Greeting earthlings. If you follow the channel,
you know that we love all things Apollo. And that during our last visit to Steve Jurvetson’s amazing
space collection, we were given the opportunity to take two holy boxes of Apollo electronics to
our lab for a deeper look. These are from the command module. We believe these are the S-band
transponder and the power amplifier. Oh boy, oh boy. These are the boxes that brought you
voice, data and live TV from the moon, and should be early masterpieces of microwave electronics,
the blackest of black arts in analog electronics. Among the many technical advances which
are the legacy of the Apollo program, three stand out that have
influenced technology until today. The first one is the most obvious:
the colossal space rocketry, which considerably expanded the US space
industry and led to the Shuttle program. The second one is arguably the Apollo guidance
computer, which took the bold decision to use the first integrated circuits, just a few
weeks after the first 30 pre-production samples became available. This probably had the
largest economic impact, jump starting the US microelectronics industry and the rise of Silicon
Valley, which legacy has carried on until today. But today we’ll take a look at a third and less
obvious one: microwave telecommunications. The Apollo program stretched the limits
of what was then thought possible, achieving live TV transmission at lunar distances,
in black and white at first, then even in color. This was done over the Unified S-Band link, as it
was called. This single 2 GHz link carried voice and an array data formats in both directions,
as well as live television, while at the same time measuring position and speed of the
spacecraft to an astounding degree of precision. How was this possibly done with 1960’s technology? Well, we are about to find out, as we
are planning on opening up these boxes and have a looksy inside. And you know
us - power them up again if we can. Unfortunately, my modern camera electronics
do not have a space level of reliability, and due to an unfortunate malfunction,
I lost all the sound from the footage I took while Mike Stewart, Master Ken and I
we were opening them for the first time. So you’ll have to endure my french accented
voiceover on a background of soothing game music. For fun, we put them on the
scale. The amplifier was 15 kg, or about 33 pounds, and the transponder
did not fare much better at 14 kg, or about 30 British Archaic Weight Units. And
that’s not because of the beefy shells, as they were made surprisingly light and thin magnesium.
It’s just the weight of the electronics inside. But before we look at these
heavy boxes in more detail, let me explain how the Unified
S-Band bidirectional link worked. At its core, the USB link, as it was
called, is primarily a ranging system. NASA JPL had previously developed it for
its early solar system exploration probes: the ranger probes in 1961, the Mariners
probes to Venus in 1962 and to Mars in 1964. Let’s say that we have a good
looking spacecraft really far away, and we want to find out exactly how
far it is, and how fast it travels. For that, we use a giant antenna, 26m in diameter,
which is 82 feet in archaic measurements units. No matter what your favorite measurement
system is, it is a big antenna. They even had a 210 ft antenna at Goldstone in CA and another one at Parkes in Australia (that’s a
whopping 64 m). Both were considered backup but came in handy on the Apollo 11 landing
when communication difficulties arose. We first send a microwave carrier
at exactly 2.10640625 GHz, as derived from an atomic clock. We’ll call this
reference frequency f0. This carrier signal is phase modulated by a pseudo random code called the
ranging code, which we’ll have to explain later. This signal is received at the CSM by the
transponder box. Because the spaceship is traveling at such high speed, and also because
the ground station is rotating on earth, the frequency it sees is not f0. It has been
shifted a little bit by the doppler effect. The shift is on the order of a few 10’s of kHz, which is small compared to the 2GHz carrier
frequency, but still fairly easy to measure. In any case, our receiver locks on, and
tracks this received frequency very exactly using a Phase Locked Loop, a clever circuit
which I have explained in another video. It then translates this frequency by the ratio
of 240 over 221, using frequency synthesis techniques which I have also explained in the same
video. The new frequency, at around 2.2875 GHz, give or take the doppler shift, is amplified
using our amplifier box, and sent back to earth. At the same time, the receiver demodulates the
ranging code from the incoming uplink, and the transmitter remodulates it on the downlink.
