Switchboard in the Sky

NASA FactsNational Aeronautics andSpace AdministrationGlenn Research CenterCleveland, Ohio 44135–3191 FS-2002-06-013-GRC

The Advanced Communications Technology Satellite (ACTS)

On September 12, 1993, a new and soon-to-be award-winning on-ramp to the informationsuperhighway was opened to heavy traffic: NASAlaunched one of the most revolutionary break-throughs in space communications history, theAdvanced Communications Technology Satellite(ACTS).

ACTS is the first high-speed, all-digitalcommunications satellite. It is the first to havesophisticated telephone-system-type switchingonboard. It can carry data at standard fiber-optic data rates with the same transmissionquality, added performance, and cost savingsthat a land-based network provides. It operatesin the uncrowded Ka-band (18 to 30 gigahertz)portion of the radio spectrum. ACTS technologyintegrates well with ground telephone networksystems, allowing for high-speed transmissionover great distances to even the remotestlocations.

For more than 6 years, ACTS was thetestbed where Glenn researchers demonstratedthe abilities and reliability of the system theydesigned and where U.S. companies andinstitutions tested their own uses of the NASAtechnologies.

The early intention was to solve anotherexpected problem: crowding of the geosynchro-nous orbit where communications satellitesfly. Satellites in geosynchronous orbit fly22,300 miles above the equator at a speed thatmatches the rotation of the Earth. Satellites ofthe same frequency must be positioned, byinternational agreement, at least 4° apart toavoid interference between signals.

Commercial companies need greater com-munications capacity to support their manyservices, such as high-speed and high-securityfinancial transactions for banks and wirelessmobile networks for consumers. Countries theworld over are clamoring for their own satellites,particularly those countries without the infra-structure for a reliable ground communicationsnetwork.

Artist's concept of ACTS.

As early as the 1970’s, Glenn researchersand their industry collaborators were awarethat the demand for geosynchronous orbitalslots would all too soon exceed capacity. Theydesigned the technology, then launched ACTSto demonstrate that their new technology couldmeet the growing communications needs of ademanding public. Well before the end of theformal demonstration project in May 2000, theanswer was a resounding yes.

SPACECRAFT DESCRIPTIONACTS, which was built and operated by

Lockheed Martin Corporation, was launchedon September 12, 1993, as the primary payloadof the Space Shuttle Discovery on its STS–51mission. After separation from the Shuttle, atransfer orbit rocket propelled it into a geo-transfer orbit. After 7 days, ACTS moved intoits geosynchronous operations orbit and stabi-lized into a nonspinning configuration. ACTSremained in this orbit at 100° west longitudethroughout the life of its demonstration project.With its solar arrays and main communicationsantennas deployed, ACTS measures 47.1 feetfrom tip to tip of its solar arrays and 29.9 feetacross from its main receiving and transmittingantenna. At the beginning of its on-orbit life, itweighed 3250 pounds.

ACTS has operated flawlessly, experiencingexpected downtimes only during the spring andfall equinoxes when the spacecraft’s solar panelsare eclipsed. Experimental studies began 12weeks after launch and continued through May2000 when Glenn’s experiments program cameto an end.

TECHNOLOGIESThe ACTS communications payload includes

several technologies through which a full rangeof voice, video, and data communications ser-vices can be provided on-demand:

• Operation in the Ka-band radio spectrumwhere the 2.5 gigahertz of available spectrumis five times that available in lower frequencybands.

• Multiple, high-gain hopping-beam antennasthat permit the use of smaller diameter Earthstation antennas.

• Onboard baseband switching, which, like atelephony switching station, directly connectssending and receiving Earth stations. A fur-ther innovation in satellite communications isthat ACTS makes the user-to-user connectionin a single satellite hop.

• A microwave switch matrix that enables high-speed (billions of bits per second) and high-volume communications and data transfer.

Ka-Band TransmissionsOne way to increase the amount of informa-

tion or data per unit of time transmitted by asatellite is to use a higher radiofrequency. UntilACTS, the higher frequency Ka-band was virtu-ally unused—most communication satellitesoperate in the lower frequency C- and Ku-bands. The Ka-band has a greater bandwidth(in other words, it has a higher capacity for datatransfer) and allows smaller spacecraft andground stations. Ka-band frequency is capableof carrying simultaneously a multiple mix of

Major ACTS spacecraft components.

voice, high-speed data, and teleconferencingtransmissions. The ACTS three transponderscan process over a billion bits of data per second.

and the ability to slow the transmission rate tocompensate for rain fade. The ACTS digitalaccess approach to sharing the radio spectrum,known as time division multiple access orTDMA, is similar to that used in many cellularphones. In TDMA, each user is allocated a verylarge portion of the available bandwidth for avery short period of time.

