Milstar (Military Strategic & Tactical Relay) 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-1 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-2
satellites extend the communications capabilities to higher data rates by adding a
medium-data-rate (MDR) payload. The Milstar-1 and -2
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. BSS 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 satellites use a unique bus, consisting of three boxes, which are folded during
launch and deploy in orbit.
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.
- LDR Payload:
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.
- MDR Payload:
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.
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.
- Crosslink Payload:
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.
Originally the full Milstar system was to contain ten satellites, which were cut to four.
Milstar-2 1 was launched on 30 April 1999, but was stranded in a into a lower orbit than had been planned, and damaged by deployment at excessive rates due to a database error affecting the attitude control system of the Centaur upper stage. It could not be raised into its operational orbit due to fuel limitations. Its orbit was raised as much as possible to increase the expected lifetime and then it was permanently turned off after 10 days.
Milstar was succeded by the AEHF (Advanced Extreme High Frequency Satellite) satellites.
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