Orbiter Communications

Orbiter Communications

Communications Windows Microwave Band Signal Characteristics Orbiter Communications Systems Orbiter S-band Communications Orbiter Ku-band Communications Orbiter UHF Communications Orbiter Audio Communications Orbiter Operational Instrumentation (Telemetry) Orbiter Payload Communications Orbiter Communications Antennas NASA’s Communications Networks

Communications Windows

Communications - Space
Space communications is limited to the microwave band because of atmospheric attenuation and scattering at most other frequencies Three windows through the Earth’s atmosphere are found at: Visible band – not always available because of clouds & rain Infrared band – too narrow and poor background signature (hot spots looking upward from the ground and downward from space) Microwave – useful from approximately 1-10 GHz

Communications through more dense atmospheres (Venus, Titan) have only microwave windows The microwave band has become a standard for space communications 1-10 GHz typical through the Earth’s atmosphere Much higher frequencies used for space-to-space communications (10-50 GHz) since there is no interfering atmosphere

Most of the Earth’s atmospheric attenuation of microwave signals comes from the oxygen and water molecules Scattering of electromagnetic radiation is most common from water droplets Lowest noise band available for Earth-space communications is roughly 1-10 GHz

Microwave Band

Communications – Microwave Band
Microwave frequencies are defined as 300 MHz–300 GHz 1 MHz = 1 Mega Hertz = 1 Million Hertz = 106 Hz 1 GHz = 1 Giga Hertz = 1 Billion Hertz = 109 Hz Common designations for the microwave bands used for spacecraft communications are: UHF MHz to 3 GHz L-band GHz S-band GHz * C-band GHz X-band GHz K, Ka, Ku-band GHz * Most commonly used space-terrestrial communications band

Communications – Microwave Band
Higher frequency X-band and Ku bands are used in space since there is no atmospheric interference The higher frequencies also have the advantage of higher data transmission rates which means higher bandwidth Higher bandwidth offers greater signal density Higher signal density allows more instruments and/or data on increasingly complex spacecraft Newer technology also allows higher signal density with lower mass and lower power consumption

Signal Characteristics

Communications – Signals
Signal characteristics The three fundamental characteristics of the communications signal are 1. Center (or carrier) frequency Frequency of transmission and reception 2. Signal bandwidth Signal data capacity 3. Modulation and encoding Used to format the data to make it compatible between the transmitter and receiver

Center frequency Orbiter communications covers three different microwave frequency bands UHF S-band Ku-band Each of the Orbiter's numerous communications systems includes two center frequencies, one to transmit signals and a separate frequency to receive signals simultaneously Known as duplex communications

Center frequency Each of the communications system operate on two frequencies called duplex links Uplink transmission from the ground station that is received at the spacecraft Downlink transmission from the spacecraft received at the ground station The exception to the duplex link is the Orbiter's S-band command data link The simpler format called half-duplex allows transmission and reception, but not simultaneously

Signal bandwidth Higher frequencies have inherently higher bandwidths unless the design incorporates a smaller bandwidth for lower noise content. The highest bandwidth in the Orbiter communications links is the highest frequency High-bandwidth Ku band is used for the TDRSS satellite link Lowest frequency link on the Orbiter which is the UHF communications links that carry voice communications and also has the smallest bandwidth

Data and signal modulation Data and data signals are modulated in two stages The first stage is data modulation Data are first encoded for easy digital conversion, transfer and identification These modulation types include Pulse code modulation (PCM) – the most common spacecraft data modulation scheme Phase modulation (PM) Phase shift key modulation (PSK) Pulse amplitude modulation (PAM)

The Orbiter communications system's data modulation is Pulse Code Modulation (PCM) Analog signals are converted into digital signals by sampling circuits that function at specific levels (8 bit, 24 bit, 32 bit, 64 bit, etc.)

