Inside the Globus INK: a mechanical navigation computer for Soviet spaceflight

The Soviet space program used completely different controls and instruments from American spacecraft. One of the most interesting navigation instruments onboard Soyuz spacecraft was the Globus, which used a rotating globe to indicate the spacecraft's position above the Earth. This navigation instrument was an electromechanical analog computer that used an elaborate system of gears, cams, and differentials to compute the spacecraft's position. Officially, the unit was called a "space navigation indicator" with the Russian acronym ИНК (INK),1 but I'll use the more descriptive nickname "Globus".

The INK-2S "Globus" space navigation indicator. Coincidentally, the latitude indicator matches the Ukrainian flag.

We recently received a Globus from a collector and opened it up for repair and reverse engineering. In this blog post, I explain how it operated, show its internal mechanisms, and describe what I've learned so far from reverse engineering. The photo below gives an idea of the mechanical complexity of this device, which also has a few relays, solenoids, and other electrical components.

Side view of the Globus INK. Click this (or any other image) for a larger version.

Functionality

The primary purpose of the Globus was to indicate the spacecraft's position. The globe rotated while fixed crosshairs on the plastic dome indicated the spacecraft's position. Thus, the globe matched the cosmonauts' view of the Earth, allowing them to confirm their location. Latitude and longitude dials next to the globe provided a numerical indication of location. Meanwhile, a light/shadow dial at the bottom showed when the spacecraft would be illuminated by the sun or in shadow, important information for docking. The Globus also had an orbit counter, indicating the number of orbits.

The Globus had a second mode, indicating where the spacecraft would land if they fired the retrorockets to initiate a landing. Flipping a switch caused the globe to rotate until the landing position was under the crosshairs and the cosmonauts could evaluate the suitability of this landing site.

The cosmonauts configured the Globus by turning knobs to set the spacecraft's initial position and orbital period. From there, the Globus electromechanically tracked the orbit. Unlike the Apollo Guidance Computer, the Globus did not receive navigational information from an inertial measurement unit (IMU) or other sources, so it did not know the spacecraft's real position. It was purely a display of the predicted position.

A close-up of the complex gear trains in the Globus.

The globe

The globe itself is detailed for its small size, showing terrain features such as mountains, lakes, and rivers. These features on the map helped cosmonauts compare their position with the geographic features they could see on Earth. These features were also important for selecting a landing site, so they could see what kind of terrain they would be landing on. For the most part, the map doesn't show political boundaries, except for thick red and purple lines. This line shows the borders of the USSR, as well as the boundaries between communist and non-communist countries, also important for selecting a landing site. The globe also has numbered circles 1 through 8 that indicate radio sites for communication with the spacecraft, allowing the cosmonauts to determine what ground stations could be contacted.

A view of the globe showing Asia.

Controlling the globe

On seeing the Globus, one might wonder how the globe is rotated. It may seem that the globe must be free-floating so it can rotate in two axes. Instead, a clever mechanism attaches the globe to the unit. The key is that the globe's equator is a solid piece of metal that rotates around the horizontal axis of the unit. A second gear mechanism inside the globe rotates the globe around the North-South axis. The two rotations are controlled by concentric shafts that are fixed to the unit, allowing two rotational degrees of freedom through fixed shafts.

The photo below shows the frame that holds and controls the globe. The dotted axis is fixed horizontally in the unit and rotations are fed through the two gears at the left. One gear rotates the globe and frame around the dotted axis, while the gear train causes the globe to rotate around the vertical polar axis (while the equator remains fixed).

The axis of the globe is at 51.8° to support that orbital inclination.

The angle above is 51.8° which is very important: this is the inclination of the standard Soyuz orbit. As a result, simply rotating the globe around the dotted line causes the crosshair to trace the standard orbit.2 Rotating the two halves of the globe around the poles yields the different 51.8° orbits over the Earth's surface as the Earth rotates. (Why 51.8 degrees? The Baikonur Cosmodrome, launching point for Soyuz, is at 45.97° N latitude, so 45.97° would be the most efficient inclination. However, to prevent the launch from passing over western China, the rocket must be angled towards the north, resulting in 51.8° (details).)

One important consequence of this design is that the orbital inclination is fixed by the angle of the globe mechanism. Different Globus units needed to be built for different orbits. Moreover, this design only handles circular orbits, making it useless during orbit changes such as rendezvous and docking. These were such significant limitations that some cosmonauts wanted the Globus removed from the control panel, but it remained until it was replaced by a computer display in Soyuz-TMA (2002).3

A closeup of the gears that drive the motion of the two halves of the globe around the polar axis, leaving the equator fixed.

This Globus had clearly suffered some damage. The back of the case had some large dents.7 More importantly, the globe's shaft had been knocked loose from its proper position and no longer meshed with the gears. This also put a gouge into Africa, where the globe hit internal components. Fortunately, CuriousMarc was able to get the globe back into position while ensuring that the gears had the right timing. (Putting the globe back arbitrarily would mess up the latitude and longitude.)

Orbital speed and the "cone"

An orbit of Soyuz takes approximately 90 minutes, but the time varies according to altitude.4 The Globus has an adjustment knob (below) to adjust the orbital period in minutes, tenths of minutes, and hundredths of minutes. The outer knob has three positions and points to the digit that changes when the inner knob is turned. The mechanism provides an adjustment of ±5 minutes from the nominal period of 91.85 minutes.3

The control to adjust the orbital period.

