## 3 Challenges for a Future Imaging Radar for Space Observation
### The Motivation for Ground-Based High-Resolution Fully Polarimetric Space Imaging Systems  
One driver of change in the near-Earth orbit is the rapid progress in the development of new technologies. In the past decades, large, complex and very expensive monolithic individual satellites were built. Currently, the trend is towards the deployment of small and cost-effective systems. The trend towards constellations of small, less expensive, and simple satellites replacing large satellites allows for much shorter development and deployment times. Also, a better global coverage can be achieved with a constellation of small satellites. This development is also supported by standardized satellite buses and falling costs for satellite launches by private companies such as SpaceX. Inevitably, the amount of space debris will increase considerably in the near future, as small satellites do not have a long operational lifetime and are usually not equipped with an active or passive de-orbiting system. This trend is accelerated by the deployment of mega-constellations. For instance, Starlink satellites for global internet access might reach 12,000 objects in 2025. In addition, the reliability and thus the operational safety of these small satellites are also expected to decrease. This raises the risk of collisions with other space objects. The trend towards smaller satellites poses a great challenge to ground-based reconnaissance systems. Moreover, the mountings and instruments are also becoming more compact for large satellites, so that their ground-based reconnaissance is also becoming increasingly difficult. This problem is heightened by the fact that some satellites have to be operated at significantly higher orbits. One reason is that LEO is already extremely populated at altitudes between 500 km and 800 km. Another reason is that it is often not possible to raise the orbit of small satellites when the orbit height decreases because of friction in the atmosphere. Figure 13: United Nations Office for Outer Space Affairs’ (2024), with major processing by Our World in Data, “Annual number of objects launched into space—NOOSA” [dataset]. United Nations Office for Outer Space Affairs” “Online Index of Objects Launched into Outer Space” [original data] [20]. Due to the future development of artificial satellites in near-Earth space described above, the reconnaissance capability of ground-based sensors has to be improved to meet the requirements of future reconnaissance systems. Future ground-based space reconnaissance systems must operate with a significantly higher spatial and radiometric resolution in order to be able to image small satellites at a far range together with their instruments and small attachments in order to perform technical analyses. Additional information about space objects can be gained by using polarimetric ISAR. Especially in the case of a fully polarimetric space radar, the full electromagnetic vector field is available as opposed to a single polarized ISAR image. Fully polarimetric radar imaging takes advantage of illuminating objects with both horizontally (H) and vertically (V) oriented electric field vectors and also record both polarized returns. This acquires all four components of the scattering matrix and allows us to investigate the structure of space objects due to the interaction with the polarized electromagnetic waves [21]. Indeed, depending on the transmitted and received polarization directions, structures (e.g., bars) of an object might be clearly visible in the radar image or suppressed. This allows us to separate different areas or components of a space object in radar images in order to investigate them independently of each other in more detail. Moreover, very bright and prominent scatterers which outshine faint nearby structures could be suppressed to make adjacent faint scatterers visible. A well-known consequence for ISAR imaging is the layover effect. This means that spatially separated resolution cells of a 3D space object can superimpose in the processed 2D radar image.