There has been exponential growth of space objects, including orbital debris that has increased the in-orbit collision risk. NASA estimates there are 21,000 objects orbiting Earth that are larger than 10 cm, 500,000 between 1 and 10 cm, and more than 100 million that are less than 1 cm. Orbital debris of even 1cm size, travelling at an average speed of about 11 km/sec can cause partial or complete destruction of satellite. These objects pose a major threat to satellites in orbit that power everything from smartphones and weather prediction to national security and global financial markets.
Space Fence is an advanced ground-based radar system that will use a highly sensitive S-band radar to identify and track these objects, which will help prevent satellites and the International Space Station from colliding with debris.
Space Fence, currently under construction on Kwajalein Atoll by Lockheed Martin, will revolutionize Space Situational Awareness (SSA) using advanced phased array radar technology in two of the largest phased arrays to ever be constructed. “Once complete, Space Fence will deliver revolutionary capability to the U.S. Air Force with a flexible system capable of adapting to future missions requiring new tracking and coverage approaches.” It would detect, track, and catalog about 200,000 orbital objects in space more than 1.5 million times a day to predict and prevent space-based collisions. That site is expected to be fully operational by the end of 2018.
Currently the USAF only has the ability to track space objects in low earth orbit (LEO) that are roughly the size of a basketball, Elaine Doyle, the Space Fence programme manager said. Space Fence, she said, will provide much greater sensitivity and allow the service to detect, track, and characterise objects to the size of a softball (9.7 cm diameter) in uncued, or passive, search.
For cued, or much more detailed tracking, Doyle said the USAF will be able to track marble-sized (roughly 1 cm diameter) objects. Objects the size of a marble can pose threats to satellites and other spacecraft because they travel at high velocities.
Threat of Space debris
The rise of space junk orbiting the Earth could ‘provoke armed conflict’ as damage to military satellites could be misconstrued as an attack, a new report warned. Researchers at the Russian Academy of Sciences in Moscow said the debris had a “special political danger” because it is difficult to determine whether an operational satellite had been hit by the fragments or was intentionally attacked by another country.
The warning comes after a Russian satellite, Blits, was damaged in 2013 after colliding with debris created when China shot down an old weather satellite in 2007. The destruction of the satellite left 3,000 more pieces of debris in orbit.
Professor Adushkin warns unless something is done to clean up this area of the Earth’s orbit it could lead to more space junk forming as pieces of debris crash into each other and produce smaller fragments. Data from the Russian Space Agency last year shows the International Space Station was forced to take evasive action to avoid space wreckage five times in 2014.
Space agencies in the US and Russia track thousands of pieces of space junk larger than 10cm but estimate there could be trillions of smaller pieces.
Space militarization and weaponization
Space is also becoming another domain of conflict due to enhanced militarization and weaponization of space. Gen. John E. Hyten, Space Command commander, said Chinese anti-satellite weapons are still under development but “close to fruition.”
China continues to develop a variety of capabilities designed to limit or prevent the use of spacebased assets by adversaries during a crisis or conflict, including the development of directed-energy weapons and satellite jammers. “As China’s developmental counterspace capabilities become operational, China will be able to hold at risk U.S. national security satellites in every orbital regime,” says 2015 Report to Congress.
Since 2005, China has conducted eight anti-satellite tests. Tests conducted in 2010, 2013, and 2014 were labelled “land-based missile interception tests.”
“There have been additional tests that didn’t destroy a satellite since that time.” Secretary of the Air Force Deborah Lee James said at the Space Symposium in Colorado Springs, Colorado: “The testing has continued, so that is an ongoing concern, something that we are watching.”
Russia is also a cause for concern, she added. In May 2014, it launched three communication satellites, along with a fourth spacecraft that is maneuvering between higher and lower orbits and sidling up to other objects.
Last year, the Defense Department conducted a strategic review of the space portfolio. One conclusion was that current space systems were designed in an era when space was not contested or congested, “This is no longer the case.”
Lockheed Martin’s Space Fence System will offer revolutionary capability
The new space object tracking site will give satellite operators a clearer picture of the debris that could damage their networks, and how they can avoid potential collisions. That data will then be quickly and accurately delivered to customers allowing them to manoeuvre satellites and prevent collisions.
The space is also becoming increasingly militarized many countries are developing killer microsatellites and other antisatellite weapons (ASAT) that could be used to damage other satellites. There is also thrust on space robots which can perform repair of satellites and which could also put to deorbit adversary’s satellites. The space radar such as this could provide complete awareness of adversary’s activities in space so that one can take counter actions.
The Space Fence program will replace the earlier VHF based radar by S-band radar system, leading to much higher sensitivity that will allow the Air Force to track baseball-sized objects, microsatellites or debris, as far out as 1,900 kilometers in space. The new system would have a maximum coverage area of 40,000 kilometers, compared to 22,000 kilometers maximum of earlier systems.
