Author Topic: Apollo Landing Radar  (Read 2580 times)

Online catdlr

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Apollo Landing Radar
« on: 12/07/2017 03:55 am »
The How and Why ~ 1968 Ryan Aeronautical; Moon Landing Guidance Radar

Jeff Quitney
Published on Dec 6, 2017

"Being manufactured here is the landing radar system for the Apollo Lunar Module, a spacecraft that will place America's astronauts on the Moon..."

APOLLO EXPERIENCE REPORT - LUNAR MODULE LANDING RADAR AND RENDEZVOUS RADAR...

The landing radar, which is located in the LM descent stage, is packaged in two replaceable assemblies. The antenna assembly forms directs, transmits, and receives four narrow microwave beams. To perform these functions, the antenna assembly is composed of two interlaced phase arrays for transmission and four space-duplexed planar arrays for recep­tion. The transmitting arrays form a platform; four quadrature-pair balanced microwave mixers, four dual audio­ frequency preamplifiers, two solid-state microwave transmitters, a frequency mod­ulation ( FM) modulator, and an antenna pedestal tilt mechanism are mounted on the platform. The electronics assembly contains the circuitry that is required to track, process, convert, and scale the Doppler and FM/continuous wave ( cw) re­ turns, which provide the velocity and slant range information to the LGC and to the display panels.

The transmitting antenna radiates the cw microwave energy from the solid-state velocity-sensor transmitter to the moon. Three separate receiving antennas accept the reflected energy. The received Doppler-shifted energy, which is split into quad­rature pairs, is mixed with a portion of the transmitted energy by microwave diodes that function as balanced mixers. The output of the crystal balanced mixers gives the frequency difference between the received signals and the transmitted signals. This frequency difference is the Doppler shift, which is directly proportional to the LM ve­locity with respect to the lunar surface along the detected microwave beam. The output of the altimeter transmitter ( a sawtooth waveform) is frequency mod­ulated at 130 hertz and is transmitted by a second antenna. The reflected energy received by the receiving antenna is split to form a quadrature pair and, with a sample of the transmitted signal, is coupled to balanced microwave mixers. The frequency difference at the output of the balanced mixers is proportional to the time difference between the transmission and the reception of the modulated energy, plus a Doppler­ shift factor. The undesired Doppler-shift factor is compensated for in the range computer.


The Apollo Lunar Module (LM), originally designated the Lunar Excursion Module (LEM), was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman Aircraft to carry a crew of two from lunar orbit to the surface and back. Designed for lunar orbit rendezvous, it consisted of an ascent stage and descent stage and was ferried to lunar orbit by its companion Command/Service Module (CSM), a separate spacecraft of approximately twice its mass, which also took the astronauts home to Earth. After completing its mission, the LM was discarded. It was capable of operation only in outer space; structurally and aerodynamically it was incapable of flight through the Earth's atmosphere. The Lunar Module was the first manned spacecraft to operate exclusively in the airless vacuum of space. It was the first, and to date only, crewed vehicle to land on a natural object in the solar system other than the Earth.

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Originally a public domain film, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

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Update: Alternate video source to replace Jeff Quitney former YT account that was suspended.
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« Last Edit: 05/11/2019 04:07 am by catdlr »
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Offline jimothytones

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Re: Apollo Landing Radar
« Reply #1 on: 01/19/2024 04:21 am »
Reviving the thread to ask if we have any RF experts on the forum to help explain the LM altimeter signal to me. I have a family and as of recently professional connection to this part of Apollo history and I'm considering getting some representation of the radar altimeter waveform tattooed, but I'd like it to be as accurate as possible. I've heard both 13 GHz and 130Hz as operating frequencies, so that's confusing enough, but I also have no idea what an appropriate amplitude for the wave might be, or if an accurate depiction of the waveform would actually fit on a tattoo. Any help is appreciated.

Offline laszlo

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Re: Apollo Landing Radar
« Reply #2 on: 01/19/2024 12:25 pm »
Reviving the thread to ask if we have any RF experts on the forum to help explain the LM altimeter signal to me. I have a family and as of recently professional connection to this part of Apollo history and I'm considering getting some representation of the radar altimeter waveform tattooed, but I'd like it to be as accurate as possible. I've heard both 13 GHz and 130Hz as operating frequencies, so that's confusing enough, but I also have no idea what an appropriate amplitude for the wave might be, or if an accurate depiction of the waveform would actually fit on a tattoo. Any help is appreciated.

The LM altimeter carrier frequency was 9.58 GHz. It was frequency modulated with a 130 Hz sawtooth signal. Since it was an FM signal, the amplitude when it left the transmitter antenna was constant. The frequency deviation ranged from 0 to either +/-4MHz or +/-20 MHz depending on whether it was in low altitude or high altitude mode.

The problem with accurately representing this on a tattoo is that the maximum deviation is only 0.2% of the carrier frequency. So if you want the individual carrier waves visible, the tattoo is going to have to be some 500 times as long as the carrier wavelength to show just one cycle of the 130 Hz sawtooth modulation. That is, if the carrier wave length (peak-to-peak) on the tattoo is 3 mm (1/8") the tattoo would have to be 1.5 meters (59 inches) long to show one modulation cycle.

If you want to go the other way and reduce the carrier wavelength spacing so you could fit the tattoo into a reasonable space, you wouldn't be able to see the individual carrier waves. A 15 cm (6") tattoo would need the carrier waves to be .3mm (1/80) of an inch apart to be at accurate scale. Maybe this could be represented by a gray-scale background, I don't know. Sounds like something to discuss with your tattoo artist.

I've attached an image of a carrier frequency modulated by a sawtooth. The scale is wrong for the LM altimeter but it's the correct general shape. It shows 3 and a half modulating cycles. For it to be an accurate representation of the LM signal, there should be some 500 waves in each modulating cycle in the high altitude range and 2000 in the low altitude range. But if instead of perfect accuracy you'd be happy with a non-scale representation, something like that image would be accurate in all other respects. If you want to try the gray-scale representation, it would be black where the waves are closest together, white where they're farthest apart and changing intensity linearly from lightest to darkest. And your tattoo artist would have to advise you on if it would work with your skin color.

For anyone who wants to geek out on the LM landing radar, here's a link to a 1972 NASA document: https://www.nasa.gov/wp-content/uploads/static/history/alsj/ApolloLMRadarTND6849.pdf
« Last Edit: 01/19/2024 12:27 pm by laszlo »

Offline jimothytones

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Re: Apollo Landing Radar
« Reply #3 on: 01/20/2024 05:30 pm »
Wow, thank you. That's amazing information, much more detailed and thorough than I ever hoped for. If you're ever around the FL Space Coast I'd love to buy you a drink or something in return.

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