Over the next decade, more than twenty space borne SAR missions are being planned or proposed by many space agencies and commercial entities. Understandably each mission is designed and optimized (orbit, crossing time, coverage, frequency, polarization, etc.) to meet the user needs and objectives of the sponsoring organization. An ideal situation will be to have an integrated capability to provide multi-frequency, multi-polarization, well calibrated interferometric global coverage at very frequent rate to monitor both slow and rapid changing surface features. Even though this will be hard to achieve by one or two missions, it would be possible to do if the dozens of planned missions can be coordinated as a “constellation”. By coordinating the characteristics of the different missions and properly selecting their exact orbit and nodal crossing, a very powerful integrated capability for users can be achieved that far exceed what can be achieved one or a combination of uncoordinated mission.
The idea about an international coordination of future SAR missions was first explored in a workshop held in May 2018 at Caltech and attended by 50 participants from all the agencies flying, or that will shortly launch space borne radar missions. There was a strong consensus that a coordinated effort will be of great value and a number of specific recommendations. Subsequently, in May 2019, a session on the same topic was organised at the ESA Living Planet Symposium (LPS-19) in Milano, Italy, attended by more than 250 persons [1] indicating the strong interest for this type of activities. The second “International Coordination of future Spaceborne SAR Missions” was supposed to take place in 2020/21 but has been postponed due to COVID-19 to September 2022 and will be organised by ESA at ESRIN, Frascati, Italy.
The work has been organised in three international working groups (WG) covering:
• WG-1: Present and future data‐visibility and access
• WG-2: Future imaging systems, challenges and opportunities
• WG-3: Data exploration, Cal/Val, fusion and assimilation.
The outcome of the activities of the 3 WG’s are regularly reported in workshops since 2018 as indicated in the previous sections. Since 2021, three thematic areas (TA) have been added to further deepen the collaboration across the WG topics and cover the following domains:
• TA-1: Whole-earth system science and data mining
• TA-2: Targeted science and applications
• TA-3: Programme coordination
The presentation will go in the details and the main results obtained so far in the frame of this activity to coordinate future Spaceborne SAR missions.
On May 30, 31 and June 1, 2018, a workshop was held at the California Institute of Technology to explore the interest in, and value of a more coordinated approach between the different organizations to achieve higher value to the user community. One of the main topic of this workshop is to make a recommendation to improve data visibility and accessibility of spacebornes SAR under the international coordination.
To understand the issues related to data discovery and data access, a working group 1 (WG-1) was established and compiled two tables. Table 1 relates to the discovery/accessibility of past archive data and Table 2 relates to discovery, tasking, and access to present and future data. WG-1found that it is feasible to construct a search engine to discover most data, past and present. Through this exercise, WG-A found that all agencies flying spaceborne SAR systems either provide all the data free of cost, or subsets of them for specific purpose or by inter agency agreements. However, if all the data has standard geometric and radiometric formats, their value will be significantly enhanced. As this result, WG-1 recommend that archival, present and future data should be easily accessible with a standard and common format, and can be easily requested and acquired electronically, free or at minimal cost. On the other hand, because the discovery of planned acquisitions is more problematic, WG-1 recommend developing a mechanism to coordinate future data acquisition and coverage by present and planned systems, as well as ground reception and processing systems for mutual benefit. In several cases, coordination between systems have led to significant benefit particularly in case of polar ice studies (Polar Space Task Group) and rapid response for natural hazards. It will be of great value if a mechanism can be put in place to extend this coordination to a larger number of applications which can benefit from expanded coverage, shorter repeat time and multiple frequency/polarized observations.
On the other hand, a critical element is to develop common standards for data formatting, geodetic projection and radiometric calibration. Another important element is to have the ability to simply search for data available from all the systems in a common app, as well as insight of planned future data acquisitions and, with appropriate credentials, be able to request future new acquisitions. In addition, there is a strong need for a high resolution, high accuracy elevation model with accurate time stamp for improved SAR processing and data inter-comparison.
This paper explains about the overview of this WG-1 analysis and recommendations to improve data visibility and accessibility of spacebornes SAR under the international coordination
A number of national and international space agencies and space organisations that operate Synthetic Aperture Radar (SAR) sensors have come together to improve coordination between SAR missions with interoperable and/or complementary characteristics. In the shorter-term the coordination focuses on currently operational and near future missions (e.g. Sentinel-1, ALOS-2/4, SAOCOM-1, NISAR, BIOMASS, TSX-NG, CSK-2G), and for the next decade, takes into consideration missions still to be defined. Working groups are tasked to address the visibility and access to SAR data products (WG1); opportunities and challenges of future imaging systems (WG2); and the exploration of multi-mission SAR data, including Cal/Val, data fusion and assimilation aspects (WG3).
Three Thematic Area (TA) subgroups have been established to support the working group activities with cross-cutting topics relating to science and applications:
• TA-1 – Polarimetric and Multi-frequency SAR – covers applications where polarimetric or multi-frequency backscatter intensity and/or polarimetric phase constitute the main measurements. TA-1 addresses applications such as Forestry, Agriculture, Wetland and Other Land Uses (i.e. the IPCC “AFOLU” themes), plus such relating to Ocean and Sea Ice.
