Moon Based SAR (Synthetic Aperture Radar)
The Moon Based SAR (MBSAR) is an emerging concept that offers unique perspectives and insights for Earth observation. Unlike a conventional airborne or spaceborne SAR system, the MBSAR operates on the lunar surface, enabling it to observe the rotating Earth from a considerable distance.
The MBSAR leverages the Earth–Moon distance to create a large beam footprint and the Earth–Moon relative motion to synthesize a substantial aperture, facilitating high-resolution imaging with an extensive swath width; this, in effect, maximizes the Earth observation capabilities by the MBSAR.
Lunar explorations have received increasing attention in recent years with tremendous application values, including using the Moon as a remote sensing platform for Earth observation. As an active sensor, the Synthetic Aperture Radar (SAR) can detect changes in the atmosphere, terrain, and ocean. Moon-based SAR, complementary to the spaceborne SAR systems, expands our capabilities of watching and understanding the Earth.
Moon-based SAR Potential Features
- Uses the Moon as a remote sensing platform for Earth observation.
- Explores how to obtain a high spatial resolution with a short revisit time using the Moon-based SAR.
- Observation geometry, range &signal models, two-dimensional signal spectrum, & focusing algorithms for the Moon-based SAR
- Analysis of sources of phase errors in the Moon-based SAR signal.
- Global case studies and conceptual ideas for further research.
Why Moon-based SAR?
Presently, the spaceborne SAR has been offering constant mapping of Earth’s surface. Still, the SAR technologies necessitate further advancements to meet the needs of diverse applications, a core on two extremes — the ‘small’ and the ‘large’. The small pole concerns the augmentation of SAR spatial resolution, which is pivotal for detecting ground objects and quantitatively inverting surface parameters by remote sensing data. On the other hand, the large pole pertains to the substantial spatio-temporal coverage performance required to monitor the Earth’s surface proficiently.
This aspect is gaining importance as scientific inquiry increasingly recognizes Earth as an integrated entity. Hence, augmenting the SAR system’s swath width and spatial-temporal resolution in tandem can effectively tackle the geoscience challenges posed by the Earth’s interconnected systems. However, the existing spaceborne SAR poses some constraints in line with the spatio-temporal coverage, which, in effect, impedes the data applications from multiple perspectives, as briefly outlined below.
Constraints of Space Borne SAR
Research tasks necessitate acquiring geoparameters from SAR sensors spanning the Earth’s disk while maintaining temporal and spatial continuity at a consistent level. The current space- borne SAR missions are confronted with the daunting challenge of fulfilling this exacting requirement as they encounter the inherent trade-off between azimuthal resolution and swath width. Various SAR imaging modes have been suggested to compromise the coverage and resolution. Nevertheless, none of those imaging modes can attain extensive coverage while maintaining a fine spatial resolution simultaneously.
In addition, due to the LEO altitude, the revisit period of spaceborne SAR is restricted to several to tens of days. The coarse temporal resolution, coupled with the coverage versus resolution restriction, limits some applications that require uncompromised spatiotemporal resolution, such as, but not limited to:
Rapid Disaster Response
Fast detection and swift reaction are paramount in the face of calamitous events, such as deluges, seismic activities, or oil spills. Such use cases necessitate more frequent monitoring with extensive coverage at the temporal resolution of hours to days.
The intervals between successive flyovers of spaceborne SAR systems, and their narrow swath width, restrict the capacities of spaceborne SAR systems to detect and respond to events across vast territories promptly.
This limitation might be why most SAR systems available today still find it challenging to offer a quick and timely response for efficient emergency response applications, This evaluating the damage in the aftermath of earthquakes, hurricanes, or volcanic eruptions. In this regard, spaceborne SAR systems may not be the optimal solution for fulfilling the requirement of swift disaster response.
Ecosystem Disturbance Monitoring
Numerous ecological disturbances, such as deforestation, necessitate high-resolution, wide-area SAR imaging with frequent revisits to identify and track changes. To illustrate, detecting deforestation and forest degradation demands frequent monitoring to capture alterations between observations.
Nonetheless, existing SAR systems encounter difficulties in achieving broad coverage and high temporal resolution. Consequently, many cases involving illegal logging or forest clearing may evade detection by the spaceborne SAR. Moreover, high- spatial resolution could be vitally helpful in detecting small-scale forest loss.
Glacier & Ice Sheet Monitoring
Regular and repetitive observations are necessary to consistently monitor glaciers’ movement and melting rates throughout the ablation season. To accurately monitor the rapid changes occurring in Glacier regions, it is essential to utilize SAR imaging with both high temporal resolution and wide coverage. Indeed, it demands tracking fast- moving glacier flow rates by SAR interferometry.
In this regard, the temporal resolution of current spaceborne SAR is usually insufficient to capture the dynamics of glaciers. Further, the swath width limits an extensive coverage of ice sheets and ice fields. Due to the long revisit times, the ice dynamics might be overlooked without operational ice monitoring and forecasting. The long temporal baselines and sparse acquisitions usually deteriorate the measurement accuracy.
