Advancements in Earth Observation: Synthetic Aperture Radar (SAR) Satellite Constellations

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Introduction

Remote sensing, the science of collecting information about Earth’s surface without direct contact, has made tremendous strides in recent years. One of the most significant innovations in remote sensing technology is Synthetic Aperture Radar (SAR) satellite constellations. These constellations are revolutionizing our ability to observe and understand our planet, with applications ranging from geology and agriculture to disaster monitoring and military surveillance.

In this article, we will explore the latest developments in SAR technology, applications, challenges, and the critical role of data exploitation in harnessing the full potential of SAR satellite constellations. We will also explore the recent commercial constellations, and military SAR satellites from major nations like Russia, the United States, China, and India. Additionally, we will delve into design considerations, hardware challenges, and the crucial role of data exploitation in making SAR satellite constellations a reality.

 

Understanding SAR Technology

Synthetic Aperture Radar (SAR) is a radar imaging technique that employs microwave signals to create high-resolution images of Earth’s surface. Unlike optical satellites, SAR sensors are not affected by weather conditions or daylight, making them invaluable for both day and night observations. SAR works by emitting microwave pulses towards the Earth’s surface and capturing the signals reflected back. These signals are then processed to create detailed images.

Satellites equipped with Synthetic Aperture Radar (SAR) sensors follow a specific orbital pattern that allows them to capture data with unique capabilities. Here’s an explanation of their orbit and how it enables interferometric SAR (InSAR):

1. Sun-Synchronous Low Earth Orbit (LEO) Polar Orbit:
   • Orbit Type: SAR satellites typically orbit the Earth in what is known as a sun-synchronous low Earth orbit (LEO) polar orbit. This type of orbit is chosen for several reasons:
     • Polar Orbit: The satellite passes over the Earth’s polar regions, which provides global coverage over time.
     • Sun-Synchronous: The orbit’s geometry is synchronized with the position of the sun. This means that the satellite consistently passes over the same region of the Earth at approximately the same local solar time during each orbit. For example, it may pass over a specific location at 12:00 PM and 12:00 AM every day.
2. Advantages of Sun-Synchronous Orbit:
   • Constant Lighting Conditions: By orbiting in a sun-synchronous manner, SAR satellites ensure that they pass over the Earth’s surface under consistent lighting conditions. This is particularly important for SAR imaging because it allows for accurate comparisons of data acquired at different times.
   • All-Day Observations: SAR satellites can acquire data at any time of day or night. This capability is crucial for applications that require 24/7 monitoring, such as disaster management or military surveillance.
   • Cloud-Independence: SAR sensors are not affected by cloud cover or adverse weather conditions. By orbiting in a sun-synchronous manner, they can capture data even when optical sensors might be hindered by clouds or darkness.
   • Global Coverage: The polar orbit provides global coverage over time, ensuring that the entire Earth’s surface is observed at regular intervals.
3. Interferometric SAR (InSAR):
   • Repeating Paths: SAR satellites follow repeating paths as they orbit the Earth. This means that they revisit the same geographic locations at regular intervals.
   • Two-Phase Datasets: InSAR relies on acquiring two-phase datasets for the same location at different times. These datasets are acquired when the satellite passes over the same area during separate orbits. Due to the satellite’s consistent orbit and the repeating paths, it’s possible to capture data for the same location at different moments.
   • Relative Ground Displacements: InSAR leverages the phase differences in the radar signals between the two datasets acquired at different times. These phase differences are sensitive to ground displacements, such as subsidence or uplift. By analyzing these phase changes, InSAR can provide valuable information about relative ground movements along the direction of the radar beam.

In summary, SAR satellites in sun-synchronous low Earth orbit offer consistent lighting conditions, 24/7 data acquisition capabilities, and global coverage. These features, combined with their repeating orbits, enable interferometric SAR (InSAR), a powerful technique for monitoring ground displacements and other changes on the Earth’s surface over time. This makes SAR technology invaluable for a wide range of applications, from geological studies to infrastructure monitoring and disaster response.

 

SAR Applications

SAR technology has a multitude of applications, making it indispensable in various domains:

In desert environments, SAR can distinguish surface deformations as small as tire marks left behind by vehicles crossing the sand. This is because SAR is sensitive to changes in the surface roughness of the ground. When a vehicle drives across the sand, it creates a disturbance in the sand surface. This disturbance can be detected by SAR and used to create an image of the tire marks.

