Hunchbacked - An Incoherent Apollo Denier

For some reason, Youtube is a hotbed of conspiracy paranoia, including my personal favorite: people who still argue, that NASA faked the Apollo lunar landing missions. Yes, they're still at it 40+ years after the fact despite overwhelming evidence that the missions were real and no evidence to the contrary that withstands even minimal scrutiny.

I refer to these people with the straightforwardly descriptive term "Apollo deniers". Most of them merely parrot the long-debunked claims of others without really understanding why one shouldn't see stars in daytime lunar photographs, craters under the lunar modules, etc. They exhibit obvious and extreme ignorance of basic scientific and technical principles.

But one Apollo denier, Youtube user hunchbacked, stands out in several ways that personally fascinate me. He is an extremely prolific source of highly original claims [1] of what he calls "incoherences" in the Apollo record. Many involve that staple of Apollo denial, the huge archive of pictures returned by the astronauts from the moon. But hunchbacked also asserts that many Apollo technologies and procedures could not have worked as designed. He further claims that the otherwise competent Apollo designers resented being forced to produce nonworking equipment so they left "whistleblowing" clues in documentation such as block diagrams. [2] [3] And it's his purpose in life to find them!

I was 12 when Apollo 11 landed on the moon. It helped inspire me to get my ham and commercial FCC licenses, two electrical engineering degrees and to pursue a rewarding career in computer and radio communication R&D. And while I don't work professionally in the space field, I have been an AMSAT volunteer for over 30 years and remain very interested in all aspects of space flight. Over the years I've studied a fair number of Apollo systems, understood most of what I read, and never once saw anything that obviously could not work as advertised. So, never being one to pass up a good argument, hunchbacked's strident claims to the contrary naturally intrigued me.

In this note I will examine a representative example of hunchbacked's many absurd claims about Apollo technology: that the FM (frequency modulation) modulator on the Apollo Lunar Module (LM) could not have worked as designed. I will show that not only did it work, it applies standard principles long used outside the Apollo program, specifically FM transmitters in broadcasting and amateur ("ham") radio.

The Apollo Unified S-Band (USB) System

To understand how Apollo used this device, a brief description of the Apollo "Unified S-band" (USB) system is necessary. [4]

The term "S-band" dates to World War 2 and refers to radio frequencies between 2000 and 4000 MHz (2 to 4 GHz) near the low end of the "microwave" region.

The Apollo USB system, operating just below 2300 MHz, was used for all space-to-earth and earth-to-space communications beyond low earth orbit including the S-IVB upper stage of the Saturn IB and Saturn V rockets; the Command and Service Module (CSM); the Lunar Module (LM); the Apollo Lunar Surface Equipment Packages (ALSEPs); the Lunar Communications Relay Unit (LCRU) on the Lunar Rover (LRV) on Apollos 15-17; and the lunar subsatellites on Apollo 15-16. [5]

As the name suggests, the Apollo USB combined several functions onto a single radio link in each direction. Its primary mode used Phase Modulation (PM) with a low modulation index to carry intermittent distance measurements (ranging) and continuous Doppler velocity tracking, voice communications, biomedical and vehicle telemetry.

But the Apollo PM link budget could not handle one important form of communication ‐ television ‐ even with huge receiving antennas. [6] For TV, Apollo used wideband frequency modulation (FM) as did TV on communication satellites until their eventual switch to digital. Apollo also used FM for high speed tape recorder "dumps", another wideband signal with properties similar to video.

Wideband FM could still carry voice and telemetry but not Doppler tracking because of its "noncoherent" nature. The CSM carried separate PM and FM transmitters so it could send TV (or dump its tape recorder) in flight without loss of tracking. The LM had only one transmitter able to transmit PM or FM but not both at the same time. Because the LM sent TV only from the lunar surface, the associated loss of tracking was not a serious drawback.

FM had another drawback in that it required a large receiving antenna even when TV wasn't being transmitted, so the LM (and later the LRV) generally switched back to PM whenever TV wasn't being sent. PM also had to be used (without TV) when transmitting with a low gain antenna, as from the LRV in motion and from Apollo 16's LM Orion due to the failure of its steerable S-band antenna.

The LM's PLL FM Modulator

This is the block diagram of the Apollo LM FM modulator that hunchbacked claims cannot work. [7]

This is a wholly conventional Phase-Locked Loop (PLL)-type FM modulator just like those used in many commercial FM broadcast radio transmitters and amateur (ham) FM communication transceivers for at least the past 40 years.