Permafrost Monitoring
Monitoring permafrost is vital in studying and comprehending the ramifications of climate change in the Arctic and similar regions. In doing so, employing high- spatial resolution wide-area SAR with frequent revisits is necessary, as the time window for permafrost monitoring is quite short. To attain the objective of capturing variation in permafrost coverage during this time window necessitates SAR systems that provide high-spatial resolution imaging and possess a wide-swath width to cover vast areas.
Supposing that a SAR system can capture such observations, it would enhance our comprehension regarding the influence of climate change on permafrost and its corresponding ecological systems, particularly during the fleeting summer season. Unfortunately, most present-day spaceborne SAR systems encounter difficulty in supporting such observation.
Oceanic Environment Monitoring
Monitoring the oceanic environment involves the observation of physical, chemical, and biological parameters, as well as marine pollutants, disasters, and climate change. High-spatial resolution and continuous monitoring with frequent coverage are necessary to depict the state and changes of the oceanic environment accurately. Further, the estimation of wind fields in terms of wind speeds and directions over the ocean requires frequent SAR imaging.
However, the long revisit time and narrow swath of most spaceborne SAR systems hinder their use for operational monitoring. Specifically, they restrict the ability to obtain high spatiotemporal resolution data and detect transient processes or high-frequency environmental change signals, which may result in missing critical information, such as environmental changes or emergencies in local sea areas.
Additionally, the discontinuous observation and finite operational lifespan of spaceborne SAR pose constraints on monitoring longterm slow changes in the marine environment within specific geographic locations. Furthermore, the low spatial resolution makes capturing small-scale changes or details in the marine environment difficult, if not impossible. To enhance monitoring of the oceanic environment, it is imperative to secure SAR time series with high precision and long life-cycle, broad coverage, and satisfactory spatial resolution.
Potential Solutions
In the context of a single spaceborne SAR, it is highly challenging to achieve both high azimuthal resolution and wide-swath width simultaneously during SAR data acquisition. The two factors are subject to a trade-off, and optimizing one comes at the cost of the other. Meanwhile, the temporal resolution is also subjected to the LEO altitude, resulting in less frequent revisit observation of the target region, making it difficult to track dynamic geo-phenomena. To surmount the constraints of spaceborne SAR for Earth observation, we may deploy the distributed spaceborne SAR system or employ a high-orbit SAR system.
Implementing a distributed SAR system is reliant on a constellation comprising multiple satellites. This approach is characterized by its high reliability, ease of implementation, and cost effectiveness. Moreover, the distributed SAR system has the potential to collaborate with pre-existing spaceborne SAR systems. To enhance the performance of Earth observation, specifically launched or upcoming spaceborne SAR missions integrate satellite constellations and advanced imaging modes.
On the other hand, deploying the distributed SAR amounted on the constellation of satellites can potentially bring about unexpected technical intricacies, such as system synchronization, inter-satellite and satellite-ground station communications, formation design and relative navigation, data acquisition, and signal processing. The other challenge in the distributed SAR pertains to its spatiotemporal coverage performance. Specifically, it remains a difficult task to ensure the temporal consistency and spatial continuity in the coverage of distributed SAR. In this regard, such a system is unfavorable for observing dynamic, large-scale geophenomena, a distributed SAR technology breakthrough shall be required.
An alternative is to employ high-orbit SAR for Earth observation to ensure uninterrupted monitoring of an extensive region for a sustainable period, when the orbital altitude increases, the trade-off between the swath width and azimuthal resolution becomes more flexible as the threshold for restricting both increases. By deploying a SAR on a platform orbiting at a high altitude, it is feasible to attain both the high spatial resolution and wide-swath imaging simultaneously, thereby ensuring the spatiotemporal congruity of the observed region. Moreover, the high orbit also confers the advantage of generating data with satisfactory temporal resolution.
An exemplary high-orbit SAR is the Geosynchronous Earth Orbit (GEO) SAR, which operates in an inclined geosynchronous orbit at an altitude of around 36,000 km. The GEO SAR possesses the potential to cover an extensive area with high temporal resolution, thus facilitating Earth observation with a shorter revisit period.
Despite the advantages of GEO SAR, the required substantial antenna puts an enormous weight burden on the artificial satellite, given that the satellite’s payload capacity is typically constrained. Furthermore, it is worth noting that the GEO SAR can only attain a partial view of Earth, leaving a large portion of Earth’s surface area unobservable. Therefore, although the GEO SAR represents a promising technology on the horizon, pursuing a more efficient high-orbit SAR system remains to be ongoing.
As the global fascination with lunar exploration gains renewed momentum, establishing a sustainable lunar base has emerged as an inevitable trend. Establishing a lunar base has further stimulated the curiosity to deploy radar, referred to as Moon- Based SAR MBSAR), for Earth observation.
In contrast to the artificial satellite platform, the Moon provides a distinctive opportunity to conduct SAR Earth observation without many limitations of installing large antennas. In this regard, the MBSAR is endowed with the formidable ability to monitor Earth with an extensive swath width, as illustrated in Figure, owing to the relative motion between the Moon and Earth, the sensors (including radar) installed on a lunar base can provide a holistic view of our planet comprehensively while also revisiting it in a shorter period approximately 24.8 hours.
Source:
Moon-Based Synthetic Aperture Radar: A Signal Processing Prospect