SAR can also be used to measure terrain elevations to an accuracy of a few millimeters. This is done by using the phase differences in the SAR data. The phase of the SAR signal is sensitive to the distance between the radar antenna and the ground. By comparing the phases of the SAR signals from different images, it is possible to calculate the changes in elevation between the images.

Continuous monitoring over time with SAR can reveal subtle ground changes that may indicate clandestine underground tunneling and excavation. This is because SAR can detect changes in the surface roughness of the ground, even if the changes are very small.

SAR can also be used to differentiate water, snow, and ice. This is because the radar signal interacts differently with different materials. Water, for example, reflects the radar signal much better than snow or ice. This difference in reflection can be used to create images that show the distribution of water, snow, and ice on the ground.

The SAR signal intensity can also be interpreted to calculate the thickness of ice. This is because the amount of attenuation of the radar signal is related to the thickness of the ice. By measuring the attenuation of the radar signal, it is possible to calculate the thickness of the ice.

SAR is also sensitive to moisture. This means that SAR can be used to track agricultural growth to assess crop health and forecast harvest yields. Crops that are healthy and well-watered will reflect the radar signal more strongly than crops that are unhealthy or stressed. This difference in reflection can be used to create images that show the health of crops.

SAR is a valuable tool for monitoring a variety of environmental and security applications. Its ability to image through clouds and darkness makes it a valuable tool for monitoring desert environments.

Geology: SAR satellites can detect ground deformations, fault lines, and volcanic activity, contributing to earthquake prediction and volcano monitoring.

Crop Monitoring: Farmers benefit from SAR’s ability to track crop growth, assess soil moisture levels, and forecast harvest yields, enhancing food security.

Sea Ice Measurement: SAR satellites play a crucial role in monitoring sea ice thickness and extent, vital for understanding climate change impacts on polar regions.

Disaster Monitoring: Rapid disaster response relies on SAR’s real-time data. It aids in assessing the extent of natural disasters such as floods, wildfires, and earthquakes. Rapidly changing natural disasters, such as floods, wildfires, and volcanic eruptions, require ongoing, frequent temporal monitoring, and this is an inherent strength of SAR technology. Before the clouds and smoke have cleared, SAR data acquisitions can occur that enable users to produce preliminary products and information thus enabling first responders to assess the disaster situation.

Vessel Traffic Surveillance: SAR constellations contribute to maritime security by tracking ships, detecting illegal fishing, and assisting in search and rescue operations.

New high-definition satellite radar can detect bridges at risk of collapse from space: Researchers from the NASA Jet Propulsion Laboratory (JPL) and the University of Bath have developed a cutting-edge satellite-based early warning system capable of detecting minuscule movements in bridges that may indicate an impending collapse. By combining data from new-generation satellites with advanced algorithms, this system offers a potential safety net for large-scale infrastructure projects.

The team, led by NASA’s JPL and engineers from the University of Bath, validated the technique using 15 years of satellite imagery of the Morandi Bridge in Genoa, Italy, part of which tragically collapsed in August 2018, causing 43 fatalities. The research, published in the journal Remote Sensing, revealed that the bridge exhibited warping signs in the months leading up to the disaster.

Traditional structural monitoring focuses on specific sensor-equipped points but lacks comprehensive real-time coverage. In contrast, this innovative approach enables continuous monitoring of an entire structure, providing unprecedented accuracy in detecting ground deformation changes across infrastructure.

Dr. Pietro Milillo, the lead author from the Jet Propulsion Laboratory, emphasized the technique’s potential to enhance structural health assessments, offering detailed insights not achievable with existing methods.

Advanced satellite technology, including the Italian Space Agency’s COSMO-SkyMed constellation and the European Space Agency’s Sentinel-1a and 1b satellites, has played a crucial role in acquiring highly accurate data. Precise Synthetic Aperture Radar (SAR) data, obtained from multiple satellites at varying angles, enables the creation of detailed 3D models of structures such as buildings and bridges.

Dr. Giorgia Giardina, a lecturer at the University of Bath, highlighted the technology’s improved accuracy, akin to transitioning from standard to Ultra-HD TV quality, providing the necessary level of detail for effective structural monitoring.

This technology’s versatility extends to monitoring structural movements during underground excavation projects, offering valuable insights and confirmation of progress.

Dr. Giardina has already been approached by UK infrastructure organizations interested in implementing this monitoring approach for roads and rail networks, illustrating the wide-reaching potential and significance of this innovative satellite-based early warning system.