Data and signal modulation The second modulation stage is signal modulation Data that are communicated between spacecraft and ground stations are handled at a much lower frequency than the 2-3 GHz frequency used to transmit and receive the signals Therefore, the data must be mixed with the carrier (center) frequency that is in the 2-3 GHz frequency range The two types of Orbiter RF signal modulation are FM frequency modulation) and PM (phase modulation)

Following the two modulation stages in the transmitter and two demodulation stages in the receiver, the signal output from the receiver will be approximately the same as the signal input into the transmitter The difference in the two signals is a function of the quality of the transmitter and receiver, and the influence of external and internal noise Reproduced signal quality is determined by the communications system design

Orbiter Communications Systems

UHF Voice Duplex and simplex S-band Data, voice Duplex Ku-band Video, data

Orbiter Communications Data Types
Telemetry Downlink data of the Orbiter's operating conditions and configurations, systems, payloads and crew biotelemetry measurements Command Uplink data directed to the Orbiter systems to perform functional or configuration changes Rendezvous and tracking Onboard radar and communications system for tracking and performing rendezvous with orbiting satellites/spacecraft Video Video imaging is used onboard, or relayed to ground from the crew cabin or on EVA activities, or from the payload bay, or from the remote manipulator arm Voice communications Intracommunications between the flight crew members, and between the flight crew and ground Documentation Printed data from the Orbiter's thermal impulse printer system

Orbiter Communications Data Types
The Orbiter communications system frequency bands include: 1. S-band PM (Phase Modulation) FM (Frequency Modulation) Payload 2. Ku-band TDRSS data & video communications Rendezvous radar 3. UHF voice Ground EVA Note: Voice communications are also available through the military TACAN unit Other frequencies are used for the Orbiter's navigation subsystems and include C-band for the radar altimeter, L-band for the GPS and TACAN units, and Ku-band for the MSBLS landing system

Orbiter S-band Communications

S-band S-band communications are the most versatile of the Orbiter's communications bands Payload data, telemetry, commands, voice, and some video are handled with the multiple S-band units The versatile functions of the S-band communications include two modulation types Phase modulation (PM) Frequency modulation (FM)

S-band The Orbiter's S-band communications are used for Inter-Orbiter communications TDRS satellite uplink and downlink Payload communications Telemetry to/from ground Video and audio to/from ground DoD payloads (discontinued)

S-band PM The Orbiter's S-band Phase Modulation unit is the primary communications system which provides a duplex link between the Orbiter and ground, either through the STDN stations or through the TDRSS relay satellite S-band PM is the most versatile of the Shuttle's communications modes, providing communication channels for four primary functions

S-band PM Command channel - used to send commands from ground control to the Orbiter Voice channel - used for one-way and two-way voice communications between ground and Orbiter. Also used for the thermal impulse printer system Telemetry channel - carries real-time Orbiter and payload operational telemetry data to ground Turnaround tone ranging channel - used to aid in tracking the orbiter A precise RF carrier is transmitted to the Orbiter for timing and Doppler measurements

S-band PM S-band PM uplink The Orbiter's S-band duplex forward (up) link operates through the STDN or TDRS Carrier frequency is at either MHz (primary) or MHz (secondary) for the NASA networks S-band PM downlink The S-band duplex return (down) link also operates through the STDN or TDRSS Phase modulation center carrier frequency is at MHz (primary) or MHz (secondary) S-band PM Department of Defense S-band link (discontinued)

Transponders Dual S-band PM transponders operate as multipurpose, multi mode transmitter/receivers Each can simultaneously transmit and receive, or transmit only, or receive only, although only one transponder operates at one time Transponders allow commands, telemetry and voice data through the communications network The S-band transponders provide coherent (stable, timed) measurements on the PM up and down links for two-way Doppler data for spacecraft velocity data, and two-way tone ranging for spacecraft slant-range distance data

Transponders Doppler and ranging signals are available for tracking while in line-of-sight from the NASA Spaceflight and Tracking Data Network (STDN) ground stations during launch, lift-off, ascent, or landing, or when it is in view of Space-Ground Link System (SGLS) ground stations The third tracking dimension comes from the ground station's antenna elevation and azimuth The two-way Doppler function operates through the TDRSS, but the two-way ranging does not