The orbital speed feature is implemented by increasing or decreasing the speed at which the globe rotates around the orbital (horizontal) axis. Generating a variable speed is tricky, since the Globus runs on fixed 1-hertz pulses. The solution is to start with a base speed, and then add three increments: one for the minutes setting, one for the tenths-of-minutes setting, and one for the hundredths-of-minutes setting.5 These four speeds are added (as shaft rotation speeds) using obtain the overall rotation speed.

The Globus uses numerous differential gears to add or subtract rotations. The photo below shows two sets of differential gears, side-by-side.

Two differential gears in the Globus.

The problem is how to generate these three variable rotation speeds from the fixed input. The solution is a special cam, shaped like a cone with a spiral cross-section. Three followers ride on the cam, so as the cam rotates, the follower is pushed outward and rotates on its shaft. If the follower is near the narrow part of the cam, it moves over a small distance and has a small rotation. But if the follower is near the wide part of the cam, it moves a larger distance and has a larger rotation. Thus, by moving the follower to a particular point on the cam, the rotational speed of the follower is selected.

A diagram showing the orbital speed control mechanism. The cone has three followers, but only two are visible from this angle. The "transmission" gears are moved in and out by the outer knob to select which follower is adjusted by the inner knob.

Obviously, the cam can't spiral out forever. Instead, at the end of one revolution, its cross-section drops back sharply to the starting diameter. This causes the follower to snap back to its original position. To prevent this from jerking the globe backward, the follower is connected to the differential gearing via a slip clutch and ratchet. Thus, when the follower snaps back, the ratchet holds the drive shaft stationary. The drive shaft then continues its rotation as the follower starts cycling out again. Thus, the output is a (mostly) smooth rotation at a speed that depends on the position of the follower.

Latitude and longitude

The indicators at the left and the top of the globe indicate the spacecraft's latitude and longitude respectively. These are defined by surprisingly complex functions, generated by the orbit's projection onto the globe.6

The latitude and longitude functions are implemented through the shape of metal cams; the photo below shows the longitude mechanism. Each function has two cams: one cam implements the desired function, while the other cam has the "opposite" shape to maintain tension on the jaw-like tracking mechanism.

The cam mechanism to compute longitude.

The latitude cam drives the latitude dial, causing it to oscillate between 51.8° N and 51.8° S. Longitude is more complicated because the Earth's rotation causes it to constantly vary. The longitude output on the dial is produced by adding the cam's value to the Earth's rotation through a differential gear.

Light and shadow

The Globus has an indicator to show when the spacecraft will enter light or shadow. The dial consists of two concentric dials, configured by the two knobs. These dials move with the spacecraft's orbit, while the red legend remains fixed. I think these dials are geared to the longitude dial, but I'm still investigating.

The light and shadow indicator is controlled by two knobs.

The landing location mechanism

The Globus can display where the spacecraft would land if you started a re-entry burn now, with an accuracy of 150 km. This is computed by projecting the current orbit forward by a partial orbit, depending on how long it would take to land. The cosmonaut specifies this value by the "landing angle", which indicates this fraction of an orbit as an angle. An electroluminescent indicator in the upper-left corner of the unit shows "Место посадки" (Landing place) to indicate this mode.

The landing angle control.

To obtain the landing position, a motor spins the globe until it is stopped after rotating through the specified angle. The mechanism to implement this is shown below. The adjustment knob on the panel turns the adjustment shaft which moves the limit switch to the desired angle via the worm gear. The wiring is wrapped around a wheel so the wiring stays controlled during this movement. When the drive motor is activated, it rotates the globe and the swing arm at the same time. Since the motor stops when the swing arm hits the angle limit switch, the globe rotates through the desired angle. The fixed limit switch is used when returning the globe's position to its regular, orbital position.

The landing angle function uses a complex mechanism.

The landing location mode is activated by a three-position rotary switch. The first position "МП" (место посадки, landing site) selects the landing site, the second position "З" (Земля, Earth) shows the position over the Earth, and the third position "Откл" (off) undoes the landing angle rotation and turns off the mechanism.

The rotary switch to select the landing angle mode.

Electronics

Although the Globus is mostly mechanical, it has an electronics board with four relays and a transistor, as well as resistors and diodes. I think that most of these relays control the landing location mechanism, driving the motor forward or backward and stopping at the limit switch. The diodes are flyback diodes, two diodes in series across each relay coil to eliminate the inductive kick when the coil is disconnected.

The electronics circuit board.

A 360° potentiometer (below) converts the spacecraft's orbital position into a voltage. Sources indicate that the Globus provides this voltage signal to other units on the spacecraft. My theory is that the transistor on the electronics board amplifies this voltage, but I am still investigating.

The potentiometer converts the orbital position into a voltage. To the right is the cam that produces the longitude display. Antarctica is visible on the globe.

The photo below shows the multiple wiring bundles in the Globus, at the front and the left. The electronics board is at the front right. The Globus contains a surprising amount of wiring for a device that is mostly mechanical. Inconveniently, all the wires to the box's external connector (upper left) were cut.7 Perhaps this was part of decommissioning the unit. However, one of the screws on the case is covered with a tamper-resistant wax seal with insignia, and this wax seal was intact. This indicates that the unit was officially re-sealed after cutting the wires, which doesn't make sense for a decommissioned unit.

This view shows the back and underside of the Globus. The round connector at the back left provided the interface with the rest of the spacecraft. The black wires under this connector were all cut.

