIDE the cameras, then erased the light-flood staturated image so each image had an essentially perfectly repeatable residual image after erasure. The noisy Mariner 10 data are from the Mercury flybys, where the range to Earth was right a the limit of the experimental 114,000 (or 148,000 or whatever then number was) bits / sec transmission rate. They traded off getting a LOT of salt-and-pepper speckled images at Mercury for fewer but clean images. The highest resolution images during the first flyby were tape recorded and played back later, the tape recorder failed before either the second (I think) or third flyby. DSN failure at the third flyby limited imaging to the central 1/4 or so of each frame due to a forced lower data rate, losing contiguous targeted high resolution mapping of selected targets <damn>. |
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