Military and Intelligence Applications: SAR technology serves critical roles in reconnaissance, surveillance, and target identification. It ensures national security by monitoring adversarial activities.

GEOINT stands for Geospatial Intelligence, which is the exploitation and analysis of imagery and geospatial information to describe, assess and visually depict physical features and geographically referenced activities on the Earth. Satellite-based SAR data has been used for GEOINT applications for many years.

Design Considerations and Challenges

Developing a SAR satellite constellation is a complex endeavor with several design considerations and hardware challenges.

Current SAR systems offer remarkable versatility in their imaging capabilities, achieved through precise control of the antenna radiation pattern. This control allows for the creation of various combinations of swath width and resolution, catering to specific imaging requirements. At the core of these imaging modes lies the Stripmap mode, representing the fundamental SAR imaging mode.

In the Stripmap mode, the SAR system illuminates the ground swath with a continuous sequence of radar pulses while maintaining a fixed orientation of the antenna beam. This results in the creation of a long, narrow strip of imagery that runs parallel to the flight direction. This mode is highly effective and provides a balanced compromise between resolution and coverage.

To achieve finer azimuth resolution, SAR systems employ what is known as the Spotlight mode. However, this heightened resolution often comes at the cost of reduced spatial coverage. In this mode, the SAR-equipped satellite captures images at an impressive 1-meter resolution, but the scene size is constrained to a relatively small 5 × 5 km² area.

Conversely, when a broader swath is desired, SAR systems switch to the ScanSAR mode. While this mode allows for the capture of larger areas, it comes with a trade-off as the azimuth resolution is somewhat degraded compared to the Stripmap mode.

It’s worth noting that ongoing developments in SAR technology are focused on enhancing the capabilities of ScanSAR mode. For instance, a reference SAR-equipped small satellite is currently in the developmental phase, with the aim of achieving a 100-meter resolution and an expansive 350 km swath. This improvement in ScanSAR mode bridges the gap between the need for extensive coverage and the demand for sharper imaging.

In summary, SAR systems offer a range of imaging modes, each tailored to specific requirements. From the fundamental Stripmap mode that strikes a balance between resolution and coverage to the high-resolution Spotlight mode and the wide swath capabilities of ScanSAR, these modes empower SAR-equipped satellites to adapt to diverse imaging scenarios and serve a multitude of applications, from environmental monitoring to disaster management and beyond.

Orbital Altitude: Choosing the right orbital altitude is crucial to balance coverage, revisit times, and resolution. Different applications may require different altitudes. The orbital altitude of a SAR satellite is a key design consideration. Higher orbits provide better coverage of the Earth’s surface, but they also require more powerful radars and larger antennas. Lower orbits provide less coverage, but they require less power and smaller antennas.

Phased-Array Radar Antenna: Developing large phased-array radar antennas capable of surviving launch and space conditions that can be deployed in space is a significant technical challenge. These antennas must be capable of generating precise radar beams and managing power efficiently.

Data Exploitation: SAR constellations generate vast amounts of data that need to be efficiently processed and analyzed. Advanced algorithms are essential for distinguishing between moving targets and background clutter, as well as extracting meaningful information.

However, it’s essential to acknowledge that while the benefits of satellite-based SAR technology are clear, there are significant challenges associated with handling the vast amount of data generated. Archiving, processing, and analyzing SAR data can be complex and resource-intensive tasks, especially with the increasing volume of data expected from small satellite constellations. Effective data management and analysis solutions are critical to fully harness the potential of SAR technology for GEOINT applications.

Choosing the right orbit and optimizing radar parameters are crucial design considerations. The miniaturization of SAR satellites for micro- and nano-satellite platforms has been challenging but is now feasible thanks to technological advancements.

Pulse repetition frequency (PRF): The PRF is the rate at which the radar transmits pulses. A higher PRF can provide better resolution, but it also requires more power.

Antenna size: The size of the antenna is also a key design consideration. A larger antenna can provide better resolution, but it also requires more power and is more difficult to deploy in space.

Radar frequency: The radar frequency also affects the design of a SAR satellite. Higher frequencies (such as 8-12 GHz) can provide better resolution, but they also require more power and smaller antennas. Lower frequencies (such as 1-2 GHz) can provide better penetration of clouds and vegetation, but they have lower resolution.

