Boeing, a major supplier of sophisticated, state-of-the-art radio frequency (RF), microwave, and antenna subsystems, has been part of the Milstar satellite team since 1982.
Milstar is the tactical and strategic multiservice satellite system designed to provide survivable communications for U.S. forces worldwide. The program is managed by the U.S. Air Force Space and Missile Center.
The Milstar I satellites carry a secure, robust low-data-rate (LDR) communications payload, and a crosslink payload that allows the satellites to communicate globally without using a ground station. The Milstar II satellites extend the communications capabilities to higher data rates by adding a medium-data-rate (MDR) payload. The Milstar I and II satellites are fully interoperable for LDR communications and crosslinks.
Boeing has the prime responsibility for supplying the MDR and crosslink payloads to prime contractor Lockheed Martin Missiles and Space Company. Boeing subcontracts with TRW Space and Electronics Group for the MDR antennas and digital subsystem, and provides portions of the LDR payload under subcontract to TRW.
Milstar MDR meets the needs of multiple users by supporting the connectivity of tactical and conventional forces. Milstar LDR supports strategic and tactical requirements for high anti-jam and nuclear scintillation protection. Using the crosslinks, the constellation supports multiple users simultaneously without reliance upon physically vulnerable nodes. Milstar's space assets provide a "communications switch in the sky."
Communications features of the MDR and LDR payloads are compared in the table below.
| ||MDR ||LDR
||4.8 kbps - 1544 kbps
||75 bps - 2400 bps
||8 high-gain narrow
|2 narrow spot beams
1 wide spot beam
1 earth coverage antenna
1 UHF transmit antenna
1 UHF receive antenna
5 earth coverage uplink
1 earth coverage downlink
Nuller - active
The Milstar system provides uplink communications at extremely high frequency (EHF), 44 GHz, and ultrahigh frequency (UHF), 300 MHz, and downlink communications at super-high frequency (SHF), 20 GHz, and UHF, 250 MHz. The crosslinks operate in the 60 GHz region. Survivability and endurability requirements are satisfied by anti-jam, hardening and system autonomy features.
The MDR payload provides secure, jam-resistant communications services through unique onboard signal and data processing capabilities. It sends real-time voice, video and data to military personnel in the field at rates up to 1.5 Mbps. The payload uses a 32-channel EHF uplink and an SHF downlink.
The MDR payload dynamically sorts incoming data and routes them to the proper downlinks to establish networks and provide bandwidth on demand. If necessary it passes the data on to another satellite via crosslink.
As the supplier of the MDR payload, BSS integrates antennas and electronic units onto the MDR structure, called the -X (minus X) wing, provided by Lockheed Martin. (see satellite diagram above)
The MDR antenna coverage subsystem consists of eight narrow spot beam antennas provided by TRW: two narrow spot beams with nulling capabilities (nuller antennas) and six distributed user coverage antennas (DUCAs), each supporting two-way communications.
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. The nuller antennas resist jamming from within their respective coverage areas by changing their gain patterns when a jamming signal is detected. The DUCAs provide high gain/low sidelobes for distributed users.
Radio Frequency Subsystem (RFSS)
The RF subsystem includes the processing and receiving components and the downlink group. The processing and receive group performs the following four payload functions:
- amplifies, dehops, and downconverts the EHF waveform to the first intermediate frequency (IF) via the low-noise amplifier/downconverter;
- receives, amplifies, downconverts, and switches the first IF to the second IF for input to one of four demodulator groups of eight channels each;
- employs a differential phase shift key (DPSK) to modulate and upconvert onto a hopped SHF carrier for input to the downlink group; and
- generates and distributes the hopping and fixed local oscillators for the antenna coverage subsystem, digital subsystem and RFSS.
The downlink group amplifies, filters and switches, on a hop-by-hop basis, the SHF waveform to any of the eight antennas. The SHF amplifiers are triple-redundant traveling wave tube amplifiers. Switching capability is provided by a high speed/high power beam select switch.
The digital subsystem consists of the demodulator, the data router and control processor, the crosslink processor and the Milstar Advanced Processor. The subsystem has five major functions:
- demodulate uplink baseband data;
- provide the switching and routing of the incoming uplink and crosslink baseband data to the downlink and crosslink destinations;
- provide the uplink and downlink acquisition and tracking control protocols;
- provide on-board resource allocation enabling dynamic user connectivity; and
- provide the interface to the spacecraft.
Together the MDR payload weighs about 1100 pounds and uses 860 watts.
Like a handshake in space, crosslinks provide rapid, ultra-secure communications by enabling the satellites to pass signals to one another worldwide while requiring only one ground station on friendly soil. The crosslink payload provides V-band (60 GHz) data communications between Milstar satellites for both the MDR and LDR payloads. This includes modulation and demodulation of the data, upconversion, amplification for transmission and downconversion.
On Dec. 15, 1995, Milstar was the world's first satellite constellation, government or commercial, to employ crosslinks. A message was uplinked from the national Military Command Center at Fort Belvoir, Va., through the Milstar F-1 satellite. It was crosslinked to the Milstar F-2 spacecraft, then downlinked to commanders at Pacific Command at Camp H.M. Smith, Hawaii, and U.S. Atlantic Command at Norfolk, Va.
The major crosslink payload elements are as follows:
- Millimeter wave assemblies - include the antenna group with autotracking modulator, transmit and receive downconverter and telemetry/command unit. There are two assemblies, one on the -X wing and one on the +X wing.
- I-Panel assembly - includes receivers, acquisition unit and autotrack unit.
Together this equipment weighs approximately 800 pounds and uses 260 watts.
The LDR payload offers nearly 200 user channels and relays coded teletype and voice messages at data rates of 75 to 2400 bits per second.
Boeing provides five major elements for the LDR payload, as follows:
- Time and frequency reference subsystem -- includes time-of-day clock for system timing and synchronization for spectrally pure frequency generation, and for dehopping of EHF uplink frequencies and hopping of SHF downlink frequencies.
- SHF subsystem transmit equipment group -- provides downlink modulation and power amplification at 20 GHz.
- Synthesizer select switch -- provides connectivity between wideband frequency synthesizers and EHF downconverters.
- EHF downconverters -- receive 44 GHz uplink, dehops it and downconverts to an intermediate frequency.
- Antenna positioner equipment includes beam steering mechanisms and control electronics for three shared-aperture EHF/SHF spot beam antennas.
Together these elements weigh approximately 600 pounds and use 350 watts.