Unified S-Band
Encyclopedia
The Unified S-band system was a tracking and communication system developed for the Apollo program by NASA
and the Jet Propulsion Laboratory
(JPL). It operated in the S band
portion of the microwave spectrum, combining ("unifying") voice communications, television
, telemetry
, command
, tracking
and ranging
into a single system to save size and weight and simplify operations. The USB ground network was managed by the Goddard Space Flight Center
(GSFC). Commercial contractors included Collins Radio, Blaw-Knox
, Motorola
and Energy Systems
.
and Gemini
, had separate radio systems for voice, telemetry, and tracking. Uplink voice and command, and downlink voice and telemetry data were sent via UHF and VHF systems. The tracking capability was a C band
beacon interrogated by a ground-based radar. With the much greater distance of Apollo, passive ranging
was not feasible, so a new active ranging system was required. Apollo also planned to use television transmissions, which were not supported by the existing systems. Finally, the use of three different frequencies complicated the spacecraft systems and ground support. The Unified S-band (USB) system was developed to address these concerns.
The USB system did not completely replace all other radio transmitters on Apollo. While it was the sole mode for deep space communications, Apollo still used VHF for short range voice and low rate telemetry between astronauts and the LM
and lunar rover
during EVA
; between the LM
and CSM
; and between the CSM and earth stations
during the orbital and recovery phases of the mission. The CM
had a backup capability to range the LM over its VHF voice links.
Apollo also carried several radars that operated independently of the USB on their own frequencies, including the landing and rendezvous radars on the LM and a C-band radar transponder on the CM.
and operations.
Each Apollo spacecraft was assigned a frequency pair. For certain phase modulation
(PM) downlinks, the uplink to downlink frequency ratio was exactly 221/240. The ALSEP lunar surface experiments shared a common uplink and did not,
insofar as is known, implement a coherent transponder. (The passive laser retroreflectors
left by the Apollo 11, 14 and 15 missions provide much greater accuracy, and have far outlived the active electronics in the other ALSEP experiments.) The Lunar Communications Relay Unit (LCRU) on the Lunar Rover
had its own downlink frequency (to avoid interference with the LM) but shared the LM's uplink frequency as it did not implement a coherent transponder. To keep the VHF transmitters on the LM and LCRU from both trying to relay uplink voice and interfering with each other, separate voice subcarriers were used on the common S-band uplink: 30 kHz for the LM and 124 kHz for the LCRU.
The CSM had two separate transmitters, one PM and one FM. The LM had only one S-band transmitter that could operate in PM or FM, but not both simultaneously.
The S-IVB
upper stage had its own USB transponder so it could be tracked independently after Apollo spacecraft separation until the stage either flew past the moon (Apollos 8, 10, 11, 12) or, starting with Apollo 13
, hit the moon. This tracking data greatly aided the analysis of the impact as recorded by the seismometers left by earlier Apollo crews.
The S-IVB shared its S-band frequency pair with the LM. This created no problem in a normal mission as the LM remained dormant until lunar orbit, by which time the S-IVB had already hit the moon or flown off into orbit around the sun. However, it created a operational problem during the Apollo 13
mission when the LM had to be used as a lifeboat well before Apollo and the S-IVB reached the moon.
The LM frequency pair was also used by the subsatellites left in lunar orbit by the later J-missions. They were deployed by the CSM shortly before leaving lunar orbit and after the LM had completed its mission.
The use of two separated frequency bands made full duplex operation possible. The ground and the spacecraft transmitted continuously. Microphone audio was keyed either manually or by VOX
, but unlike ordinary half duplex two-way radio both sides could talk at the same time without mutual interference.
(PM). PM, like FM, has a constant amplitude (envelope) regardless of modulation. This permits the use of nonlinear RF amplifiers that can be considerably more efficient than RF amplifiers that must maintain linearity.