• TA-2 – Interferometric SAR – covers applications where interferometric phase constitutes the main measurement. TA-2 covers the traditional InSAR driven applications such as Solid Earth (incl. crustal deformation, volcanoes), Glaciers/Ice Caps, Geo-hazards, and Permanent Scatterers
• TA-3 – Program and mission coordination.
The TA subgroups are to work closely with the relevant science communities to identify SAR coordination actions that could be taken in the near/mid-term (in 2020s) that would improve science/applications overall, as well as to identify gaps and missing critical elements with current and near-future missions. Looking beyond the missions currently in operation or development, the TA subgroups are to identify and prioritize the goals and objectives for SAR coordination that would vastly improve science by coordinating long-term (2030+) Earth observing SAR missions.
A first cut gap study was undertaken in 2021 to assess the SAR information requirements for six application areas: Glaciers and Ice Caps; Solid Earth Science; Hazards; Forest and Biomass; Wetlands; Agriculture and Soil Moisture [1]. The study resulted in a number of observations:
1. The most important requirement highlighted in all cases is the need to reduce observation revisit times to the order of days, or less. For the applications that rely on SAR interferometry, temporal decorrelation constitutes a major limiting factor, preventing, e.g., the tracking of fast-moving glacial or seismic events. Temporal decorrelation is also the main obstacle to the use of InSAR for forestry applications.
2. For the glacier and solid Earth applications, it was noted that systematic observations from at least 3 viewing angles are required to characterise the displacement field in three dimensions, while maintaining a zero interferometric baseline for each imaging geometry in the temporal stack. While such data can be obtained by observations from ascending and descending orbits, and alternating right- and left-looking platform roll, no mission currently comprises such plan.
3. For forestry applications, a variety of interferometric baselines would enable tomographic retrieval of forest structural parameters. This can be achieved by relaxing the orbit control, which however affects other InSAR applications.
4. Polarimetric (full scattering matrix) observations constitute a critical requirement for measurements of soil moisture, and is desired also for agriculture (crop identification). It was acknowledged that PolSAR and Pol-InSAR applications are under-developed due to the lack of global polarimetric time-series data, and that basic research in this field should be stimulated.
5. Each SAR frequency contributes with unique and complementary measurements, but multi-frequency applications also are under-developed due to lack of data for research. Simultaneous or near-simultaneous multi-frequency observation campaigns, in particular for land cover related applications, are therefore strongly encouraged.
6. It was finally noted that open data policies and free public access is vital for data democracy and science development.
The TA activity for 2022 includes undertaking a more comprehensive SAR user survey and a public open online questionnaire is available on the International SAR Coordination group website [2]. The results will be summarised to identify information gaps associated with each thematic application area, and subsequently, provide recommendations to the working groups on how these gaps can be mitigated by coordination of current and already planned missions, and for the next decade, with a vision for a comprehensive constellation system that would address the outstanding scientific requirements.
References:
[1] Rosenqvist A., Jones C., Rignot E., Simons M., Siqueira P. and Tadono T., 2021. A Review of SAR Observation Requirements for Global and Targeted Science Applications. International Geoscience and Remote Sensing Symp. (IGARSS’21). FR3.O-5.3, Virtual, 16 July, 2021.
[2] https://nikal.eventsair.com/NikalWebsitePortal/second-workshop-on-international-coordination-for-spaceborne-synthetic-aperture-radar/esa/ExtraContent/ContentPage?page=10
International coordination and collaboration as an essential element of the Earth Observation Service Continuity
Guennadi Kroupnik, Éric Dubuc, Mays Ahmad, Patrick Plourde, Geneviève Houde, Daniel De Lisle.
The Earth Observation Service Continuity (EOSC) initiative aims to identify successor solutions for Canada’s next generation RADARSAT program. Due to the wide range of user needs, the EOSC initiative is considering a diversified portfolio of access to imagery: free and open data, commercial purchase of data, international cooperation, and a dedicated Synthetic Aperture Radar (SAR) system. This paper will present an overview of the options analysis that led to the identification of international partners and their respective Earth Observation (EO) programs, highlight the current status, and path forward.
It is anticipated that certain user department needs can be met through existing or planned international space-based data assets. Potential collaboration scenario(s) to meet Canada’s Harmonized User Needs (HUN) can refer to, but is not limited to, bartering of data, harmonize requirements/technical solutions for missions, harmonization of data products, prototyping with sample Areas of Interest (AOI) to address downlink feasibility, access to infrastructure, etc.
In leveraging international capabilities for the purposes of augmenting Canada’s earth observation capabilities the following key benefits arise:
• Strengthen international relationships with key partners
• Facilitation of the harmonization between nations and their respective EO programs
• Stimulation of social and environmental benefits from the use of EO data
• Increase resilience through access to multiple source of data
• Improve compliance to end user requirements by providing the optimal mix of multi-frequency data.
