Lincoln Laboratory demonstrates
airborne communication with Milstar

Lincoln Laboratory has added protected military satellite communications (MilSatCom) to the list of capabilities of the Paul Revere, a heavily modified Boeing 707 the Laboratory operates for the Air Force as a communications and sensor testbed used in the development of integrated networking and airborne sensing. A new system mounted on the Paul Revere affords the government an airborne platform to test secure extremely high frequency (EHF) satellite communications.

Photo of 707 testbed The Milstar radome is the large teardrop-shaped radome just aft of the wings of the Paul Revere testbed.

"We now have the infrastructure in place to support present and future MilSatCom airborne terminal and antenna development," says Doug Marquis, leader of the Laboratory's Netcentric Integration Group, which directed this effort.

In August 2008, Lincoln Laboratory ruggedized and installed on the Paul Revere an Advanced EHF Universal System Test–Terminal (AUST-T), which was developed by the Laboratory as a "gold standard" terminal for use by contractors to test advanced EHF (AEHF) satellite payloads. Along with an antenna/pedestal combination, the terminal provides Milstar connectivity in the air.

Photo of antenna and pedestalAlong with the AUST-T, the antenna and pedestal system seen above is used on the Paul Revere to provide airborne connectivity to Milstar.

Successful flight tests with the system were conducted in August and September 2008.

"Our goal was to provide the government with an airborne EHF satellite communications test capability. In particular, we want to reduce risk for the
FAB-T program," says Kevin Kelly, assistant leader of the Netcentric Integration Group.

The Air Force–sponsored Family of Advanced Beyond-line-of-sight Terminals (FAB-T) program is developing next-generation protected military satellite communication terminals for fixed and mobile platforms. "The terminals will be capable of communicating with the government's present Milstar satellite constellation and the soon-to-be-launched Advanced EHF satellites," says Marquis.

This new AEHF system will add higher data-rate links called extended data rate (XDR) to the current Milstar low-data-rate (LDR) and medium-data-rate (MDR) links. Both the Milstar and AEHF systems operate in frequency bands reserved for government use. The terminals listen to the satellites at 20 GHz and transmit to them at 44 GHz. The complicated nature of the system and the high operating frequency make communications from a moving platform particularly challenging.

While the Laboratory has participated in Milstar flight tests before, this is the first airborne terminal that is AEHF capable. The system has the ability to use the XDR waveform coming from the new AEHF satellites. Since the waveforms are defined in software, with a software upgrade the terminal will be compatible with future transformational satellite communications (TSAT) waveforms as well.

In early 2009, the AUST-T will be moved to make room for the first FAB-T system. "The experience gained from these exercises will be invaluable to making the FAB-T flight tests successful," says Marquis. The AUST-T isn't done yet, though. It will stay on the Paul Revere, but move across the aisle and act as the terminal for next-generation FAB-T antennas for fighter aircraft.

The August test flights originated at Hanscom Air Force Base in Massachusetts. The flight pattern consisted of a straight flight from Hanscom to Lansing, Michigan, and back again. Once back in the area, two 15-mile-radius loops around the base were completed.

Map of the August and September flight paths In the August and September tests of the AUST-T and associated antenna on the Paul Revere, two very different flight paths were used.

From takeoff to landing, the antenna/pedestal system continually tracked a Milstar satellite. An LDR network was maintained for the entire flight. For the last 90 minutes, the highest-order LDR modulation was used and the link remained error-free, even during the two loops at the end of the flight. A terminal at the Laboratory was also able to join the network and listen to the data traffic. "We were ecstatic over the results. The system worked extraordinarily well," says lead engineer Tim Gallagher.

In September, the Paul Revere participated in the Capstone II exercise at Pax River Naval Air Station in Maryland. Capstone II was the second in a series of flight events sponsored by the Air Force's Electronic Systems Center to test and evaluate technologies for a future airborne wireless Internet protocol (IP) network. The AUST-T was one of many systems run during the exercise. The flight patterns for these tests forced the antenna to point primarily out the front and rear of the radome. The curvature of the radome, as seen from the antenna at the front and rear, makes maintaining a link more difficult.

"The curvature of the radome can cause the electromagnetic waves to refract, plus the tail of the aircraft just plain gets in the way," says Gallagher.

No uplink testing was conducted, but the system successfully tracked both LDR and MDR downlink beams through the most difficult parts of the radome.

Posted January 2009

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