The drive solenoids

The unit is driven by two ratchet solenoids: one for the orbital rotation and one for the Earth's rotation. These solenoids take 27-volt pulses at 1 hertz.3 Each pulse causes the solenoid to advance the gear by one tooth; a pawl keeps the gear from slipping back. These small rotations drive the gears throughout the Globus and result in a tiny movement of the globe.

One of the driving solenoids in the Globus. The wheels to indicate orbital time are underneath.

The other driving solenoid in the Globus.

Apollo-Soyuz

If you look closely at the globe, it has a bunch of pink dots added, along with three-letter labels in Latin (not Cyrillic) characters.8 In the photo below, you can see GDS (Goldstone), MIL (Merritt Island), BDA (Bermuda), and NFL (Newfoundland). These are NASA tracking sites, which implies that this Globus was built for the Apollo-Soyuz Test Project, a 1975 mission where an Apollo spacecraft docked with a Soyuz capsule.

North America as it appears on the globe. The US border is marked in red. The selection of cities seems a bit random, such as El Paso as the only western city until the coast.

Further confirmation of the Apollo-Soyuz connection is the VAN sticker in the middle of the Pacific Ocean (not visible above). The USNS Vanguard was a NASA tracking ship that was used in the Apollo program to fill in gaps in radio coverage. It was an oil tanker from World War II, converted postwar to a missile tracking ship and then used for Apollo. In the photo below, you can see the large tracking antennas on its deck. During the Apollo-Soyuz mission, Vanguard was stationed at 25 S 155 W for the Apollo-Soyuz mission, exactly matching the location of the VAN dot on the globe.

The USNS Vanguard with a NASA C-54 plane overhead. (source).

History

The Globus has a long history, back to the beginnings of Soviet crewed spaceflight. The first version was simpler and had the Russian acronym ИМП (IMP).9 Development of the IMP started in 1960 for the Vostok (1961) and Voshod (1964) spaceflights.

The Globus IMP. Photo from Francoisguay (CC BY-SA 3.0).

The basic functions of the earlier Globus IMP are similar to the INK, showing the spacecraft's position and the landing position. It has an orbit counter in the lower right. The latitude and longitude displays at the top were added for the Voshod flights. The large correction knob allows the orbital period to be adjusted. The main differences are that the IMP doesn't have a display at the bottom for sun and shade and doesn't have a control to set the landing angle.9 Unlike the INK, the mode (orbit vs landing position) was selected by external switches, rather than a switch on the unit.

The more complex INK model (described in this blog post) was created for the Soyuz flights, starting in 1967. It was part of the "Sirius" information display system (IDS). The Neptun IDS used on Soyuz-T (1976) and the Neptun-M for Soyuz-TM (1986) modernized much of the console but kept the Globus INK. The photo below shows the Globus mounted in the upper-right of a Soyuz-TM console.

The Neptun-M IDS for the Soyuz-TM (source).

The Soyuz-TMA (2002) upgraded to the Neptun-ME system3 which used digital display screens. In particular, the Globus was replaced with the graphical display below.

A computer display from the Neptune-ME display system used in the Soyuz-TMA spaceship. The Soyuz consoles are much simpler than the Apollo or Space Shuttle consoles, and built with completely different design principles. From Information Display Systems for Soyuz Spaceships.

Conclusions

The Globus INK is a remarkable piece of machinery, an analog computer that calculates orbits through an intricate system of gears, cams, and differentials. It provided cosmonauts with a high-resolution, full-color display of the spacecraft's position, way beyond what an electronic space computer could provide in the 1960s.

Although the Globus is an amazing piece of mechanical computation, its functionality is limited. Its parameters must be manually configured: the spacecraft's starting position, the orbital speed, the light/shadow regions, and the landing angle. It doesn't take any external guidance inputs, such as an IMU (inertial measurement unit), so it's not particularly accurate. Finally, it only supports a circular orbit at a fixed angle. While the more modern digital display lacks the physical charm of a rotating globe, the digital solution provides much more capability.

I plan to continue reverse-engineering the Globus and hope to get it operational, so follow me on Twitter @kenshirriff or RSS for updates. I've also started experimenting with Mastodon recently as @oldbytes.space@kenshirriff. Many thanks to Marcel for providing the Globus. Thanks to Stack Overflow for orbit information and my Twitter followers for translation assistance.

I should give a disclaimer that I am still reverse-engineering the Globus, so what I described is subject to change. Also, I don't read Russian, so any errors are the fault of Google Translate. :-)

With the case removed, the complex internals of the Globus are visible.