The PM modulation index
is small, on the order of 1 radian, so the modulated signal more closely resembled double sideband amplitude modulation
(AM) except for the carrier phase. A PM signal can be approximated for analysis purposes as an AM signal with the carrier (and only the carrier) rotated 90 degrees from its original phase. One important difference is that in AM, the carrier component has a constant amplitude as the sidebands vary with modulation while in PM the total signal (the envelope) is constant amplitude. This means that PM shifts power from the carrier to the information-carrying sidebands with modulation, and at some modulation indices the carrier can disappear completely. This is why Apollo uses a low modulation index: to leave a strong carrier that can be used for highly accurate velocity tracking by measurement of its Doppler shift.
, multiplied the uplink carrier frequency by the ratio 240/221 to produce its own downlink carrier signal.
When no uplink was detected, the transponder downlink carrier was generated from a local oscillator at the nominal frequency.
This "two-way" technique allowed extremely precise relative velocity measurements (in centimeters/sec) by observing the Doppler shift of the downlink carrier without a high accuracy oscillator on the spacecraft, although one was still needed on the ground.
The uplink had subcarriers at 30 kHz and 70 kHz. The 30 kHz subcarrier was modulated with uplink (Capcom) voice using narrowband FM
(NBFM) and the 70 kHz carrier was modulated with command data for the onboard computer. This latter capability, which could be blocked by the astronauts, was used primarily to update the state vector
s maintained by the computers with accurate values determined by ground tracking. It was also used to execute maneuvers in an unmanned spacecraft, e.g., deorbiting the lunar module after it had been jettisoned in lunar orbit.
Either or both subcarriers could be turned off when not needed, e.g., the voice subcarrier could be turned off during astronaut sleep periods. This improved the signal margins for the other information streams such as telemetry data.
The downlink normally had subcarriers at 1.25 MHz (NBFM voice) and 1.024 MHz (telemetry data). The telemetry could be at one of two rates, 1.6 kilobits/sec (low rate, 1/640 of the subcarrier frequency) and 51.2 kilobits/sec (high rate, 1/20 of the subcarrier frequency). High rate was used unless low rate was forced by poor link conditions, e.g., the use of a small earth receiving antenna, an omni
spacecraft antenna, or the need to conserve spacecraft power by turning off its RF power amplifier. (The S-band transponder on the S-IVB had no voice subcarrier.)
A "backup voice" mode was available that shut off the 1.25 MHz NBFM voice subcarrier and transmitted voice at baseband on the main PM S-band carrier. This provided a few more dB of margin when the link was unusually degraded but worse voice quality than the normal voice mode when conditions were good.
The two modes can be easily distinguished by how they react to signal fades. In the normal (NBFM subcarrier) voice mode the audio SNR is usually very high. But as the link degrades below threshold, impulse or "popcorn" noise appears suddenly and builds up rapidly until it overwhelms the astronauts' voices. An excellent example occurred during the Apollo 11 lunar landing when the lunar module structure occasionally obstructed the antenna's view of earth.
The backup voice mode behaves more like AM; there is a constant background "hiss" and the astronauts' voices vary with signal strength. This mode was used extensively during the Apollo 13
emergency to conserve battery power in the LM Aquarius and during Apollo 16
because of the failure of the steerable S-band antenna on the lunar module Orion.
if the downlink were too severely degraded to support even the backup voice mode. Although this mode had been tested (on Apollo 7
) and most of the astronauts were trained in its use, this mode was never actually needed during any Apollo mission. (Apollo 13 made extensive use of the "backup voice" mode, as did the Apollo 16 lunar module Orion due to a failed high gain antenna).
A similar uplink capability did not exist because the uplink budget had far more margin than the downlink. A typical Apollo S-band spacecraft transmitter produced 20 watts; a typical uplink transmitter produced 10 kW, a ratio of 27 dB.
sequence at 994 kilobit/s and added it to the baseband signal going to the PM transmitter. The transponder echoed this PN signal back to earth on the downlink, and by correlating
the received and transmitted versions the precise round trip light time to the spacecraft could be determined very accurately (typically to within a few meters).
The PN sequence, although deterministic, had the properties of a random bit stream. Although the PN sequence was periodic, its period of about 5 seconds exceeded the largest possible round trip time to the moon so there would be no ambiguity in its received timing.
