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6 Small Spacecraft Communications Technology BACKGROUND AND STATUS The present infrastructure for command, control, communications, and data recovery from NASA spacecraft consists of a number of facilities, such as the Tracking and Data Relay Satellite System (TDRSS); the Deep Space Network; and others, including commercially available services. This infrastructure is old and has been developed over many years. It is massive and costly in proportion to its presently envisioned uses with low-cost small spacecraft systems. Several studies examining ways to update these facilities have been performed in the past, but these study concepts did not consider the use of small spacecraft in conjunction with these facilities. The development of low-cost enabling technologies can greatly contribute to the overall effort in using small spacecraft for future NASA missions. The infrastructure is discussed further in Chapter 2 of this report. A second important area in communications covers application of commercial spacecraft to normal, every day, high-capacity voice and data communications in conjunction with the national and international public-switching networks. All aspects of every day life have developed a dependence on these communications services. Computer-dependent services, manufacturing facilities, financial institutions, health care services, entertainment, TV, etc., are utilizing today's spacecraft communications that have become an integral part of the national and international communications and data transmission infrastructure. Spacecraft communication systems also have been utilized for dedicated, specialized services as well as for government and military use. A multibillion dollar segment of private and government-owned industry has been developed, which is of vital importance in the overall economic structure of every nation, including the United States. With the rapid expansion of the wireless communications networks and cellular systems, and the initiation of worldwide personal communications networks, a number of innovative approaches recently have been proposed utilizing constellations of lightweight spacecraft. These proposed new systems utilize both low Earth orbit and higher-altitude orbits. Table 6-! lists a few of the recently proposed mobile systems. In recent years, with the exception of the Advanced Communications Technology Satellite (ACTS) spacecraft, NASA has not been involved in the new developments in 50
Small Spacecraft Communications Technology satellite communications. Private industry and DoD, however, have invested substantially in this field, in both technology as well as operational capabilities. Still, there are several areas where NASA could provide unique technological and operational contributions to enhance the pnvate-sector efforts. Launch vehicles and launch operations technology is an area where industry could benefit from NASA operations. Command, control, and tracking of space assets is another area where NASA experience could be very useful. TABLE 6-! Some Recently Proposed Mobile Satellite Systems 51 COMPANY SYSTEM Iridium, Inc. (Motorola) Loral/Qualcomm Constellation Communications Ellipsat Orbital Sciences Corporation Starsys Global Positioning IRIDIUM_'SM Globalstar Aires Ellipso Orbcomm Starsys One of the more important NASA contributions to industry will be the experimental development and evaluation of advanced technologies for use by modern high-capacity voice and data satellite communications systems. Examples of technologies that can contribute to future, low-cost small spacecraft missions are as follows: . . . satellite-to-satellite communication technology; new multiple access techniques such as Code Division Multiple Access; signal interference and other effects (channelization, error correction techniques, bandwidth compression, rain attenuation at higher frequencies, etc.) on quality of transmission; effects on communications and data transmission due to nonstationary spacecraft (handover from one Earth station to the next Earth station, Doppler frequency shift, etc.~; efficient utilization of the radio spectrum for mobile low-Earth-orbit satellite constellations; spacecraft antennas; and optical communications.
52 Technology for Small Spacecraft NASA PROGRAMS Since NASA has not been active in communications technology development in recent years, while industry and DoD have been very aggressive in promoting new systems and new technology, the opportunities for NASA to contribute significantly to small spacecraft communications technology in the near term are limited. For example, the NASA ACTS program for developing and space testing advanced communications concepts was initiated in the 1970s and was only recently launched aboard the Space Shuttle to flight test the technologies that were proposed at the beginning of the program. Meanwhile new concepts and new needs have been developed. OACT, in conjunction with JPL and LeRC, is overseeing the NASA communications technology program. The program addresses the following areas: ACTS experiments; commercial fixed and broadcast satellite communications; commercial mobile and personal satellite communications; NASA near-Earth missions communications; and NASA deep-space mission communications. The ACTS spacecraft is now operating in orbit and performing a variety of tests, such as spot-beam tests, on-board switching, and propagation at 20 and 30 GHz. Both JPL and LeRC are involved in this activity. Among other functions, ACTS serves as a testbed for mobile satellite communications technology programs, which also involves both fixed and mobile terminals at the Ka-frequency band which is being used more often since most of the lower frequencies are allocated. The direct utility of the ACTS technologies to near-term, small spacecraft systems is modest. The JPL communications technology program addresses both the technology needs for planetary space communication and the critical technologies for commercial satellite communications (such as optical [laser] communications and power amplifiers), which could be used for small spacecraft. JPL, in addition to identifying needs for future NASA missions, is envisioning the use of industry partnerships for identifying future commercial applications and for technology development and demonstration, including ground test programs for technology venf~cation. The LeRC program, besides the ACTS involvement, addresses space communications technologies. In fiscal year 1994, LeRC has budgeted $2 million for work on traveling wave tubes and on solid-state, gallium arsenide/indium phosphicie power amplifiers (Giffin, 19931. NASA also has a number of relatively modest development programs in place to support its optical communications technology needs at both GSFC and JPL, and those are aimed at some future generation of TDRSS. These systems could be useful for small spacecraft systems that require intersatellite links, but there is little likelihood that they will reach technological readiness in time for decisions on, for example, the commercial IRIDIUM_/SM system
