NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

NASA SP-504: Space Shuttle Avionics System

Section 4  System Mechanization/Operation

Communications and Tracking

The major functions performed by the communications and tracking (C&T) system include the following.

The RF links maintained by the system during on-orbit operations are shown in figure 4-34. Direct ground/ spacecraft/ground S-band links including voice, command, and a variety of data are available with both the NASA GSTDN and the USAF SGLS. Both S-band and Ku-band links are maintained with the NASA TDRS system of geosynchronous satellites; S-band command and data links are also possible with detached payloads. Ultrahigh frequencies are used for voice and data communications with extravehicular astronauts, and an S-band video link is provided from the astronaut to the Orbiter.

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Figure 4-34. - Orbital communication links.

The RF links maintained during atmospheric flight are shown in figure 4-35. In addition to the S-band direct and TDRS links, ultrahigh frequency (UHF) voice communications coverage is provided for ATC purposes. Three navaid systems are included for use after blackout: the tacan system at L-band, the MSBLS at Ku-band, and the radar altimeters at C-band.

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Figure 4-35. - Atmospheric flight links

The hardware associated with the various communications links and the other functions of the system can be grouped as shown in figure 4-36. Each of these groupings is discussed in the paragraphs to follow. The multiple antennas used in the system are shown in figure 4-37. All are flush-mounted and overlaid with thermal protective material except for the UHF airlock and the Ku-band deployable antennas. The antenna locations were chosen to optimize coverage to the extent possible within the constraints of available mounting space.

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Figure 4-36. - Hardware groupings.

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Figure 4-37 - Antenna locations.

S-Band Network System

Figure 4-38 contains a block diagram of the S-band network system, which provides tracking and two-way communications by way of phase modulated (PM) links directly to the ground or through the TDRS, and transmission of wide-band data directly to the ground by way of a frequency modulated (FM) link. The system is dually redundant, except for the RF contacts in the antenna switch, the diplexers in the preamplifier, and the antenna and associated RF cables. Either redundant LRU's are provided as shown or dual, electrically isolated, internal redundancy is used within boxes. As indicated in figure 4-38, the PM and FM systems are functionally independent except for the antenna switch assembly, which provides RF signal routing services for both. The antenna switch, controlled automatically by the DPS or manually by the crew, is used to select the antenna that provides the best coverage in a given situation. When operating the PM system with the TDRS, the preamplifier and the power amplifier are used to augment the signal available at the transponder. These components are not required for direct links. The transponders, the basic functioning units of the PM system, support full duplex operation, provide a specified phase-coherent turnaround ratio, and have the capability to retransmit range tones. A Costas detector is employed in the receiver and a spread-spectrum processor is activated in TDRS modes. The network signal processor (NSP) provides for interface of the S-band PM system with the audio system, with the instrumentation system, with the data processing system, and, when security is required, with the communications security (comsec) units. The NSP receives voice from the audio system, digitizes it using a delta modulation process, and multiplexes it with telemetry data from the PCMMU using time-division multiplexing (TDM). Then, depending on the operational mode, the signal is routed through or bypasses the convolutional encoder or the comsec unit or both and is finally sent to the transponder. The inverse of these functions is applied to data received from the ground. The FM system provides the capability for the transmission of data not suitable for incorporation into the limited-rate PM system. Included are main engine data, television, payload data, and playbacks of recorded telemetry.

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Figure 4-38. - S-band network equipment.

The system provides for several modes and data rates as shown in figure 4-39 for both the forward and the return links. The "forward" link as referred to here means the link from the ground to the Space Shuttle whether direct or through the TDRS. "Return" refers to the link from the Space Shuttle to the ground, again either direct or through the TDRS. Convolutional encoding/Viterbi decoding are used in the TDRS modes to improve bit error rates. The forward link receiving equipment is capable of handling data at two different rates as shown, with or without spectrum spreading, transmitted on any of four frequencies. A spread-spectrum technique, using a pseudorandom noise (PN) code rate of 11.232 megachips/sec, is used in the TDRS forward link to reduce interference with ground-based communications by spreading the power flux density impacting the Earth's surface over a wide bandwidth (BW). The four forward link frequencies are related to two return link frequencies and two turnaround ratios (ratios of Orbiter transmit to receive frequencies), NASA at 240/221 and Department of Defense (DOD) at 256/ 205. Two return link frequencies are used to minimize interference with payload communications, which may operate anywhere in the 1.7- to 2.3-gigahertz band. High and low data rates are available on both forward and return links, selectable as required to use the link performance margins available.

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Figure 4-39. - S-band network services.