Modern GPS receivers work somewhat similarly in that they also correlate a received PN bit stream (at 1.023 Mb/s) with a local reference to measure distance. But GPS is a receive-only system that uses relative timing measurements from a set of satellites to determine receiver position while the Apollo USB is a two-way system that can only determine the instantaneous distance and relative velocity. However, an orbit determination program can find the unique spacecraft state vector
or orbital element set
that most closely matches (usually in a least squares
sense) a set of range, range-rate (relative velocity) and antenna look angle observations made over a period of time by one or more ground stations assuming purely ballistic spacecraft motion over the observation interval.
Once the state vector has been determined, the spacecraft's future trajectory can be fully predicted until the next propulsive event.
Transponder ranging turn-around had to be manually enabled by an astronaut. It consumed an appreciable fraction of the downlink capacity and it was only needed occasionally, typically during handover from one ground station to the next. After the new uplink station achieved a 2-way coherent transponder lock in which the spacecraft generated its carrier at 240/221 times the received uplink frequency, the new ground station ranged the spacecraft. Then the ranging signal was turned off and the range measurement was continually updated by Doppler velocity measurements.
If for some reason a ground station lost lock during a pass, it would repeat the ranging measurement after re-acquiring lock.
EVA, are much wider in bandwidth than the other Apollo downlink signals. The PM link margin
simply could not provide an acceptable picture, even when the largest available dishes were used.
A means was also needed to transmit wideband engineering and scientific data, such as that recorded on a tape recorder and played back at high speed.
The answer to both needs was wideband FM - frequency modulation. FM with a large modulation index
exhibits a capture
or threshold effect. The output signal-to-noise ratio
(SNR) can be significantly greater than the RF channel SNR provided that the RF SNR remains above a threshold, typically around 8-10 dB.
This enhancement comes at a price: below the FM threshold, the output SNR is worse than the RF channel SNR. Reception is "all or nothing"; a receiving antenna too small to capture the video cannot capture the narrowband elements either (e.g., voice).
The CSM carried separate FM and PM transmitters that could operate simultaneously, so voice and telemetry continued to be transmitted by PM while the video came down by FM. The LM only carried a single transmitter that could operate in either FM or PM, but not both. FM cannot be used for Doppler tracking, so the LM always transmitted PM during flight, reserving FM for when video was required (e.g., during a surface EVA).
Until the transition to digital, satellite television
also used wideband FM.
or any other non-Western (or non-aligned) country at the time chose to intercept any of the Apollo telemetry is unclear. Amateur radio and affiliated telecommunications sector persons could listen to the Apollo telemetry the world over—provided they could afford the reception equipment.
Within the territory of the US it was legally possible for Amateur Radio
operators to monitor the telemetry, but the FCC did issue a directive that required all disclosure of Apollo telemetry interception be cleared by NASA. Paul Wilson and Richard T. Knadle Jr. received voice transmissions from the Command Service Module of Apollo 15 in lunar orbit on the morning of August 1, 1971. In an article for QST magazine they provide a detailed description of their work, with photographs.
At least two different radio amateurs, W4HHK and K2RIW, reported reception of Apollo 16 signals with home-built equipment.
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
and the Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
(JPL). It operated in the S band
S band
The S band is defined by an IEEE standard for radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF and SHF at 3.0 GHz. It is part of the microwave band of the electromagnetic spectrum...
portion of the microwave spectrum, combining ("unifying") voice communications, television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
, telemetry
Telemetry
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
, command
Command (computing)
In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is a directive to some kind of command line interface, such as a shell....
, tracking
Tracking
Tracking can refer to:*Tracking , separating children into different classes according to their academic ability*Tracking, in computer graphics, a vital part of match moving...
and ranging
Ranging
Ranging is a process or method to determine the distance from one location or position to another location or position. Another term for this method is lateration, see unilateration...
into a single system to save size and weight and simplify operations. The USB ground network was managed by the Goddard Space Flight Center
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
(GSFC). Commercial contractors included Collins Radio, Blaw-Knox
Blaw-Knox
Blaw-Knox is one of the most prominent manufacturers of road paving equipment in the world. After Ingersoll-Rand divested of their road-making equipment operations in 2007, the company became part of Volvo of Sweden's construction equipment division....
, Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
and Energy Systems
Energy systems
There are three sources of Adenosine triphosphate , the body's main energy source on the cellular level.*ATP-PC system - This system is used only for very short durations of up to 10 seconds. The ATP-PC system neither uses oxygen nor produces lactic acid if oxygen is unavailable and is thus said...
.
Basis
The previous programs, MercuryProject Mercury
In January 1960 NASA awarded Western Electric Company a contract for the Mercury tracking network. The value of the contract was over $33 million. Also in January, McDonnell delivered the first production-type Mercury spacecraft, less than a year after award of the formal contract. On February 12,...
and Gemini
Project Gemini
Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966....
, had separate radio systems for voice, telemetry, and tracking. Uplink voice and command, and downlink voice and telemetry data were sent via UHF and VHF systems. The tracking capability was a C band
C band
The C band is a name given to certain portions of the electromagnetic spectrum, including wavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE C-band - and its slight variations - contains frequency ranges that are used for many satellite communications...
beacon interrogated by a ground-based radar. With the much greater distance of Apollo, passive ranging
Ranging
Ranging is a process or method to determine the distance from one location or position to another location or position. Another term for this method is lateration, see unilateration...
was not feasible, so a new active ranging system was required. Apollo also planned to use television transmissions, which were not supported by the existing systems. Finally, the use of three different frequencies complicated the spacecraft systems and ground support. The Unified S-band (USB) system was developed to address these concerns.
The USB system did not completely replace all other radio transmitters on Apollo. While it was the sole mode for deep space communications, Apollo still used VHF for short range voice and low rate telemetry between astronauts and the LM
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...
and lunar rover
Lunar rover
The Lunar Roving Vehicle or lunar rover was a battery-powered four-wheeled rover used on the Moon in the last three missions of the American Apollo program during 1971 and 1972...
during EVA
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...
; between the LM
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...
and CSM
Apollo Command/Service Module
The Command/Service Module was one of two spacecraft, along with the Lunar Module, used for the United States Apollo program which landed astronauts on the Moon. It was built for NASA by North American Aviation...
; and between the CSM and earth stations
Deep Space Network
The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...
during the orbital and recovery phases of the mission. The CM
Apollo Command/Service Module
The Command/Service Module was one of two spacecraft, along with the Lunar Module, used for the United States Apollo program which landed astronauts on the Moon. It was built for NASA by North American Aviation...
had a backup capability to range the LM over its VHF voice links.
Apollo also carried several radars that operated independently of the USB on their own frequencies, including the landing and rendezvous radars on the LM and a C-band radar transponder on the CM.
Technical summary
From a NASA technical summary:
The design of the USB system is based on a coherent doppler and the pseudo-random range
system which has been developed by JPL. The S-band system utilizes the same techniques as
the existing systems, with the major changes being the inclusion of the voice and data channels.
A single carrier frequency is utilized in each direction for the transmission of all tracklng
and communications data between the spacecraft and ground. The voice and up-date data are
modulated onto subcarriers and then combined with the ranging data [...]. This composite
information is used to phase-modulate the transmitted carrier frequency. The received and transmitted carrier frequencies are coherently related. This allows measurements of the carrier doppler frequency by the ground station for determination of the radial velocity of the spacecraft.
In the transponder the subcarrìers are extracted from the RF carrier and detected
to produce the voice and command information. The binary ranging signals, modulated
directly onto the carrier, are detected by the wide-band phase detector and translated to
a video signal.
The voice and telemetry data to be transmitted from the spacecraft are modulated
onto subcarriers, combined with the video ranging signals, and used to phase-modulate
the down-link carrier frequency. The transponder transmitter can also be frequency-
modulated for the transmission of television information or recorded data instead of ranging signals.
The basic USB system has the ability to provide tracking and communications data for two
spacecraft simultaneously, provided they are within the beamwidth of the single antenna. The
primary mode of tracking and communications is through the use of the PMPhase modulationPhase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...
mode of operation.
Two sets of frequencies separated by approximately 5 megacycles are used for this purpose [...].
In addition to the primary mode of communications, the USB system has the capability of receiving data on two other frequencies.