Notes and references

1. In Russian, the name for the device is "Индикатор Навигационный Космический" abbreviated as ИНК (INK). This translates to "space navigation indicator." The name Globus (Глобус) seems to be a nickname, and I suspect it's more commonly used in English than Russian. ↩

2. To see how the angle between the poles and the globe's rotation results in the desired orbital inclination, consider two limit cases. First, suppose the angle between is 90°. In this case, the globe is "straight" with the equator horizontal. Rotating the globe along the horizontal axis, flipping the poles end-over-end, will cause the crosshair to trace a polar orbit, giving the expected inclination of 90°. On the other hand, suppose the angle is 0°. In this case, the globe is "sideways" with the equator vertical. Rotating the globe will cause the crosshair to remain over the equator, corresponding to an equatorial orbit with 0° inclination. ↩

3. A detailed description of Globus in Russian is in this document, in Section 5. ↩↩↩↩↩

4. Or conversely, the altitude varies according to the speed. ↩

5. Note that panel control adjusts the period of the orbit, while the implementation adjusts the speed of the orbit. These are reciprocals, so linear changes in the period result in hyperbolic changes in the speed. The mechanism, however, changes the speed linearly, which seems like it wouldn't work. However, since the period is large relative to the change in the period, this linear approximation works and the error is small, about 1%. It's possible that the cone has a nonlinear shape to correct this, but I couldn't detect any nonlinearity in photographs. ↩

6. The latitude is given by arcsin(sin i * sin (2πt/T)), while the longitude is given by λ = arctan (cos i * tan(2πt/T)) + Ωt + λ₀, where t is the spaceship's flight time starting at the equator, i is the angle of inclination (51.8°), T is the orbital period, Ω is the angular velocity of the Earth's rotation, and λ₀ is the longitude of the ascending node.3

   The formula for latitude is simpler than longitude because the latitude repeats every orbit. The longitude, however, continually changes as the Earth rotates under the spacecraft. ↩

7. The back of the Globus has a 32-pin connector, a standard RS32TV Soviet military design. The case also has some dents visible; the dents were much larger before CuriousMarc smoothed them out.

   

   The back of the Globus.

    ↩↩

8. The NASA tracking sites marked with dots are CYI (Grand Canary Island), ACN (Ascension), MAD (Madrid, Spain), TAN (Tananarive, Madagascar), GWM (Guam), ORR (Orroral, Australia), HAW (Hawaii), GDS (Goldstone, California), MIL (Merritt Island, Florida), QUI (Quito, Ecuador), AGO (Santiago, Chile), BDA (Bermuda), NFL (Newfoundland, Canada), and VAN (Vanguard tracking ship). Most of these sites were part of the Spacecraft Tracking and Data Network. The numbers 1-7 are apparently USSR communication sites, although I'm puzzled by 8 in Nova Scotia and 9 in Honduras. ↩

9. Details on the earlier Globus IMP are at this site, including a discussion of the four different versions IMP-1 through IMP-4. Wikipedia also has information. ↩↩

Talking with the Moon: Inside Apollo's premodulation processor

The Apollo missions to the Moon required complex hardware to communicate between Earth and the spacecraft, sending radio signals over hundreds of thousands of miles. The premodulation processor was a key component of this system, combining voice, scientific data, TV, and telemetry for transmission to Earth.1 It was built from components that were welded together and tightly packed into a 14.5-pound box.2 In this blog post, I look inside the premodulation processor, examine its construction, and describe how each module worked.

The premodulation processor with its case removed, showing some of the circuitry. (Click any image for a larger version.)

The communications systems in the Apollo Command Module were very complex, as shown in the block diagram below.3 The premodulation processor (PMP, yellow) played a central role: most of the audio (red), data (orange), and TV (purple) went through the premodulation processor, where the signals were combined for transmission by the S-band (blue) radio systems. The premodulation processor also handled most of the voice and data signals received from Earth or from the Lunar Module via the VHF (green) or S-band radio systems.

Block diagram of the Apollo communications system. From Apollo Operations Handbook: Telecommunications System page 3.

One reason for the complexity of the premodulation processor was that the audio system had to support a variety of communications configurations. The diagram below illustrates one configuration, when astronauts were walking on the Moon (i.e. extra-vehicular activity, EVA). They communicated with the Lunar Module on the Moon's surface via VHF/AM radio, which relayed their audio to Earth via the Unified S-Band (USB) radio. Meanwhile, the Command and Service Module (CSM) orbiting the Moon also communicated with Earth via S-Band. These voices were conferenced together so the astronauts and ground could all hear each other. The need for redundancy added to the complexity; for example, signals from the Moon could be relayed through the Command Module in the event of an equipment failure.

Typical Apollo communication for lunar surface operations. From Apollo Experience Report.

Construction

Like much of the Apollo electronics, the premodulation processor was packaged in a drab bluish metal case. The case has four round military-style connectors on top that linked the various audio, RF, and control signals to other components of the spacecraft.

This photo shows the premodulation processor inside its case.

We opened the case by removing the screws and inside we found 11 rectangular modules packed together tightly, from the power supply at the top to the "SCO & diff ampl" (subcarrier oscillator and differential amplifier) at the bottom, conveniently labeled with their functions. The modules were plugged into a thin backplane,5 at the right, connected by D-Sub connectors, similar to vintage RS-232 connectors but in a variety of sizes. Bundles of wires connected the backplane to the round connectors. This construction technique made it easy for us to remove the modules and inspect them individually.

A side view of the premodulation processor, showing the labeled modules.

The modules themselves don't use printed-circuit boards, but instead are built from components that are spot-welded to metal pegs, as shown below.6 These resistors, diodes, capacitors, and transistors are tightly packed with a jumble of overlapping wiring. Most of the wiring consists of the component leads, but point-to-point wiring provided additional connection. The wiring is a combination of color-coded insulated wires, bare wires, and bare wires in clear insulating tubes. The components are liberally covered in what looks like hot glue. I suspect that the hot glue was only used in equipment for ground testing, while modules for spaceflight were fully encapsulated to prevent short circuits.

A closeup of the wiring in the aux bi-phase modulator module. Most of the connections are spot-welded, although a few seem to have solder.

