Over the next decade, more than a dozen 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 by one or a combination of
uncoordinated mission.

Such a coordinate effort will still allow each Agency/partner to achieve its own objectives. However, it does require using common data formatting, map projections, calibration,
coordinated data acquisition and receiving stations, etc., and the use of common test sites. By doing so, in addition to the benefit to the users, some cost savings might be achieved.

A workshop was organized 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 were made. To follow up on these recommendations, three international
working groups were organized covering 1) Present and future SAR data‐visibility and access, 2) future imaging systems, challenges and opportunities, 3) Data exploration, Cal/Val, fusion and assimilation.

The outcome of the workshop will be presented as well as the recommendations proposed from the three working groups to illustrate the benefits of such a coordinate international
approach.

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.

During the commissioning of Sentinel-1A a campaign of coordinated acquisitions with RADARSAT-2 took place in order to investigate the mutual interference impact between the two C-band SARs whilst operating simultaneously. The conclusions were that Radio Frequency Interference existed, and although it does not pose a threat to either space segment, the image products are affected. Since Sentinel-1A launch, many other interferences between both satellites have been observed and some of them documented.

Due to the upcoming SAR constellations the number of interferences is expected to increase. These interferences cannot be avoided, being the satellites in the down dusk orbital plane at different altitudes, it is inevitable that they will be crossing over the same location regularly. However, the location of the crossing points between two satellites is not random, but it depends on their orbits. The orbital parameters are optimized for each mission and usually cannot be modified, but an appropriate selection of the ground-track and, or a slight tuning on the mean local solar time allows to place the possible interference's between the two SAR satellites away from areas of interest.

The geometry of the problem impose some limitations on the optimization. Two satellites will cross several times within a cycle and the locations of these crossing follow certain rules, meaning that when one location is selected the others are automatically fixed.

It shall be noted that before and after the crossing of the satellites they will be observing the same targets in a short time interval, creating opportunities for synchronized observations. A full optimization should not only avoid interference over areas of interest but also allow short time interval SAR acquisitions over selected locations in a systematically way.

Definitely, coordination between SAR missions is needed to achieve these objectives. The purpose of the presentation is to give an overview of the problem, the coordination needed to optimize the location of the crossing between SAR satellites and the limitations that exist.

Italian Space Agency (ASI) invests more than 50% of its budget in the Earth Observation (EO) field, considering both National and ESA contribution. In the framework of Space Systems for EO, the COSMO-SkyMed (COnstellation of small Satellites for Mediterranean basin Observation) Program represents the main Italian investment. The program was funded by Italian Ministry of Research (MIUR) and Italian Ministry of Defence (MoD) and developed by the National Industry, with the involvement of national small and medium enterprises (SMEs).
The most important EO Italian space asset was designed to satisfy a wide variety of user needs in both civilian (institutional and commercial) and defense domains exploiting four satellites that host on board a Synthetic Aperture Radar (SAR) operating in X-band. In the last decade, thanks to its features, the Italian constellation has provided a significant contribution in several applications domains providing data for scientific, commercial and strategic applications. The four spacecraft’s were stepwise deployed from 2007 to 2010 and they are still operating at full performance, even if all of them reached the end of the nominal operational life.
To ensure the continuity of the SAR EO servicing, providing at the same time a significant improvement of performances and to consolidate and enlarge the Italian leadership on SAR observation from space keeping a strategic prevalence on an increasing market of high technology, a second generation of satellites and a significant Ground Segment upgrade have been designed and developed. This is known as COSMO Seconda Generazione (CSG) program and it has been taking place since 2010 including two additional satellites, planned to be launched starting from 2019. A smooth transition from first to second generation of satellites will be ensured through an integrated system accounting of both capabilities (CSK and CSG), meaning that COSMO-SkyMed services provided to the users will be granted as if a unique constellation is operating. More than 10 years of operative servicing constitute a great return of experiences and lesson learned that have been capitalised in defining the new CSG system architecture and operative concepts aimed at greatly improving efficiency and reliability of the on-demand services that will be provided to a large variety of users, in a complex context as a dual-use priority-based system, as COSMO system actually is.
This work aims to present the status and future plans of the Italian space asset COSMO-SkyMed, highlighting the evolution of the functionalities and providing an overview of the new scenarios for the users community and potential cooperation in relation to the second generation age.

This paper provides an overview of two different concepts for spaceborne SAR systems: full-fledged multi-channel spaceborne SAR systems with digital beamforming and small, low-cost systems based on a NewSpace mission concept. The capabilities of these systems will be summarized by means of two exemplary missions: Tandem-L and MirrorSAR. While Tandem-L is a proposal for an innovative L-band SAR mission for the systematic observation of dynamic processes on the Earth’s surface with unprecedented accuracy and requires advanced technology as digital beamforming to enhance and imaging swath and imaging capability, MirrorSAR is a mission study based on a new multistatic SAR concept with the goal to generate a global DEM with 4 m horizontal spacing and 2 m height accuracy. It is also shown, that the imaging capabilities of NewSpace-based SAR concepts can be largely increased by the use of advanced techniques. Finally, the paper summarizes the user needs for each type of mission concept and provides a recommendation for future developments.

The Tandem-L mission concept is based on two SAR satellites flying in close formation featuring latest digital beamforming techniques in combination with a large deployable reflector for increasing the swath width and imaging resolution. This enables innovative operation modes such as polarimetric SAR tomography for determining the vertical structure of vegetation and ice. With novel imaging and processing techniques and the vast recording capacity of up to 8 Terabyte/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere and will make an essential contribution for a better understanding of the Earth system and its dynamic. Important mission goals are the global measurement of forest biomass and its variation in time for a better understanding of the carbon cycle, the systematic monitoring of deformations of the Earth’s surface on a millimeter scale for the investigation of earthquakes, volcanos and risk analysis, the quantification of glacier motion and melting processes in the polar regions, the fine scale measurement of variations in the near-surface soil moisture as well as observations of the ocean surface dynamics and ice drift. Tandem-L will simultaneously measure 7 essential climate variables in a single satellite mission.

The MirrorSAR concept is a new multistatic approach for robust single-pass interferometry and generation of digital elevation models. Compared to previously published bi- and multistatic SAR systems, the receiver satellites are considerably simplified, as their main functionality is reduced to a kind of microwave mirror which routes the radar echoes towards the transmitter. The forwarded radar signals are then coherently demodulated within the transmitter by using the same oscillator that had been used for radar pulse generation. In this way, the MirrorSAR principle eliminates the need for units for demodulation, digitization, storage and downlinking of the radar data by the companion satellites. Instead, these processing stages are carried out by the main satellite, which receives the ground-reflected radar signals of the companion satellites via a phase-preserving radio link. In other words, the companion satellites work in a similar way to transponders that relay the ground-backscattered radar signal to the main satellite. Since at least three companions are deployed, multiple interferometric images with different baselines can be acquired simultaneously from the same area. The combination of multiple baselines during DEM processing is essential for robust phase unwrapping to enhance the accuracy of the final product.

It is concluded that full-fledged, multi-channel SAR systems with digital beamforming capability will pave the way for future SAR missions for the global monitoring of dynamic processes on the Earth surface, while NewSpace-based SAR concepts will play a major role for missions with a focused application area which requires a very short revisit time. We foresee a bright future in both cases since spaceborne SAR is the only technology which can provide high-resolution images and timely geospatial information on a global scale independent of weather conditions and day time.