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{{Short description|Constellation of American military satellites}}
{{Multiple issues|
{{one source|date=April 2020}}
{{Update|date=May 2021}}
}}
{{Infobox spacecraft class
|image=Milstar.jpg
|caption=Artist's impression of a Milstar Block I spacecraft
|name = Milstar
|manufacturer = [[Lockheed Martin]] (prime, formerly Lockheed Missiles and Space)<br/>[[Northrop Grumman]] (formerly [[TRW Inc.|TRW]])<br/>[[Boeing]] (formerly [[Hughes Aircraft|Hughes]])
|country = United States
|bus = [[Milstar Block I]]<br/>[[Milstar Block II]]
|applications = Military communications
|orbits = [[Geosynchronous orbit|Geosynchronous]]
|operator = [[United States Space Force|U.S. Space Force]]
|lifetime = 10 years
|status = Out of production<br/>Active
|built = 6
|launched = 6
|operational = 5{{citation needed|date=June 2019}}<!-- no source indicating that all of these are still operational in 2019, 16 to 25 yrs after they launched -->
|failed = 
|lost = 1
|first = [[USA-99]], 1994-02-07
|last = [[USA-169]], 2003-04-08
|retired = 
|mass = {{convert|4500|kg}} |autoconvert = off 
}}
'''Milstar''' (Military Strategic and Tactical Relay)<ref>{{Cite web|url=https://www.globalsecurity.org/military/library/budget/fy2000/dot-e/airforce/00milstar.html|title = MILITARY STRATEGIC AND TACTICAL RELAY (MILSTAR) SATELLITE SYSTEM - FY00 Activity}}</ref> is a [[satellite constellation|constellation]] of [[Military satellite|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.

== History ==
Milstar Block I spacecraft, or Milstar Developmental Flight Satellite (DFS)-1 and -2, were designed with a Low Data Rate (LDR) payload in the {{nobreak|+X}} wing of the satellite that broadcast in the [[Super High Frequency]] (SHF) and [[Extremely High Frequency]] (EHF) ranges, and also a classified communication payload in the {{nobreak|-X}} wing. The DFS-1 satellite was launched on 7 February 1994 aboard the first [[Titan IV|Titan IV(401)A]] rocket, but with the classified {{nobreak|-X}} wing payload deactivated. It was followed by the DFS-2 spacecraft on 7 November 1995. DFS-2 was similar to DFS-1, but the classified payload was replaced by [[ballast]] in the form of a precision machined [[aluminum]] block to maintain the weight and balance characteristics of the satellite. Both Block I satellites (USA-99 and USA-115) are still operational as of March 2025, over 25 years since they were launched.

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|Titan IV(401)B]] rocket. Due to a database error affecting the attitude control system of the [[Centaur (rocket stage)|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.<ref>{{Cite web |title=NASA - NSSDCA - Spacecraft - Details |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1999-023A |access-date=2023-03-23 |website=nssdc.gsfc.nasa.gov}}</ref><ref>{{Cite web |title=Milstar-2 1, 2, 3, 4 (Milstar 3, 4, 5, 6) |url=https://space.skyrocket.de/doc_sdat/milstar-2.htm |access-date=2023-03-23 |website=Gunter's Space Page |language=en}}</ref> 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 Air Force Base|Hanscom AFB]] is responsible for the US Air Force portion of the terminal segment development and acquisition. The [[4th Space Operations Squadron]] at [[Schriever Space Force Base|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.{{citation needed|date=June 2019}}

==Characteristics==
Milstar satellites provide secure, jam resistant, worldwide communications to meet the requirements of the United States military. They were built by [[Lockheed Martin|Lockheed Martin Missiles and Space Corporation]], at a cost of [[US dollar|US$]]800 million each. Each satellite has a design life of 10 years. Six were built, of which five reached their operational [[geosynchronous orbit]]s, and remain in service. Launches were made using [[Titan IV]] rockets with Centaur upper stages, and all six occurred from [[Cape Canaveral Air Force Station Space Launch Complex 40|Space Launch Complex 40]] at the [[Cape Canaveral Air Force Station]]. The satellites are designed to provide communications which are hard to detect and intercept, and to be survivable in the event of [[nuclear warfare]].

The spacecraft have a mass of {{convert|4500|kg}}, and are equipped with [[solar panel]]s which generate eight [[kilowatt]]s of [[electric power]] to power its [[transponder (satellite communications)|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&nbsp;kbit/s and 1.544&nbsp;Mbit/s. The satellites' uplinks operate in the [[Q band]], while their downlinks operate within the [[K band (IEEE)|K band]]. The uplink corresponds to the [[extremely high frequency]] band while downlink corresponds to the [[super high frequency]] radio band.{{citation needed|date=June 2019}}

===Spacecraft===
{{redirect|USA-99|the World Cup|1999 FIFA Women's World Cup}}<!--often referred to as "USA 99"-->
{| class="wikitable"
|-
! USA ID
! Name
! Block
! Launch date/time (UTC)
! [[International Designator|COSPAR ID]]
! Rocket
! Remarks
|-
|USA-99
|DFS-1
|Block I
|1994-02-07, 21:47:01
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1994-009A 1994-009A]
|[[Titan IV|Titan IV(401)A]]
|
|-
|USA-115
|DFS-2
|Block I
|1995-11-06, 05:15:01
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1995-060A 1995-060A]
|[[Titan IV|Titan IV(401)A]]
|
|-
|USA-143
|DFS-3M
|Block I/II hybrid
|1999-04-30, 16:30:00
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1999-023A 1999-023A]
|[[Titan IV|Titan IV(401)B]]
|Launch failure
|-
|USA-157
|DFS-4
|Block II
|2001-02-27, 21:20
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2001-009A 2001-009A]
|[[Titan IV|Titan IV(401)B]]
|
|-
|USA-164
|DFS-5
|Block II
|2002-01-16, 00:30:00
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2002-001A 2002-001A]
|[[Titan IV|Titan IV(401)B]]
|
|-
|[[USA-169]]
|DFS-6
|Block II
|2003-04-08, 13:43:00
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2003-012A 2003-012A]
|[[Titan IV|Titan IV(401)B]]
|
|-
|}

==See also==
*[[Advanced Extremely High Frequency]]
*[[Defense Satellite Communications System]] 
*[[Transformational Satellite Communications System]]
*[[Wideband Global SATCOM system]]

==References==
<references />
* King, Mak and Riccio, Michael J. (Spring 2010). "Military Satellite Communications: Then and Now". ''Crosslinks Magazine''. Aerospace Corp.

==External links==
*[https://www.losangeles.spaceforce.mil/?id=5328 U.S. Air Force fact sheet on MILSTAR]
*[https://web.archive.org/web/20110826180043/http://satsin.com/milstar-3-advanced-extremely-high-frequency-aehf/ MILSTAR 3 / Advanced Extremely High Frequency (AEHF)]

{{USAF space vehicles}}
{{Lockheed Martin}}
{{USAF system codes}}

[[Category:Communications satellites]]
[[Category:Post–Cold War military equipment of the United States]]
[[Category:Lockheed Martin satellites and probes]]
[[Category:Military space program of the United States]]
[[Category:Equipment of the United States Space Force]]
[[Category:Military equipment introduced in the 1990s]]

[[ja:アメリカ軍の衛星通信#MILSTAR]]