It does what is called a data turnaround. The box that does this combination of receiving,
frequency shifting, and retransmitting the same data it received, is called a transponder, hence
the name of our holy Apollo box. Think of it as an assisted radar technique, where the target
helps in reflecting the interrogating signal. The ground now receives the
shifted signal, and locks onto it. Note that once again, the signal has been
shifted by doppler. So the signal it gets has what is called a two-way doppler shift,
which frequency is indicated in the formula. The earth station has now everything
it needs to track the spacecraft: - it gets the distance by figuring out the
time delay in the pseudo random ranging code - it gets the radial speed by
measuring the two-way doppler shift - and it gets the position
by the antenna pointing. This ranging system is truly the backbone of the
whole thing. Once two-way locking is accomplished (and you’ll hear it being called out on the voice
loops), voice and data is added on subcarriers in both directions, also using complicated
methods that we’ll have to get into later. But that’s not all. A third signal is transmitted
down at yet another frequency, on 2.2725 GHz. This carrier uses the more
traditional FM modulation technique, and transmits the famous TV signal as well as
recorded data and voice from the Command Module and also recorded data and voice
relayed from the LM. It is used to retransmit the data that was gathered while
the spacecrafts were not in contact with earth, when they were behind the moon for example, or
if the LM could not contact the earth directly. Now if this is not complicated enough, multiply
this by three, as there was another S-Band link for the LM, and another for the rocket, operating
on their own set of frequencies. All three could be operated at the same time. No wonder it
took a full control center to manage all that. If there is just one thing to remember
from this complicated explanation, it’s that there are 3 frequencies involved,
and two carrier modulation techniques: A PM uplink at 2.1064 GHz A PM downlink at 2.2875 GHz
An FM downlink at 2.2725 GHz These will be coming up all the time when
we look at the detail of these boxes, and God forbid, try to make them work again. But for now, let’s open up these bad boys. There
are many screws involved. The reason is that the two boxes are hermetic. They were filled with
dry nitrogen gas, and each box hads a fill nipple. So here is the one for the transponder, and here is another corresponding
one on the amplifier right here. And we were please to hear the hiss
of the nitrogen getting out when we depressurized both of them - the protective
nitrogen had stayed in there to this day, and the chances that the electronics will
be in pristine condition were very high. Both of our units were conspicuously
adorned with the NOT FOR FLIGHT stickers. But these were definitely the
real NASA stuff used until 1976. The amplifier had the early serial number
of 6, so these were likely early units used for and qualification, and then kept on the
ground for further testing and experimentation. Master Ken, working on the amplifier was
the first one to cross the finish line, as, believe it or not, he
had less screws than Mike. So here's the big reveal. Ooh,
voila! And here's the full amplifier, big transformer, danger high voltage, don't stick
your fingers in there! A whole maze of coaxes and the connectors to the
antenna and the transponder and immediately you - oh there's one that's
connected to nothing, but actually that's normal. And first we suspected the traveling wave
tubes, which are the core amplifying element in the amplifier, were hidden in this "danger high
voltage" thing and powered by the big transformer, but as you'll see it turned
out to be quite different. It was then Mike’s turn for the
big transponder reveal. Ooh, aah, serious microwave electronic goodness in there.
The many RF coax connections between the different modules are immediately apparent. But you can
also spot immediately that the transponder is redundant. There is two of most everything,
although not all. The amplifier is redundant too, we’ll get to it, but it was less obvious
when looking at it for the first time. The complexity of the wiring is a
little bit overwhelming at first, but we eventually figured out pretty
much what goes on every one of those coax cables. But for now let's admire this.
This is made by Motorola, the transponder that is. The other box, the amplifier, is
made by Collins of today Rockwell Collins. Before we dive into more explanations
about what we see, let’s give a break to our neurons and just admire this hardware.