Small Spacecraft Communications Technology Recently, an intercenter (IPL, LeRC, Langley Research Center, GSFC) systems- analysis team performed a study to identify priorities for technology development in support of OACT's small spacecraft technology program (Budinger et al., 19931. The team prepared a communications technology summary indicating that: electronically steered, (phased array) Ka-band antennas; Ka-band solid-state amplifiers; and Ka-band power modules were the highest priority, followed by source/channel coding, optical communications, and low-mass antennas. On-board processing was categorizes! as the next highest priority. DoD PROGRAMS DoD programs for lightweight communications subsystems and components are mainly directed toward the development of space defense systems. Developments are concentrated in the extremely high radio (EHF, 60 GHz) and laser frequencies, where over $550 million has been spent over the past ~ ~ years on military optical communications technology (Munro, 19931. In the 60 GHz range, both transmitters and receivers have been developed and demonstrated in a working link. Substantial work has been directed toward the application of millimeter-wave integrated circuit components to solid-state power amplifiers. In addition, work on 40-watt traveling wave tube power amplifiers is sponsored by the Navy. Work on electronically steerable, phased array antennas for use on spacecraft remains to be completed. Work in digital programmable modems has been sponsored by BMDO. In the laser area, work on laser sources, beam formation ant! control, and other components is underway. Two types of laser systems are under development, heterodyne systems and laser diode systems. Heterodyne systems require much less power than other laser systems for the same performance. The above technologies are applicable in spacecraft-to-spacecraft crosslinks. Work on these technologies is performed by the U.S. Air Force Phillips Laboratory with industry support. Some of these technologies will be very useful for small spacecraft programs of NASA and industry. 1NDUS TRY PROGRAMS Industry has extensively supported both the DoD and NASA programs. Starting with the ACTS program and continuing in the Military Satellite (MilSat) program and the BMDO work, a large number of major contractors, as well as small ones, have made substantial contributions to communications technology. In addition, industry has carried out proprietary company developments for application in commercial programs. A substantial effort has been expended in developing small spacecraft low-Earth-orbit systems concepts for commercial communication purposes. The technology utilized is a mix of the results from the government-sponsored developments and corporate proprietary efforts. 53
54 Technology for Small Spacecraft SPACECRAFT-TO-SPACECRAFT COMMUNICATIONS TDRSS is the first operational system to utilize spacecraft-to-spacecraft crosslinks. Each geostationary TDRSS spacecraft has the capability to communicate with as many as 22 spacecraft. Ground tracking and computation determines the position in space of each of the spacecraft. This information is transmitted to the TDRSS spacecraft and through the spacecraft's multi-element, electronically controlled antenna, the proper beam is formed in order to establish a link with the other spacecraft. Due to the relative motion between the two communicating spacecraft, a Doppler frequency shift takes place, which must be recognized and compensated for. In the general case, the frequency shift and the establishment of the communications link result in complexities and difficulties that limit the capabilities of a system. These problems are more complex in systems with constellations of many spacecraft in a nonstationary orbit, especially if there is a requirement for each spacecraft to communicate with several others simultaneously. The utilization of optical communications is expected to be very beneficial for the space crosslinks, since lasers are highly directive and can accommodate high data rates. Development of laser technology for spacecraft-to-spacecraft communications is currently underway bv NASA and DoD. -I - J In addition, technology utilizing radio frequency communications is currently available commercially for the most simple cases. The effort has been concentrated in direct digital synthesizers, solid-state amplifiers, and low-weight antennas. In addition to the industry-sponsored developments, the U.S. Air Force Phillips Laboratory has~also been active in these technologies. MULTIPLE ACCESS When simultaneous transmissions from a number of transmitters are received by the same receiver, a protocol is required in order for the receiver to reconstruct each message correctly. There are two multiple access protocols frequently in use in satellite communications systems: Frequency Division Multiple Access and Time Division Multiple Access. For security and radiation-hardening purposes or in case of lack of adequate bandwidth or for other reasons, other multiple access schemes have been devised. For example, some of the proposed low-Earth-orbit wireless telephone systems plan to use Code Division Multiple Access techniques. With radio frequency bandwidths becoming scarce due to overcrowding, and with the need for low-power, lightweight, mobile receive/transmit hand sets, the need for proven, efficient, multiple access techniques becomes pressing. This is another area in which advanced technology could have a high payoff for small spacecraft.
