I have a friend who insists that while the moon landing itself was real, the live broadcast from the moon was faked because NASA did not have the technology to transmit the video signal that far, due to a lack of power.

Does anyone know anything about this theory?

I'm not inclined to believe it, partly because I haven't seen any evidence to back this up, and partly because he's the conspiracy type. Most of my friends like the idea of the Monolith. It comforts them to think someone's out to get them.

This page at Clavius.org may help explain things. In essence, the picture quality was severly cut in order to send it out through the lower-power antenna aboard Eagle. Apollo 12 (I think) had a way bigger antenna, but then Alan Bean went and pointed the camera at the sun #-o .

Fred

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There is a great essay on lunar tv written by one of the guys who worked at Goldstone during Apollo inthe ALSJ:
http://www.hq.nasa.gov/alsj/alsj-TVEssay.html
Some of the old technical documentation can be found at:
http://www.hq.nasa.gov/alsj/alsj-TVDocs.html

In general, you don't need that much power for transmission, when the receiving dish is big enough. During Apollo, transmission of tv signals was nothing new and the bandwidth was much lower than a normal NTSC tv signal.

Harald

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Yup. After switching on the tv, it was decided that the quality when the signal was sent through the steerable s-band antenna (the small dish on the starboard side of the LM) was sufficient and they canceled setting up the deployable s-band, though Armstrong had trained for that and they carried one with the LM (the gallery at http://www.apolloarchive.com/ has plenty of training shots on this subject)
With 12, there was more time for such things and so they set up one. With 15, 16 and 17, tv signals were transmitted through the dish on the rover.

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"Flying in space is risky business, but just staying on this planet is risky business too." - John Young, astronaut

I went through this theory on GLP with IDW (shutter!!!).

Basically people don't understand why commerical TV stations need a megaWatts of power to send a signal 100 miles while the lunar module could send a signal 250,000 miles for about 20 Watts. The answers in a nutshell are:

1) Commerical stations are broadcasting - that mean they are sending a signal in almost all direction so that anyone can pick it up. They have very little gain on their transmitting antenna. The lunar transmissions used point-to-point transmission. This means that both the transmitting and receiving antennae had a lot of gain (power up sent only in the direction required).

2) NASA used specialized transmitting and receiving equipment with, at that time, up to date engineering. The specifications for TV receivers were set in the 1950's for lowest cost of manufactering. Power was cheap to the broadcaster so the receiver did not need to be able to receive really low power signals. The antenna and receivers for the lunar mission were big and expensive. You can not put a 225 foot dish on your roof and you can not cool the front end receiver electronics in your TV ("sorry dear, you can't watch Desperate Housewives - I used up all the liquid Nitrogen watching football" [-X ).

3) NASA also took out all the unnecessary information in the video signal to minimize the bandwidth. This increases the effective signal to noise ratio and increases the range.

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We all should find it hard to believe that televesion was sent from the moon. It is truly amazing. Television from the moon was one of the promises the JPL engineers made for requesting a 210-foot dish at Goldstone.

One way to get your mind around the power issue is to consider that broadcast television signals transmitted from satellites in geosynchronous orbit to dishes on Earth. Analog video signals were typically transmitted from a satellite with a power level somewhere in the 10-watt range over a distance of 22,000 miles to a 12-foot dish on Earth.

The Lunar Module transmitted 18.6 watts of power, which for our purposes is in the same range as many satellites. The Moon is roughly 10 times farther away than the satellite, which means for the same received signal power, you need a dish at the receiving end roughly 10 times bigger. The idea here is to "scoop" the same amount of signal out of the air. 12-feet times 10 means we need a 120-foot dish. Apollo video was often received over 210-foot dishes, so there was adequate signal strength for a reasonably good picture. Sometimes the video had to be picked up by an 85-foot dish. There was still enough margin for a usable television picture, but it meant the picture was noisier.

The transmission via S-band used a parabolic type dish. Unfortunately I don't have the specs on hand, but put in very simple terms, this then _focuses_ the radio wave into a very narrow, yet very "powerful" beam. Of course this is powerful in relation to the original power of the signal. Similar to the way a larger domestic dish allows for more reliable TV reception, so too did the size of the tracking station dish.

Compare the size of the Parkes 90m (I believe) dish to the standard 9m used for regular TV reception in fringe footprint areas. Funny how the conspiracy folk never have a problem with the fact that an 80cm home dish can still pickup satellite TV. In such applications 9m is already considered too big to be economically practical for commercial TV.

Furthermore the tracking stations were purosely built in rural areas to significantly reduce interferance from surrounding electronic devices and extraneous carrier waves. Also take into consideration the Australian tracking stations were in areas where weather for the most part is reliably clear. It come to no surprise either that automobiles and the like were not permitted within a set radius of the recieving dish.

Of all the claims about faking the landings, this one takes the cake in my book, mainly because I work with satellite signals on a daily basis, and ABSOLUTELY nothing from the Apollo TX is a practical and technical impossibilty. Of course anyone who doubts this can happily pick up the phone and more than likely be laughed at by _ANY_ professional satellite communications operator.

What's next from the hoax camp? Claiming the TV was fake because there is no way a cathode ray gun can cause phosphorous screens to glow? Or is EVERY SINGLE tv station both private and national on the planet in on NASA's technological hoodwink?

Dwight
RTL Transmission Control

where's everyone's manners? welcome to the board, Phoenix Feet! (okay, part of why I'm saying this is that I have nothing relevant to contribute.)

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"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"

"You can't erase icing."

