by Bill Anderton
Video images of the historic first steps on the Moon were important for NASA and for history. Capturing, transmitting and distributing real-time video images from the Moon presented unique engineering challenges for NASA.
The resulting system and its components that were used on Apollo were remarkable.
In the early planning of the Apollo program, there was a call for a new communication concept wherein all voice, telemetry, television and ranging information would be transmitted over a single frequency system. This would be the primary link for both near-earth and lunar distances. The resulting system was called the Unified S-Band (USB) system wherein the voice and biomedical data would be carried on a 1.25 MHz. FM subcarrier, telemetry data on a 1.024 MHz. bi-phase modulated subcarrier and pseudo-random ranging code would use a common phase-modulated S-band downlink frequency. There was a separate USB for the Command and Service Module and Lunar Module, each on a different frequency; 2,287.5 MHz. was used for the CSM and 2,282.5 MHz. for the LM.
To provide video on the single LM uplink, the ranging code was removed and the modulation changed from Phase to FM. This left 700 kKz of clear bandwidth available below the subcarriers for a narrow band television signal on the S-band uplink.
A 320 horizontal progressive line, 10-frame per second, format was chosen to fit in this space. This slow-scan format used only one-tenth of the 5 MHz. bandwidth of the 525 interlaced line, 30 frames per second format that was standard for television in the U.S.A. at the time.
Atop the LM ascent stage was a 26-inch parabolic-dish called the Steerable S-Band Antenna that transmitted the USB signal to Earth. This was a high-gain antenna (20.5 dB on the transmit leg) that had to maintain its pointing toward Earth. The antenna had to be continuously re-aimed because the motions of the Earth and Moon caused the Moon to drift out of the antenna’s field. The antenna could be manually steered ("slew" mode) or autonomously steered ("auto" mode) by its onboard equipment. The Steerable S-Band Antenna was relatively small and required a larger dish on Earth to make the video usable. The LM also had lower-gain omnidirectional antennas on the fore and aft sides of the LM but the lower gain of these antennas were not suitable for television transmission.
Potential difficulties receiving weak signals from the relatively small Steerable S-Band Antenna were enough of an issue for NASA that engineers also created an Erectable S-Band Antenna to be used on the lunar surface. The Erectable S-Band Antenna could be unstowed from one the MESA bays of the LM and erected on the lunar surface by the astronauts. The Erectable S-Band Antenna was first flown on Apollo 11 and was intended to provide a stronger television signal for the first lunar moon walk. However, because the time allotted for the Apollo 11 EVA was so brief and precious, the expected 19-minute deployment of the antenna would have a major impact on productivity. Consequently, an assessment was made for the black and white TV signal should come through the LM's Steerable S-Band Antenna. The signal was deemed adequate, so the Erectable S-Band wasn't deployed or used on Apollo 11 although it was employed on later missions.
The Westinghouse Apollo Lunar Surface Camera was used on Apollo 11.
The camera used on the lunar surface had a challenging set of design criteria. It had to be designed to:
The contract to develop the camera was given to Westinghouse which produced a camera with the following specifications.
|NASA Component No.||SEB16101081-701|
|Sensor||Westinghouse WL30691 Secondary Electron Conduction Tube (SEC)|
|Sensor size||1/2 inch tube|
|Field Scan type||progressive scan|
|Frame rate||10 fps at 320 lines, 0.625 fps at 1280 lines|
|Frame size||320 scan lines (10 fps) and 1280 scan lines (0.625 fps)|
|Resolution||200 lines (10 fps), 500 lines (0.625 fps)|
|Power Consumption||6.5 watts @ 24—31.5 volts DC|
|Weight||3.29 kilograms (7.3 lb)|
|Dimensions||269 mm × 165 mm × 86 mm (10.6 in × 6.5 in × 3.4 in) LxHxW|
|Lens mount type||Bayonet|
The camera used a "slow scan television" (SSTV) technique that produced only ten frames per second (fps) in order to meet the specifications, primarily to fit within the LM's limited S-band communications path. This was a design compromise that understood that motion fidelity from such an SSTV system would be degraded but it decided that it would be sufficient because astronauts would not be moving quickly in their bulky moon suits, even in the weaker lunar gravity on the Moon's surface.
The Westinghouse Apollo Lunar Surface Camera was carried in the Lunar Module's (LM) descent stage, in one of the Modularized Equipment Stowage Assembly (MESA) bays. The camera was attached to a shock mounting upside down (the camera's only flat surface) so then the MESA was opened, the camera was in the proper position to photograph the images of Neil Armstrong descending the LM's ladder and setting foot on the Moon. It was from its mounted position in the MESA where the Lunar Surface Camera captured man's first step onto the Moon.
The camera could also be unstowed and detached from its MEAS mounting and mounted on a camera stand away from the LM to give a better view of the moon walk. The camera was attached to the LM by a cable.