In addition, the new System will be based active phased array radar system made up of a large array of elements that, together, make an integrated beam. This allows the new system to be smarter, able to be steered in specific directions whenever necessary, unlike the legacy system which continually stares off into the same area of space.
A Lockheed Martin-led team won a $915 million contract last June to construct new Space Fence system, and the total value of the eight-year contract is valued at more than $1.8 billion. Space Fence system is a multiphase programme seeking delivery of two, globally positioned, S-band radars to enable accurate detection, tracking, measurement and recording of objects and debris orbiting the Earth. Once construction is complete, Space Fence will go through testing and validation before its initial operating capability occurs in late 2018.
Space Fence includes up to two minimally manned radar sites. The first radar site is under construction on Kwajalein Atoll in the Pacific Ocean near the equator and is expected to become operational in late 2018. The second site, currently an unfunded contract option, is located in Western Australia. The sensor sites provide assured coverage for objects in LEO and are integrated through an operations center located in Huntsville, Ala.
Lockheed Martin’s new test site representative of the larger system under construction on the remote Kwajalein Island is now complete. The U.S. Air Force’s Space Fence marked a major accomplishment Jan. 30, 2016, after a scaled-down version of the end-item system recorded its first track of a satellite. “First track is major milestone for us and represents that we have a functioning radar,” says Bruce Schafhauser, Space Fence Program Director for Lockheed Martin. “It’s the first time the end-to-end radar loop is closed and we track real objects in space.
The test facility is used for early validation of hardware, firmware and software that will enable the Space Fence system to detect, track, and catalog orbital objects that facilitates the prediction and prevention of collisions in space. The test site will also provide early lessons learned on installation of the S-band ground-based radar, support maintenance training and allow engineers to test verification procedures
General Dynamics (GD) SATCOM Technologies has completed the construction of a 7,000ft² radar array structure for the US Air Force’s (USAF) Space Fence programme. The 12m-tall structure has been designed to withstand earthquakes, hurricane force winds and extremes in temperature and humidity. General Dynamics Mission Systems vice-president and general manager Mike DiBiase said: “The ground-based receive array is an elegant merger of a huge physical structure built with the precision of a complex scientific or medical instrument.
Australia breaks ground on second space tracking site
The Space Fence will only reach its full capability with the completion of second site in Australia by 2022. The second site will “fill in the gaps” in the system, allowing the Air Force to see some objects more often, according to Lockheed Martin.
Ground has been broken on an Australian facility that will track space debris. The facility is part of the Optical Space Services (OSSTM) network, collaboration between Lockheed Martin and Australia’s Electro Optic Systems (EOS). It is designed to complement radar-based tracking such as the U.S. Air Force’s Space Fence, according to a Lockheed Martin news release.
The network developed by EOS and Lockheed Martin, called Optical Space Services (OSSTM), was formed in August 2014. Sensor systems like OSSTM serve as a complement to radar-based systems like the U.S. Air Force’s Space Fence, which will sweep the sky tracking 200,000 objects.
“The strategic collaboration with Lockheed Martin has allowed a critical mass of sensors, data and services to be assembled, enabling OSSTM to deliver the suite of asset protection services requested by customers,” said Dr. Ben Greene, EOS Chief Executive Officer. “This new tracking capacity will provide unique data which is exclusively available to EOS and Lockheed Martin, enabling each organisation to offer both data and services to meet global market needs. Based on current contracts and active negotiations, EOS expects to commence the delivery of data and services by late 2016.”
Sensors, lasers and optic systems will be fused together by software enabling OSSTM to hone-in on, characterise and track human-made objects orbiting the depths of space. That data will then be quickly and accurately delivered to customers allowing them to manoeuvre satellites and prevent collisions. The system can also predict the paths of debris, giving operators advance warning of potential collisions.
The expansion of space debris tracking by EOS and Lockheed Martin is expected to make a significant contribution to the preservation of the space environment, by providing data which will enable cost-effective debris manoeuvre for satellites,” said Mark Valerio, Lockheed Martin vice president and general manager of Military Space. “The accuracy of our optical sensor network, combined with an ability to reschedule tracking operations according to commercial priorities, will provide a trusted source of critical space data to commercial and government operators.”
Space Fence will provide catalog completeness, accuracy and timeliness with vastly improved performance in Low Earth Orbit (LEO) and capability to support missions in Geosynchronous Earth Orbit (GEO). Attaining detection and tracking performance within the large coverage volume necessitated developing advanced technologies including long-pulse high-duty factor Gallium Nitride (GaN) transmit modules, low-cost dual-polarized Radio Frequency Integrated Circuit (RFIC) receivers, and element-level digital beamforming across 86,000 receive elements. These technologies have been matured to Technology Readiness Level (TRL) 7 and Manufacturing Readiness Level (MRL) 7 based on end-to-end scaled prototypes, says Joseph Haimerl, Member IEEE.