Here's how the LM circuit works. The 4.7 MHz crystal oscillator (actual frequency 4 755 208.333 Hz), isolation amplifier, converter and ÷512 divider produces a reference frequency at 9 287.516 276 042 Hz that is fed to the first input port of the phase detector.

The 76 MHz voltage controlled oscillator (actual nominal frequency 76 083 333.33 Hz), amplified by the power limiter amplifier and passed through the impedance-matching network at the top, becomes the FM-modulated output to the S-band transmitter, not shown. In the S-band transmitter, this VCO signal is frequency-multiplied by a factor of 30 to produce the actual S-band LM transmitted carrier frequency of 2282.5 MHz.

The VCO output is also buffered by an isolation amplifier, frequency divided by 4, buffered and frequency divided again by 2048 for an overall frequency division ratio of 8192 to produce a nominal 9 287.516 276 042 Hz for the second port of the phase detector. Note that this is exactly the same frequency generated by the crystal oscillator and its divider and fed to the first port of the phase detector.

So when the loop is locked without any modulation, the VCO runs at exactly 16 times the frequency of the crystal oscillator to produce a stable, unmodulated S-band carrier at 2282.5 MHz. Any static phase difference between the two input ports of the phase detector produces a DC voltage at the output that is amplified and low-pass-filtered before being fed back to the control input of the VCO to keep it exactly on the correct frequency, locked to the crystal by a frequency ratio of 8192÷512 = 16:1.

Any difference in frequency (which can only be when the loop is out of lock) results in a low frequency AC signal appearing at the phase detector output. The low pass filter must also pass this signal to the VCO for the loop to achieve initial lock.

This is the main consideration in the choice of cutoff frequency for the LPF; it must ensure that the loop locks up at power-up despite worst case tolerances in the VCO's frequency-determining components. Because the VCO frequency is divided by a large factor (8192:1) any frequency error is decreased by the same large ratio so even a fairly large error appears as a relatively low frequency in the phase detector output, allowing a low cutoff frequency to be chosen for the LPF. Once the loop is locked, the LPF only need pass DC to keep the loop in lock even if the VCO components drift further.

Frequency Modulating the PLL

The TV signal to be transmitted to earth enters this diagram in the lower left corner. It occupies a "baseband" frequency range from just above 0 Hz to about 1 MHz. Not shown in this diagram are the same subcarrier oscillators and modulators used for downlink voice and telemetry in the PM mode; the voice FMs (frequency modulates) a 1.25 MHz subcarrier and the telemetry BPSKs (binary phase shift keys) a 1.024 MHz subcarrier. These two modulated subcarriers are filtered and summed with the baseband video and sent to the control input of the VCO where they are summed with the DC control voltage from the phase detector and low pass filter.

Because the modulating signals are fed directly to the VCO, they vary its output frequency to produce the desired FM. The frequency deviation need not be large because it will be multiplied by 30x along with the carrier frequency in the S-band transmitter.

But there's a complication ‐ the FM modulation, divided by 8192, also appears at the phase detector and at the phase detector output. If it passes the low pass filter (LPF) and reaches the VCO, it will be treated as unwanted drift in the VCO and the loop will track it out. This is not what we want.

But if a low enough cutoff frequency for the LPF is chosen, then only very low frequencies in the modulating signal can get through the LPF to the VCO. The rest of the modulating signal is blocked by the LPF so they are not removed from the VCO. Since low frequency signals won't get through the modulator anyway, they are removed by the high pass filter at the modulator input.

Voice and telemetry are completely unaffected by the LPF because they're on subcarriers (1.25 MHz for voice, 1.024 MHz for telemetry) far above the LPF cutoff. The video signal may contain a small amount of very low frequency power that will be removed by the highpass filer (or tracked out by the PLL) but this is not a problem because those very low frequencies are easily restored at the receiver by use of a circuit called, naturally enough, a "DC restorer" or "clamper" that uses the video sync pulses as references to restore (or clamp) the correct DC levels in the output signal. They are common in analog TV broadcast receivers because their AM modulators, for various reasons, don't pass modulation all the way down to 0 Hz either.

The ARRL FM Modulator

To show the totally conventional nature of this circuit, I now refer to the classical ham radio technical reference, The ARRL Radio Amateur's Handbook.