Synthetic Aperture Radar (SAR) technology possesses a unique ability to “see” through various types of cover, such as smoke, vegetation, snow, or sand, depending on the satellite’s designated operating band. This capability is influenced by the sensor’s associated frequency and wavelength, which vary across different SAR bands. Let’s delve into this concept:

1. SAR Bands and Penetration Strength:
   • Frequency and Wavelength: SAR systems operate at specific microwave frequencies, each corresponding to a particular wavelength. The frequency and wavelength of SAR signals determine how they interact with different materials and cover types on Earth’s surface.
   • Penetration Strength: The penetration strength refers to the ability of SAR signals to pass through or interact with materials. It depends on the frequency and wavelength. Higher-frequency bands have shorter wavelengths and are less penetrating, while lower-frequency bands have longer wavelengths and can penetrate certain materials more effectively.
2. Categorizing SAR Bands:
   • X-band (eXtreme-band): X-band SAR operates at a relatively high frequency and shorter wavelength. It has low penetration capabilities, making it suitable for applications where surface details and small objects are of interest. X-band SAR is commonly used for urban monitoring, forest analysis, and infrastructure assessments.
   • C-band (Common-band): C-band SAR operates at a moderate frequency and wavelength. It strikes a balance between penetration and detail resolution. C-band SAR is versatile and finds applications in agriculture, forestry, and land-use mapping.
   • L-band (Long-band): L-band SAR operates at a lower frequency and longer wavelength. It has high penetration capabilities, allowing it to “see” through various types of cover, including vegetation, smoke, and sand. L-band SAR is commonly used for applications such as soil moisture monitoring, ice characterization, and forest canopy analysis.
3. Examples of SAR Satellites in Different Bands:
   • Germany’s TanDEM-X: TanDEM-X is a SAR satellite that operates in the X-band. It offers high-resolution imagery suitable for urban and terrain mapping.
   • Canada’s RCM (Radarsat Constellation Mission): The RCM includes SAR satellites operating in the C-band. These satellites provide versatile data for applications like agriculture, disaster management, and maritime surveillance.
   • Japan’s ALOS-2: ALOS-2 is an SAR satellite that operates in the L-band. Its high-penetration capabilities make it valuable for monitoring forests, detecting subsidence, and studying natural disasters.
4. Selecting the Right Band for the Application:
   • Choosing the appropriate SAR band depends on the specific application’s requirements. For instance, if you need to monitor changes beneath a forest canopy, an L-band SAR would be preferable due to its penetration capabilities. Conversely, if you’re interested in urban planning and building analysis, an X-band SAR might be more suitable.

Data rate: The data rate is the amount of data that the SAR satellite can transmit back to Earth. A higher data rate is required for applications that require high-resolution imagery, such as disaster monitoring and target tracking.

Power consumption: The power consumption of a SAR satellite is a major design constraint. The radar, antenna, and other components all consume power, and the satellite must have enough power to operate for its intended mission.

Launch and deployment: The launch and deployment of a SAR satellite is a complex and challenging process. The satellite must be able to survive the harsh environment of space, and it must be able to deploy its antenna and other components correctly.

The development of new technologies is making it possible to overcome some of the challenges of space-based SAR. For example, new radar technologies are being developed that can provide high resolution at lower frequencies, which would allow for smaller and lighter satellites. New antenna technologies are also being developed that can be more compact and efficient.

SAR Interferometry and Coherent Change Detection

SAR Interferometry (InSAR) and Coherent Change Detection (CCD) are advanced techniques that exploit SAR data’s coherence. InSAR can measure ground deformation with millimeter-level precision, while CCD can identify subtle changes in the environment. These techniques are invaluable for monitoring infrastructure stability, land subsidence, and detecting illegal construction activities.

Coherent Change Detection workflows using SAR data offer precise 3-D reconstruction capabilities. It can detect subtle ground changes, such as underground tunneling, surface deformations, and changes in water levels.

It’s worth noting that miniaturizing SAR satellites has presented unique challenges when compared to the miniaturization of optical satellites. Small SAR satellites require larger antennae and higher power throughputs, making their development more complex. However, recent strides in the miniaturization of electronic components and other technological advancements have paved the way for SAR technology to become compatible with small satellite platforms.

The appeal of small satellites has grown substantially due to their versatility, driven by advancements in space engineering over the past decade. These compact satellites come with a significantly lower price tag compared to their larger counterparts. This cost-effectiveness opens the door to constellation-based operations, involving multiple satellites working in concert. This collaborative approach empowers these constellations to achieve high revisit rates and short revisit times.

