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The current satellites mostly operate at lower frequencies like C- and X-bands each have 500 MHz of bandwidth and they are already crowded with users. Today, naval C- and X-band shipboard SATCOM terminals require supplemental EMI rejection filters to allow them to operate in a battle group environment, especially in close proximity with Aegis surface combatants.
Now the military is moving to higher millimeter-wave region of the electromagnetic spectrum that corresponds to radio band frequencies of 30 GHz to 300 GHz and is sometimes called the Extremely High Frequency (EHF) range. The AEHF (Advanced Extreme High Frequency Satellite), a.k.a. AWS (Advanced Wideband Satellite), program is the next generation of highly secure, high capacity, survivable communications to the U.S. warfighters during all levels of conflict, and will become the protected backbone of the Department of Defense’s military satellite communications architecture. The system will provide military communications and jointly serve armed forces of the US and international partners, including Netherlands, the UK and Canada.
One of the greatest and most important uses of millimeter waves is in transmitting large amounts of data. The high frequency of millimeters waves as well as their propagation characteristics (that is, the ways they change or interact with the atmosphere as they travel) make them useful for a variety of applications including transmitting large amounts of computer data, cellular communications, and radar.
Use of extremely high frequency (EHF) also enable satellite communications systems to achieve a high degree of survivability under both electronic warfare and physical attack. Unlike systems dependent on lower frequencies, EHF satellite communications recover quickly from the scintillation caused by a high-altitude nuclear detonation. The use of higher frequencies offers a number of advantages — assurance of reliable communications in a nuclear environment, minimal susceptibility to enemy jamming and eavesdropping, and the ability to achieve smaller secure beams with modest-sized antennas.
Another military benefit is that K/Ka-band SATCOM terminals are smaller in size hence can provide benefit in reduced radar cross section (RCS) of the platform carrying them. Smaller antennas with reduced RCS also reduce the highly reflective hot spots in a warship’s topside that is of paramount importance in the face of threats from increasingly sophisticated sea-skimming anti-ship cruise missiles.
Certain characteristics of the earth’s atmosphere pose both problems and solutions for millimeter wave applications. For example, at 60 GHz (5 mm or 0.2 inches wavelength) oxygen molecules will interact with electromagnetic radiation and absorb the energy. Since there is almost no oxygen in space at the geosynchronous altitudes of 43,000 km or 26,000 miles), 60 GHz works just fine for communication between satellites and between satellites and earth.
AEHF System
The AEHF operational system is composed of three segments: space, terminals, and mission control. The space segment consists of a cross-linked constellation of satellites to provide worldwide coverage. The terminal segment includes fixed and mobile ground terminals, ship and submarine terminals, and airborne terminals. The mission control segment controls satellites on orbit, monitors satellite health, and provides communication system planning and monitoring. This segment is also survivable, with both fixed and mobile control stations.
Milstar Satellites
Earlier, the Military, Strategic, and Tactical Relay Satellite (Milstar), joint service satellite communications system was used to provide secure, jam resistant worldwide communications to meet essential wartime requirements for high priority military users. The multi-satellite constellation linked command authorities with a wide variety of resources, including ships, submarines, aircraft and ground stations. Milstar was the first major space-based communications effort using EHF technology (30-300 gigahertz) to overcome crowding and interference in other frequencies.
MILSTAR used a variety of new technologies, onboard signal processing, adaptive antennas, uplink nulling, steerable downlinks, and cross-links to provide satellite-to-satellite inter-connectivity. It was the first defense communication satellite system to use frequency hopping on the uplink to frustrate enemy eavesdropping and jamming. Additional protection against jammers was using a phased-array antenna on the satellite that can minimize sensitivity in the direction of a jamming signal.
The cross-link network routes the appropriate communication traffic from terminals in view of one satellite to another terminal located at other parts of the world not covered by that satellite’s field of view. The cross-link capability provides near-real-time connectivity without extensive relay and circuit patching.
Military Capability
AEHF will replace the older Milstar system and will operate at 44 GHz Uplink (EHF band) and 20 GHz Downlink (SHF band). AEHF provides greater capacity and more flexible coverage than its predecessor, Milstar, while assuring operational continuity through compatibility with the Milstar constellation. A single AEHF satellite provides greater total capacity than the entire legacy five-satellite Milstar constellation.
AEHF allows the National Security Council and Combatant Commanders to control their tactical and strategic forces at all levels of conflict up to and including general nuclear war, and it supports the attainment of information superiority. Advanced Extremely High Frequency Satellite (AEHF) is a joint service satellite communications system that provides global, survivable, secure, protected, and jam-resistant communications for high priority military ground, sea, and air assets. The AEHF system provides joint, interoperable, assured connectivity for warfighters in operations in all levels of conflict–a capability not available through other planned military communication networks.
