Artist's concept of a Milstar communications satellite orbiting Earth. Photo: U.S. Air Force

Hughes Space and Communications Co. (HSC) has shipped to Lockheed Martin Space Systems the last in a series of powerful medium-data-rate (MDR) communications payloads for the U.S. Air Force Milstar satellites.

The MDR payload provides jam-resistant communications to the military through unique onboard signal and data processing capabilities.

Milstar is the tactical and strategic multiservice satellite system designed to provide secure, survivable communications for U.S. forces worldwide. The constellation can transmit voice, data and imagery, in addition to offering video teleconferencing capabilities. The program is managed by the U.S. Air Force Space and Missile Center.

HSC is responsible for supplying the MDR communications and crosslink payloads on Milstar. HSC subcontracts with TRW for the MDR antennas and digital subsystem. Lockheed Martin Space Systems Co. (LMSSC), the prime contractor, is responsible for the space and mission control segments of the program.

The MDR payload dynamically sorts incoming data and routes the data to the proper downlinks to establish networks and provide bandwidth on demand. If necessary, it crosslinks the data between satellites. These crosslinks provide rapid, ultra-secure communications by enabling the satellites to pass signals to one another without assistance from ground stations.

The payload HSC has just delivered is for the F-6 spacecraft, scheduled for launch in 2002. After conducting a series of thermal-vacuum and antenna range tests, HSC integrated antennas and electronic units onto the MDR structure, called the -X (minus X) wing. The F-6 wing is HSC's fourth MDR payload and the sixth crosslink payload delivered to LMSSC.

HSC is a major supplier of state-of-the-art radio frequency, microwave and antenna subsystems on Milstar. In addition to the crosslink payloads on the first two satellites, HSC provided elements of a low-data-rate (LDR) payload under subcontract to TRW. The MDR payload was incorporated with Milstar beginning with the F-3 spacecraft, also called the first Milstar 2 satellite.

Technicians make final adjustments to the minus-X wing of a Milstar satellite, which contains the medium-data-rate (MDR) communications payload and a portion of the crosslink payload. Hughes recently delivered the wing for the Milstar F-6 to prime contractor Lockheed Martin. It is identical to the wing shown here, which is for Milstar F-5. Photo: Hughes

Unfortunately, that satellite was lost in a launch vehicle failure last year. Two Milstar 1 satellites are in orbit. The remaining three in the Milstar 2 series will be launched over the next two years, with the next one scheduled for this fall.

In addition, HSC, LMSSC and TRW have formed a national team to build the follow-on to Milstar, called the Advanced EHF (Extremely High Frequency) system. Production is planned to begin next year, with first launch scheduled for 2004.

"Our work on Milstar is complete, but the legacy continues," said Tig H. Krekel, HSC president and chief executive officer. "We are leveraging our systems integration, manufacturing and digital processing experience on both commercial and government new programs. We look forward to continuing our team effort with Lockheed and TRW for the Milstar follow-on, the Advanced EHF system."

The MDR payload uses a 32-channel extremely high-frequency (44 GHz) uplink and a super-high-frequency (20 GHz) downlink. It sends real-time voice, video and data to military personnel in the field at rates that range from 4.8 Kbps to 1.5 Mbps.

That is up to 50 times faster than a common PC modem. The crosslinks provide communications capability at 60 GHz between Milstar satellites for both the MDR payload and the 2.4 Kbps LDR communications payload on the satellites.

The MDR antennas consist of eight narrow spot-beam antennas: six distributed user coverage antennas (DUCAs), and two narrow spot beams with nulling capabilities, known as nuller antennas. In contrast to commercial communications satellites, whose beams can cover entire continents, Milstar's beams are very narrow, providing less opportunity for enemy detection and penetration.

HSC is the world's leading manufacturer of commercial communications satellites, having built about 40 percent of those satellites in operation. It also is a major supplier of spacecraft and equipment to the U.S. government, and a builder of weather satellites.