Enter Micro- and nano-satellite SAR constellations:

Nonetheless, the current capabilities of SAR satellite systems fall short when it comes to rapidly monitoring floods in the context of disaster management. For effective flood monitoring, disaster management professionals require up-to-date flood extent information with minimal delay. However, one significant obstacle to collecting this critical information is the limited satellite revisit time. The dynamic nature of flooding, which can both expand and recede, poses a challenge when attempting to track these changes within the constrained timeframes currently available.

These innovative constellations of small satellites, equipped with synthetic aperture radar (SAR) payloads, offer a promising solution. They enable frequent observations with short intervals, regardless of daylight or weather conditions, and represent an emerging technology in its early developmental stages.

In essence, the emergence of micro- and nano-satellite SAR constellations holds great promise for addressing the limitations of current flood monitoring capabilities. These constellations have the potential to provide timely and detailed flood information, making them invaluable assets in disaster management and other applications that demand frequent and agile Earth observation.

 

SAR Constellation Growth

The proliferation of SAR satellite constellations is a significant trend in Earth observation. Several companies, such as Capella Space, ICEYE, Synspective, and Umbra Lab, have launched or planned SAR constellations. These constellations offer high-resolution imagery and frequent updates, catering to diverse customer needs.

Rapid Disaster Monitoring

SAR technology excels in monitoring rapidly changing natural disasters, such as floods and wildfires. Its ability to provide real-time data enables first responders to assess the situation quickly, even in adverse weather conditions.

SAR in Maritime Surveillance

SAR technology is instrumental in maritime surveillance due to its wide swath capabilities. However, achieving high resolution for ship identification remains a challenge. Special algorithms are required to distinguish ships and their wakes effectively.

ICEYE: ICEYE is a Finnish company that launched the world’s first SAR microsatellite constellation in 2018. The constellation currently consists of 18 satellites and is expected to grow to 24 satellites by 2023. ICEYE’s satellites are used for a variety of applications, including disaster monitoring, maritime surveillance, and land use change detection.

In 2020, the Finnish New Space company ICEYE  introduced the capability to achieve incredibly high-resolution imaging using SAR technology, with an impressive resolution of 25 centimeters. This achievement marked a major milestone in the field, as it allowed ICEYE’s SAR data to reach the same resolution class as larger, conventional commercial SAR satellites operating at their peak performance.

To understand this achievement better, it’s important to clarify the resolution parameters. In standard industry terms, SAR data have two key resolution measurements: azimuth direction and range direction. The native slant plane resolution of ICEYE’s SAR data was announced to be 25 centimeters in the azimuth direction and 50 centimeters in the range direction. These measurements represent the level of detail that ICEYE’s SAR technology can capture before any ground-plane adjustments are applied.

ICEYE’s imaging process involves a satellite staring at a specific location for a duration of 10 seconds. During this time, the SAR sensor captures a wealth of data that forms the high-resolution image. The finest resolution data, which boasts a remarkable 25-centimeter level of detail, is then made accessible to customers in ICEYE’s standard product formats. These formats are designed to be compatible with widely used Geographic Information System (GIS) tools, making it easier for customers to integrate and analyze the SAR data in their geospatial workflows.

Additionally, ICEYE has demonstrated its capabilities in interferometric imaging, a technique that leverages phase differences in SAR data to create highly accurate 3D reconstructions of terrain and objects. This technology has a wide range of applications, from monitoring ground deformations to infrastructure assessment.

Moreover, ICEYE has introduced a product referred to as SAR video. This innovative product allows for the capture of multiple images of a single location in a single satellite pass. This capability is invaluable for real-time monitoring and analysis, especially in scenarios where rapid updates and change detection are crucial, such as disaster response and infrastructure management.

Capella Space: Capella Space is an American company that launched its first SAR satellite in 2020. The company plans to launch a constellation of 20 satellites by 2023. Capella’s satellites are used for a variety of applications, including oil and gas exploration, agriculture monitoring, and border security.

Capella Space is known for its sub-meter resolution capabilities, which means it can distinguish objects and features on the ground with a high level of precision. This level of detail is particularly useful for urban planning, change detection, and defense applications.

Capella’s constellation of SAR satellites is designed to provide frequent revisits to specific areas of interest. This rapid revisit capability allows for real-time monitoring of dynamic situations, such as disaster response, vessel tracking, and agricultural monitoring.