These are used primarily for the transmission of FM data from the spacecraft.
Frequencies
The Unified S-Band System used the 2025-2110 MHz band for uplinks (earth to space transmissions) and the 2200-2290 MHz band for downlinks (space to earth transmissions). Both bands are allocated internationally for space researchand operations.
| Apollo S-band frequency assignments | ||
|---|---|---|
| Spacecraft | Uplink (MHz) | Downlink (MHz) |
| Command Module PM Phase modulation Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions... |
2106.40625 | 2287.5 |
| Command Module FM Frequency modulation In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant... |
2272.5 | |
| Lunar Module | 2101.802083 | 2282.5 |
| S-IVB S-IVB The S-IVB was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine... PM |
2101.802083 | 2282.5 |
| S-IVB S-IVB The S-IVB was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine... FM |
2277.5 | |
| Lunar Rover Lunar rover The Lunar Roving Vehicle or lunar rover was a battery-powered four-wheeled rover used on the Moon in the last three missions of the American Apollo program during 1971 and 1972... |
2101.802083 | 2265.5 |
| Apollo 11 Apollo 11 In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was... Early ALSEP |
2119 | 2276.5 |
| Apollo 12 Apollo 12 Apollo 12 was the sixth manned flight in the American Apollo program and the second to land on the Moon . It was launched on November 14, 1969 from the Kennedy Space Center, Florida, four months after Apollo 11. Mission commander Charles "Pete" Conrad and Lunar Module Pilot Alan L... ALSEP |
2119 | 2278.5 |
| Apollo 14 Apollo 14 Apollo 14 was the eighth manned mission in the American Apollo program, and the third to land on the Moon. It was the last of the "H missions", targeted landings with two-day stays on the Moon with two lunar EVAs, or moonwalks.... ALSEP |
2119 | 2279.5 |
| Apollo 15 Apollo 15 Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous... ALSEP |
2119 | 2278.0 |
| Apollo 15 subsatellite | 2101.802083 | 2282.5 |
| Apollo 16 Apollo 16 Young and Duke served as the backup crew for Apollo 13; Mattingly was slated to be the Apollo 13 command module pilot until being pulled from the mission due to his exposure to rubella through Duke.-Backup crew:... ALSEP |
2119 | 2276.0 |
| Apollo 17 Apollo 17 Apollo 17 was the eleventh and final manned mission in the American Apollo space program. Launched at 12:33 a.m. EST on December 7, 1972, with a three-member crew consisting of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, Apollo 17 remains the... ALSEP |
2119 | 2275.5 |
Each Apollo spacecraft was assigned a frequency pair. For certain phase modulation
Phase modulation
Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...
(PM) downlinks, the uplink to downlink frequency ratio was exactly 221/240. The ALSEP lunar surface experiments shared a common uplink and did not,
insofar as is known, implement a coherent transponder. (The passive laser retroreflectors
Lunar laser ranging experiment
The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the moon during the Apollo program, and the time for the reflected light to return is determined...
left by the Apollo 11, 14 and 15 missions provide much greater accuracy, and have far outlived the active electronics in the other ALSEP experiments.) The Lunar Communications Relay Unit (LCRU) on the Lunar Rover
Lunar rover
The Lunar Roving Vehicle or lunar rover was a battery-powered four-wheeled rover used on the Moon in the last three missions of the American Apollo program during 1971 and 1972...
had its own downlink frequency (to avoid interference with the LM) but shared the LM's uplink frequency as it did not implement a coherent transponder. To keep the VHF transmitters on the LM and LCRU from both trying to relay uplink voice and interfering with each other, separate voice subcarriers were used on the common S-band uplink: 30 kHz for the LM and 124 kHz for the LCRU.
The CSM had two separate transmitters, one PM and one FM. The LM had only one S-band transmitter that could operate in PM or FM, but not both simultaneously.
The S-IVB
S-IVB
The S-IVB was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine...
upper stage had its own USB transponder so it could be tracked independently after Apollo spacecraft separation until the stage either flew past the moon (Apollos 8, 10, 11, 12) or, starting with Apollo 13
Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
, hit the moon. This tracking data greatly aided the analysis of the impact as recorded by the seismometers left by earlier Apollo crews.