S-band FM The Orbiter's S-band FM system is used exclusively to downlink telemetry data from as many as seven different sources Limited to one source at a time S-band FM downlink operates at a center frequency of MHz and is available through the STDN or Air Force ground stations S-band FM downlink does not operate through the TDRSS system

S-band FM selection

S-band FM The S-band FM telemetry data sources include : Real-time SSME data from the engine interface units from prelaunch through MECO (ME) Real-time video (TV) Operations recorder dumps of high- or low-data-rate telemetry at kbps (OPS RCDR) Payload recorder at 25.5 kbps or kbps (PL RCDR) Payload analog at 300 Hertz or 4 MHz (PL ANLG) Payload digital data at 200 bps or 5 Mbps (PL DIGITAL) DoD data at 16 kbps or 256 kbps in real time or 128 kbps or kbps of playback (DOD)

Orbiter Ku-band Communications

Ku-band The Orbiter's Ku-band system is a dual-function unit Communications system Tracking/rendezvous radar system Not available simultaneously Ku-band high-frequency, high-bandwidth unit operates from MHz to MHz, with carrier frequencies of GHz from the TDRSS (return/uplink) and GHz from the Orbiter (forward/downlink) Being increased to 22.5 to 27.5 GHz for new TDRSS capabilities Ku-band can be used for TDRSS space-to-space communications since there is no atmospheric interference that effects space-to-ground links

Ku-band Ku-band frequencies are roughly six times higher than the Orbiter's S-band center frequency which offers a much larger bandwidth Advantages of the Ku-band link are found in its high-bandwidth video capability which is extremely limited in S-band Because the Ku-band antenna is located in the payload bay, the system can only be operated while on orbit and while the payload bay doors are open

Ku-band unit The 1-meter single antenna can be rotated 360o in roll and 162o in pitch Some pointing positions can be blocked by the Orbiter depending on its attitude and orbit position with respect to the TDRS satellites Pointing for TDRSS communications or for the radar tracking can be made manually, or are automated using the General Purpose Computer's background SM software

Ku-band block schematic

Ku-Band rendezvous radar The Orbiter's Ku-band rendezvous radar functions as a traditional radar Uses skin reflections and signal path timing for distance measurements It also has transponder capability Tracks other spacecraft or payloads in orbit by the identification of their unique signal reply Like aircraft, transponder coding offers a much better return signal which increases target identification distance and improves range accuracy, provided the spacecraft has a compatible transponder

Ku-Band rendezvous radar Manual or automated search routines are available to gimbal the Ku-band antenna to search for orbital hardware Rendezvous radar identification is a function of the radar sensitivity, and the range, reflective cross section, surface reflectivity, and the transponder performance Ku-band radar range is approximately: 30.5 m to 27.8 km (100' to 15 nm) for passive (reflection) targets 30.5 m to 555 km (100' to 300 nm) for active (active transponder) targets

Ku-band radar and communications switches

Orbiter UHF Communications

UHF band Voice communications between crew members on the Orbiter, and launch control and mission control personnel is backed up with the narrow-band UHF communications system Orbiter's UHF system is used as primary EVA crew communications between the cabin crew UHF can also be used in the half-duplex or simplex mode for communications through the STDN or SGLS ground stations UHF communications may also be used for voice communications during approach and landing through the TACAN

UHF band UHF can be used as a two-way audio link with the Shuttle Training Aircraft during launch UHF signals (uplink and downlink) are routed through the external UHF antenna on the Orbiter's bottom forward fuselage UHF voice communications are available through: Cabin-EVA link Airlock-EVA link TDRS (backup) STDN and SGLS (simplex, backup) TACAN (backup)