Capella Space employs interferometric SAR techniques, which involve comparing SAR data from multiple passes over the same area. InSAR enables the measurement of ground displacement with millimeter-level accuracy, making it valuable for applications like monitoring ground subsidence and deformation.

Capella Space offers customizable SAR data products to meet the specific needs of its customers. Users can request data with different parameters, including resolution, polarization, and revisit frequency, tailored to their application requirements.

SAR satellites can capture data in different polarization modes. This multi-polarization capability provides additional information about the surface properties of objects and materials, enhancing the versatility of SAR data for various applications.

Capella Space emphasizes rapid data delivery to its customers, ensuring that users can access SAR imagery promptly for timely decision-making. This feature is particularly important in time-sensitive scenarios like disaster response and security monitoring.

Synspective: Synspective is a Japanese company that is developing a constellation of 30 SAR satellites. The company’s first satellite is scheduled to launch in 2023. Synspective’s satellites are designed to be used for a variety of applications, including disaster monitoring, climate change research, and urban planning.

IQPS, or iQPS Inc., is a Japanese company that develops and operates SAR (synthetic aperture radar) satellites. The company was founded in 2016 and is based in Tokyo. IQPS’s first satellite, QPS-SAR 1, was launched in November 2019. QPS-SAR 6 was launched in June 2023 and is also in orbit. It has a ground resolution of 1.2 meters and a swath width of 120 kilometers. The company plans to launch a constellation of 36 SAR satellites by 2025. These satellites will be used to create a constellation of SAR satellites that can image any point on Earth every 10 minutes. The main purpose of the IQPS constellation is to provide high-resolution SAR imagery for a variety of applications, such as disaster monitoring, maritime surveillance, and environmental monitoring. The constellation is also expected to be used for commercial applications, such as crop monitoring and insurance.

SAR-Lupe: SAR-Lupe is a German SAR satellite constellation that was launched in 2006. The constellation consists of five satellites and is used for a variety of applications, including military surveillance, disaster monitoring, and environmental monitoring.

The SAR-Lupe satellites are equipped with a synthetic aperture radar (SAR) payload. The satellites have a ground resolution of 1.2 meters, which means that they can image objects on the ground that are 1.2 meters in size. The satellites have a swath width of 100 kilometers, which means that they can image a strip of land that is 100 kilometers wide. The satellites are placed in a sun-synchronous orbit, which means that they orbit the Earth at the same time each day. This allows the satellites to image the same area of the Earth at regular intervals.

SAR-Lupe is a valuable asset for the German government. The satellites are used for a variety of applications, including:

Disaster monitoring: The satellites can be used to monitor natural disasters, such as floods and earthquakes.
Environmental monitoring: The satellites can be used to monitor environmental changes, such as deforestation and desertification.

The satellites are expected to continue operating until 2025. However, the German government is planning to replace the SAR-Lupe constellation with a new constellation called SARah. The SARah constellation will consist of three satellites and is expected to be operational by 2023.

TerraSAR-X/TanDEM-X: TerraSAR-X/TanDEM-X is a German-European SAR satellite constellation that was launched in 2007 and 2010, respectively. The constellation consists of two satellites and is used for a variety of applications, including land use change detection, urban planning, and disaster monitoring.

The satellites are flying in close formation to acquire SAR data of unique geometric accuracy. Operating together, the missions deliver data that are used to generate Digital Elevation Models of Earth. The satellites are equipped with a C-band SAR payload. This means that they can image objects on the ground that are 3 meters in size. The satellites have a swath width of 250 kilometers. This means that they can image a strip of land that is 250 kilometers wide.
The satellites are placed in a sun-synchronous orbit at an altitude of 514 kilometers. This means that they orbit the Earth at the same time each day.

The data from the mission is used by researchers, governments, and businesses around the world. The mission is expected to continue operating until at least 2030.

These are just a few of the many recent SAR satellite constellations that have been launched or are in development.

 

Military Applications

Military surveillance: The satellites can be used to monitor military activity, such as troop movements and weapons deployments.

Synthetic Aperture Radar Imaging: One primary mission of Space Radar involves synthetic aperture radar (SAR) imaging. SAR utilizes transmitted microwaves to create detailed images of the Earth’s surface, somewhat analogous to optical photographs.

