The early days of image processing : the 1960s lunar probes

Some people probably think image processing was designed for digital cameras (or to add filters to selfies), but in reality many of the basic algorithms we take for-granted today (e.g. improving the sharpness of images) evolved in the 1960s with the NASA space program. The space age began in earnest in 1957 with the USSR’s launch of Sputnik I, the first man-made satellite to successfully orbit Earth. A string of Soviet successes lead to Luna III, which in 1959 transmitted back to Earth the first images ever seen of the far side of the moon. The probe was equipped with an imaging system comprised of a 35mm dual-lens camera, an automatic film processing unit, and a scanner. The camera sported a 200mm f/5.6, and a 500mm f/9.5 lens, and carried temperature and radiation resistant 35mm isochrome film. Luna III took 29 photographs over a 40-minute period, covering 70% of the far side, however only 17 of the images were transmitted back to earth. The images were low-resolution, and noisy.

The first image obtained from the Soviet Luna III probe on October 7, 1959 (29 photos were taken of the dark side of the moon).

In response to the Soviet advances, NASA’s Jet Propulsion Lab (JPL) developed the Ranger series of probes, designed to return photographs and data from the moon. Many of the early probes were a disaster. Two failed to leave Earth orbit, one crashed onto the moon, and two left Earth orbit but missed the moon. Ranger 6 got to the moon, but its television cameras failed to turn on, so not a single image could be transmitted back to earth. Ranger 7 was the last hope for the program. On July 31, 1964 Ranger 7 neared its lunar destination, and in the 17 minutes before it impacted the lunar surface it relayed the first detailed images of the moon, 4,316 of them, back to JPL.

Image processing was not really considered in the planning for the early space missions, and had to gain acceptance. The development of the early stages of image processing was led by Robert Nathan. Nathan received a PhD in crystallography in 1952, and by 1955 found himself running CalTech’s computer centre. In 1959 he moved to JPL to help develop equipment to map the moon. When he viewed pictures from the Luna III probe he remarked “I was certain we could do much better“, and “It was quite clear that extraneous noise had distorted their pictures and severely handicapped analysis” [1].

The cameras† used on the Ranger were Vidicon television cameras produced by RCA. The pictures were transmitted from space in analog form, but enhancing them would be difficult if they remained in analog. It was Nathan who suggested digitizing the analog video signals, and adapting 1D signal processing techniques to process the 2D images. Frederick Billingsley and Roger Brandt of JPL devised a Video Film Converter (VFC) that was used to transform the analog video signals into digital data (which was 6-bit, 64 gray levels).

The images had a number of issues. First there was the geometric distortion. The beam that swept electrons across the face of the tube in the spacecraft’s camera moved at nonuniform rates that varied from the beam on the playback tube reproducing the image on Earth. This resulted in images that were stretched or distorted. A second problem was that of photometric nonlinearity. The cameras had a tendency to display brightness in the centre, and a darkness around the edge which was caused by a nonuniform response of the phosphor on the tube’s surface. Thirdly, there was an oscillation in the electronics of the camera which was “bleeding” into the video signal, causing a visible period noise pattern. Lastly there was scan-line noise, which was the nonuniform response of the camera with respect to successive scan lines (the noise is generated at right-angles to the scan). Nathan and the JPL team designed a series of algorithms to correct for the limitations of the camera. The image processing algorithms [2] were programmed on JPL’s IBM 7094, likely in the programming language Fortran.

  • The geometric distortion was corrected using a “rubber sheeting” algorithm that stretched the images to match a pre-flight calibration.
  • The photometric nonlinearity was calculated before flight, and filtered from the images.
  • The oscillation noise was removed by isolating the noise on a featureless portion of the image, created a filter, and subtracted the pattern from the rest of the image.
  • The scan-line noise was removed using a form of mean filtering.

Ranger VII was followed by the successful missions of Ranger VIII and Ranger IX. The image processing algorithms were used to successfully process 17,259 images of the moon from Rangers 7, 8, and 9 (the link includes the images and documentation from the Ranger missions). Nathan and his team also developed other algorithms which dealt with random-noise removal, Sine-wave correction.

Refs:
[1] NASA Release 1966-0402
[2] Nathan, R., “Digital Video-Data Handling”, NASA Technical Report No.32-877 (1966)
[3] Computers in Spaceflight: The NASA Experience, Making New Reality: Computers in Simulations and Image Processing.

† The Ranger missions used six cameras, two wide-angle and four narrow angle.

  • Camera A was a 25mm f/1 with a FOV of 25×25° and a Vidicon target area of 11×11mm.
  • Camera B was a 76mm f/2 with a FOV of 8.4×8.4° and a Vidicon target area of 11×11mm.
  • Camera P used two type A and two type B cameras with a Vidicon target area of 2.8×2.8mm.

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