Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 50
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
OCR for page 51
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.
OCR for page 52
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
OCR for page 53
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
OCR for page 54
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.
OCR for page 55
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
OCR for page 56
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.
Representative terms from entire chapter:
multiple access