Multiple Hopping Beam AntennasUntil ACTS, communications satellites

broadcast a single beam or “footprint” over alarge part of the Earth. This is highly efficientfor large-scale, one-way communication such astelevision broadcasts but not for on-demandtwo-way communications.

To accommodate real-time two-way commu-nications for multiple users, ACTS uses 5 tightlyfocused spot beams directed to 51 sites. Eachbeam has a diameter on the Earth of 150 to200 miles, and each can be “hopped” from onesite to another in less than 1 microsecond. Thesmall spot beams allow the same frequency tobe used over and over because the spatialisolation between spots prevents interferencebetween beams.

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More information can be transmitted at higher frequencies in thesame period of time

High frequency means short wavelengths,though. And the millimeter wavelength Ka-bandsignals are easily degraded by rain, a problemknown as rain fade. Using detailed studies from75 years of weather patterns, rain amounts, andtheir effects on lower frequency radio signals,Glenn scientists designed ACTS with highersignal strength, digital error correction codes,

necessary, and forwards the message to theproper spot beam and address—all of this inmicroseconds.

The microwave switch matrix interconnectsfixed beams using three transponders, eachwith an available bandwidth of 900 megahertz(the total bandwidth of conventional C- andKu-band satellites is 250 and 500 megahertz).The switch is capable of high-speed, high-volume transmission to terminals, operating at622 million bits per second, and supportedapplications, such as linking to supercom-puters via fiber-optic-satellite networks. Theswitch also proved well suited to transmittingdata to and from mobile terminals with small,14-inch antennas.

GROUND SEGMENT

Master Ground Station and SatelliteOperations Center

The ground control segment includes themaster ground station at Glenn, the satelliteoperations center in Newtown, Pennsylvania,and experimenters terminals at industry, uni-versity, and government locations across theUnited States.

The master ground station controls thespacecraft and the network, manages experi-ments, and records spacecraft and system data.

High data rate terminal

Very small aperture terminal

Ultrasmall aperture terminal

Experimenters terminals.

The tighter, more concentrated beams alsopermit smaller and less expensive Earth sta-tions. They penetrate rain better and mitigaterain fade. ACTS, with its multiple hoppingbeams and other technologies, uses less power,provides three times the communicationscapacity, and can transmit data 20 times fasterthan conventional satellites of similar mass.

Onboard Digital Processing andSwitching

In order to sort and route data transfer athigh volumes and speeds, ACTS is equippedwith a baseband processor and a microwaveswitch matrix. The baseband processorinterconnects hopping spot beams. It receivesdata from one spot beam, stores the data as

Conventional satellitesingle-beam broadcast.

ACTS spot beam and switching.

Traffic requests are processed and set up, andtraffic channels are assigned on-demand.Command, ranging, and telemetry informationis sent to and from ACTS via the master groundstation.

The satellite operations center monitorsspacecraft health and status. The center alsohandles orbital maneuvering and housekeepingfunctions. It is linked by terrestrial line to themaster ground station at Glenn.

Experimenter TerminalsNASA Glenn researchers developed three

special ground stations, or terminals, for certaintypes of user applications or experiments. Inaddition, some ACTS experimenters designedterminals suited to their particular needs. In all,53 terminals were built and used by more than100 experimenters to test ACTS abilities.

The high-data-rate terminals, equipped withan 11-foot antenna, handled high-volume, high-speed traffic to and from the ACTS transpon-ders. The terminals connected seamlessly toterrestrial fiber-optic systems and supercom-puters at rates up to 622 million bits per sec-ond. They could transport high-definition videoat up to 520 million bits per second.

Very small aperture terminals were devel-oped to work with the on-demand traffic to andfrom the baseband processor. Using a 4-footantenna, these terminals can provide single hopdata rates up to 1.8 million bits per second forvoice, video, and data.

Ultrasmall aperture terminals are about thesame size as today’s 18-inch direct broadcasttelevision terminals, but they can uplink data(send data to ACTS) as fast as 1.5 million bitsper second and downlink (receive from ACTS) asfast as 45 million bits per second. The terminalsare ideal for high-speed Internet service, elec-tronic banking, and real-time videoconferencing.