Here is the Collins amplifier. Collins was the prime contractor for the Apollo communication
system. And here is the Motorola transponder. So unfortunately, I lost the sound of the
footage that we took when we actually opened these boxes live. But what are those, Mike, what
are we looking at? [Mike] So first on the table closest to me is the S-band transponder
from an Apollo Block 2 Command Module, and then on the table right behind
that is the S-band power amplifier. [Marc] This is the main radio, right, that
did all the communications from the moon back to earth. [Mike] Yep! [Marc] So this is
the transponder, means transmitter-receiver. Because it's a weird animal: in order to gain
propagation distance when they do ranging, they send the signal, it gets received, it gets
shifted in frequency, and gets retransmitted. [Mike] Right. [Marc] And you gain an enormous
amount of dB when you do that rather than simple reflection. And the box behind it is just
a power amplifier, so it just works in transmit. And raises power to the incredible
level of 2.8 Watts in low power mode, and we said 11 point something... [Mike]
Yeah 11.6 Watts minimum. [Marc] 11.6 Watts at full power. So they just used 11.6 Watts to
retransmit TV from the moon - which blows my mind. And of course the reason they do that - well,
first is that the amplifier back there, is a tube based animal. [Mike] Right [Marc] And there was no
solid state device that could amplify microwave - this is in the microwave band at 2 gigahertz -
and there was just nothing available but tubes. And then it gets heavy I think. And they just
stopped when it was too heavy. [Mike] Right, yeah, both of these boxes are right around 32
pounds, I think. [Marc] Yeah, we weighed them right? Actually when you weigh them you
don't expect them to be that heavy. [Mike] Right, yeah. [Marc] They feel super heavy. So,
they beat it by putting an absolutely giant antenna on the ground and also
blasting it with power going up. So those two are linked by this cable. There
[are] three cables and I can't remember, there's a one that's the TV and the two other must be the phase modulation or whatever? I have to look it
up, actually it's over here. They have... Oh no! One is PM at this frequency, 2.28
gigahertz, one is FM, that's only the TV. So the main mode is phase modulated with all the
data and voice, and the TV is just - it's, I would say, it's not an afterthought, but it's
not redundant as the main mode. So we have the PM: that's J3. So that's the main output of the
radio and towards the amplifier. Then J2, that's when they do the TV in FM, goes over here,
transmitted on a different frequency. [Mike] And J4 is here. [Marc] That's the receive. Okay,
so that goes [to the] receive over here. Because although the receive frequency is
not amplified at all by this box, it goes through the triplexer. So there's
an output for the antenna, that must be J5,. And the antenna goes in the triplexer, which
is this big box. It says "danger high voltage", and that threw us off, because we thought it was
high voltage in the box. [Mike] Right. [Marc] But there is not! [Mike] Right! [Marc] This is just
a frequency filter. And just based on frequency, it kicks off the the received
signal straight to the receiver. And it gets powered by the output of the
traveling wave tubes, which are the amplifiers, and [it] goes back to the antenna in the other
direction. So if you see it in the diagram, here are the antennas in the aircraft. It comes here, so this is the directional. The
triplexer will kick the receive frequency this way, and it will ingest the FM and the PM
frequency that way. And it's all combined because they are all at different frequencies. So
the receive is not done at the same frequency as the transmit, right? [Mike] Right. [Marc]
So they don't interfere with each other. On the receiver, same thing, you
have a combination of redundant mains, and you have a single FM. And this has
actually the transmitter and the receiver. So the receives comes - this is receive, starts here. I
disconnected it because I wanted to find out what kind of connector this was. Fortunately this is
an SMA so we're going to be able to work with it. Then it goes into two identical receivers. And then on the transmitter, you have a
redundant PM section. So this [two are] the same. I think the transmitter is just this and
this. And then you have a single FM section, which is the FM exciter and the FM power
amplifier, which only puts out 100 milliwatts. So if something goes wrong in the FM that's it,
you can't reuse it. If something goes wrong with the PM you can switch to the other bank. And
then if all else fails there's a morse key, it goes tut tut tutut tut tut on the front panel.
No, that's not on the front panel! That's on them, it's on the push to talk. They can transmit morse. Alright, I think this is heavy enough for a first
episode. The Apollo communications system is such a complicated thing, it’s going to take us a while
to explain all of that. We’ll go into the details of each of these boxes, explain the rats nest of
coax cables in the amplifier, reverse engineer the transponder, and try to make everything work
again over the next episodes. See you then!
Not quite on topic, sorry, just too awesome to pass.
Icom 7300 & MFJ PSU makes a quick cameo at the end of the video as part of the test equipment setup :)