The S-IVB shared its S-band frequency pair with the LM. This created no problem in a normal mission as the LM remained dormant until lunar orbit, by which time the S-IVB had already hit the moon or flown off into orbit around the sun. However, it created a operational problem during the Apollo 13
Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
mission when the LM had to be used as a lifeboat well before Apollo and the S-IVB reached the moon.
The LM frequency pair was also used by the subsatellites left in lunar orbit by the later J-missions. They were deployed by the CSM shortly before leaving lunar orbit and after the LM had completed its mission.
The use of two separated frequency bands made full duplex operation possible. The ground and the spacecraft transmitted continuously. Microphone audio was keyed either manually or by VOX
Vox
Vox is Latin for Voice. It may refer to:* Vocals, a common abbreviation, especially in pro audio-Music:* "Vox" , a song by Sarah McLachlan* Vox Records, an American record label* Vox Records , a German record label...
, but unlike ordinary half duplex two-way radio both sides could talk at the same time without mutual interference.
Modulation
The S-band uplinks and downlinks usually (but not always) used phase modulationPhase modulation
Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...
(PM). PM, like FM, has a constant amplitude (envelope) regardless of modulation. This permits the use of nonlinear RF amplifiers that can be considerably more efficient than RF amplifiers that must maintain linearity.
The PM modulation index
Modulation index
The modulation index of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level. It is defined differently in each modulation scheme. See:*Amplitude modulation index...
is small, on the order of 1 radian, so the modulated signal more closely resembled double sideband amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
(AM) except for the carrier phase. A PM signal can be approximated for analysis purposes as an AM signal with the carrier (and only the carrier) rotated 90 degrees from its original phase. One important difference is that in AM, the carrier component has a constant amplitude as the sidebands vary with modulation while in PM the total signal (the envelope) is constant amplitude. This means that PM shifts power from the carrier to the information-carrying sidebands with modulation, and at some modulation indices the carrier can disappear completely. This is why Apollo uses a low modulation index: to leave a strong carrier that can be used for highly accurate velocity tracking by measurement of its Doppler shift.
Coherent Transponders and Doppler Tracking
Allocating uplink/downlink frequency pairs in a fixed ratio of 221/240 permitted the use of coherent transponders on the spacecraft. The spacecraft tracked the uplink carrier with a phase locked loop and, with a series of frequency dividers and multipliersFrequency multiplier
In electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal...
, multiplied the uplink carrier frequency by the ratio 240/221 to produce its own downlink carrier signal.
When no uplink was detected, the transponder downlink carrier was generated from a local oscillator at the nominal frequency.
This "two-way" technique allowed extremely precise relative velocity measurements (in centimeters/sec) by observing the Doppler shift of the downlink carrier without a high accuracy oscillator on the spacecraft, although one was still needed on the ground.
Subcarriers
As mentioned above, the uplink and downlink carriers played a critical role in spacecraft tracking. Sidebands generated by the information also carried by the system had to be kept away from the carriers to avoid perturbing the phase locked loops used to track them. This was done through the use of various subcarriers.The uplink had subcarriers at 30 kHz and 70 kHz. The 30 kHz subcarrier was modulated with uplink (Capcom) voice using narrowband FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(NBFM) and the 70 kHz carrier was modulated with command data for the onboard computer. This latter capability, which could be blocked by the astronauts, was used primarily to update the state vector
State vector
*A state vector in general control systems describes the observed states of an object in state space, e.g. in variables of the degrees of freedom for motion *A state vector in general control systems describes the observed states of an object in state space, e.g. in variables of the degrees of...
s maintained by the computers with accurate values determined by ground tracking. It was also used to execute maneuvers in an unmanned spacecraft, e.g., deorbiting the lunar module after it had been jettisoned in lunar orbit.
Either or both subcarriers could be turned off when not needed, e.g., the voice subcarrier could be turned off during astronaut sleep periods. This improved the signal margins for the other information streams such as telemetry data.
