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Milstar (Military Strategic and Tactical Relay) is a constellation of military communications satellites in geosynchronous orbit, which are operated by the United States Space Force, and provide secure and jam-resistant worldwide communications to meet the requirements of the Armed Forces of the United States. Six spacecraft were launched between 1994 and 2003, of which only five were operational after launch; the third launch failed, both damaging the satellite and leaving it in an unusable orbit.
The four later satellites were Block II spacecraft, which featured an additional medium data-rate payload. The first Block II satellite (DFS-3m, a hybrid mix of largely Block I support systems and LDR payload and a MDR (Medium Data Rate) Block II payload) was launched on 30 April 1999, using a Titan IV rocket. Due to a database error affecting the attitude control system of the Centaur upper stage of its carrier rocket, it was placed into a lower orbit than had been planned, and damaged by deployment at excessive rates. 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. It was the third consecutive, and last, failure of a Titan IV rocket. The remaining three satellites (DFS-4, -5, and -6) were launched on 27 February 2001, 15 January 2002, and 8 April 2003.
The Milstar system consists of three segments; the space segment which consists of the six satellites, ground terminals and users, and stations to command and control the satellites. The Military Satellite Communications Systems Wing (MCSW) division of the Space and Missile Systems Center, located at Los Angeles AFB was responsible for development and acquisition of the Milstar space and mission control segments. The Electronic Systems Center at Hanscom AFB is responsible for the US Air Force portion of the terminal segment development and acquisition. The 4th Space Operations Squadron at Schriever SFB and the 148th Space Operations Squadron at Vandenberg SFB are responsible for providing real-time satellite control and communications payload management.
In August 2010 control of the Milstar system was transferred to the Advanced Extremely High Frequency program, in preparation for the launch of the first AEHF satellite, USA-214. Advanced Extremely High Frequency satellites are intended to replace Milstar.
The spacecraft have a mass of , and are equipped with which generate eight of electric power to power its transponders. Both Block I and Block II satellites provide low data-rate communications at bandwidths between 75 bit/s and 2,400 bit/s, whilst the Block II spacecraft can also provide medium data-rate communications between 4.8 kbit/s and 1.544 Mbit/s. The satellites' uplinks operate in the Q band, while their downlinks operate within the K band. The uplink corresponds to the extremely high frequency band while downlink corresponds to the super high frequency radio band.
| USA-99 | DFS-1 | Block I | 1994-02-07, 21:47:01 | 1994-009A | Titan IV | |
| USA-115 | DFS-2 | Block I | 1995-11-06, 05:15:01 | 1995-060A | Titan IV | |
| USA-143 | DFS-3M | Block I/II hybrid | 1999-04-30, 16:30:00 | 1999-023A | Titan IV | Launch failure |
| USA-157 | DFS-4 | Block II | 2001-02-27, 21:20 | 2001-009A | Titan IV | |
| USA-164 | DFS-5 | Block II | 2002-01-16, 00:30:00 | 2002-001A | Titan IV | |
| USA-169 | DFS-6 | Block II | 2003-04-08, 13:43:00 | 2003-012A | Titan IV | |
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