# Recent Advances of the BIRALET System about Space Debris Detection
## 3 Results and Discussion  
Between 13 December 2018 and 10 October 2019, the BIRALET system was able to observe a total of 27 objects. The Italian Air Force (Aeronautica Militare--AM) has provided the list of objects to observe, making sure to include a wide variety of samples in terms of RCS and range. All the observations were made in a variant of the beam-park mode. Usually, the beam-park mode (or stare mode) is employed for the survey of space debris, illuminating a specific portion of the sky and waiting for the objects to cross the field of view (FOV) of the antenna. It is employed very often to detect objects that are not part of a catalogue. In this case, the aim was not to detect uncatalogued objects, but to assess the capabilities of the radar system. Furthermore, the characteristics of the BIRALET system are not suitable for survey purposes, owing to the narrow beam. Thus, the antenna was pointed each time in a different direction, dictated by a specific object in the list, and waited for its passage. Before the observations, a forecasting simulation was made in order to obtain the azimuth and elevation pointing coordinates and to estimate the Doppler shift values. An algorithm based on the reading of the TLEs provides these pieces of information. The results of the campaign are summarized in Figures 6 and 7, representing the RCS as a function of the range and the altitude as a function of the elevation pointing angle, respectively, for the 27 detected objects. Figure 6: Scatter plot of RCS vs. range for the detected objects with respect to the minimum detectable size by the BIRALET system. SNR, signal-to-noise ratio. As expected, with reference to the scatter plot in Figure 6, the detected objects are located above the curve, representing the predicted minimum detectable size by the BI-RALET system. This threshold value was obtained accounting for a minimum SNR of 10 dB (see Figure 3). It is worth noting that the SNR depends on many factors such as rotation of the object, materials, uncertainty in the values of the RCS and trajectory provided by the TLEs, pointing accuracy, and possible unwanted losses in the receiving chain. Moreover, especially in our case, where the transmitter is handled by a third part, the actual SNR can be difficult to estimate owing to unknown fluctuations of the transmitted power. For the above reasons, in this work, SNR was employed only for a first estimate of the system ability to detect a particular object. The RCS of the detected objects varies between 0.13 m\({}^{2}\) (\(-8.86\) dBsm) and 13.4 m\({}^{2}\) (11.27 dBsm), whereas the range varies between 459 and 1224 km. However, the majority of detections are clustered between 500 and 1000 km. From this image, it is clear that the BIRALET system would be capable of detecting even smaller objects, especially for low range values. Notwithstanding, the object with an RCS of 0.13 m\({}^{2}\) is the smallest object ever detected by the bistatic radar, so far. During the reception of the scattered signals from the larger and/or closer objects, the receiving system did not suffer from saturation problems, thanks to an accurate setting of the digital step attenuator. From Figure 7, it can be inferred that all the spotted objects are located in the low Earth orbit (LEO), which includes altitudes between 200 and 2000 km. Detections on higher orbits are not feasible, owing to the relatively low power of the transmitter. However, almost every radar space debris sensor is focused on LEO measurements, as it is the most populated one. As mentioned in Section 2, the data recorded by the back-end are acquired in a row format that can be later post-processed. These refined data are displayed in a spectrogram fashion, showing the duration of the passage, frequency, and power. An example of a time-frequency spectrogram is reported in Figure 8, for the object 4256, which is characterized by an RCS of 0.51 m\({}^{2}\), an altitude of 918.1 km, and a range of about 941.5 km. Figure 7: Scatter plot of altitude vs. elevation for the detected objects.