UHF EVA operations UHF duplex communications employed on Orbiter EVA includes the following features: Biotelemetry and suit data transmitted to Orbiter at MHz Biomedical data are replaced with suit telemetry data every 2 minutes for 15 seconds Audio transmit frequency is MHz for EVA-A astronaut and MHz for EVA-B astronaut Receive frequencies are (A) and MHz (B) Airlock communications are available through the airlock antenna

UHF-band schematic

Orbiter Audio Communications

Audio Communications Voice communications between crew members and ground are furnished by the S-band, UHF and Ku-band links Multiple audio links provide near-continuous communications between ground and Orbiter crew Interrupted during the Zone of Exclusion (ZOE) region that is dictated by the TDRSS orbit coverage Communications as well as navigation signals are also interrupted during the during the high-temperature phase of reentry

Audio Communications Audio on the Orbiter is transferred between communications points by the Audio Distribution System (ADS) which integrates the signal sources for distribution throughout the Orbiter, and the S-band and Ku-band links with ground Audio communications are available on the following links: Downlink to launch control and mission control are through the Ku-band and the S-band links Interconnect with launch control and mission control on the launch pad are made through the T-0 launch umbilical panel TACAN

Audio communications schematic

Orbiter Operational Instrumentation
Vehicle Telemetry

Orbiter Operational Instrumentation (OI) system Orbiter systems are closely monitored by the instrumentation system which consists of Sensors, or transducers Signal conditioners that bring sensor voltages/currents to digital circuitry levels on the MDM inputs Multiplexer/Demultiplexers (MDMs) Pulse Code Modulation Master Units (PCMMUs) Operational recorders Payload recorders Master timing equipment Onboard checkout equipment The OI system monitors more than 3,000 parameters for processing and display, either through downlink telemetry or onboard readouts

Orbiter Operational Instrumentation (OI) system Operational instrumentation data begins with the sensor acquisition Weak transducer signals are converted into digital logic levels for input into the Mulitplexer/Demultiplexers for initial processing Instrumentation data are then transferred to the Pulse Code Modulation Master Unit for data formatting Data are then routed to the Network Signal Processor There the data are interleaved with audio, video, and other telemetry for transmission through the S-band and Ku-band downlinks, and the operational recorders for later downlink

Simplified block diagram of the Orbiter Instrumentation System

Orbiter instrumentation system component characteristics

Orbiter Payload Communications

Payload Communication System (PCS) The Payload Communication System (PCS) is used to transfer data to and from Orbiter's various payloads Transferred over hardwire lines or on dedicated S-band payload data links Payload patch panel in the flight deck is used to direct payload data in one or more paths Data from the payloads can be routed directly to the downlink without processing, or through the Pulse Code Modulation Master Unit and processing Data can also be recorded for later downlink

Payload Communication System (PCS) PCS is also used to activate, deactivate, and check out attached and deployed payloads S-band payload data are transferred through a hemispherical antenna located on the upper forward section of the Orbiter The link can be used for communications with attached and free-floating payloads

Payload data flow schematic

Orbiter Communications Antennas

Orbiter antennas Except for the Ku-band system, the antennas on the Orbiter are placed on the vehicle exterior, but protected from the extreme heat of reentry by the surface insulation A wide variety of antenna designs on the Orbiter are matched to the operating frequency and directional requirements Antennas include S-band Ku-band deployable antenna (radar & communications) UHF L-band antennas for the TACAN and GPS systems C-band antennas for the radar altimeter Ku-band antennas for the MSBLS landing system

Orbiter antennas S-band antennas are designed to cover either a broad-beam 180o and placed on the top and bottom of the vehicle, or a narrower 90o beam and placed on four points around the Orbiter Both provide a full 360o coverage Broad-beam antennas used for the S-band FM communications, called hemisphere antennas, are located on the top and bottom of the forward fuselage

Orbiter antennas 90o S-band PM antennas are called quadrature antennas because they provide a full 360o coverage with four antennas at four quadrant points Two at the top and two at the bottom of the forward fuselage UHF and S-band payload communications antennas are also placed on the forward fuselage beneath HRSI or LRSI/FRSI surface tiles/blankets Like the S-band FM and PM antennas