"I can't believe it doesn't work! I found it on the internet, man!"

This one really is not rocket science.

Consider a flashlight. It employs a parabolic reflector to direct a great deal of the emitted energy along the axis of the parabola. The same bulb allowed to emit in all directions (i.e., without the reflector) will not illuminate any given spot on the wall with as great intensity as that produced by directing the flashlight toward that spot.

Similarly most visitors to science fairs are familiar with the parabolic dishes placed a great distance apart, allowing people standing at the foci of the respective parabolas to converse over that great distance using much less acoustic energy than would otherwise be indicated for that distance. The Mormon Tabernacle is a building-sized example of this property; I can stand in the balcony and have a conversation with someone in the choir loft more than 50 meters away using nothing louder than my normal speaking voice.

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Clavius Moon Base

antenna gain is inversely proportional to the square of the beamwidth in radians... for a 1 degree beamwidth, the gain due to directivity is 10*log10(4/bw^2) = 41 dBi (1 degree = 2pi/360)... for 0.1 degree beamwidth, the gain is 61 dBi. this means that a 1 W transmitter with a 0.1 degree beamwidth will behave as if it is a 1 MW transmitter due to gain.

of course, there are other factors at play, such as efficiency.

taks

The same folks that think we couldn't have sent an image from the moon think nothing of watching the signal from their tiny Dish TV receiver.

In essence, the picture quality was severly cut in order to send it out through the lower-power antenna aboard Eagle. Apollo 12 (I think) had a way bigger antenna,

Jay's excellent TV primer describes mission planning from early in the program when it was assumed that only 85-foot dishes would be available for receiving TV signals. By the time of Apollo 12, 210-foot dishes were available (except at the Madrid station). Those monstrosities provided a strong enough signal, even with color TV, that the plan was for Apollo 12 to transmit its first EVA through the smaller, steerable antenna. That saved having to send Al Bean back into the LM to switch the transmitter to the larger, erectable antenna.

One can get the impression from Jay's page that color TV through the smaller antenna was not practical, but that was only true when the 85-foot dishes were in consideration. The purpose of that page, of course, is to teach basics and not to wander off into a technical history of Apollo TV.

Further to the other points raised here (which settle the issue decisively), I'd just mention briefly in passing that the Voyager spacecraft are still transmitting data from around 90 AU (about 35,000 times Earth-Moon distance) using 1970's technology and the same receiving dishes.

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Yep. The dishes were beefed up during the 80's to compensate for the Voyagers' decreasing signal strength. They are now 70-meter instead of 64-meter (210-foot) dishes.

The relationship between directivity and gain is exactly the same for loudspeakers, if you assume a perfect conical beam with no "spill" outside the coverage angle.

Real-world speakers, of course, never have brick-wall coverage control, so instead of calculating directivity from coverage angles, sound system designers work with a directivity factor, or Q, derived from measurements made on the specific product in question.

In a free field, you can calculate the on-axis sound intensity from the distance, loudspeaker Q and the acoustic power output of the speaker:

Lp = Lw + 10 log (Q/(4*pi*r^2)

where Lp= SPL in dB referred to 10^-12 W/m^2
Lw = acoustic power output in dB referred to 10^-12W
r= distance in meters

I remember the GLP thread that BillEE mentioned. The examples he gave of how he would calculate the signal power at the input to a receiver from the transmitter power, antenna gains and path loss were an awful lot like the methods I've used to predict what sound levels I could expect from my PA at a given distance.

These aren't terribly difficult ideas- not if I can grasp them. :wink:

Well, Joe, to be honest I think the TV page on Clavius really needs to be augmented with a discussion about the receiver dish diameters.

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Clavius Moon Base

Thanks for all the additional equations behind the way TV signals get "beamed" around. With morning shift and late bed to brain work not good. But now I'm wide awake and sending TV with the very same physical constraints that were used during Apollo, except the TV signal I am sending is made up of 0s and 1s.

Which actually leads me to another related theme. In the days of analogue TV, there was a large amount of tolerance if one did not correctly point the dish at the incoming signal. Digital tends to be alot more unforgiving, ie drift off the correct angle only slightly and you tend to get absolutely no useable TV. Such moments as when Gene Cernan parks the rover and we can see him driving would probably _not_ happen if the video signal was sent today.

Dwight
RTL Transmission Control

PS The updated TV section by Mr Wood over at ALSJ is well worth a read!

known as the waterfall affect. digital is either coming in or not. once the decoder can't verify a packet due to too many errors, it's dropped. that's why PCS cell phones (CDMA) drop out and the older analog phones (TDMA) get staticy (word?) in poor coverage areas.

so, are we geeks because we like this stuff or do we like this stuff because we're geeks?

taks

I can't really say, but considering I get paid to "watch" television I feel a certain superiority to the folk who can't claim cable TV subscription on their income tax form. Actually looking at that comment I feel a certain kindred association with the Comic Book guy out of the Simpsons.

Dwight
RTL Transmission Control Geek

Oh, and welcome, Bill EE. I'm glad _something_ good came out of that nasty mess over at GLP.

I hate the word "static." The public seems convinced that it is a meaningful technical term, and the only acceptable way to describe any sub-par performance of an audio system. Every time I'm called in to fix a problem with the house audio system, or the backstage monitor system, or the Clear-Com communications system, or the radio microphones, or the infared hearing-enhancement system, or even the ancient "Bogen" intercom, the detailed description of the problem is delivered to me with aplomb; "There was static." If I could just get them to let go of that word I might actually get a description I could use.

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