The downlink normally had subcarriers at 1.25 MHz (NBFM voice) and 1.024 MHz (telemetry data). The telemetry could be at one of two rates, 1.6 kilobits/sec (low rate, 1/640 of the subcarrier frequency) and 51.2 kilobits/sec (high rate, 1/20 of the subcarrier frequency). High rate was used unless low rate was forced by poor link conditions, e.g., the use of a small earth receiving antenna, an omni
Omnidirectional antenna
In radio communication, an omnidirectional antenna is an antenna which radiates radio wave power uniformly in all directions in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often...
spacecraft antenna, or the need to conserve spacecraft power by turning off its RF power amplifier. (The S-band transponder on the S-IVB had no voice subcarrier.)
A "backup voice" mode was available that shut off the 1.25 MHz NBFM voice subcarrier and transmitted voice at baseband on the main PM S-band carrier. This provided a few more dB of margin when the link was unusually degraded but worse voice quality than the normal voice mode when conditions were good.
The two modes can be easily distinguished by how they react to signal fades. In the normal (NBFM subcarrier) voice mode the audio SNR is usually very high. But as the link degrades below threshold, impulse or "popcorn" noise appears suddenly and builds up rapidly until it overwhelms the astronauts' voices. An excellent example occurred during the Apollo 11 lunar landing when the lunar module structure occasionally obstructed the antenna's view of earth.
The backup voice mode behaves more like AM; there is a constant background "hiss" and the astronauts' voices vary with signal strength. This mode was used extensively during the Apollo 13
Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
emergency to conserve battery power in the LM Aquarius and during Apollo 16
Apollo 16
Young and Duke served as the backup crew for Apollo 13; Mattingly was slated to be the Apollo 13 command module pilot until being pulled from the mission due to his exposure to rubella through Duke.-Backup crew:...
because of the failure of the steerable S-band antenna on the lunar module Orion.
Emergency Key
The Apollo USB downlink also provided an "emergency key" capability consisting of a manually on-off keyed subcarrier oscillator at 512 kHz. Presumably this would have been used by the crew to transmit Morse CodeMorse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
if the downlink were too severely degraded to support even the backup voice mode. Although this mode had been tested (on Apollo 7
Apollo 7
Apollo 7 was the first manned mission in the American Apollo space program, and the first manned US space flight after a cabin fire killed the crew of what was to have been the first manned mission, AS-204 , during a launch pad test in 1967...
) and most of the astronauts were trained in its use, this mode was never actually needed during any Apollo mission. (Apollo 13 made extensive use of the "backup voice" mode, as did the Apollo 16 lunar module Orion due to a failed high gain antenna).
A similar uplink capability did not exist because the uplink budget had far more margin than the downlink. A typical Apollo S-band spacecraft transmitter produced 20 watts; a typical uplink transmitter produced 10 kW, a ratio of 27 dB.
Ranging
The Apollo S-band system provided for accurate range (distance) measurements. The ground station generated a pseudo-random PNPN
PN may refer to:* p-n junction, a type of junction in electronics* Pitcairn Islands, ISO 3166-1 country code** .pn, the country code top level domain for Pitcairn Islands* Pacific National, a rail freight company in Australia...
sequence at 994 kilobit/s and added it to the baseband signal going to the PM transmitter. The transponder echoed this PN signal back to earth on the downlink, and by correlating
Correlator
Correlator may refer to:* Correlation function * An optical correlator* A leak noise correlator* A correlation function...
the received and transmitted versions the precise round trip light time to the spacecraft could be determined very accurately (typically to within a few meters).
The PN sequence, although deterministic, had the properties of a random bit stream. Although the PN sequence was periodic, its period of about 5 seconds exceeded the largest possible round trip time to the moon so there would be no ambiguity in its received timing.
