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 is also becoming another domain of conflict due to enhanced militarization and weaponization of space. China has been testing kinetic kill, directed energy, electromagnetic, cyber and other systems in an effort to develop methods for crippling American satellites during a conflict. “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.”
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. Recent CSIS Space Threat Report described this work by the Russians: “On several occasions the country has maneuvered space objects in LEO and GEO that were initially identified (incorrectly) as debris in the U.S. Space-Track catalog. These objects later appeared to maneuver and conduct proximity operations.” Defense Department strategic review of the space portfolio also concluded was that current space systems were designed in an era when space was not contested or congested, “This is no longer the case.”
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.
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.
Space situational awareness (SSA)
Space situational awareness (SSA) encompasses surveillance of all space objects, activities, and terrestrial support systems (satellites & debris), more detailed reconnaissance of specific space objects assets (mission identification, capabilities, vulnerabilities, etc.), discerning the intent of others who operate in space, knowing the status of our own forces in real-time, and analysis of the space environment and its effects (solar storms, meteor showers, etc.).
The US Strategic Command defines SSA as “The requisite current and predictive knowledge of space events, threats, activities, conditions and space system (space, ground, link) status capabilities, constraints and employment — to current and future, friendly and hostile– to enable commanders, decision makers, planners and operators to gain and maintain space superiority across the spectrum of conflict.”
It also entails determining the various threats to space assets such as the growing number of debris, space weather, meteorites, orbital collisions and intentional attacks including directed energy attack, direct ascent and orbital ASAT, HAND events, etc. It involves the collection, processing, fusion and assessment of data and information from many different sources and dissemination of information to decision makers and various users.
Comprehensive SSA requires a networked system of radars and electro-optical sensors. Low altitude debris is usually observed by radar ground stations while high altitude debris is observed by optical ground stations. Bistatic, multistatic and phased array radars are widely used. The optical telescopes have some disadvantages like they can only track objects that are illuminated while the telescopes are in darkness. Recently the trend is to use space based sensors to provide timely detection, collection, identification and tracking of man-made space objects from deep space to LEO orbits.
To ensure safety of satellites in orbit it is essential to know their precise orbits as well as the orbits of debris that could cause damage. This information is also essential for the U.S. to maintain a SSA (Space Situation Awareness) capability to protect its space assets from potential hostile actions. The U.S. Air Force has developed the Space Fence System as part of its SSN (Space Surveillance Network) to provide unprecedented detection sensitivity, coverage and tracking accuracy.
Lockheed Martin’s Space Fence System became fully operational in 2020
A radar system known as Space Fence, which can track material in space as small as 10 centimeters, is fully operational, the U.S. Space Force announced in March 2020. Using enhanced S-band radar, the Space Fence improves on previous capabilities of the Space Surveillance Network in tracking objects such as commercial and military satellites, depleted rocket boosters, and space debris in low, medium, and geosynchronous Earth orbit regimes.
In June 2014, the Lockheed Martin industry team was awarded the contract by the USAF for the EMDPD (Engineering, Manufacturing, Development, Production and Deployment) of the Space Fence System ($915 million). 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. The objective is to monitor hundreds of thousands of objects in orbit, allowing the military to detect and track ten times more space junk than possible with existing decades-old technology
The first radar site is on Kwajalein Atoll in the Pacific Ocean near the equator and 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. Once construction is complete, Space Fence will go through testing and validation before its initial operating capability occurs in late 2018.
SFS (Space Fence System) will enable the decommissioning of the aging U.S.-based AFSSS (Air Force Space Surveillance System), originally installed in 1961. 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.
With a total anticipated value of around $6.1 billion over its lifetime, Space Fence will deliver a system of 2-3 geographically dispersed ground-based radars to provide timely assessment of space objects, events, and debris. Space Fence will use S-band ground-based radars to provide the Air Force with uncued detection, tracking and accurate measurement of space objects, primarily in low-earth orbit. The geographic separation and the higher wave frequency of the new Space Fence radars will allow for the detection of much smaller microsatellites and debris than current systems. Additionally, Lockheed Martin’s Space Fence design will significantly improve the timeliness with which operators can detect space events which could present potential threats to GPS satellites or the ISS (International Space Station).
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.
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. Space Fence, 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. A second radar site is planned to go online in 2021.
According to officials, the SSN has tracked 26,000 objects already accounted for in space, and the new system is expected to vastly increase that figure, essentially offering a catalog and location of every object in space. The radar system’s infrastructure is located on Kwajalein Atoll in the Republic of the Marshall Islands, with headquarters at the Space Fence Operations Center in Huntsville, Ala. 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.
The Space Fence System is net-centric (interconnected by the Global Information Grid, more recently referred to as the DoDIN (Department of Defense Information Network) and will seamlessly integrate into the existing SSN, providing services to external users—such as the JSpOC (Joint Space Operations Center) — and coordinating handoffs to other SSN sites. The military’s JSpOC (Joint Space Operations Center) is in charge of tracking space traffic and issuing collision threat notices to global satellite operators, which can move their spacecraft out of the way if notified in time.
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.
A cornerstone of Lockheed Martin’s risk-reduction effort has been to stand up an integration test bed in Moorestown, New Jersey. There, Lockheed Martin built a small piece of the large array that is on Kwajalein. The test bed provided a place to conduct early integration and testing of Space Fence’s hardware, software, and firmware 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 facility has its own 10-megawatt power plant. 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.