Antenna placement - forward fuselage

Antenna placement including navigation signals

NASA’s Communications Networks

NASA communications networks NASA's space communications network is called the Spaceflight Tracking and Data Network (STDN) The STDN consists of the following: Ground station network (GN) originally established for the Mercury program Space network (SN) that consists of the TDRSS satellites and the ground station in White Sands, New Mexico A network link system called the NASA Communications Network (NASCOM) NASCOM is an amalgamation of national and international communications channels that interconnect the NASA and USAF launch and control sites, control centers, tracking sites, and support functions and locations.

NASA communications networks NASA's primary mission launch and control sites at the Kennedy Space Center and the Johnson Space Center are but two of the main centers that manage mission data on NASCOM. The primary network switching center at the Goddard Space Flight Center (GSFC) directs worldwide network operations including those at other communications and mission centers located at the Jet Propulsion Lab in Pasadena, California, and the three Deep Space Network sites Additional network facilities and support are provided by the Air Force communications centers at Cape Canaveral, Florida and Vandenberg AFB, California

TDRSS – Tracking and Data Relay Satellite System NASA's Tracking and Data Relay Satellite System was developed to overcome the spotty communications coverage by NASA's STDN ground tracking stations used for manned spacecraft missions and orbiting satellites The Mercury-era ground station network offered only partial communications service, and had to be manned continuously because of the many satellites that were in orbit at any one time

The final design configuration for NASA's space-based communications network employed two satellites in geostationary orbit with a fixed ground station link This arrangement provided coverage for at least 85% of low Earth orbit spacecraft A third geostationary spacecraft designed to serve as a spare is also used to relay data between the two active communications satellites and the fixed ground station through a Ku-band link

TDRSS satellite placement

TDRSS Ku-band is used for the ground-satellite link located at White Sands Ku-band has marginal all-weather propagation through the atmosphere in moderate-to-heavy precipitation White Sands, New Mexico was selected for the ground site because of its high annual clear day average and its low precipitation climate White Sands site is also Federal land established originally for flight tests, and the launch and tests of suborbital rockets

TDRSS satellites TDRSS satellites support data relay communications on two primary bands, the S-band and the Ku-band Telemetry and command data that supports the TDRS satellites from the ground station is available in C-band on a separate antenna and receiver/transmitter system The S-band and Ku-band communications are carried by two sets of antennas partitioned in frequency and accessibility Two types of data access are available SA – single access MA – multiple access

TDRSS satellites SA – single access Ku-band service on the TDRS is carried through two 4.9 m (16') graphite steerable parabolic reflector antennas Data use on the Ku-band antennas is dedicated to specified users, and includes spacecraft tracking and ranging information Normally allocated to high-bandwidth users such as the ISS, HST, Landsat satellites, and when in orbit, the Space Shuttle Each of the two parabolic antennas has two access frequencies Ku-band S-band

TDRSS satellites MA - multiple access S-band multi-access antennas available for up to 20 users Multi-user S-band service called MA includes 30 helical antennas that are not steerable Does not provide tracking or ranging information

TDRSS satellite

TDRSS Ground Station The TDRSS ground element includes the dual-redundant ground station links and the data distribution and control network Ground operations also include the TDRS satellite control and maintenance operations that support the spacecraft and maintain their orbital position

Two separate communications installations are collocated at the site Both have duplicate network data handling and services Cacique, the name for the first White Sands Ground Station was completed in 1978 Danzante was completed in 1991

TDRSS Coverage Geometric coverage of the typical circular
LEO orbit is greater than 88% as shown

TDRSS Coverage and Zone of Exclusion (ZOE)

TDRS Spacecraft Payload 17.4 m span including solar arrays
14 m from SA antenna edge to antenna edge 2,270 kg (5,000 lb) at launch Minimum expected lifetime is 10 years on-orbit Built by TRW

The End

Orbiter Communications

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