The modules have circuitry on both sides, which increased the density. About half of the metal pegs provide connection to the other side, while half have plastic stubs on one side. As will be seen below, many of the modules also contain rectangular metal sub-units that implement functional blocks such as oscillators or filters. It appears that these standardized functions could be bought "off-the-shelf", not as integrated circuits, but as blocks containing discrete components.

In the following sections, I'll discuss each module in more detail, starting with the power supply.

Power supply module

The premodulation processor contains a power supply that converted the spacecraft's 28-volt DC supply to 18 volts. For efficiency, it is a switching power supply, a buck converter that chops up the input power at a high frequency to drop it to the lower voltage. Although switching power supplies are now ubiquitous, in everything from phone chargers to PC power supplies, switching power supplies were expensive and rare in the 1960s, used in aerospace applications that required a compact, high-efficiency power supply.

The block diagram below shows that the power supply was implemented redundantly, with a normal regular and an auxiliary regulator. A relay switches between the two regulators, controlled by the PMP NORM/AUX switch.

Diagram of the power supply module. From Command/Service Module Systems Handbook p63.

You may know of the Apollo 12 incident where the spacecraft was hit by lightning seconds after launch, scrambling the telemetry. The problem was resolved by the famous "set SCE to AUX" switch.7 The PMP's power switch is next to the SCE switch but never played a dramatic role.8

The power switches for the signal conditioning equipment (SCE) and the premodulation processor (PMP) are in the lower-left corner of the Command Module's control panel. Each switch has positions for NORM, OFF, and AUX.

The photo below shows the power supply module. The redundant halves of the power supply are visible with the lower circuitry a mirror image of the upper circuitry. The relay to switch between the two is the black box in the center-left. The power switching transistors are above and below the relay, fastened down with screws. To the right of the transistors are cylindrical tan inductors, storing energy across each pulse. Large silver filter capacitors are between the inductors. The right half of the module is the control circuitry: resistors, capacitors, transistors, and diodes. The connector at the far right connects the power supply to the other modules via the backplane.

The power supply module for the premodulation processor.

Flipping the power supply over reveals the high-frequency power transistors, in large metal packages to dissipate heat. These packages are square, unlike the typical two-tab (TO-3) power transistor packaging. Note the second layer of discrete component circuitry on this side of the module. This illustrates how the modules have two layers of circuitry, one on each side. You can also see the tops of the smaller transistors that are wired on the other side.

Underside of the power supply with 2N3137 power transistors.

Voice and data detector module

The data and voice detectors handle signals transmitted to the spacecraft over the S-band. The S-band transceiver receives these signals, demodulates them, and passes the signal to the premodulation processor. The data and voice detectors appear as one module on the block diagram below but are implemented as two modules physically.

Diagram of the data and voice detector modules. From Command/Service Module Systems Handbook p63.

The photo below shows the voice detector module. Voice is transmitted to the spacecraft, frequency modulated onto a 30-kilohertz subcarrier. The voice detector extracts this signal through a 30-kilohertz bandpass filter, demodulates it with an FM discriminator, amplifies it, and sends it to the Audio Center, which provides it to the astronauts. The largest component of the module is the 30-kilohertz bandpass filter at the center top. This module was built by Bulova Electronics, a division of the watch company that produced quartz crystals, oscillators, filters, servo amplifiers, and other components. Two gray transformers are also visible; these coupled the audio signals. The black relay in the lower right was controlled by the "Up Voice Relay" console switch. (Don't be confused by the two completely different definitions of "relay".)

The circuitry of the voice detector module. The connector is on the left.

More circuitry is on the other side of the voice detector. The transformers, relay, and bandpass filter are visible through openings in the module's metal frame. The discrete components are arranged in orderly columns, unlike the other modules.

The other side of the voice detector module.

The data detector module operated similarly to the voice detector, except that it extracted the data link signals from ground. From the data detector module, data was processed by the Up-Data Link box, giving the ground control over multiple spacecraft systems. For instance, commands could be entered into the Apollo Guidance Computer. The spacecraft clock (CTE, Central Timing Equipment) could be set. Various relays could be controlled, overriding some of the switches on the console.

The data detector module. It contains a 70-kilohertz bandpass filter produced by Bulova.

The implementation of the data detector module (above) is similar to the voice detector module, but simpler since it doesn't have the summing and switching circuitry. It uses a 70-kilohertz bandpass filter module, rather than the voice detector's 30-kilohertz filter. In case of a malfunction with the voice detector, backup voice communication could be transmitted to the spacecraft over the 70-kilohertz subcarrier, and extracted by the data detector module. This mode was controlled by the "Up-voice backup" switch.

Bi-phase modulator modules

The role of the bi-phase modulator modules was to modulate telemetry data using bi-phase modulation. In total, data had three layers of modulation. First, data was digitally encoded using pulse-code modulation (PCM). Next, this module applied bi-phase modulation to the bits at 1.024 MHz. Finally, the S-band transceiver used FM or PM (frequency or phase modulation) for the communication to Earth.

Bi-phase modulation encodes a bit using a sine wave for a 1 and an inverted sine wave for a 0 (i.e. a phase shift of 180°). Bi-phase modulation is a type of phase-shift keying. The PCM data was at 51.2 kilobits per second ("high bit rate") or 1.6 kilobits per second ("low bit rate"). Since the data was modulated at 1.024 MHz, a bit was encoded by at least 20 cycles of the waveform. This gave the receiver plenty of time to determine the phase and distinguish a 0 from a 1.