Ground Moving-Target Indication (GMTI): Space Radar is equipped with specialized radar techniques for detecting moving targets on Earth’s surface. Radar signals reflected off objects in motion exhibit a Doppler shift different from those reflected off stationary surfaces. By processing these Doppler shifts, GMTI can identify and locate moving targets. This capability is essential for surveillance of large areas and can overlay detected moving targets onto maps or images.

High-Resolution Terrain Information: Space Radar also provides precise measurements of surface elevation. By collecting two observations of the same terrain from slightly different angles, differences in timing between returning radar signals are used to estimate terrain height. This technique, known as interferometric SAR, allows for the creation of high-resolution elevation models of the Earth’s surface.

Open-Ocean Surveillance: Another significant application is open-ocean surveillance, where Space Radar monitors vast oceanic areas to track the movement of ships. The radar signals are sensitive to the roughness of water surfaces, enabling the detection of ship wakes. This capability is particularly useful for maritime monitoring.

In the context of maritime surveillance, SAR satellites have a wide swath, covering several hundred kilometers. This broad coverage allows them to locate ships over a large area, regardless of weather conditions. However, their wide swath modes typically offer lower resolution, often measured in tens of meters, making ship identification challenging. To address this, higher-resolution systems or additional satellite passes in high-resolution mode may be required. Additionally, ship motion can affect image quality, so specialized algorithms are employed to detect both the ships themselves and their wakes, enhancing the system’s ship-detection capabilities.

Recent Military SAR Satellites

The race to deploy advanced radar satellites for intelligence and military applications has intensified in recent years. Various nations have been operating Synthetic Aperture Radar (SAR) imaging satellites for national security and scientific purposes since the late 1970s. These radar satellites serve critical roles in military and intelligence operations, and several notable developments have emerged:

United States:

• The United States operates the Lacrosse system, consisting of six spacecraft capable of high-resolution SAR imaging regardless of weather conditions or time of day. It provides monitoring within a wide stripe, up to 100 kilometers wide.
• The proposed U.S. Space-based Radar (SBR) constellation is still in development, but it is expected to be operational in the early 2030s. The SBR constellation will consist of a network of 24 satellites that will be able to detect and track aircraft, ships, and potentially land vehicles from space. This information would be relayed to command centers.

Russia:

• Russia developed the radar intelligence system named “Liana,” comprising space vehicles like “Lotos-S” and “Pion NKC.” These satellites can monitor small objects on Earth’s surface, even as small as a car.
• Liana’s global coverage and ability to penetrate clouds make it a powerful tool for real-time battlefield information sharing in network-centric warfare.

Canada:

• Canada launched three next-generation radar satellites in 2019, using synthetic aperture radar (SAR) for applications like maritime surveillance, disaster management, and ecosystem monitoring. These satellites gather data, offering daily revisits to Canada’s territory and significant global coverage.
• In 2023, Canada plans to launch its fourth radar satellite, which will further enhance its SAR capabilities.

United Kingdom:

• The UK government initiated Project Oberon in 2021, involving a cluster of military radar satellites. These small spacecraft are designed to provide high-resolution, all-weather imaging and radio transmission detection for target identification.
• The first satellite in the Project Oberon constellation is expected to be launched in 2025.

China:

• China’s Zhuhai-1 constellation includes various remote-sensing satellites, including radar and infrared satellites, enhancing its Earth observation capabilities.
• The Zhuhai-1 constellation is still under development, but it is expected to be operational in the early 2020s.

India:

• India has been actively deploying radar imaging satellites, such as RISAT-2B, which utilizes synthetic aperture radar (SAR) for all-weather surveillance and observation. A constellation of such radar satellites allows comprehensive monitoring of the country, aiding in various applications, including crop estimation and disaster management.
• India plans to launch its fifth radar satellite in 2023, which will further enhance its SAR capabilities.

These radar imaging satellites play a pivotal role in military reconnaissance, disaster response, and various civilian applications due to their ability to provide high-resolution, all-weather, and day-and-night imaging capabilities, overcoming limitations posed by cloud cover and darkness.

The development of radar imaging satellites is a rapidly evolving field. As new technologies are developed, these satellites will become even more capable and versatile. This will allow them to be used for a wider range of applications, both military and civilian.

Technologies

In the rapidly evolving field of micro- and nano-satellite missions, the development of compact Synthetic Aperture Radar (SAR) systems relies on a range of critical technologies. These technologies are instrumental in miniaturizing SAR payloads while preserving or even enhancing their capabilities for Earth observation and remote sensing applications.