Apollo 11 would be the first and last time this particular camera design was used on the lunar surface because it was replaced by a 30 fps color camera on Apollo 12 and all later missions. Later flights had longer EVA periods so there was sufficient time to set up the Erectable S-Band Antenna which was larger and could carry a signal with higher capacity. This allowed the use of a 30 fps color NTSC-banded camera. However, SSTV camera also flew as a backup camera on the Apollo missions from Apollo 13 to Apollo 16, in case the new color cameras failed as it did on Apollo 12 when the color camera was accidentally pointed to the Sun that resulted in damaging its video tube.
One of the main restrictions that caused the use of slow-scan video was the limited bandwidth available via the Unified S-Band link and its small Steerable S-band Antenna on the LM.
The SSTV signal from the Lunar Surface Camera was uplinked from the surface of the Moon in its analog format via the Unified S-Band link and the LM's Steerable S-band Antenna. In the design phase, digital transmission was considered, but in the technology of the day, it required more bandwidth than was available so analog was used.
The strength of the signals, when received on Earth, was also an issue. The Steerable S-Band Antenna on the LM was much smaller that the one on the command service module (CSM) and naturally resulted in a weaker lower-gain signal originating from the lunar surface. With its larger antenna, signals received from the CSM were normally received by 26-meter antennas.
To receive the signals from the LM, NASA arranged for the use of the 64-meter radiotelescope dishes at Goldstone, California, and Parkes, Austraila. The larger dishes improved the Apollo downlink signal by 8-10 dB, an important margin.
When the video signal from the LM was received, the ground stations received its raw unconverted SSTV signal and split it into two paths. The video output was from the Unified S-Band Signal Data Demodulator (SDDS). One signal path from the SDDS without any additional processing was sent to an Ampex FR-1400 fourteen-track analog data tape recorder where it was recorded onto one-inch-wide magnetic tape on fourteen-inch diameter reels. The primary purpose of these recordings was to serve as a backup in case the scan converter failed. The analog data was recorded at the rate of 120 inches per second to capture the full bandwidth of the downlinked video signal. The recorders were loaded with 9,600 feet of instrumentation-grade recording tape.
A reel of tape recorded about 15-16 minutes of video so two recorders were used to allow continuous recording. The second unit could be started before the first recorder was stopped to be unloaded and reloaded.
The other raw SSTV signal branch was sent to an RCA scan converter (more below) where it would be processed into an NTSC broadcast television signal.
The processed NTSC signal was sent via multiple microwave and satellite links to the receiving ground stations in Houston, Texas. The signal in Houston was split into a pool feed for various television networks and sent via microwave relay to New York and the world. Ultimately, the pool feed from NASA to the networks was broadcast using analog techniques to your home via their local broadcast affiliates.
The video seen on home television sets was further degraded by its very long and noisy analog transmission path. The entire NASA network (NASCOM) was state of the art for its day but many of its links were analog.
The television broadcasting standard is standardized by the National Television System Committee (NTSC) in North America and also used in Japan. Since the SSTV camera on the Moon used a non-NTSC format different from that of the commercial television, it could not broadcast to the public until the signals were converted.
The conversion process was accomplished via devices made for this purpose the RCA Astro Electronics Division.
RCA Apollo Slow Scan Converter
The conversion process started when the SSTV signal was sent to the RCA converter's high-quality 10-inch slow-scan cathode-ray-tube-based monitor inside a dark chamber inside the unit. The image from the screen was re-photographed by a conventional vidicon-tube-based RCA TK-22 monochrome television camera. The TK-22 used the NTSC broadcast standard of 525 scanned lines interlaced at 30 fps. In literal terms, the converted image was simply a picture of a picture. Since the monitor being re-photographed used long-persistence phosphors, it allowed the image to remain visible from frame to frame. Overall, the rig acted as a primitive analog frame buffer.
This type of crude scan conversion did degrade the quality of the resulting image. Re-photographing a screen, called optical transfer, resulted in overall quality that was similar to the old kinescope recordings of the 1950s and their inherent loss of detail.The images were considerably lower in quality than the broadcast norms of the day.
Above, the photo on the left was taken directly off a Fairchild 320 line, 10 frames per second monitor located at Sydney Video control room using Polaroid PN-55, 4- x 5-inch instant film. Look at the reflection in Buzz Aldrin’s visor and compare it with the after conversion photo.
The photo on the right was taken off an NTSC monitor after scan conversion, also at Sydney Video control room, very close to the time of the unconverted photo on the left was taken. This shows the loss of resolution and shadow detail that occurred during the conversion process.
This was the time before digital frame buffers and digital scan converters that processed the signal itself and eliminated the optical transfer limitation so the resulting degradation of the Apollo 11 video is understandable.
The NTSC output of the scan converter was also recorded for later playback in case of a microwave circuit failure occurred. The video was recorded on Ampex VR-660B professional recorders using a helical scan head onto 2-inch video tape that was recording at the rate of 3.7 inches per second. Tape reels were 12.5 inches in diameter that held 5,540 feet of tape, sufficient for five hours of recording.