Diagram of the normal bi-phase module. From Command/Service Module Systems Handbook p63.

The premoduation processor contains two modulator modules: the "normal" module and the "auxiliary" module. The normal module transmits real-time data over PM, while the auxiliary module is more flexible. The normal bi-phase modulator module (below) contains a complex tangle of circuitry. The 1.024 MHz bandpass filter is the large metal package at the right, limiting the output signal to a narrow frequency range around 1.024 Mhz.

Inside the bi-phase modulator module.

The auxiliary bi-phase modulator (below) is roughly the same as the normal modulator, but with a bit more circuitry to switch between modes, transmitting either recorded PCM data from tape or real-time PCM data, using the PM or FM transmitters. Curiously, the different modes are selected by switching the power supply between NORM and AUX. In NORM mode, the auxiliary module transmits recorded data over FM. In AUX mode, the auxiliary module transmits real-time data over both PM and FM, providing a backup in case the normal module fails.

Diagram of the auxiliary bi-phase module. From Command/Service Module Systems Handbook p63.

The output signals from the bi-phase modulators are processed either by the FM mixer / LM PCM limiter module or the PM mixer / key / TV module; these are discussed later.

Underside of the auxiliary bi-phase modulator. The bandpass filter is at the left.

Voice clipper module

Voice communication from astronauts to the ground ("downlink voice") went through multiple stages of processing to improve quality. The design standard for the Apollo audio system was 90% word intelligibility for the main links and 70% for the backup links.9 This standard seems surprisingly poor, with one out of 10 words unintelligible, but achieving this standard was challenging due to the extreme distance to the Moon. Moreover, the spacecraft had a lot of ambient noise that interfered with communication. To maximize voice intelligibility over the available radio link, the voice signal was pre-emphasized and clipped. The voice clipper module (below) implemented the pre-emphasis and clipping of the voice signal.

Diagram of the voice clipper module. From Command/Service Module Systems Handbook p63.

The photo below shows the voice clipper module. It has two gray audio transformers at the left. The remainder of the module is filled with circuitry.

The voice clipper module.

The voice signal next goes to the voice relay module, discussed below. (The backup voice signal, however, went directly to the S-band transceiver for transmission to ground.)10

Voice relay module

The voice relay module permitted voice communication from the Lunar Module to be relayed through the Command Module instead of being transmitted directly to Earth from the Lunar Module. If the S-band mode switch was set to "RELAY", the voice and biomedical data from the Lunar Module would be mixed in with the Command Module's voice signal and sent to Earth. This module also optionally applied a low-pass filter to the Command Module's voice signal, under the control of the VHF duplex switch. (I think this is so voice and biomed data can be sent over the same channel without frequency conflict.)

Diagram of the voice relay module. From Command/Service Module Systems Handbook p63.

The photo below shows the voice relay module circuitry. There are three audio transformers, along with circuitry liberally encased in goo.

The voice relay module.

Flipping the module over, the upper right corner is completely covered in plastic. The reason for this is unclear. That corner holds one of the transformers, but I don't see a reason why this one in particular would be covered.

The other side of the voice relay module.

Voice modulator module

Next, the voice signal went to the voice modulator module, which used a complicated circuit to apply frequency modulation. First, the voice signal controls a 113-kilohertz voltage-controlled oscillator (VCO), yielding an FM signal at 113 kilohertz. Next, this signal is mixed with a 512-kilohertz signal from the central timing equipment (CTE), yielding signals at the sum and difference frequencies (399 kHz and 625 kHz). The bandpass filter passes the 625-kilohertz FM signal. The signal frequency is doubled and filtered to produce the final 1.25 MHz FM signal.

Diagram of the voice modulator module. From Command/Service Module Systems Handbook p63.

Three large modules are visible inside: the voltage-controlled oscillator and the two bandpass filters.

Inside the voice modulator module.

The other side of the module has the circuitry, wired to the larger modules. The frequency-doubler may be implemented by a varactor diode, but I haven't located it. From the voice modulator, the voice signal passed to the PM mixer / key / TV module.

Another view of the voice modulator module.

PM mixer/key/TV module

As the name suggests, the PM mixer/key/TV module had multiple functions. In the top part of the diagram below, the mixer combines three data sources: voice with data, emergency keying, and voice. The voice and data combination consists of PCM data at 1.024 megahertz with voice data at 1.25 megahertz; the PCM data is provided by one of the bi-phase modulator modules, while the voice data is provided by the voice modulator module. The next mixer input is the emergency key signal. The purpose of emergency key is that if voice communication failed, an astronaut could send Morse code by using the XMIT key on their communication cable. This key signal might be able to get through to Earth even if voice communication fails or is unintelligible. This module produces the emergency key signal at 512 kHz along with a 400 Hz feedback tone for the astronauts. The final mixer source is voice. The sum of these signals is sent to the S-band PM transponder for phase modulation and transmission. This module also includes a TV isolation amplifier to supply a TV signal to ground support equipment (GSE) before launch.

Diagram of the PM mixer / key / TV module. From Command/Service Module Systems Handbook p63.

The photo below shows this module. On the front right is a component that looks a bit like a power transistor. However, it is an adjustable component (note the screw in the middle), probably a variable resistor.

The PM mixer / key / TV module.