Other experimenters designed mobile termi-nals small enough to be placed on ships, air-planes, and land vehicles. Still others usedreceive-only terminals for studying rain fade.

EXPERIMENTS PROGRAMSince its launch, ACTS has successfully

validated NASA’s vision of future satellite

communications and has demonstrated theability of its technology to meet the needs of usersapplications. Experiments have been conductedin such diverse areas as computer networking,medicine, industry, education, defense, business,emergency response, mobile communications,and astronomy.

TelemedicineACTS helped physicians provide high-

quality, low-cost health care to patients inremote areas. From their offices at the MayoClinic in Rochester, Minnesota, physiciansdiagnosed and evaluated patients at the PineRidge Indian Reservation in South Dakota.ACTS real-time voice, video, and data transferallowed the physicians to see and speak topatients as well as receive diagnostic informa-tion from stethoscopes, ultrasound sonograms,and electrocardiograms. In one case, a childwho had suffered for years from a misdiagnosedskin condition was cured after Mayo physiciansdiagnosed the condition as leprosy and pre-scribed appropriate treatment.

Early detection and diagnosis of breastcancer is another medical area benefited byACTS. NASA Glenn, the Cleveland Clinic Foun-dation, and the University of Virginia conducteda clinical study in which mammograms weregiven to women in rural regions where there arefew or no breast cancer specialists. In the firstpart of the study, mammogram images thatwere digitally compressed and transmitted werejudged to be equal in quality and resolutionto standard x-ray mammogram films. In thesecond phase, patients’ mammograms weretransmitted in real time to specialists for inter-pretation. Images taken in a local hospital can

Experimenter viewing digitally compressed images.

be interpreted elsewhere and reported back tothe local physician immediately at very littleadditional cost.

IndustryThe demand for new sources of fuel has

caused offshore oil exploration to move intodeeper ocean waters and to remote regions ofthe Earth, sending costs soaring. ACTS hasplayed a vital role in helping U.S. oil companiesdemonstrate how high-speed satellite communi-cations could make them more cost competitive.In effect, ACTS helped the petroleum industryremove time and distance obstacles from itsoperations.

Oil and gas companies use seismic informa-tion to chart the subsurface structure of theEarth. Done from sea vessels, the resultingdata, sometimes amounting to billions of bytes,are recorded on magnetic tape and sent toanalysts at distant facilities. Often, by the timethe analysts realize that more data are needed,the collection ships have traveled miles from thesite and must be sent back. The process ofaccurately charting an area can take months.

EducationThe ACTS small terminals and higher data

rates improved distance education experiencesbecause the nearly instantaneous transmissionsmade for a more natural interaction among theparticipants. Georgetown University used ACTSto provide business education training to com-panies at universities in Bogota, Colombia, andQuito, Ecuador. By obtaining business knowl-edge via ACTS, local employees in these coun-tries are better prepared to conduct businesswith U.S. companies. Also, the university wasable to expand its educational services intointernational markets.

First ever shipboard trial was aboard the Geco Diamondexploration vessel.

Participants in the electronic field trip to the Amazon rainforest.

ACTS took U.S. elementary and high schoolstudents on a virtual tour to the far reaches ofthe Amazon rainforest and to Antarctica forPassport to Knowledge electronic field tripprograms. Through high-quality live audio andvideo links, students in their own classroomshelped perform science experiments and inter-acted with researchers at those exotic sites.Another electronic field trip, this one to NASA’sKuiper Airborne Observatory (a 1-meter infraredtelescope flown above the clouds in a modifiedC–140 aircraft), is believed to be the firstInternet transmission to and from a flyingaircraft.

DefenseACTS helped the U.S. Armed Forces gain

experience with satellite-supported communica-tions. ACTS was put to one of its toughest testsduring the deployment of U.S. Armed Forces inHaiti. During the first month of the operation,

ACTS transmitted these seismic data fromoffshore oil-drilling platforms to analysts inWashington, DC, in real time. Transferring datavia ACTS not only saved months in researchtime at a savings of $200,000 per month, butalso provided higher quality seismic data bygiving scientists access to the data in enoughtime to redirect vessels to more promising areasof the ocean.

