Artist's impression of the UHF Follow-On satellite in orbit. Photo: Hughes Space and Communications

The U.S. Navy is replacing and upgrading its ultra-high frequency (UHF) satellite communications network during the 1990s with a constellation of customized Hughes HS 601 spacecraft known as the UHF Follow-On (UHF F/O) series.

In July 1988, Hughes Space and Communications Company won the competition for a fixed-price contract awarded by the Program Executive Office for Space, Communications, and Sensors in Washington, D.C. The initial agreement called for Hughes to build and launch one satellite, with options for nine more. Options for spacecraft 2 and 3 were exercised in May 1990; for 4, 5, and 6 in November 1990; and for 7, 8, and 9 in November 1991. In January 1994, the customer extended the contract by ordering a 10th satellite and launch services, bringing the total value to $1.7 billion.

In March 1996, for an additional $150 million, the Navy ordered a high-power, high-speed Global Broadcast Service (GBS) payload to be incorporated onto the last three satellites. This Phase II GBS package will revolutionize communications for the full range of the Defense Department's high-capacity requirements, from intelligence dissemination to quality-of-life programming. The first GBS payload was put into service in 1998. Full, three-satellite operational capability is planned for mid-1999.

The UHF F/O spacecraft has proven to be a very flexible platform for the efficient evolution of critical, advanced DOD communications services. The satellites are versions of the Hughes body-stabilized, three-axis HS 601 model. The spacecraft was introduced in 1987 to meet anticipated requirements for high-power, multiple-payload satellites for such applications as UHF F/O, direct television broadcasting to very small terminals, private business networks, and mobile communications.

The HS 601 body is composed of two main modules. The bus module houses the bus electronics, propulsion subsystem, and battery packs. The payload module contains the communications equipment and antennas. The UHF Follow-On contract calls for a minimum 10-year mission.

The UHF Follow-On satellites replace the Fleet Satellite Communications (FLTSATCOM) and the Hughes-built Leasat spacecraft currently supporting the Navy's global communications network, serving ships at sea and a variety of other U.S. military fixed and mobile terminals. They are compatible with ground- and sea-based terminals already in service.

A technician at Hughes Space and Communications Company performs deployment testing of a solar wing for the UHF Follow-On F-10 spacecraft. Photo: Hughes Space and Communications

Hughes is manufacturing the UHF F/O satellites in its El Segundo, Calif., factory. Using a building-block approach, Hughes and the Navy are enhancing the constellations capabilities in stages. Satellites F-1 through 3 carry UHF and SHF (super-high frequency) payloads. Starting with F-4 there is an additional payload for EHF (extremely high frequency) communications. F-7 introduced an enhancement to the EHF package that essentially doubles capacity. The SHF payload is replaced by the GBS package on F-8 through 10.

The UHF F/O satellites offer increased communications channel capacity over the same frequency spectrum used by previous systems. Each spacecraft has 11 solid-state UHF amplifiers and 39 UHF channels with a total 555 kHz bandwidth. The UHF payload comprises 21 narrowband channels at 5 kHz each and 17 relay channels at 25 kHz. In comparison, FLTSATCOM offers 22 channels. The F-1 through F-7 spacecraft include an SHF subsystem, which provides command and ranging capabilities when the satellite is on station as well as the secure uplink for Fleet Broadcast service, which is downlinked at UHF.

The Navy has added an extremely high frequency communications package beginning with the fourth spacecraft. This addition includes 11 EHF channels distributed between an earth coverage beam and a steerable 5-degree spot beam and is compatible with Milstar ground terminals. The EHF subsystem provides enhanced antijam telemetry, command, broadcast, and fleet interconnectivity communications, using advanced signal processing techniques. The EHF Fleet Broadcast capability supersedes the need for the SHF fleet uplink. Beginning with UHF F/O F-7, the EHF package has been enhanced to provide 20 channels through the use of advanced digital integrated circuit technology.

The GBS payload replaces the SHF payload on spacecraft F-8, 9, and 10. This new package includes four 130-watt, 24 megabits-per-second (Mbps) military Ka-band (30/20 GHz) transponders with three steerable downlink spot beam antennas (two at 500 nmi and one at 2000 nmi) as well as one steerable and one fixed uplink antenna. This modification results in a 96 Mbps capability per satellite. Three spacecraft give the DOD near-global coverage. The system will transmit to small, mobile, tactical terminals.

The first seven satellites measure more than 60 feet long from the tip of one three-panel solar array wing to the tip of the other. Spacecraft F-8 through F-10 each have four solar panels on a side, making them 86 feet long. These arrays generate a combined 2500 watts of electrical power on the first three satellites, 2800 watts for F-4 through F-7, and 3800 watts for F-8 through F-10 with GBS.

	Previous launches
	UHF F-1	Mar. 25, 1993 *
	UHF F-2	Sept. 3, 1993
	UHF F-3	June 24, 1994
	UHF F-4	Jan. 28, 1995
	UHF F-5	May 31, 1995
	UHF F-6	Oct. 22, 1995
	UHF F-7	July 25, 1996
	UHF F-8	Mar. 16, 1998
	UHF F-9	Oct. 20, 1998
	* Did not reach intended orbit because of Atlas 1 first stage malfunction. Source: Spaceflight Now research

The arrays are folded against the spacecraft bus for launch, forming a cube roughly 11 feet per side. The satellites weigh an average of 2600 pounds with the UHF payload, 3000 pounds with the additional EHF payload, and 3400 pounds with the GBS payload.

Integrating the GBS payload requires several spacecraft modifications. The Ka-band traveling-wave tube amplifiers are integrated onto a larger south radiator panel with heat pipes added to accommodate the increased thermal dissipation. The GBS fixed receive antenna, together with the forward omni antenna, is mounted on the structure previously used for the SHF antennas. The GBS steerable receive antenna is mounted on a deployable boom. A new pallet structure has been added to integrate the three GBS transmit spot beam antennas. The nickel-hydrogen battery, which supports eclipse operations, has been enlarged to 164 from 126 amp-hour cells.

Hughes has chosen the Atlas rocket series to provide the launches from Cape Canaveral, Fla. The Atlas I rocket was used for the UHF-only satellites. The Atlas II was chosen for those with the additional EHF payloads and for those with GBS packages.