Modern GPS receivers work somewhat similarly in that they also correlate a received PN bit stream (at 1.023 Mb/s) with a local reference to measure distance. But GPS is a receive-only system that uses relative timing measurements from a set of satellites to determine receiver position while the Apollo USB is a two-way system that can only determine the instantaneous distance and relative velocity. However, an orbit determination program can find the unique spacecraft state vector
State vector
*A state vector in general control systems describes the observed states of an object in state space, e.g. in variables of the degrees of freedom for motion *A state vector in general control systems describes the observed states of an object in state space, e.g. in variables of the degrees of...
or orbital element set
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...
that most closely matches (usually in a least squares
Least squares
The method of least squares is a standard approach to the approximate solution of overdetermined systems, i.e., sets of equations in which there are more equations than unknowns. "Least squares" means that the overall solution minimizes the sum of the squares of the errors made in solving every...
sense) a set of range, range-rate (relative velocity) and antenna look angle observations made over a period of time by one or more ground stations assuming purely ballistic spacecraft motion over the observation interval.
Once the state vector has been determined, the spacecraft's future trajectory can be fully predicted until the next propulsive event.
Transponder ranging turn-around had to be manually enabled by an astronaut. It consumed an appreciable fraction of the downlink capacity and it was only needed occasionally, typically during handover from one ground station to the next. After the new uplink station achieved a 2-way coherent transponder lock in which the spacecraft generated its carrier at 240/221 times the received uplink frequency, the new ground station ranged the spacecraft. Then the ranging signal was turned off and the range measurement was continually updated by Doppler velocity measurements.
If for some reason a ground station lost lock during a pass, it would repeat the ranging measurement after re-acquiring lock.
FM and Video
The normal operating mode of an Apollo S-band downlink transmitter was PM. This mode provided for coherent Doppler tracking, uplink commands, downlink telemetry and two-way voice—but not television. Video signals, even that from the slow scan camera used during the Apollo 11Apollo 11
In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...
EVA, are much wider in bandwidth than the other Apollo downlink signals. The PM link margin
Link margin
In a wireless communication system, the link margin, measured in dB, is the difference between the receiver's sensitivity and the actual received power...
simply could not provide an acceptable picture, even when the largest available dishes were used.
A means was also needed to transmit wideband engineering and scientific data, such as that recorded on a tape recorder and played back at high speed.
The answer to both needs was wideband FM - frequency modulation. FM with a large modulation index
Modulation index
The modulation index of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level. It is defined differently in each modulation scheme. See:*Amplitude modulation index...
exhibits a capture
Capture effect
In telecommunication, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated....
or threshold effect. The output signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(SNR) can be significantly greater than the RF channel SNR provided that the RF SNR remains above a threshold, typically around 8-10 dB.
This enhancement comes at a price: below the FM threshold, the output SNR is worse than the RF channel SNR. Reception is "all or nothing"; a receiving antenna too small to capture the video cannot capture the narrowband elements either (e.g., voice).
The CSM carried separate FM and PM transmitters that could operate simultaneously, so voice and telemetry continued to be transmitted by PM while the video came down by FM. The LM only carried a single transmitter that could operate in either FM or PM, but not both. FM cannot be used for Doppler tracking, so the LM always transmitted PM during flight, reserving FM for when video was required (e.g., during a surface EVA).
Until the transition to digital, satellite television
Satellite television
Satellite television is television programming delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an...
also used wideband FM.
Interception
It is historically understood that the USSR did intercept the Apollo missions telemetry on the territory of the USSR, but no source in the former USSR military or intelligence services has come forth with any evidence of this happening. The USSR used different frequency bands for its own space missions, so by default its deep space network did not readily have equipment able to receive Apollo telemetry. Whether ChinaChina
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
or any other non-Western (or non-aligned) country at the time chose to intercept any of the Apollo telemetry is unclear. Amateur radio and affiliated telecommunications sector persons could listen to the Apollo telemetry the world over—provided they could afford the reception equipment.
Within the territory of the US it was legally possible for Amateur Radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operators to monitor the telemetry, but the FCC did issue a directive that required all disclosure of Apollo telemetry interception be cleared by NASA. Paul Wilson and Richard T. Knadle Jr. received voice transmissions from the Command Service Module of Apollo 15 in lunar orbit on the morning of August 1, 1971. In an article for QST magazine they provide a detailed description of their work, with photographs.
At least two different radio amateurs, W4HHK and K2RIW, reported reception of Apollo 16 signals with home-built equipment.