One of the key technologies sought after for such missions is the Highly Integrated Radar Transceiver Integrated Circuits (ICs) or SAR Transceiver ICs. These advanced electronic components combine various functions of the SAR transceiver into a single integrated circuit. By doing so, they play a pivotal role in reducing the overall size, weight, and power requirements of the SAR system. Highly integrated ICs not only contribute to the miniaturization of SAR payloads but also enhance their overall performance and reliability.

Low-Noise Amplifiers (LNAs) represent another critical technology in the context of compact SAR systems. LNAs are responsible for amplifying weak radar signals reflected from the Earth’s surface while introducing minimal noise. Given the limited power resources of micro- and nano-satellites, advanced LNAs with low power consumption are essential. They enable the detection of faint radar returns, ensuring the effectiveness of SAR missions.

On the transmitting side, High-Power Amplifiers (HPAs) are indispensable for micro- and nano-satellite SAR systems. These components are responsible for transmitting radar signals with sufficient power to illuminate and interact with the Earth’s surface. Compact and efficient HPAs are vital for achieving the desired imaging characteristics while operating within the constraints of small satellites.

Gallium Nitride (GaN) technology represents a significant advancement in SAR technology for small satellites. GaN is a semiconductor material known for its high electron mobility and power handling capabilities. GaN-based electronic components, such as amplifiers and transmitters, are increasingly adopted in SAR systems. GaN technology allows for high power output, efficiency, and reliability, making it particularly beneficial for miniaturized SAR payloads.

Additionally, the innovation of a Microelectromechanical System (MEMS)-Based Delay Line introduces an alternative approach to SAR system design. MEMS-based delay lines offer precise control over the timing and phase of radar signals, contributing to system miniaturization. MEMS-based components are known for their small size and high precision, making them suitable for micro- and nano-satellite SAR missions where space constraints are paramount.

In summary, these advanced technologies collectively address the challenges associated with developing compact SAR systems for micro- and nano-satellite missions. They enable the reduction of system size, weight, and power consumption while maintaining or even improving SAR performance. Ongoing research and development efforts focus on enhancing these technologies further to meet the specific demands of Earth observation and remote sensing in the era of small satellites.

Data Exploitation

Data exploitation is a critical aspect of the Space Radar program, encompassing several intricate technical challenges. One of the foremost challenges is the development of data processing algorithms capable of discerning moving targets from the surrounding background clutter. This challenge is particularly daunting in space-based radar observations due to the high-speed orbit of the satellite. Distinguishing a relatively slow-moving target from the rapid satellite movement demands advanced algorithms and techniques.

Synthetic Aperture Radar (SAR) data introduces unique complexities. SAR data files can be substantial in size, and certain analysis workflows require the integration of multiple temporal datasets to achieve the highest accuracy. The richness of SAR data lies in the intensity and phase of the radar signal that reflects off the Earth’s surface and returns to the sensor. The interpretation of these intensity and phase differences is pivotal for effective SAR data exploitation but necessitates intricate processing methodologies.

Moreover, the proliferation of remote sensing satellites, while advantageous in terms of frequent revisits and global coverage, presents a significant downside. The sheer volume of data generated by numerous satellites poses immense challenges in terms of data archiving, processing, and analysis. Many organizations primarily equipped to handle optical imagery are ill-prepared for the complexities of SAR data processing.

Addressing these technical challenges necessitates the continuous development of advanced algorithms and processing methodologies capable of accommodating the distinctive characteristics of SAR data. These challenges include accounting for the satellite’s rapid motion, efficiently processing large datasets, and extracting meaningful information from the intensity and phase of radar signals. As the number of SAR-equipped satellites continues to grow, organizations must adapt and enhance their data processing capabilities to fully exploit the potential benefits of SAR data across various applications.

Conclusion

Synthetic Aperture Radar (SAR) satellite constellations represent a significant leap in Earth observation capabilities. These constellations are transforming our ability to monitor our planet’s dynamic changes and provide critical data for various applications, from agriculture to disaster response and national security. As technology continues to advance, SAR technology is poised to play an even more prominent role in shaping our understanding of Earth and its evolving landscapes. Effective data exploitation and ongoing technological innovation will be key in realizing the full potential of SAR satellite constellations.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References and Resources also include:

https://spacenews.com/sar-renaissance-pandemic-slows-but-doesnt-stop-constellation-progress/

http://spaceref.com/news/viewpr.html?pid=58432