AEHF will provide connectivity across the spectrum of mission areas, including land, air, and naval warfare; special operations; strategic nuclear operations; strategic defense; theater missile defense; and space operations and intelligence. AEHF delivers the flexible connectivity-on-demand needed to achieve 21st century objectives–swift, decisive outcomes based on information dominance. On-orbit processing provides the flexibility needed to rapidly establish and reconfigure networks to meet dynamic command and control requirements.
“Protected communications means more than encryption and authentication—these systems must be the communications channel that stands when all others fail,” said Iris Bombelyn, vice president of Lockheed Martin’s Protected Communications mission area. “This is an important milestone in our support of that mission, and we continue to remain focused on anticipating changing needs and innovating new capabilities long into the future.”
The nuclear-hardened communications satellites are resistant to high-tech jammers, eavesdropping and cyberattack. Using on-board signal processing and radio frequency crosslinks, for communication between on-orbit satellites, the system also removes the vulnerability of ground relay stations from cyber and other attacks.
The risk of Nuclear Security including nuclear command and Control has increased due to many reasons including rising number of cyber-attacks, increasing nuclear threats from many countries including North Korea, proliferation of nuclear material or technology by countries like Pakistan and considering that half of [U.S. and Russian] strategic arsenals being continuously maintained on high alert. The Advanced Extremely High Frequency, or AEHF, allows the National Security Council and unified combatant commanders to control tactical and strategic forces at all levels of conflict through general nuclear war and supports the attainment of information.
“We’re proud to deliver an unparalleled leap forward in protected communications capability for both our nation’s senior leaders and also our warfighters in the field,” Lt. Gen. Samuel Greaves, the head of the service’s Space and Missile Systems Center, said
AEHF Series of satellites
The space segment will consist of six satellites in geostationary orbits, three of which have been launched. When complete, the space segment will provide coverage of the surface of the Earth between latitudes of 65 degrees north and 65 degrees south. When fully operational, It is designed to provide extremely high-frequency (EHF) range uplink/crosslink capabilities and super high-frequency (SHF) range communications.
Lockheed Martin completed on-orbit test of the US Air Force’s (USAF) advanced extremely high frequency AEHF-4 spacecraft in April 2019. Launched in October, AEHF-4 comprises payload built by subcontractor Northrop Grumman. The satellite will provide a new capability of global extended data rate (XDR) communications to support military users across the world.
The AEHF-4 on-orbit test (A4 OOT) activated the payload. According to Lockheed, the spacecraft met all of its requirements during the test. A4 OOT marks the first ever test to have all six AEHF operational terminals communicating over XDR. XDR communications enable the transmission of data to its users at rates five times higher than medium data rate (MDR) and 350 times higher than low data rate (LDR) communications. The MDR and LDR communication modes are available on Milstar, AEHF’s predecessor, to directly support troops.
Lockheed Martin Protected Communications vice-president Michael Cacheiro said: “Four AEHF satellites in orbit means protected global connectivity for those who need it most, from the president to deployed soldiers. “We offer powerful end-to-end systems so that more operational users can have assured connectivity in contested environments. “Delivering this fourth satellite in orbit will be critical to the air force, as it will connect all four satellites on orbit, forming a geostationary ring to provide uninterrupted global communications.”
Under the contract, Lockheed Martin is required to develop jam-proof communication systems that offer a low probability of detection or interception. The AEHF satellite will facilitate military communications with real-time video, battlefield maps and targeting data. According to the company, strategic command and tactical troops using the AEHF system can engage in highly secure, protected communications during their operations on ground, sea and air platforms.
In addition to the US, the jam-resistant communications system serves international partners such as Canada, the Netherlands and the UK.
The Defense Department awarded United Launch Alliance a $138 million contract modification May 31 largely to launch the fifth in a series of protected communications satellites on an Atlas 5 rocket in 2018. The first spacecraft, AEHF 1, was launched in August 2010, while the AEHF-2 and AEHF-3 were launched in May 2012 and September 2013 respectively. In July 2015, the AEHF system achieved initial operational capability and is being operated by the U.S. Air Force’s 4th Space Operations Squadron. The AEHF-4, is scheduled to launch in 2017. AEHF-5 and 6 are in production at Lockheed Martin in Sunnyvale, California, before they undergo final assembly, integration and test operations prior to launch.
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.
The terminal segment includes fixed and ground mobile terminals, ship and submarine terminals, and airborne terminals, including the Family of Advanced Beyond Line-of-Sight -Terminal (FAB-T), used by all of the Services and international partners (Canada, Netherlands and UK.). The mission control segment controls satellites on orbit, monitors satellite health and provides communication system planning and monitoring. This segment is highly survivable, with both fixed and mobile control stations.
Like the Milstar system, AEHF will be operated by the 4th Space Operations Squadron, located at Schriever Air Force Base. The Defense Department has three Raytheon-built AEHF terminals in use today: the Navy Multiband Terminal, the Humvee-mounted Secure Mobile Anti-jam Reliable Tactical Terminal and the Minuteman Minimum Essential Emergency Communications Network Program Upgrade.
