The National Aeronautics and Space Administration is making final plans for processing the pictures Mariner IV will take of Mars as it flys within 5600 miles of the planet on July 14.
The processing of the 20 photographs of Mars taken by Mariner will be many times more complex and difficult than the processing of the photographs of the Moon taken by the Ranger spacecraft.
Mariner will radio its "pictures" back to Earth from more than 134 million miles away as contrasted with Ranger's 250,0 miles - more than 500 times the distance.
As with two people trying to shout to each other the length of a football field, there will be some difficulty in understanding all of what Mariner is trying to say. As a result, the tape on which Mariner records the pictures it takes will be transmitted at least twice to Earth tracking stations.
A data analysis system has been set up at the Jet Propulsion Laboratory, Pasadena, California, which will compare the two playbacks and eliminate errors. Here is how Mars pictures will be taken and returned to Earth:
Mariner will take some 20 photographs of Mars in about 24 minutes. These will be recorded on magnetic tape. After Mariner has passed behind Mars and has re-established radio contact with Earth, it will start scanning the picture tape, converting the pictures into digital data (a series of 0's and 1's), and radioing the data to the stations of the NASA/JPL Deep Space Network.
Mariner has a 10-watt radio transmitter on board but by the time its signal is gathered by an 85-foot diameter dish and focused on to the antenna's receiver, the signal will be extremely weak - about 1/10th of a billionth of a billionth of a watt or a little less than 1-quintillionth (.00000000000000000001) of one watt.
For this reason, Mariner will "speak" very slowly. A picture which was taken in less than a minute, will take more than 8 hours to radio to Earth. Mariner will transmit only 8-1/3 bits of data each second.
Because Mariner will be sending the pictures in digital form, the stream of digits must be turned back into pictures after they reach Earth. Thus, each picture frame of Mars will be made up of some 40 thousand dots or elements. Each element is made up of 6 bits of data.
Each of the elements can be any one of 64 shades of gray ranging from white to black.
To complicate the job of understanding Mariner's picture message is the fact that engineers want to keep track of the spacecraft's condition during the picture transmission sequence so, mixed in with the digitized pictures will be other digit reporting on Mariner's temperature, its battery voltages, and other engineering performance data. There also will be time reports, picture synchronization and other information.
And because it takes so long to playback each picture, two Deep Space Network stations may receive parts of one picture. For instance, Mariner may be passing over the DSN station at Johannesburg, South Africa, when it starts playing back one picture and be over Goldstone, California, when that picture is completed.
Once a DSN station has received Mariner's message, the magnetic tape on which it is printed will be sent to Pasadena by courier or mail and the same message will be relayed by teletype.
At Pasadena, in JPL's Space Flight Operations Facility, the job of converting the teletype tapes into picture form begins. Computers and a device called a digital photographic processor will do the work.
If a picture is split, computers will take teletype messages or magnetic tapes from two DSN stations and form one picture message from them. They will then analyze the tapes and separate from them all spacecraft performance and other information leaving a tape of picture data only.
This tape then will be fed into the digital photographic processor. This device converts each digit into its proper shade of gray and electronically projects it on to a precisely controlled and very accurate cathode ray tube - a complex version of the picture tube in a home television set. As the dots are projected on the screen, they are photographed and the picture is then developed and printed.
Project officials at JPL plan to immediately release to the public the first pictures after they have gone through this process. If all goes well, this could be within 36 to 48 hours after Mariner begins taking the pictures at about 8:00 p.m. EDT July 14.
The remaining pictures will undergo further processing to improve their accuracy before they are released.
For instance there may be some errors in the electronic transmission or in the intricate processing equipment in the early pictures from 134 million miles away. Project engineers hope to correct these errors by comparing two and possibly three playback tapes. Computers will compare the tapes and select the correct elements if one is missing or is in error on one tape. After the pictures are finally processed, they will be released to the public as scientists continue their intensive investigation of what can be learned from man's first close-up look at Mars.
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