SCO (subcarrier oscillator) and differential amplifier

This module is used for transmitting three channels of analog scientific data. (This is in contrast to most of the data, which was transmitted digitally, using pulse-code modulation (PCM).) Each of the three scientific signals modulates a subcarrier oscillator on a different frequency: 95 kHz, 125 kHz, and 165 kHz. These signals are sent to the FM mixer / LM PCM limiter module, which will be discussed in the next section.

This module also contains relays so the real-time scientific data could be directed to tape for storage. The recorded data could be played back for transmission, amplified by the differential amplifiers. The mode was controlled by the S-band Aux TV/SCI switch. If set to SCI, real-time scientific data was transmitted. If the transmitter was used for TV, the scientific data was recorded to tape for later playback. The tape recorder switch was set to PCM/ANLG to play back the analog data.

Diagram of the SCO differential amplifier module. From Command/Service Module Systems Handbook p63.

Inside, the three large tan oscillator modules are visible. The three relays are the smaller grayish boxes. This module has the D-Sub connector attached with wires and rotated 90°, unlike the other modules that have the connector mounted to the end of the module.

The SCO & differential amplifier module.

On the other side of the module, the circuitry is visible. Note the 6-pin transistors (gold and green circles). These probably contain two carefully-matched transistors for the differential amplifiers. The performance of a differential amplifier strongly depends on its two input transistors; by putting the transistors in the same package, the effects of temperature are minimized.

Another view of the SCO & differential amplifier module.

FM mixer / LM PCM limiter module

The final module is the FM mixer / LM PCM limiter. Like the PM mixer module, this module combines multiple signals for transmission. But this module prepares signals for FM transmission rather than PM transmission. Specifically, the module combines the three analog scientific data inputs, digital PCM data from the Lunar Module, intercom voice from the Lunar module, and PCM data modulated at 1.024 MHz. Various switches on the console control the different modes.

Diagram of the FM mixer / LM PCM limiter module. From Command/Service Module Systems Handbook p63.

The photo below shows the module's circuitry. It has four gray transformers along with the typical transistors, diodes, resistors, and capacitors.

The FM mixer / LM PCM limiter module.

The unusual feature of this module is the encapsulated module in the upper left. This module appears to contain three transistors and five capacitors. It's unclear why these components are encased in plastic. The block diagram for this module doesn't show any special circuitry that would motivate encapsulation. I hope to reverse-engineer this module to figure this out.

The encapsulated block appears to contain three transistors and five capacitors.

Conclusion

Well, I had planned to write a quick description of what we saw inside the premodulation processor but it turned out to be much more complicated than I expected. Congratulations on making it to the end of this blog post.

The premodulation processor illustrates how analog electronics were very bulky before integrated circuits became popular. In the modules, amplifiers and other functional blocks were built from discrete components. The result was a 14.5-pound box to perform a few signal processing tasks. A decade later, many of the circuits could have been replaced with compact ICs.

The premodulation processor also shows how complex everything was in Apollo. You might think that transmitting voice, data, and TV wouldn't be too tricky, just three signals. But everything needed to be redundant. Then there were special cases, such as recording data when you're on the far side of the Moon. Or communicating between astronauts in the Command Module, in the Lunar Module, and walking on the Moon. All these cases required circuitry to switch signals and optimize the radio link for each mode. And the premodulation processor is just one of many boxes in the Apollo communications system! Apollo is like a fractal, where you find successive layers of complexity as you look closer at any system.

We haven't been able to find detailed schematics of the premodulation processor modules, so I plan to reverse-engineer some of the circuitry. I announce my latest blog posts on Twitter, so follow me @kenshirriff for updates. I also have an RSS feed. Thanks to Marcel for providing the premodulation processor and letting Mike, CuriousMarc and me disassemble it.

Front view of the premodulation processor with the case removed.

Notes and references

1. The Apollo Operations Handbook: Telecommunications System gives this description: "The premodulation processor (PMP) equipment provides the interface connection between the airborne data-gathering equipment and the RF electronics. The PMP accomplishes signal modulation and demodulation, signal mixing, and the proper switching of signals so that the correct intelligence corresponding to a given mode of operation is transmitted." ↩

2. The premodulation processor was one of many boxes of electronic circuitry packed into the spacecraft and linked by thick cables. The diagram below highlights where it was mounted in the lower equipment bay of the Apollo Command Module.

   

   The premodulation processor was one of many electronic boxes in the Command Module's lower equipment bay. Diagram from Command/Service Module Systems Handbook p212.

    ↩

3. The block diagram below shows the functions of the premodulation processor, along with the switches that control it.

   

   A block diagram of the premodulation processor. From Apollo Operations Handbook: Telecommunications System.

   The block diagram below provides a more detailed view of the premodulation processor. I split out the sub-module diagrams for the discussion, but the full diagram shows the interconnections between the modules.

   

   Block diagram of the PMP. (Click for a larger version.) From Command/Service Module Systems Handbook p63.

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4. As shown by the nameplate, the premodulation processor was built by Collins Radio in 1966, two days before Christmas. Collins Radio built much of the communications equipment for the space program from Mercury through Apollo including the Deep Space Network antenna system, microwave links, and ground support equipment (details).4

   

   The nameplate for the premodulation processor shows that it was built by Collins Radio.

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5. The backplane is a sheet of metal with D-Sub connectors for each module. The round connectors are underneath, wired to the backplane by individual wires.

   

   The premodulation processor's backplane links the modules to the external connectors.

