På taket av en MIT Lincoln Laboratory-byggnad sitter en 38 fot bred kupolformad radioantennhölje, eller radom. Inuti den klimatkontrollerade miljön, avskärmad från New England-vädret, stöder en stålkonstruktion en 20 000 pund, 20 fot diameter satellitkommunikationsantenn (SATCOM). Antennen – kallad Multi-Band Test Terminal (MBTT) – kan rotera 15 grader per sekund, vilket gör ett enda varv på 24 sekunder. Med denna hastighet kan MBTT upptäcka och spåra satelliter i medelhög och låg omloppsbana om jorden (medel och låg hänvisar till den höjd där satelliterna kretsar runt jorden).

Före installationen av MBTT 2017 förlitade laboratoriet sig på en mängd mindre antenner för SATCOM-testning, inklusive Over-the-Air Ka-band Test Terminal, eller OTAKaTT. Jämfört med OTAKaTT-antennen med nästan åtta fot diameter är MBTT sju gånger känsligare. Och till skillnad från sin föregångare, är MBTT, som namnet antyder, utformad för att lätt kunna konfigureras om för att stödja flera radiofrekvensband (RF) som används för militära och kommersiella satellit-SATCOM-system.

"Som en mycket större, kraftfullare och mer flexibel testtillgång än OTAKaTT, är MBTT en spelväxlare för att möjliggöra utvecklingen av avancerad SATCOM-teknik", säger Brian Wolf, en teknisk personal i Lincoln Laboratorys Advanced Satcom Systems and Operations Grupp.

Wolf var involverad i installationen och den första idrifttagningen av MBTT 2017. Han ledde sedan MBTT genom en rigorös certifieringsprocess med U.S. Army Space and Missile Defense Command, avslutad 2019, vilket visade att antennens sändnings- och mottagningsprestanda var tillräckligt för att den ska fungera på Wideband Global SATCOM (WGS)-systemet. En konstellation av 10 satelliter som ägs och drivs av det amerikanska försvarsdepartementet, WGS tillhandahåller anslutningar med hög datahastighet mellan olika punkter på jorden. Sedan 2019 har Wolf fungerat som huvudutredare i ett projekt som äger MBTT, som stöder utvecklingen av U.S. Space Forces Protected Anti-Jam Tactical SATCOM (PATS)-kapacitet.

"PATS utvecklar förmågan att leverera skyddade taktiska vågformer, eller PTW, tjänster över WGS, såväl som över kommersiella transpondersatelliter och nya DoD-satelliter med dedikerad inbyggd PTW-behandling", säger Wolf.

Som Wolf förklarar är en vågform signalen som sänds mellan två modem när de kommunicerar, och PTW är en speciell typ av vågform utformad för att ge mycket säker, störningsbeständig kommunikation. Jamming hänvisar till när kommunikationssignaler störs – antingen av misstag av vänliga styrkor (som till exempel kan ha felkonfigurerat sin SATCOM-utrustning och sänder med fel frekvens) eller avsiktligt av motståndare som försöker förhindra kommunikation. Lincoln Laboratory började utveckla PTW 2011, vilket bidrog till den initiala designen och systemarkitekturen. Under åren sedan har laboratoriet deltagit i prototyper och tester för att hjälpa industrin att mogna modem för att bearbeta vågformen.

"Our prototype PTW modems have been fielded to industry sites all over the country so vendors can test against them as they develop PTW systems that will be deployed in the real world," says Wolf. The initial operating capability for PTW services over WGS is anticipated for 2024.

Staff originally conceived the MBTT as a test asset for PTW. Directly underneath the MBTT is a PTW development lab, where researchers can run connections directly to the antenna to perform PTW testing.

One of the design goals for PTW is the flexibility to operate on a wide range of RF bands relevant to satellite communications. That means researchers need a way to test PTW on these bands. The MBTT was designed to support four commonly used bands for SATCOM that span frequencies from 7 GHz to 46 GHz:X, Ku, Ka, and Q. However, the MBTT can be adapted in the future to support other bands through the design of additional antenna feeds, the equipment connecting the antenna to the RF transmitter and receiver.