• Increase resilience through access to multi-frequency capabilities
The economic potential of space based data has grown significantly in recent years. The Canadian Space Agency (CSA) is committed to continuing to progress strong partnerships with international stakeholders to best deliver data that meets the needs of the community and government priorities such as climate change.
Since the 1980s, Germany has built up considerable expertise in spaceborne SAR missions. The Shuttle Imaging Radar Missions SIR-C/X-SAR in cooperation with NASA/JPL consisted of two flights in April and September 1994 aiming to demonstrate the potential of fully polarimetric radar systems in three different frequency bands for a variety of applications. Germany developed the X-SAR radar system in cooperation with Italy, the USA developed the radar systems in C and L band. The combination of L, C and X-band, as well as the different polarizations of the acquired data takes over selected test sites are unique until today. SRTM, the Shuttle Radar Topography Mission, was a highlight in Germany’s radar activities in cooperation with NASA/JPL. Space Shuttle Endeavour took off on February 11, 2000, with the goal to map the topography of the Earth’s surface using two radar systems. Secondary antennas mounted at the end of a 60 m long boom allowed a topographic mapping of 80% of the Earth’s land surface with a height accuracy of 10 m. Germany participated in the mission with an interferometric X-band radar system (X-SAR) that acquired approximately 40% of land surface with the increased accuracy of approximately 6 m.
The actual highlight of the German spaceborne radar program is the successful implementation of the TanDEM-X mission. The first formation flying radar system was built by extending the TerraSAR-X mission by a second, almost identical satellite TanDEM-X. The resulting large single-pass SAR interferometer features flexible baseline selection enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. The so-called Helix formation combines an out-of-plane (horizontal) orbital displacement by small differences in the right ascension of the ascending nodes with a radial (vertical) separation by different eccentricity vectors resulting in a helix-like relative movement of the satellites along the orbit. The primary objective of the mission, the generation of a global Digital Elevation Model (DEM) with unprecedented accuracy, was achieved back in 2016. The obtained results confirm the outstanding capabilities of the system, with an overall absolute height accuracy of just 3.49 m, which is well below the 10 m mission specification. Excluding highly vegetated and snow-/ice-covered regions, characterized by radar wave penetration phenomena and consequently strongly affected by volume decorrelation, it improves to 0.88 m. Also, the relative height accuracy, which quantifies the random noise contribution within the final DEM, is well within specifications. Finally, the product is also virtually complete with 99.89% coverage. Comparisons of the TanDEM-X DEM with SRTM or among multi-temporal TanDEM-X data revealed dramatic changes and the high dynamic in the Earth’s topography especially over ice and forests. It has been therefore decided to acquire data for a global change layer that will become available in 2022. Despite being well beyond their design lifetime, both satellites are still fully functional and have enough consumables for several additional years. Therefore, bistatic operations continue with a focus on changes in the cryosphere and biosphere. The TanDEM-X mission has been and still is the first distributed SAR system in space. It demonstrates the DLR’s capabilities in the development of highly innovative mission concepts in response to demanding mission objectives, in leading the project realization facing a number of challenges, and in directing and monitoring the entire generation process from global data acquisition through to the final digital elevation model (DEM).
TanDEM-X can be seen as a precursor for Tandem-L, a pioneering mission for climate and environmental research. Tandem-L is built on a very strong science case developed in a joint effort by eight Helmholtz research centers and an international team of more than 100 scientists. Aiming at the observation of dynamic processes in the bio-, geo-, hydro- and cryosphere, this mission requires a novel SAR instrument concept based on digital beamforming in combination with a large reflector antenna. A swath width of up to 350 km enables weekly global coverage as a precondition to observe Earth’s system dynamics. The Tandem-L mission concept is based on the two SAR satellites operating in L band allowing for innovative imaging modes like polarimetric SAR interferometry and multi-pass coherence tomography for determining the vertical structure of vegetation and ice. A unique feature and major challenge of the Tandem-L mission is the systematic generation of higher-level products, including forest height, structure and biomass, various surface deformation and displacement products, as well as digital elevation models. Additional products for applications in the hydro- and cryosphere are expected to be developed by the scientific community in the course of the mission.
Given the great success of TanDEM-X, a novel concept for an X-band SAR mission denoted as High-Resolution Wide-Swath (HRWS) mission has been proposed. It consists of a powerful main satellite acting as an illuminator as well as three much smaller receive-only relay satellites to be used in formation flight. The main satellite features up to 1200 MHz bandwidth, a frequency scanning functionality (F-SCAN) combined with multiple azimuth phase centers (MAPS) enabling high-resolution wide-swath imaging. The small satellites, following the MirrorSAR concept, operate as radar transponders and allow an effective, low-cost implementation of a multistatic interferometric system for high-resolution DEM generation and for secondary mission objectives as, for example, along-track interferometry. With HRWS, nearly 40 years of successful X-band SAR development in Germany will continue. Thus, the mission will provide data continuity for scientific, institutional and commercial users.