Antenna supports advanced satellite communications test | MIT News

On the roof of the MIT Lincoln Laboratory building is a 38-foot-wide dome-shaped radio radome, or radome. In a climate-controlled environment protected from New England weather, the steel structure supports a 20,000-pound, 20-foot-diameter satellite communications (SATCOM) antenna. The antenna—called the Multiband Test Terminal (MBTT)—can rotate 15 degrees per second, completing one revolution in 24 seconds. At this speed, MBTT can detect and track satellites in low-to-medium Earth orbits (medium-to-low refers to the altitude at which satellites orbit the Earth). Before installing MBTT in 2017, the lab relied on a variety of smaller antennas for satellite communications testing, including the Wireless Ka-band Test Terminal, or OTAKaTT. Compared to the nearly eight-foot-diameter OTAKaTT antenna, the MBTT is seven times more sensitive. Unlike its predecessor, MBTT, as its name suggests, is designed to be easily reconfigured to support multiple radio frequency (RF) frequency bands for military and commercial satellite SATCOM systems.

“As a larger, more powerful and more flexible test asset than OTAKaTT, MBTT is a game-changer for advancing advanced satcom technology,” said Brian Wolf, Senior Satcom Systems and Operations Technician for the Lincoln Laboratory Group.

Wolf was involved in the installation and initial commissioning of MBTT in 2017. He then led MBTT through the U.S. Army Space and Missile Defense Command’s rigorous certification process, which was completed in 2019, proving that the antenna’s transmit and receive performance was sufficient for operation on the Wideband Global Satellite Communications (WGS) system . WGS is a 10-satellite constellation owned and operated by the US Department of Defense that provides high data rate connectivity between points on Earth. Since 2019, Wolf has been the Principal Investigator for a project with MBTT supporting the development of a Protected Anti-Jam Tactical Satellite Communications (PATS) capability for the U.S. Space Force.

“PATS is developing the ability to provide Protected Tactical Waveform, or PTW, service over WGS as well as commercial transponder satellites and new DoD satellites with dedicated onboard PTW processing,” Wolf said.

As Wolf explains, a waveform is the signal transmitted when two modems communicate, and PTW is a special waveform designed to provide highly secure, interference-resistant communications. Jamming refers to communication signals being jammed—either accidentally by friendly forces (for example, they may have misconfigured their satcom equipment and transmitting on the wrong frequency), or intentionally by an adversary trying to block communications. Lincoln Laboratory began developing the PTW in 2011, participating in the initial design and system architecture. Since then, the lab has been involved in prototyping and testing efforts to help the industry mature modems to handle waveforms.

“Our prototype PTW modems have been deployed to industrial sites across the country so suppliers can test against them when developing PTW systems that will be deployed in the real world,” Wolf said. Initial operational capacity for WGS-based PTW services is expected in 2024.

Staff initially envisioned MBTT as a test asset for PTW. Directly below the MBTT is a PTW development lab where researchers can connect directly to the antenna to perform PTW tests.

One of the design goals of the PTW was the flexibility to operate in the various radio frequency bands associated with satellite communications. That means researchers need a way to test PTW in these bands. MBTT is designed to support four of SATCOM’s commonly used frequency bands, ranging from 7 GHz to 46 GHz: X, Ku, Ka and Q. However, MBTT can be adjusted in the future to support other frequency bands through additional design Antenna feeds, devices that connect antennas to RF transmitters and receivers.

To switch between the different supported RF bands, the MBTT had to be reconfigured with a new antenna feed that radiated signals to and collected 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 below the antenna main platform. Feeds come in a variety of sizes, with the largest being six feet long and weighing nearly 200 pounds.

To swap one feed for another, a crane inside the radome is used to lift, loosen bolts and remove the old feed; a second crane then lifts the new feed into place. Not only does the feedline 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 amplifiers used to boost satellite signals and the downconverters that convert RF signals to RF signals. Lower frequencies are better suited for digital processing – also need to be replaced. A team of skilled technicians can complete the process in four to six hours. Before scientists can perform any tests, technicians must calibrate the new feed to ensure it is functioning properly. Typically, they point the antenna at a satellite known to broadcast at a specific frequency and collect reception measurements, then point the antenna directly into free space to collect transmission measurements.

Since its installation, MBTT has supported extensive testing and experimentation involving PTW. During the PTW Modem Prototyping Protected Tactical Services Live Demonstration from 2015 to 2020, the lab tested several satellites, including the EchoStar 9 commercial satellite (which provides broadband SATCOM services nationwide, including satellite TV) and the DoD – Operational WGS satellites. In 2021, using its prototype PTW modem as a terminal modem, the lab conducted an air-test of the Protected Tactical Enterprise Service with Inmarsat-5 — a ground-based PTW processing platform satellite that Boeing is developing under the PATS program. The lab is again using Inmarsat-5 to test a prototype enterprise management and control system for resilient, uninterrupted satellite communications. During these tests, a prototype PTW modem on board a 737 communicated with MBTT via Inmarsat-5.

“Inmarsat-5 provides a military Ka-band transponder service suitable for PTW, as well as a commercial Ka-band service called Global Xpress,” Wolf explained. “Through flight testing, we were able to demonstrate resilient end-to-end network connectivity across multiple SATCOM paths, including PTW on military Ka-band and commercial SATCOM services. If you run out of bandwidth, or someone tries to interfere with it—you can switch to an alternate secondary link.”

In another 2021 demonstration, the lab used MBTT as a simulated interference source to test PTW on O3b, a constellation of satellites in medium Earth orbit owned by SES. As Wolf explains, SES supplies most of their own terminal antenna equipment, so in this case the MBTT is helpful as a test instrument to simulate various types of interference. These jams range from misconfigured users transmitting on the wrong frequency to simulating advanced jamming tactics that other countries might deploy.

MBTT also supports international outreach efforts led by Space Systems Command, part of the U.S. Space Force, to expand PATS capabilities to international partners. In 2020, the lab demonstrated PTW at X-band using MBTT on SkyNet 5C, a military communications satellite serving the British Armed Forces and NATO coalition forces.

“Our role comes in when international partners say, ‘PTW is great, but will it work on my satellite or terminal antenna?’,” Wolf explained. “The SkyNet test is the first time we have used PTW on X-band.”

The MBTT is connected to Lincoln Laboratory’s research facilities via a fiber optic link and also supports non-PTW testing. Crews tested new signal processing techniques to suppress or cancel jammer interference, new techniques for signal detection and geolocation, and new ways to connect PTW users to other DoD systems.

In the coming years, the lab looks forward to conducting more tests with the wider DoD user community. As the PTW reaches operational maturity, the MBTT serves as a reference terminal to support testing of vendor systems. With PTS satellites in orbit with onboard PTW processing, MBTT can facilitate early on-orbit inspection, 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,” Wolf said.

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