New approaches to assessing the space debris environment
The latest estimates suggest that the number of objects greater than 2 cm in size currently orbiting the Earth is in excess of 200,000. Approximately 2% of these objects are detected within the framework of the typical measurement campaigns for the analysis and characterization of small-particle space debris that are carried out on a regular basis with TIRA. Fraunhofer FHR is now investigating new approaches to increase the information content and efficiency of such measurements.
From the very beginning of space exploration, radar systems have been used to determine the paths of objects that orbit around the Earth. However, due to statistical and systematic errors, sensor-based measured values are never realistic. This restricts the precision that can be achieved during orbit determination. Over the past year, the software tool PROVE (Program for Radar Observation Vector Estimation), which is capable of simulating observation vectors for monostatic pulse radars with parabolic or array antennas, was developed at Fraunhofer FHR. PROVE will primarily be used in the development of new radar systems. For this reason, the first task involved the modeling of important error values which influence the observation vectors at the specified system parameters. In a first step, measurements taken with the TIRA target tracking radar were subsequently simulated with PROVE to verify the selected approach. To eliminate object-dependent fluctuation in the measured values, the radar calibration satellite LCS-4 was measured with TIRA. Due to its spherical shape and consistency, LCS-4 exhibited a radar cross section that was practically independent of the aspect angle. Figure 2 shows a good correlation between measurement and simulation for the distance and Doppler frequency as well as for the deposition of the observation vectors from the antenna axis, thus confirming that the selected modeling approach is, in principle, correct. PROVE can also be used to investigate the effectiveness of new operating modes for the detection of space debris with TIRA.
Closing the gaps
High-performance radar systems are necessary to characterize the space debris situation in orbits between 200 km and 2000 km. For this reason, such measurements have been carried out with TIRA in the form of beampark experiments (BPE) on behalf of ESA since 1993. Regular measurements of this kind (normally once a year) are also necessary for the investigation and calibration of the statistical distribution model.
Within the framework of the BPE, the antenna is aligned to the East with high elevation. All objects of a specified size which fly through the beam of the antenna within a predefined range are recorded over a period of 24 hours. When the antenna is aligned in an easterly direction, it is only possible to observe orbits with an inclination (angle of the orbit compared to the equatorial plane) of over 50°. For this reason, a BPE with the antenna facing south and a low elevation was carried out for the first time in December 2015 to facilitate the observation of objects in orbits with an inclination of less than 50°. The recorded data is currently being evaluated.
Further modifications for BPEs are currently being examined to enlarge the sample that can be achieved or improve the orbital parameters derived from the observations over an unchanged observation period. A mode of operation in which the antenna axis is not statically aligned in a predefined angle of vision but rotates with constant angular velocity around this angle of vision was analyzed with a view to enlarging the sample. The PROVE software referred to above was used for the first time to investigate the interdependence between the rotational speed of the antenna and the size of the sample (number of recordable objects). It was shown that compared to the static BPE, the sample size can almost be doubled at a scan/rotation rate of more than 2 Hz. If the mechanical stress limits of the TIRA antenna system – which limit the scan rate to below 0.4 Hz – are also considered, the sample can be enlarged by a factor of 1.7 (Figure 1, center). Compared to a static BPE with the antenna aligned to the South, the sample can even be enlarged by a factor of 1.8.
The steadily increasing volume of space debris also leads to an increase in measurement quality requirements. The work that is currently being carried out will help to secure Fraunhofer FHR's position as one of the world's leading institutions when it comes to characterizing the space debris environment.