No telhado de um prédio do MIT Lincoln Laboratory fica um gabinete de antena de rádio em forma de cúpula de 38 pés de largura, ou radome. Dentro do ambiente climatizado, protegido do clima da Nova Inglaterra, uma estrutura de aço suporta uma antena de comunicações por satélite (SATCOM) de 20.000 libras e 20 pés de diâmetro. A antena – chamada de Terminal de Teste Multibanda (MBTT) – pode girar 15 graus por segundo, completando uma única revolução em 24 segundos. Nessa velocidade, o MBTT pode detectar e rastrear satélites em órbita terrestre média e baixa (média e baixa referem-se à altitude em que os satélites orbitam a Terra).
Antes da instalação do MBTT em 2017, o laboratório contava com uma variedade de antenas menores para testes SATCOM, incluindo o Terminal de Teste de Banda Ka Over-the-Air, ou OTAKaTT. Comparado com a antena OTAKaTT de quase dois metros e meio de diâmetro, o MBTT é sete vezes mais sensível. E, ao contrário de seu antecessor, o MBTT, como o próprio nome sugere, foi projetado para ser facilmente reconfigurado para suportar várias bandas de radiofrequência (RF) usadas para sistemas SATCOM de satélite militares e comerciais.

"Como um ativo de teste muito maior, mais poderoso e mais flexível que o OTAKaTT, o MBTT é um divisor de águas por permitir o desenvolvimento de tecnologia SATCOM avançada", diz Brian Wolf, membro da equipe técnica do Advanced Satcom Systems and Operations do Lincoln Laboratory Grupo.

Wolf esteve envolvido na instalação e comissionamento inicial do MBTT em 2017. Ele então liderou o MBTT por meio de um rigoroso processo de certificação com o Comando de Defesa Espacial e de Mísseis do Exército dos EUA, concluído em 2019, demonstrando que o desempenho de transmissão e recepção da antena foi suficiente para operar no sistema Wideband Global SATCOM (WGS). Uma constelação de 10 satélites de propriedade e operados pelo Departamento de Defesa dos EUA, o WGS fornece conectividade de alta taxa de dados entre vários pontos da Terra. Desde 2019, Wolf atua como investigador principal em um projeto que possui o MBTT, apoiando o desenvolvimento das capacidades Protected Anti-Jam Tactical SATCOM (PATS) da Força Espacial dos EUA.

"O PATS está desenvolvendo a capacidade de fornecer serviços de formas de onda táticas protegidas, ou PTW, sobre WGS, bem como sobre satélites transponder comerciais e novos satélites DoD com processamento PTW integrado dedicado", diz Wolf.

Como Wolf explica, uma forma de onda é o sinal transmitido entre dois modems quando eles estão se comunicando, e PTW é um tipo especial de forma de onda projetada para fornecer comunicações altamente seguras e resistentes a interferências. Jamming refere-se a quando os sinais de comunicação sofrem interferência - seja acidentalmente por forças amigas (que, por exemplo, podem ter configurado incorretamente seus equipamentos SATCOM e estão transmitindo na frequência errada) ou intencionalmente por adversários que procuram impedir as comunicações. O Lincoln Laboratory começou a desenvolver o PTW em 2011, contribuindo para o projeto inicial e a arquitetura do sistema. Nos anos seguintes, o laboratório participou de esforços de prototipagem e teste para ajudar a indústria a amadurecer os modems para processar a forma de onda.

"Nossos protótipos de modems PTW foram implantados em sites da indústria em todo o país para que os fornecedores possam testá-los à medida que desenvolvem sistemas PTW que serão implantados no mundo real", diz Wolf. A capacidade operacional inicial para serviços PTW sobre WGS está prevista para 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. + Explorar mais

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