A new strategic terminal system, known as the Family of Beyond Line-of-Sight Terminals and designed for presidential and other national command authority communications, is under contract to Raytheon Space and Airborne Systems.
AEHF Payload
The AEHF payload, developed by Northrop Grumman consists of processors, antennas, radio frequency subsystems and crosslinks. The payload delivers the new XDR (Extreme Data Rate) communications services, providing data rates up to 8.192 Mbps per user, Milstar LDR (Low Data Rate) services (75 – 2,400 bits per second), and Milstar MDR (Medium Data Rate) services (4.8 Kbps – 1.544 Mbps).
On-board signal processing will provide protection and ensure optimum resource utilization and system flexibility among the Armed Forces and other users who operate terminals on land, sea and air. The AEHF system will be integrated into the legacy Milstar constellation, and will be backward compatible with Milstar’s low data rate (LDR) and medium data rate (MDR) capabilities, while providing extreme data rates (XDR) and larger capacity at substantially less cost than the Milstar system.
AEHF Propulsion : Hall thruster
The US Air Force’s top-secret X-37B spacecraft took part in a ‘Hall thruster electric propulsion test’ in May 2015, for its Advanced Extremely High Frequency (AEHF) satellites. Hall thrusters use electricity to ionize and propel xenon gas, setting them apart from conventional chemical engines. These results in a ‘whisper quiet’ thruster which uses only a fraction of the fuel and weight those current methods do.
“A more efficient on-orbit thruster capability is huge. Less fuel burn lowers the cost to get up there, plus it enhances spacecraft operational flexibility, survivability and longevity,” said Major General Tom Masiello, an Air Force Research Laboratory (AFRL) commander, as quoted by Spaceflight Now.
Aerojet Rocketdyne supports the AEHF-4 mission from the launch pad to orbit to final end-of-life decommissioning of the satellite. Starting with the launch vehicle – Aerojet Rocketdyne provides the AJ-60A solid rocket motors that helped boost United Launch Alliance’s Atlas V off the launch pad; the pressure vessels that support the first and second stages of the launch vehicle; the RL10 liquid hydrogen/liquid oxygen engine that provides thrust to power the Centaur upper stage, and MR-106 Reaction Control System thrusters, which provide pitch, yaw and roll control for the Centaur upper stage, as well as settling burns.
Onboard the AEHF-4 space vehicle, Aerojet Rocketdyne provides electric XR-5 Hall Thruster strings and monopropellant rocket engines which enable nearly all propulsive functions throughout the life of the satellite. One XR-5 Hall Thruster string consists of an XR-5 Hall Thruster, a xenon flow controller (XFC) and the power processor that drives and controls the thruster and XFC. The XR-5 Hall Thrusters on AEHF enable a propellant mass savings in excess of 2,000 lbs. when compared to performing the mission with all chemical propulsion.
USAF tested B-2 stealth bomber communication with AEHF
USAF has already tested B-2 stealth bomber communicating with the Advanced Extremely High Frequency (AEHF) satellite communications (satcom) network through its AESA antenna. The will allow the B-2 to send and receive battlefield information at significantly faster data rates compared to current satellite technology
The US Air Force will upgrade its Intercontinental Ballistic Missile (ICBM) communication systems in Launch Control Centers (LCC) to receive emergency action messages from AEHF satellite constellation Minuteman Minimum Essential Emergency Communications Network Program Upgrade (MMPU) shall provide increased data rates, as well as nuclear security architecture
China’s High Throughput Satellite Shijian-13
China’s first high-throughput communications satellite Shijian-13 was launched from Xichang Satellite Launch Center in Sichuan Province in April 2017.To be located at 110.5°E longitude orbital position, the satellite will improve internet access on planes and high-speed trains.
To be operated by China Satcom, the satellite will be designated as Zhongxing-16 (ChinaSat-16) during operational phase. Design and development China’s first high-throughput communications satellite Shijian-13 was designed and manufactured by the China Academy of Space Technology (CAST) in collaboration with China Aerospace Science and Technology Corporation.
It was developed using a number of domestic components and installed with first Chinese laser communications system. It is also the first Chinese satellite to use electric propulsion in orbit instead of chemical propellants. The satellite includes a Ka-band broadband communications system with a capacity to transmit data at a speed of approximately 20GB/s. It features 26 user beams covering China and offshore areas.
It can be used to provide airborne, maritime, and emergency communications with the help of Ka-band broadband and multimedia services. It can host multiple telecommunications payloads to provide communications, including fixed, international satellite, national, regional, wideband data, mobile, direct broadcast, and military. It also offers spacecraft tracking and data relay abilities. The satellite will also help people during natural disasters, performing space-to-ground laser communication experiments.