Page 10.11 of the 2012 edition of the Handbook contains the following diagram:

Although simplified, the overall structure is identical to the LM FM modulator.

The Reference Signal input to the first port of the Phase Detector corresponds to the 4.7 MHz crystal oscillator, amplifier, converter and ÷512 divider in the LM modulator.

The Voltage Controlled Oscillator here, as in the LM, provides both the FM output signal and the second input to the Phase Detector.

Here, as in the LM, the output of the Phase Detector is amplified and low-pass filtered in the Loop Filter before being fed back to the control input of the VCO.

Here, as in the LM, the Modulation Input is summed with the output of the Loop (low pass) Filter as it is sent to the input of the VCO.

The ARRL text gives an identical description of the modulator's operation, including the need for the loop filter to cut off below the lowest significant frequency component in the modulating signal:

Consider what happens when we want to frequency-modulate a phase-locked-loop-synthesized transmitted signal. Fig 10.13 shows the block diagram of a PLL frequency modulator. Normally, we modulate a PLL's VCO because it's the easy thing to do. As long as our modulating frequency results in frequency excursions too fast for the PLL to follow and correct ‐ that is, as long as our modulating frequency is outside the PLL's loop bandwidth ‐ we achieve the FM we seek. Trying to modulate a dc level by pushing the VCO to a particular frequency and holding it there fails, however, because a PLL's loop response includes dc. The loop, therefore, detects the modulation's dc component as a correctable error and "fixes" it.

The ARRL goes on to explain that a modulation frequency response below the loop filter cutoff can be obtained, if necessary, by modulating the reference oscillator so that the loop will track it. In that case, any modulating signals above the loop cutoff will be removed before they can reach the VCO to provide the modulation we want.

I have seen more recent designs that perform "two point modulation", i.e., modulate the VCO directly for high frequency response and the reference oscillator for low frequency response. This is only necessary for baseband NRZ data signals with significant low frequency energy. It was not necessary for the LM FM downlink as the voice and telemetry were on high frequency subcarriers and the DC information removed from the video could easily be restored at the receiver.


The bottom line is that, contrary to hunchbacked's incompetent assertion, there is absolutely nothing wrong with the FM modulator on the Apollo Lunar Module. It uses a conventional PLL design that has been widely used in FM transmitters of all kinds for decades.


[1] Two hundred and twenty nine (229) Youtube video uploads to date!

[2] Hunchbacked's "whistleblowing" claims are inconsistent with what I call the "compartmentalization pixie dust" argument popular of late with other Apollo deniers. Finally recognizing the absurdity of maintaining the secrecy of a conspiracy involving 400,000 people for over 40 years, many now claim that only a very few people actually "needed to know" that Apollo was a scam. The great majority of workers ‐ including the engineers ‐ did not have to know, and so labored in blissful ignorance. But "whistle blowing" engineers very obviously had to know they were whistleblowers!

[3] Hunchbacked claims to be a practicing industrial engineer with a degree from École Nationale Supérieure de l'Aéronautique et de l'Espace (National High School of Aeronautics and Space) in France. If true, he certainly doesn't reflect well on his alma mater.

[4] The main reference on the USB is Proceedings of the Apollo Unified S-band Technical Conference July 14-15 1965, NASA SP-87, available along with many other Apollo technical documents from the NASA Technical Reports Server. Would NASA make so many documents openly available if they revealed evidence of a hoax?

[5] Today, partly because of Apollo's pioneering use, the S-band is a hotbed of activity. Apollo's frequencies are still heavily used for space communications, and frequencies just above it in the 2400 to 2483 MHz band are now widely used by microwave ovens, cordless phones and 802.11 b/g "WiFi" computer networks. In North America, direct radio broadcasting (e.g., Sirius/XM) transmits in the 2310 to 2360 MHz band.

[6] The 64-meter Parkes radio telescope and its staff, used during the Apollo 11 EVA, were the subject of the wonderful Australian movie The Dish.

[7] Found in figure 2.7-6 of S-band Transmitter ‐ Block Diagram on page 2.7-18 of the document Apollo Operations Handbook, Lunar Module, LM 10 and Subsequent, Volume I, Subsystems Data, LMA790-3-LM 10, dated 1 April 1971. The diagram appears on page 424 of the pdf file available through the NASA Technical Reports Server.

Last modified: 31 January 2014