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 3  Communications

The Space Shuttle communications design community was faced with a variety of requirements, many conflicting, and many unique to or faced for the first time in the Space Shuttle Program. The network communications system had to accommodate voice, command, and data traffic with the NASA Ground Spaceflight Tracking and Data Network (GSTDN), with the USAF Space Ground Link System (SGLS), and with NASA Tracking and Data Relay Satellites (TDRS's) in geosynchronous orbit. The downlink data to be accommodated ranged from real-time operational telemetry to television to wide-band payload/experiment data. Because the Orbiter would operate as an aircraft in atmospheric flight, the system would have to provide for air traffic control (ATC) type interfaces from postblackout through landing. Also because of the operation in the atmosphere, all antennas had to be either flush-mounted under the thermal protection system or deployable on orbit and retractable for ascent and entry. Other factors which influenced the design included requirements for communications security, all-attitude operation, coherent Doppler for navigation, voice and data links to an extravehicular astronaut, text and graphics uplink, active and passive tracking of satellites for rendezvous, and extensive remote control capability from the ground to reduce crew workload. In addition, the always overriding requirements to minimize weight, power, volume, and complexity and to use off-the-shelf equipment were present and contributed to the system configuration.

The GSTDN and SGLS networks both operate at S-band frequencies with direct ground/spacecraft/ground link performance requirements similar to those encountered on previous low Earth orbit missions. The TDRS operates at both S-band and Ku-band frequencies but with much more stringent link performance requirements because of the distances, look angles, and dynamics involved in operations with a synchronous orbit communications terminal. Off-the-shelf S-band transponders which would have allowed operation with either ground network were available, but none could meet the TDRS link margin requirements. Because the requirement existed for communications coverage during ascent, on-orbit, and entry phases when out of sight of ground stations and operating with the flush-mounted, low-gain antennas, the decision was made to develop new S-band hardware which would provide for basic operational voice/command/telemetry traffic through either ground network or the TDRS using an integrated onboard system.

The basic Space Shuttle operational network communications requirement called for voice channels (up and down), an uplink command channel, a telemetry downlink channel, two-way coherent Doppler for ground navigation, a ground ranging capability, and provisions for communications security. The stringent TDRS link performance requirements, exacerbated by the low-gain antennas, drove the system to an all-digital signal design using time-division multiplexing (TDM) to integrate the voice and command or data channels into a common bit stream. An adaptive delta modulation technique using a modified version of the ABATE algorithm was chosen to digitize the voice channels after extensive in-house laboratory tests showed that this method maintained high word intelligibility with reasonable voice quality at minimum sampling rates in the presence of very high channel errors. To achieve optimum performance on these digital channels, a phase modulation (PM) system was developed which included one or two voice channels multiplexed with an encoded 8-kbps command channel on the uplink or with a 128- or 64-kbps pulse code modulated (PCM) telemetry bit stream on the downlink. To maintain adequate circuit margins and bit error rates at these data rates for the Space Shuttle/TDRS link, it proved necessary to develop a 100-watt traveling wave tube power amplifier transmitter and a low-noise preamplifier receiver, both of which pushed the state of the art, and to employ sophisticated error-correcting channel-encoding techniques. After a series of tradeoff studies, convolutional encoding and Viterbi decoding were selected to optimize the link. Phase-shift keying (PSK) was also selected to optimize channel performance with a Costas loop to provide for carrier reconstruction and data recovery on both forward and return link signals. To provide accurate Doppler data which would allow the ground control center to determine and maintain the Space Shuttle ephemeris, the S-band uplink and downlink signals were made coherent. Range tone turnaround was incorporated to provide a direct ranging capability at GSTDN stations for ascent and postblackout entry state vector determination. An additional constraint on the system was imposed by international agreements which specify the maximum allowable power flux density received at the Earth's surface from an orbiting satellite. To avoid exceeding this limit on the TDRS to Orbiter link, it proved necessary to develop a direct-sequence spread-spectrum signal design using pseudorandom noise (PN) code modulation.

The Space Shuttle/ground direct S-band link was also required to accommodate wide-band telemetry data from the main engines during ascent, data dumps from onboard recorders, payload data (analog data up to 4 megahertz, digital data up to 5 Mbps), and video from the onboard television system. Because these types of data were not amenable to incorporation into the limited-rate PCM telemetry data stream described previously, a separate S-band system was developed for this purpose using frequency modulation (FM) and an independent signal processor, transmitter, and antennas.

Initially, the communications and data requirements of the payload community were either unknown or appeared totally open ended. A major mission of the Space Shuttle was to service, deploy, or retrieve a wide, and largely unknown and unpredictable, variety of satellites. Therefore, it posed a major engineering challenge to develop a communications system with some chance of longevity which could generate commands having payload-compatible formats, data rates, and carrier frequencies; monitor telemetry signals having various standard formats, data rates, and subcarrier and carrier frequencies; and relay nonstandard telemetry to the ground without onboard subcarrier demodulation and bit synchronization. A series of meetings was held in the early seventies with various involved government and commercial organizations in an attempt to develop a real and manageable set of requirements. This activity culminated in a major conference in 1974 at which all prospective payload developers were invited to make suggestions as to how the Space Shuttle could best serve their needs for command and data rates, formats, modulation schemes, and carrier and subcarrier frequencies. It soon became apparent that, to satisfy the entire community, the Space Shuttle would have to provide all the functions and capabilities of all the satellite ground stations and support networks developed to that time. Although technically feasible, the provision of an essentially open-ended service would have been prohibitively expensive; therefore, the decision was made to provide only normal baseband signal processing functions for a limited number of modulation schemes, subcarriers, bit rates, and PCM formats; and to implement a wide-band, transparent- throughput, indirect-transmission (bent pipe) capability to relay "nonstandard" payload signals through the TDRS Ku-band link for ground monitoring and analysis.

The requirement for wide-band (50 Mbps) Ku-band data transmission through the TDRS dictated the use of a high-gain deployable antenna with tracking capability over a wide angular range. Early in the design phase, it became apparent that this antenna, its control elements, and major portions of the associated communications hardware could also serve the Orbiter-to-satellite radar tracking requirement with significant savings in weight for the inconvenience of having to interrupt wide-band TDRS communications when performing the radar function. This concept was baselined and the combined Ku-band radar/communications system design which evolved included an antenna assembly, an antenna controller, a transmitter, a receiver, and associated microwave components common to both functions.

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