Sul tetto di un edificio del MIT Lincoln Laboratory si trova un involucro di antenna radio a forma di cupola di 38 piedi di larghezza, o radome. All'interno dell'ambiente climatizzato, al riparo dalle intemperie del New England, una struttura in acciaio supporta un'antenna per comunicazioni satellitari (SATCOM) da 20.000 libbre e 20 piedi di diametro. L'antenna, denominata Multi-Band Test Terminal (MBTT), può ruotare di 15 gradi al secondo, completando un singolo giro in 24 secondi. A questa velocità, l'MBTT può rilevare e tracciare satelliti in orbita terrestre media e bassa (media e bassa si riferiscono all'altitudine alla quale i satelliti orbitano attorno alla Terra).

Prima dell'installazione dell'MBTT nel 2017, il laboratorio faceva affidamento su una varietà di antenne più piccole per i test SATCOM, incluso il terminale di test in banda Ka over-the-air o OTAKaTT. Rispetto all'antenna OTAKaTT di quasi otto piedi di diametro, l'MBTT è sette volte più sensibile. E a differenza del suo predecessore, l'MBTT, come suggerisce il nome, è progettato per essere facilmente riconfigurato per supportare più bande di radiofrequenza (RF) utilizzate per i sistemi SATCOM militari e commerciali.

"In quanto risorsa di test molto più grande, più potente e più flessibile di OTAKaTT, l'MBTT è un punto di svolta per consentire lo sviluppo della tecnologia SATCOM avanzata", afferma Brian Wolf, un membro dello staff tecnico di Advanced Satcom Systems and Operations del Lincoln Laboratory Gruppo.

Wolf è stato coinvolto nell'installazione e nella messa in servizio iniziale dell'MBTT nel 2017. Ha quindi guidato l'MBTT attraverso un rigoroso processo di certificazione con il comando di difesa spaziale e missilistica dell'esercito americano, completato nel 2019, dimostrando che le prestazioni di trasmissione e ricezione dell'antenna erano sufficiente per funzionare sul sistema Wideband Global SATCOM (WGS). Una costellazione di 10 satelliti di proprietà e gestiti dal Dipartimento della Difesa degli Stati Uniti, WGS fornisce connettività ad alta velocità di trasmissione dati tra vari punti sulla Terra. Dal 2019, Wolf è stato ricercatore principale in un progetto proprietario dell'MBTT, supportando lo sviluppo delle capacità Protected Anti-Jam Tactical SATCOM (PATS) della US Space Force.

"PATS sta sviluppando la capacità di fornire forme d'onda tattiche protette, o PTW, servizi su WGS, nonché su satelliti transponder commerciali e nuovi satelliti DoD con elaborazione PTW a bordo dedicata", afferma Wolf.

Come spiega Wolf, una forma d'onda è il segnale trasmesso tra due modem durante la comunicazione e PTW è un tipo speciale di forma d'onda progettata per fornire comunicazioni altamente sicure e resistenti ai disturbi. Il jamming si riferisce a quando i segnali di comunicazione vengono interferiti, accidentalmente da forze amiche (che, ad esempio, potrebbero aver configurato in modo errato le loro apparecchiature SATCOM e stanno trasmettendo alla frequenza sbagliata) o intenzionalmente da avversari che cercano di impedire le comunicazioni. Lincoln Laboratory ha iniziato a sviluppare PTW nel 2011, contribuendo alla progettazione iniziale e all'architettura del sistema. Negli anni successivi, il laboratorio ha partecipato a sforzi di prototipazione e test per aiutare i modem industriali a maturare per l'elaborazione della forma d'onda.

"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. + Esplora ulteriormente

Flight testing validates waveform capability

Questa storia è stata ripubblicata per gentile concessione di MIT News (web.mit.edu/newsoffice/), un popolare sito che copre notizie sulla ricerca, l'innovazione e l'insegnamento del MIT.