Space fence Radar
“Each radar site features a design with closely spaced but separate transmit and receive phased array antennas, prime power and liquid cooling. The transmit array building houses a 36,000 element transmit phased array antenna beneath an air supported low loss Kevlar environmental radome. The receive building supports an 86,000 element array, also under a low loss Kevlar radome. Both arrays are provided power and cooling through the common services building,” write Justin Gallagher, and others in Microwave Journal.
Radar data processing and control of the apertures is performed off-array in commercial off-the-shelf (COTS) processing equipment located within the operations building. Both transmit and receive arrays are automatically calibrated with horns that are mounted on calibration towers and can transmit or receive test signals.
The extremely large phased arrays are optimized for high availability and low lifetime support costs and use GaN HPAs for transmit amplification, providing unprecedented sensitivity to detect small objects. On receive, digital beam forming (DBF) at the element level permits thousands of simultaneous beams instantaneously in any direction. This enables the system to provide persistent LEO surveillance coverage while simultaneously tracking hundreds of objects, performing cued search tasks in other surveillance regimes (including MEO and GEO) and supporting user-defined flexible surveillance volumes. Transmit and receive arrays are oriented to face straight up and are designed integrally with the building.
A scalable facility structure supports liquid cooled cold plates, which house the radar electronics. Radiator tiles are mounted on the top of the cold plates while “radar-on-a-board” digital transmit and receive line replaceable units (LRU) are mounted on the sides.
Each transmit LRU incorporates digital waveform generation, up-conversion to S-Band and high power GaN amplification for eight transmit radiating elements. Mounting the LRUs on the sides of the cold plates provides the GaN HPAs with a direct and efficient thermal path. To provide high system availability, the LRUs are serviceable from beneath the array and can be removed and replaced in less than 1½ minutes while the array is operating.
GaN high power amplification was one of the critical enabling technologies for the Space Fence solution. Relative to other technologies, the high output power of GaN reduces the number of transmit elements to achieve the required sensitivity for the target size, which reduces overall acquisition cost. GaN’s high efficiency also reduces power consumption and heat dissipated, which reduces operational costs for the sensor site. In order to effectively support the LEO orbital regime (and tasking up to GEO) and get sufficient energy back for detection, transmit pulse lengths need to be long. Previous technologies, such as GaAs or Si BJT didn’t support these pulse lengths at the required output power.
The long pulse capability of GaN in the transmit array also enables extremely efficient timeline utilization of the radar when combined with element level DBF in the receive array. Space Fence has a receiver connected to each array element within the receive array to digitize the returned signals. Unlike subarrayed antennas, which combine multiple elements in microwave electronics prior to digitizing to reduce the number of receivers, the beams in an element level DBF system can be simultaneously placed anywhere in the field-of-regard (FoR) of the array. Subarrayed approaches limit the digitally formed beams to constrained volumes and require changing analog phase shifters to move the volume from one radar event to the next.
“Space Fence is able to use its flexibility along with frequency multiplexed functions within the receiver band to form thousands of beams simultaneously. This allows many functions that would have been performed sequentially to be performed simultaneously, reducing the Space Fence array sizes along with the associated acquisition cost and operating costs. Use of GaN HPAs are needed to support the resulting concatenated “machine gun” like transmit sequence, which is longer and transmit higher duty factor than supported by other technologies,” write Justin Gallagher, and others in Microwave Journal.
Gallium Nitride (GaN) based monolithic microwave integrated circuit technology
It uses Gallium Nitride (GaN) based monolithic microwave integrated circuit technology that provides significant advantages for active phased array radar systems including higher power density, greater efficiency and significantly improved reliability over previous technologies.
GaN supports higher output power, higher transmit duty factor and longer pulse lengths than previous technologies, such as GaAs and Si BJT. These allow smaller aperture sizes and reduce overall system acquisition costs. Since GaN operates at higher efficiency, operational costs are also reduced, as less prime power is consumed and less heat is dissipated, reducing the need for active cooling. Lastly, GaN has higher reliability than previous technologies. Higher reliability reduces operational costs through reduced maintenance and spare parts.
The MMIC chip has been developed by CREE at the Wolfspeed facility in Research Triangle Park. Recently, the technology reached a major design milestone, when Lockheed Martin’s team confirmed its long-term reliability. It was a process that took more than 5,000 hours of accelerated stress testing – among the first major milestones of the collaboration. Then Lockheed Martin builds the chips into transmit-receive modules of the phased array radar.
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