Advanced Extremely High Frequency
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 Armed Forces, the Royal Netherlands Armed Forces and the Australian Defence Force. The system will consist of six satellites in geostationary orbits, five of which have been launched. AEHF is backward compatible with, and 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.
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|
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.
It extends the "cross-links" among AEHF of earlier MILSTAR satellites, which makes it much less vulnerable to attacks on ground stations. As a geosynchronous satellite over the Equator, it still needs to be supplemented, with additional systems optimized for polar coverage in high latitudes.
In the April 2009 Defense Department budget request, Secretary of Defense Robert Gates said he planned to cancel the Transformational Satellite Communications System, still in the design phase, in favor of additional AEHF capacity. Individual AEHF satellites, exclusive of launch expenses, cost US$850 million.
Prior to the AEHF, U.S. and allied military satellite communications systems fell into one of three categories:
- Wideband: maximum bandwidth among fixed and semifixed earth stations
- Protected: survivable against electronic warfare and other attacks, even if bandwidth is sacrificed
- Narrowband: principally for tactical use, sacrificing bandwidth for simplicity, reliability, and light weight of terrestrial equipment.
AEHF, however, converges the role of its wideband Defense Satellite Communications System and protected MILSTAR predecessors, while increasing bandwidth over both. There will still need to be specialized satellite communications for extremely high data rate space sensors, such as geospatial and signals intelligence satellites, but their downlinked data will typically go to a specialized receiver and be processed into smaller amounts; the processed data will flow through AEHF.
Launch and positioningEdit
AEHF satellites are sent into space using an Evolved Expendable Launch Vehicle (EELV). The payload weight at launch is approximately 14,500 pounds (6,600 kg); by the time it expends propellants to achieve proper orbit, its weight is approximately 9,000 pounds (4,100 kg). The satellites will operate in geosynchronous earth orbit (GEO) orbit; it takes over 100 days for the orbital adjustments to reach its stable geo-position after launch.
Uplinks and crosslinks are in the extremely high frequency (EHF) while the downlinks use the super high frequency (SHF). The variety of frequencies used, as well as the desire to have tightly focused downlinks for security, require a range of antennas, seen in the picture:
- 2 SHF downlink phased arrays
- 2 satellite-to-satellite crosslinks
- 2 uplink/downlink nulling antennas
- 1 uplink EHF phased array
- 6 uplink/downlink gimbaled dish antenna
- 1 uplink/downlink earth coverage horns
Phased array technology is new in communications satellites, but increases reliability by removing the mechanical movement required for gimbaled, motor-driven antennas.
The low gain earth coverage antennas send information anywhere in the third of the earth covered by each satellite's footprint. Phased array antennas provide super high-gain earth coverages, enabling worldwide unscheduled access for all users, including small portable terminals and submarines. The six medium resolution coverage antennas (MRCA), are highly directional "spot" coverage; they can be time-shared to cover up to 24 targets. The two high resolution coverage area antennas enable operations in the presence of in-beam jamming; the nulling antennas are part of the electronic defense that helps discriminate true signals from electronic attack.
Another change from existing satellites is using solid-state transmitters rather than the traveling wave tubes used in most high-power military SHF/EHF applications. TWTs have a fixed power output; the newer devices allow varying the transmitted power, both for lowering the probability of intercept and for overall power efficiency.
AEHF provides individual digital data streams from rates of 75 bit/s to approximately 8 Mbit/s. These include and go beyond MILSTAR's low data rate (LDR) and medium data rate (MDR) as well as the actually fairly slow high data rate (HDR) for submarines. The faster links are designated extended data rates (XDR).
While there are a number of ground terminals, the airborne terminal has been part of the Family of Advanced Beyond Line-of-Sight-Terminal (FAB-T) project. Other ground stations include the Single-Channel Antijam Man-Portable Terminal (SCAMP), Secure Mobile Antijam Reliable Tactical Terminal (SMART-T), and Submarine High Data Rate (Sub HDR) system.
With Boeing as the prime contractor and L-3 Communications and Rockwell as major subcontractors, the first FAB-T (Increment 1)was delivered, for use on the B-2 Spirit aircraft, in February 2009. It is planned for other aircraft including the B-52, RC-135, E-4, and E-6 aircraft. Other installations will go into fixed and transportable command posts. It successfully interoperated with legacy communications using an command post terminal and the Army Single Channel Anti-jam ManPortable Terminal,
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 have taken place 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.
The Atlas V launch vehicle successfully placed the satellite into a supersynchronous-apogee transfer orbit with 50,000 km (31,060 mile) apogee, 275 kilometer (170 mile) perigee, 22.1° inclination,
Failure of the kick motor, and recovery using the Hall-effect thrustersEdit
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.
AEHF-4 (USA 288)Edit
The sixth AEHF satellite is projected for launch in 2019.
As of this edit, this article uses content from "Advanced Extremely High Frequency (satellite)", which is licensed in a way that permits reuse under the Creative Commons Attribution-ShareAlike 3.0 Unported License, but not under the GFDL. All relevant terms must be followed.
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