Small Spacecraft Communications Technology COMMUNICATIONS COMPONENT TECHNOLOGY Both spacecraft-to-TDRSS and spacecraft-to-ground links require near- hemispherically steerable, efficient antennas. Flat-plate, phased array antennas with a 10- dB antenna gain appear readily available. NASA is developing a three-dimensional phased array that can be electronically steered approximately 60 degrees off perpendicular and with a 24-dB on-axis gain. The projected weight is 4.5 kilograms, which could be excessive for some small spacecraft missions but may be amenable to weight reduction through additional research and development. On-board spacecraft computers have often lagged behind the state of the art. As a part of the GSFC Small Explorer program, an 80386/80387 processor has been qualified and flight tested on the Solar Anomalous ant! Magnetospheric Particle Explorer spacecraft. Data storage is provided by a high-density solid-state recorder. Technology developed by ARPA has been adopted and modified by NASA to produce a 1.4 gigabit- per-card solicI-state recorder with latch-up protection. Error-detection codes and correction codes are employed to eliminate other errors with approximately a 12 percent coding burden. Military Standard 1553 and 1773 data buses are available. Programming is done in C language. Cabling occupies a significant part of a spacecraft's mass budget. GSFC is working with DoD on the Fiber Optics Data Bus project to reduce this burden. NASA is responsible for low-data-rate systems, while DoD is addressing high-data-rate systems. The base of expertise for the development of solid-state spacecraft transmitters rests with industry. NASA has in the past contributed to the development of high-power traveling-wave tubes and has internally built a number of solid-state amplifiers. The laboratories associated with DoD have been a source of space-qualified parts for NASA. Currently, however, all high electron mobility transistors made of gallium arsenide/indium phosphide for use in solid-state amplifiers, are supplied by one of two Japanese companies: NEC or Fujitsu. A core problem has been the lack of an economic incentive for private semiconductor and electronics firms to maintain the capability to provide space-qualified parts and systems, which are only purchased in small lot sizes. Other less demanding opportunities exist in the commercial market, where lot sizes are many orders of magnitude larger. SPECTRUM UTILIZATION With the explosive growth of the communication needs, which demand more and more transmission bandwidth, the available radio spectrum has become overcrowded. The increasing need for transmission of data at very high speeds and very low bit-error rates has aggravated this problem. In addition, the need for low-power, low-weight transmitters adds to the problems. Several existing techniques are being continuously improved, while new ones are being invented for the solution of this problem. Examples of these techniques are (~) new, more spectrum-efficient modulation and multiple access 55
56 Technology for Small Spacecraft technologies; (2) new multiplexing techniques; (3) frequency reuse; (4) signal coding techniques and forward error correction; ant! (5) more efficient antennas. The rapid growth of optical communications will have significant impact on this area by freeing radio frequency spectrum from the present demands. The panel expects that by freeing up the radio frequency spectrum, substantial new opportunities will present themselves for mobile and remote area telecommunications. NASA should become the technical leader in this expected future re-apportionment of frequencies and open new possibilities for space communications. ENDINGS AND PRIORITIZED RECOMMENDATIONS Communications technology is fundamental to the global economic infrastructure. Except for the ACTS program, NASA has not significantly invested in communications technology or monitored industry developments. Small spacecraft technology could play a substantial role in the development of the global communications infrastructure as well as the economic development of many geographical areas. In order to enhance communications technology for small spacecraft, the pane! makes the following recommendations for NASA: ~J 1. Development of the following technologies should be supported: . an electronically steered Ka-band phased array antenna; a Ka-band solid-state amplifier; and a Ka-band power module. 2. Optical frequency (laser) communications systems and components (e.g., electronically controlled antennas and signal processing) should be developed for space to-space links. 3. Radio frequency space-to-space links, the associated components, and spacecraft antenna systems for complex spacecraft constellations in both low Earth orbit or other orbits should be developed. 4. New, multiple access schemes and the associated critical components should be developed, as well as optimization of bandwidth utilization in the mobile satellite frequencies for low-Earth-orbit systems. 5. NASA should be the technical leader in developing the rationale for radio frequency reassignments in view of the new optical communications developments.