Because the SFS uses element level digital beam forming for both transmit and receive, the radar is capable of performing both un-cued surveillance and cued tracking simultaneously.
The SFS is net-centric (interconnected by the Global Information Grid (GIG), more recently referred to as the Department of Defense Information Network (DoDIN)) and will seamlessly integrate into the existing SSN, providing services to external users—such as the Joint Space Operations Center (JSpOC)—and coordinating handoffs to other net-centric SSN sites.
The Space Fence seems to be on track but the Joint Space Operations Center may not be ready for it. The command and control system that uses the data has “critical deficiencies” and lacks cyber security, according to the Pentagon’s annual Operational Test and Evaluation report.
Space Fence can now connect directly with the Space Force’s Unified Data Library, reported in April 2022
The U.S. Space Force announced its cloud-based data environment can now ingest observation information directly from the Space Fence radar, demonstrating its ability to connect with sensor nodes in the Space Surveillance Network. The Unified Data Library is a key component of the Space Force’s digital architecture, built to collect and integrate space object tracking data drawn from Department of Defense sensors as well as commercial, intelligence community and foreign systems.
The Space Fence and UDL integration represents the library’s first direct sensor connection with the SSN. Lt. Col. Daniel Kimmich, chief of Space Systems Command’s cross-mission ground and communications enterprise data branch, told C4ISRNET in a recent interview that prior to the direct-connect to UDL, Space Fence used a system called the Non-traditional Data Pre-processor to reformat its data. That system would then send the data to another processing system called CAVENet, which would ingest the data into the legacy Space Defense Operations Center.
The UDL connection “takes CAVENet processing, CAVENet ingestion completely out of the picture” for Space Fence observation data,” he said. It also paves the way for the service to make similar connections for other Space Fence capabilities, including object identification and characterization.
Beyond Space Fence, Kimmich said, the service is working to ensure that all Space Force sensors “within the realm of the possible” can integrate with the UDL. That includes the Geosynchronous Space Situational Awareness Program, a largely classified constellation of six satellites tasked with monitoring space activities. “That is on our roadmap in terms of a specific direct-connect,” he said.
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. Space Fence prime contractor Lockheed Martin is preparing to start a site survey in June 2018 for the second radar location that would bring the space-observing radar system to full operational capability.
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.”
SENSOR SITE 2 Benefits
The second Sensor Site will provide geographic diversity from Sensor Site 1, improve RSO positional accuracy, decrease on-orbit event detection timeliness, and enable greater custody through increased revisit rates. By having a second site, it increases the tracking opportunities per day as well the opportunities on low attitude objects.
Further, it increases southern hemisphere coverage and enhances coverage in Deep Space. Lastly, Sensor Site 2 provides a backup for Sensor Site 1 and facilitates future mission CONOPS. It improves Space Surveillance Network (SSN) resilience, balances mission load, and enables either site to focus on SSA while the other performs dedicated tasking. The second sensor site FOC is achievable as early as 2021.
Space fence Radar technology
Since array size is directly proportional to cost, our solution attempted to minimize it through use of digital array technology and separate Transmit (Tx) and Receive (Rx) phased arrays. Digital arrays, particularly those that use element-level digital beamforming, are capable of many independent beams to support simultaneous functions, reducing required array size relative to doing the functions sequentially. These functions are performed simultaneously at different frequencies within the operating band.
Use of separate Tx and Rx arrays minimizes losses during transmit and receive, which also reduces the required size. The solution also employed GaN (Gallium Nitride) high power amplifier technology for use in the Tx array to provide high power, long pulses needed for long range operations, and high efficiency for low operating costs. Even with these key affordability trades considered, the requirements still drove the Tx array sizes excessively large (78 K radiating elements and Rx array size to 300 K radiating elements, about three times as large as the final SS).
Throughout the SDR Phase, detailed hardware prototyping of our array technology building blocks refined the performance and cost models. This led to a downsizing of our arrays to 65 K Tx and 217 K Rx elements for affordability. As the project effort entered the PDR phase, cost models were used by the USAF to adjust requirements and remove drivers such as the continuous MEO mission sensitivity over such a large uncued volume. Updated requirements focused on uncued LEO as the driver for radar sensitivity, with MEO as a tasked function that did not drive larger array sizes. This simplification and additional detailed performance modeling allowed further reduction of array sizes., writes Lockheed Martin.
The new requirements also reduced the number of sensor sites to 2 and adjusted the coverage to optimized contours to provide ‘assured coverage’, as shown in Figure. Assured coverage provides optimized fence angular width as a function of altitude to guarantee one detection opportunity per pass (where the fence intersects the orbital plane) for circular orbits. Assured Coverage contours were also traded off with sensor site locations (Kwajalein Atoll and Australia, vs. antipodal sites such as Kwajalein and Ascension Island)
Each radar site features closely-spaced, but separate, Transmit and Receive Arrays that are mission-optimized for high availability and low lifetime support costs, including 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.
Coverage is optimized to provide assured coverage at IOC (Initial Operational Capability) down to 800 km altitude with the Kwajalein Atoll site and improved lower altitude assured coverage to 550 km at FOC(Full Operational Capability) with the addition of the Australian site. Both sites support cued tasking support to all altitudes including GEO.
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. Coverage is optimized to provide assured coverage at Initial Operational Capability (IOC) down to 800 km altitude with the Kwajalein Atoll site and improved lower altitude assured coverage to 550 km at Full Operational Capability (FOC) with the addition of the Australian site. Both sites provide cued tasking support to all altitudes including GEO.
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.
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.
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.
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