To switch between the different supported RF bands, the MBTT must be reconfigured with a new antenna feed, which emits signals onto and collects signals from the antenna dish, and RF processing components. When not in use, antenna feeds and other RF components are stored in the MBTT command center, located underneath the main platform of the antenna. The feeds come in a range of sizes, with the largest registering six feet in length and weighing nearly 200 pounds.

To swap out one feed for another, a crane inside the radome is used to lift up, unbolt, and remove the old feed; a second crane then lifts the new feed up into place. Not only does the feed on the front of the antenna need to be replaced, but all of the RF processing components on the back of the antenna—such as the high-power amplifier for boosting satellite signals and the downconverter for converting RF signals to a lower frequency more suitable for digital processing—also need to be replaced. A team of skilled technicians can complete this process in four to six hours. Before scientists can run any tests, the technicians must calibrate the new feed to ensure it is operating properly. Typically, they point the antenna onto a satellite known to broadcast at a specific frequency and collect receive measurements, and point the antenna straight up into free space to collect transmission measurements.

Since its installation, the MBTT has supported a wide range of tests and experiments involving PTW. During the Protected Tactical Service Field Demonstration, a PTW modem prototyping effort from 2015 to 2020, the laboratory conducted tests over several satellites, including the EchoStar 9 commercial satellite (which offers broadband SATCOM services, including satellite TV, across the country) and DoD-operated WGS satellites. In 2021, the laboratory used its PTW modem prototype as the terminal modem to conduct an over-the-air test of the Protected Tactical Enterprise Service—a ground-based PTW processing platform Boeing is developing under the PATS program—with the Inmarsat-5 satellite. The laboratory again used Inmarsat-5 to test a prototype enterprise management and control system for enabling resilient, uninterrupted SATCOM. In these tests, the PTW modem prototype, flying onboard a 737 aircraft, communicated through Inmarsat-5 back to the MBTT.

"Inmarsat-5 provides a military Ka-band transponded service suitable for PTW, as well as a commercial Ka-band service called Global Xpress," explains Wolf. "Through the flight tests, we were able to demonstrate resilient end-to-end network connections across multiple SATCOM paths, including PTW on military Ka-band and a commercial SATCOM service. This way, if one satellite communications link is not working well—maybe it's congested with too many users and bandwidth isn't sufficient, or someone is trying to interfere with it—you can switch to the backup secondary link."

In another 2021 demonstration, the laboratory employed the MBTT as a source of modeled interference to test PTW over O3b, a medium-Earth-orbit satellite constellation owned by the company SES. As Wolf explains, SES provided much of their own terminal antenna equipment, so, in this case, the MBTT was helpful as a test instrument to simulate various types of interference. These interferences ranged from misconfigured users transmitting at the wrong frequencies to simulation of advanced jamming strategies that may be deployed by other nation states.

The MBTT is also supporting international outreach efforts led by Space Systems Command, part of the U.S. Space Force, to extend the PATS capability to international partners. In 2020, the laboratory used the MBTT to demonstrate PTW at X-band over SkyNet 5C, a military communications satellite providing services to the British Armed Forces and coalition North Atlantic Treaty Organization forces.

"Our role comes in when an international partner says, "PTW is great, but will it work on my satellite or on my terminal antenna?'" explains Wolf. "The SkyNet test was our first using PTW over X-band."

Connected via fiber-optic links to research facilities across Lincoln Laboratory, the MBTT has also supported non-PTW testing. Staff have tested new signal processing technology to suppress or remove interference from jammers, new techniques for signal detection and geolocation, and new ways of connecting PTW users to other Department of Defense systems.

In the years ahead, the laboratory looks forward to performing more testing with more user communities in the Department of Defense. As PTW reaches operational maturity, the MBTT, as a reference terminal, could support testing of vendors' systems. And as PTS satellites with onboard PTW processing reach orbit, the MBTT could contribute to early on-orbit checkout, measurement, and characterization.

"It's an exciting time to be involved in this effort, as vendors are developing real SATCOM systems based on the concepts, prototypes, and architectures we've developed," says Wolf. + Utforska vidare

Flight testing validates waveform capability

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