## 2 Radar Techniques for Space Observation and Products
### Separation of Multiple Objects  
In certain situations, space objects are orbiting very closely to each other. This can happen, for example, during the release of satellites, during robotics missions, or during a spying attack. In this case, it is important to detect and monitor the maneuver from the ground and to estimate the distance between the objects. This can be achieved using the imaging radar. However, for the current imaging radar, the distance between the objects should be less than 100 m in range and 500 m in cross range at a range of 1000 km. This strong constraint restricts the use of the imaging radar only to close proximity maneuvers. It is worth mentioning that if the range/Doppler histories of the objects differ from each  
Figure 3: Sentinel\(-\)3B (27 January 2021) [6].  
other, the objects cannot be all focused simultaneously. Indeed, different processing filters are required to image the objects. Depending on the processing, one of the objects will be sharply imaged, while all the others will be blurred.  
It is possible to discriminate objects with the tracking radar without the use of radar imaging. Even if space objects are located jointly in the antenna beam and in the same range/Doppler resolution cell after matched filtering, coherent processing can be applied to the multi-channel data acquired by the monopulse system in order to discriminate the objects in the 4D space range rate/range-rate rate (the rate of change of the range rate)/azimuth angle/elevation angle. The separation in angle can be achieved, e.g., by using the monopulse dechirping method [6], which is an extension of the method presented in [9].  
This multi-channel coherent processing can be applied to discriminate objects in different orbital regimes. The example of an Astroscale exercise [10] during which an encounter maneuver was performed between two LEO objects, ELSA-D (larger object), and ELSA-D CLIENT (smaller object, see Figure 4), is shown in Figure 5. Figure 5 plots the measured radar cross section (RCS). During a limited time, both objects were located in the same radar beam. This corresponds to the region indicated by the orange box in Figure 5, which presents an oscillating pattern revealing the presence of several close objects. Applying coherent processing enables us to identify both objects (e.g., at time B and C), whereas only one object can be detected in the green regions (e.g., at time A and D). The right picture shows the relative location of the two objects in the antenna beam after monopulse dechirping processing. It has to be noted here that the TIRA system uses an amplitude monopulse to estimate the angular position. Contrary to a phase processing, it is not possible to correct for the object motion within the antenna beam during the integration time. The fast object motion in LEO, the required beam tracking, and the variation in the relative position of the object within the antenna beam cause an amplitude modulation in the received signals affecting the estimation of the angular position. Nevertheless, the relative position of the objects in the antenna beam can be coarsely followed over time.  
The same type of processing can be applied to the geosynchronous equatorial orbit (GEO) orbital regime. One major difference is that the single pulse SNR is so low that space objects cannot be directly detected. Integrating over several pulses is mandatory for target detection. GEO objects orbit at the same angular velocity as the Earth. They thus appear to be stationary for an observer on the Earth and their location in the antenna beam is fixed. Figure 6 shows the results of an observation of the NILESAT cluster. All four satellites could be successfully detected, and their parameters estimated.  
Figure 4: ELSA-D and ELSA-D CLIENT [10].  
Figure 5: Astroscale exercise (8 April 2022) [11].