astronomy

Milstar

Milstar is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Milstar rather than just read about it. In short: 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 t…

Key takeaways

Milstar belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Milstar to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Milstar from memory before moving on to harder problems.

Reference excerpt

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.

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 +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 -X wing. The DFS-1 satellite was launched on 7 February 1994 aboard the first Titan IV(401)A rocket, but with the classified -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 30 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(401)B 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.

Characteristics Milstar satellites provide secure, jam resistant, worldwide communications to meet the requirements of the United States military. They were built by Lockheed Martin Missiles and Space Corporation, at a cost of US$800 million each. Each satellite has a design life of 10 years. Six were built, of which five reached their operational geosynchronous orbits, and remain in service. Launches were made using Titan IV rockets with Centaur upper stages, and all six occurred from 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 4,500 kilograms (9,900 lb), and are equipped with solar panels which generate eight kilowatts 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.

Spacecraft

See also Advanced Extremely High Frequency Defense Satellite Communications System Transformational Satellite Communications System Wideband Global SATCOM system

References

King, Mak and Riccio, Michael J. (Spring 2010). "Military Satellite Communications: Then and Now". Crosslinks Magazine. Aerospace Corp.

External links U.S. Air Force fact sheet on MILSTAR MILSTAR 3 / Advanced Extremely High Frequency (AEHF)

Illustrations

Worked examples

Example 1 — a first encounter with Milstar

Start with the simplest possible case. Write down what Milstar claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Milstar before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Milstar ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Milstar

In research: Milstar appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Milstar in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Milstar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Communications satellites, Equipment of the United States Space Force, Lockheed Martin satellites, so understanding it makes those chapters shorter.
In everyday life: Look for Milstar outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Milstar in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Milstar means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Milstar out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Milstar in simple terms?

Why does Milstar matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Milstar?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Milstar.