   The four round military-style connectors are shown below. Two connectors have individual pins, while two connectors each have tiny coaxial connections.

   

   The premodulation processor had four connectors for its numerous audio, RF, and control signals.

    ↩

6. We've examined several different Apollo electronics boxes and surprisingly they use completely different manufacturing techniques, even for boxes built by the same manufacturer. Techniques we've seen include printed-circuit boards, surface-mount components, cordwood modules, "dead-bug" components on a ground plane, point-to-point components, and encapsulated hybrid modules. I expected that there would be a standard manufacturing technique (like PCBs are standard now), but everything is different. ↩

7. The story of "Set SCE to Aux" is a well-known Apollo incident where disaster was averted. In brief, Apollo 12 was struck twice by lightning just seconds after launch. Inside the spacecraft, so many warning lights lit up that astronaut Conrad thought "the whole board looks like a Christmas tree". On the ground, consoles started displaying nonsense telemetry. Everyone was mystified until engineer John Aaron recalled seeing similar garbled telemetry during a test. He knew the solution and gave the puzzling command "Try SCE to Auxiliary". This switch was so obscure that astronaut Conrad responded, "What the hell is that?" Fortunately, astronaut Bean flipped the switch, bringing the SCE unit back to operation and restoring telemetry. There were other consequences of the lightning strike, but after the fuel cells were brought back online and the inertial guidance system was realigned, the spacecraft continued uneventfully to the Moon.

   The underlying problem was that the lightning strike caused the spacecraft's fuel cells to go offline. The DC voltage bus was supposed to be at 28 volts, but the loss of the fuel cells caused the voltage to sag to about 18 volts. Within milliseconds, the voltage climbed to 24 volts under battery power, still low. The low voltage caused the primary power supply of the SCE (signal conditioning equipment) to shut down. Since the SCE's role was preparing dozens of analog sensor voltages for telemetry, this caused the telemetry values to Mission Control to be garbled. Flipping the SCE switch to Aux caused the SCE to use its auxiliary power supply, restoring the SCE to operation.

   The published descriptions of this incident are vague on exactly why the auxiliary power supply worked when the primary didn't, so I looked at the SCE diagram (below) to fill in a few details. Power enters at the left and passes through the SCE's famous power switch, which has three positions: NORM, OFF, and AUX. Inside the SCE, there are two power supplies (red) for redundancy, along with some control circuitry at the top. One of the two power supplies is active at a time, unless both power supplies are deactivated for an overvoltage or undervoltage condition.

   

   Diagram of the SCE power supply and the switch. From Command/Service Module Systems Handbook p118.

   The SCE has a flip flop (purple) that selects a power supply by disabling (blue) the unused one. When you switch SCE to AUX, one action is that it toggles the flip flop, switching from supply #1 to #2, or #2 to #1. But I don't think that was important for Apollo 12. AUX mode also blocks the undervoltage signal via an AND gate (green). That is, if the input voltage was still too low, both power supplies would be shut down in NORM mode but either one could function in AUX mode. This, I think, is why "SCE to AUX" powered up the SCE.

   Another interesting feature is the automatic failover (orange). In NORM mode, the SCE will automatically switch power supplies if an internal voltage is bad for 200 ms. However, the failover logic is blocked by the undervoltage detector, so it would not have taken place in Apollo 12. But otherwise, if one of the power supplies failed, the SCE would transparently switch to the other one.

   Curiously, the official NASA report Analysis of Apollo 12 Lightning Incident barely has two sentences on the SCE in its 94 pages. Although the SCE gets all the public attention in this incident, it seems like NASA didn't really care about it since the telemetry wasn't critical to the mission. NASA was much more interested in other effects of the lightning strike: the fuel cell shutdown, the effects on the computer and guidance systems, 9 failed sensors, and potential effects on the pyrotechnics. For more on the Apollo 12 incident, see the transcript, the detailed Scott Manley video, and an Apollo Flight Journal post.

   Note that the SCE's power supply logic is different from other units. Most units (such as the transponder, TWT amplifier, and premodulation processor) have primary and secondary power supplies, with a switch to explicitly select one or the other. However, in the SCE, the Aux switch toggles between power supplies, rather than selecting a specific auxiliary power supply. ↩

8. Astronauts used multiple switches on the control console to control the premodulation processor. These switches were grouped in the lower-right corner of the console with other communications switches. The diagram below shows the relevant switches, highlighted in yellow.

   ↩

   The Command Module console contains switches to control the premodulation processor. These switches are highlighted in yellow. Diagram based on from Command/Service Module Systems Handbook p208.

9. For detailed information on the voice communication system, see Apollo Experience Report - Voice Communications Techniques and Performances. It discusses the performance requirements for the Apollo communications system and how the system was designed to achieve the intelligibility requirements. ↩

10. The idea of backup voice was to provide a voice channel for emergencies that used less power, at the cost of garbling up to 30% of the words. After the explosion, Apollo 13 used the backup voice system so they could turn off the Lunar Module's power amplifier and conserve electrical power. (See Apollo 13 Mission Operations Report pages N-2 and N-7, as well as the transcript.) Backup voice was also used at times during Apollo 16 due to a failure of the Lunar Module's steerable S-band antenna; see Apollo 16 Mission Report page 7-3, which calls this mode "down voice backup". (I should point out that these backup voice incidents involved the Lunar Module, not the Command Module's premodulation processor.) ↩