ACTS videoconferencing technology was theprimary command and control system. ACTSprovided the secure audio-video link allowingfield commanders to talk to commanders in theUnited States including the Army Chief of Staffand even to then President Clinton. ACTS alsofunctioned in the secondary role of connectingsoldiers in Haiti to their families in the UnitedStates through live video contact between them.

The U.S. Navy used ACTS to improve com-munications with ships at sea. A link wasestablished between the USS Princeton and theSan Diego Naval Base using a tracking mobileantenna developed by the Jet Propulsion Labo-ratory, Pasadena, California. Unexpectedly, thiscapability was needed for a telemedicine appli-cation when the captain of a Greek merchantship became ill and was taken aboard thePrinceton. He was remotely diagnosed while stillat sea and stabilized until he could be broughtashore for lifesaving surgery.

Other ApplicationsSome of the other significant demonstra-

tions of ACTS technologies include

• Huntington Bank connected branch offices tothe central computers to observe serviceperformance during emergency communica-tions restoration via satellite.

• The National Communications System demon-strated a Public Switch Network Restoral

system using easily transportable and mobileterminals to restore phone service interruptedby a natural disaster.

• Cray supercomputers at the National Centerfor Atmospheric Research in Colorado and theOhio Supercomputer Center in Ohio wereconnected for experiments that combinedmeteorological and marine forecasting.

• The Keck telescope at the Mauna Kea observa-tory on Hawaii, was linked to a CalTech dataprocessing facility for experiments in remotefacility control and data analysis.

• Supercomputers at Boeing and Glenn wereconnected to carry out computer-simulatedwind tunnel tests.

• Eighteen organizations from U.S. computingand satellite industries, academia, and gov-ernment agencies participated in an effort toovercome technical issues in networking high-speed satellite links with the terrestrial net-work over the Internet. Data from this efforthas already been incorporated into improvingcommercial products and expanding thereaches of the Internet.

INTO TOMORROWACTS technology has shown the world new

ways to use orbital space and radio spectrumresources. It offers alternatives in voice, data,and video communications networking to placeswhere there is little or no ground infrastructure.It enables two-way, satellite communication atcable-modem speeds using antennas evensmaller than today’s 18-inch dish.

With its overwhelming success, the ACTSproject has set the stage for the continuingevolution of the information era into space byproving the reliability and fidelity of Ka-bandtransmissions. The Deep Space 1 technologydemonstrator, a robot probe whose solar systemtour included a rendezvous with a comet, usedKa-band to communicate data to and receiveinstructions from Earth. Since then, NASA hasupgraded its tracking and data relay system forspace communications to use Ka-bandtechnologies.

So significant and valuable have been theACTS contributions to the commercialization ofspace communications, that the ACTS projectwas inducted into the Space Technology Hall of

U.S. Armed Forces supported with satellite communications.

B–0991–1June 02

For more information visit: http://acts.grc.nasa.gov

Or contact theInformation and Publications Office

NASA Glenn Research CenterCleveland, Ohio 44135–3191

216–433–5573

Fame in April 1997. Research and DevelopmentMagazine selected ACTS as one of the 100 mostsignificant technologies of 1995 with its R&D100 Award. In 2002, the Society of SatelliteProfessionals International recognized NASA forproving the viability of a number of first timesatellite technologies using ACTS.

The ACTS satellite itself, though no longerused by NASA, is now an educational tooland testbed run by the Ohio Consortium forAdvanced Communications Technology.Consortium members already have found manyuses for it. Engineering schools use it to givestudents experience in satellite communicationsand spacecraft operations and control with anactual satellite. College education departmentspropose to evaluate distance teaching tech-niques using ACTS. Commercial consortiummembers will use ACTS to test, refine, anddemonstrate new Ka-band equipment.

Given all these accomplishments and thepromise, one wonders why we aren’t using thesenew capabilities for our banking transactions orInternet downloads. Even as Glenn scientistswere launching ACTS, improvements in terres-trial technologies, such as fiber-optic cables andcell phones, were introduced. The higher capac-ity of fiber and the cost competitiveness of cellphones made the ACTS new Ka-band technologyless attractive to commercial developers. What ismore, the many options consumers have inbusiness, entertainment, and personal commu-nications make it very hard to predict how orwhen these NASA technologies will be neededand find their way into our lives.

But when the need is truly there, we willhave the technology to meet that need. ACTShas already been a benefit to us all.

Space Technology Hall of Fame medal.