Advanced Extremely High Frequency
Artist's impression of an AEHF satellite
|Country of origin||United States|
|Operator||US Air Force|
|Design life||14 years|
|Launch mass||6,168 kg (13,598 lb)|
|First launch||USA-214, 2010-08-14|
Advanced Extremely High Frequency (AEHF) is a series of communications satellites operated by the United States Air Force Space Command. They will be used to relay secure communications for the Armed Forces of the United States, the British Armed Forces, the Canadian Forces and the Royal Netherlands Armed Forces. The system will consist of six satellites in geostationary orbits, three of which have been launched. AEHF will replace the older Milstar system and will operate at 44 GHz Uplink (EHF band) and 20 GHz Downlink (SHF band). AEHF systems is a joint service communications system that will provide survivable, global, secure, protected, and jam-resistant communications for high-priority military ground, sea and air assets. It is the follow-on to the Milstar system. AEHF systems' uplinks and crosslinks will operate in the extremely high frequency (EHF) range and downlinks in the super high frequency (SHF) range. 
AEHF satellites use a large number of narrow spot beams directed towards the Earth to relay communications to and from users. Crosslinks between the satellites allow them to relay communications directly rather than via a ground station. The satellites are designed to provide jam-resistant communications with a low probability of interception. They incorporate frequency-hopping radio technology, as well as phased array antennas that can adapt their radiation patterns in order to block out potential sources of jamming.
AEHF incorporates the existing Milstar low data-rate and medium data-rate signals, providing 75–2400 bit/s and 4.8 kbit/sec–1.544 Mbit/s respectively. It also incorporates a new signal, allowing data rates of up to 8.192 Mbit/s. When complete, the space segment of the AEHF system will consist of six satellites, which will provide coverage of the surface of the Earth between latitudes of 65 degrees north and 65 degrees south. For northern polar regions, the Enhanced Polar System acts as an adjunct to AEHF to provide EHF coverage.
The initial contract for the design and development of the AEHF satellites was awarded to Lockheed Martin Space Systems and Northrop Grumman Space Technology in November 2001, and covered the System Development and Demonstration phase of the program. The contract covered the construction and launch of three satellites, and the construction of a mission control segment. The contract was managed by the MILSATCOM Program Office of the United States Air Force Space and Missile Systems Center. Like the Milstar system, AEHF will be operated by the 4th Space Operations Squadron, located at Schriever Air Force Base.
The first satellite, USA-214, was successfully launched by an Atlas V 531 rocket on 14 August 2010, from Space Launch Complex 41 at the Cape Canaveral Air Force Station. This occurred four years behind schedule; when the contract was awarded in 2000 the first launch was expected to occur in 2006. The program was restructured in October 2004, when the National Security Agency did not deliver key cryptographic equipment to the payload contractor in time to meet the launch schedule.
Failure of the kick motor, and recovery using the Hall-effect thrusters
The satellite vehicle's Liquid Apogee Engine (LAE) provided by IHI failed to raise the orbit after two attempts. To solve the problem, the perigee altitude was raised to 4700 km (2900 miles) with twelve firings of the smaller Aerojet-provided Reaction Engine Assembly thrusters, originally intended for attitude control during the LAE engine burns. From this altitude, the solar arrays were deployed and the orbit was raised toward the operational orbit over the course of nine months using the 0.27 Newton Hall thrusters, also provided by Aerojet, a form of electric propulsion which is highly efficient, but low thrust. This took much longer than initially intended due to the lower starting altitude for the HCT maneuvers. This led to program delays, as the second and third satellite vehicle LAEs were analyzed. The investigation into the propulsion anomaly has been completed (but not publicly released as of June 2011[update])[needs update] and the remaining satellites were declared flight ready.
A Government Accounting Office report released in July 2011 stated that the blocked fuel line in the Liquid Apogee Engine was most likely caused by a piece of cloth inadvertently left in the line during the manufacturing process.
Like the first AEHF satellite, the second (AEHF-2) was launched on an Atlas V flying in the 531 configuration. The launch from Space Launch Complex 41 at Cape Canaveral took place on 4 May 2012. After three months of maneuvering, it reached its proper position and the testing procedures were started. Completion of checkout of AEHF-2 was announced on 14 Nov. 2012 and control turned over to the 14th Air Force for operations for an expected 14-year service life through 2026.
The third AEHF satellite was launched from Cape Canaveral on 18 September 2013 at 4:10 a.m. EDT. The two-hour window to launch the satellite opened at 3:04 a.m. EDT and the launch occurred as soon as weather-related clouds and high-altitude winds cleared sufficiently to meet the launch criteria.
The launch of the fourth AEHF satellite is scheduled for the fall of 2018.
The sixth AEHF satellite is projected for launch in 2019.
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