S-band Payload System

Figure 4-40 contains a block diagram of the payload communications system, which provides the capability to communicate with a wide variety of satellites. The payload interrogator (PI) contains both a receiver and a transmitter. All signal processing is performed in the PSP. The PI provides 851 duplex channels for simultaneous reception and transmission of information with a noncoherent-frequency turnaround ratio of 205/256 in the SGLS mode (20 channels), and 221 / 240 in the GSTDN (808 channels) and Deep Space Network (DSN) (20 channels) modes. In addition, it provides four receive-only and six transmit-only RF channels in the DSN mode. If a payload and/or a mission requires nonstandard services, the capability exists either to route the signals to/from payload-unique processors through the payload station distribution panel (PSDP) in the Orbiter payload station, or to transmit them to the ground indirectly through the TDRS using the Ku-band bent-pipe capability.

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Figure 4-40. - S-band payload communications.

Ku-band Communications/ Radar System

The Ku-band system, shown in figure 4-41, serves a dual purpose — determining the range and angle to detached satellites for rendezvous missions, and providing two-way communications through the TDRS network. In both radar and communications modes, it uses a 0.9-meter (3 foot) parabolic monopulse tracking antenna, mounted inside the front of the Orbiter payload bay and deployed by rotation about a single axis after the payload bay doors are opened on orbit. In the radar mode, the system uses pulse Doppler, frequency-hopping techniques providing range, range rate, angle, and angle rate information on uncooperative, skin-tracked targets to a maximum range of 22.2 kilometers (12 nautical miles). In the Ku-band communications mode, the system provides various data rates and formats as shown on the figure. The digital rates extend continuously from 16 kbps to 50 Mbps; on the 4-megahertz analog channel, the rates extend down to dc.

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Figure 4-41. - Ku-nad radar/communication subsystem.

UHF System

Ultrahigh frequency transceivers are provided for voice communications with ATC facilities and chase aircraft during landing operations and for transmission of voice to and reception of voice and telemetry data from extravehicular astronauts while on orbit. Two antennas are provided, one in the airlock and one on the bottom of the Orbiter. A two-way voice interface with the Orbiter audio system is included, giving astronauts performing extravehicular activity (EVA) access to Orbiter voice communications on as many as three voice channels. Availability of three channels allows direct voice contact with the ground or the Orbiter crew, and provides for recording of the astronauts' conversations.

Extravehicular Maneuvering Unit Television System

A wide-band S-band FM receiver is provided for reception of video transmitted from the EVA helmet camera. The S-band hemispheric antennas and a spare port of the switch assembly of the S-band network communications equipment (fig. 4-38) are used to route the video signal to a 40-megahertz wide-band FM receiver. This receiver demodulates the video signal and routes it to the television (TV) system.

Audio Distribution System

The audio distribution system (ADS), shown in figure 4-42, provides intercom and radio access functions for the various crew stations and hardline "subscribers" involved in a mission. It includes facilities for audio processing, mixing, amplification, volume control, isolation, switching, and distribution. It provides paging capability, communication over various alternative bus circuits, distribution of caution and warning signals, and communication with the ground crews during preflight checkout. The ADS includes six audio terminal units (ATU's) distributed as indicated in the figure, two speaker microphone units, and an audio central control unit (ACCU).

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Figure 4-42. - Audio distribution system.

Television System

The TV system includes as many as nine onboard cameras, two large-screen monitors, two portable viewfinder monitors, and the associated switching and control logic. Three inputs are provided for TV signals from payloads and one output for viewing in an attached manned payload. The cameras, either color or black and white depending on the lens assembly installed, may be located in the cabin, at various locations in the payload bay, and on the RMS arm. All externally mounted cameras may be controlled remotely from the cabin. The capability is included to record TV data onboard and/ or to transmit it to the ground as indicated previously.


Three navaid systems are installed on the Orbiter for use during postblackout through landing phases (fig. 4-43). The tacan units, used from an altitude of approximately 21.3 kilometers (70 000 feet) to final approach, are versions of units widely used in military aircraft, modified slightly to interface with Orbiter systems. They provide slant range and bearing to a selected ground station. The MSBLS, used from an altitude of approximately 3 kilometers (10 000 feet) to touchdown, also a modified version of a military system, provides precise range and angle data with respect to antennas located near the landing runway. Data from both these systems are used in the Space Shuttle navigation and guidance software to provide steering commands during the approach and landing phases. The radar altimeters provide height above the local terrain from 1.5 kilometers (5000 feet) to touchdown. The data are used for display and crew monitoring purposes only.

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Figure 4-43. - Navigation aids.

Ground Command Interface Logic

Control and monitoring of the C&T system is a generally routine but continuous, time-consuming task. The ground command interface logic (GCIL) provides the capability for the ground controllers to assume much of this burden and thus to free the crew for other tasks. Ground-originated commands, sent through either the S-band or the Ku-band links, are decoded in the NSP and sent to the DPS, which interfaces with the GCIL. The GCIL includes logic which allows the flightcrew to block or supersede ground commands.

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