# Active Control Loop of the BOROWIEC SLR Space Debris Tracking System
## 3 Active Control Loop
### Photon Event Active Control Loop  
The active control correction of the telescope's movement behind the orbital object according to the proposed algorithm can also be made by quantifying the photon trace scattered from the orbital object. In this case, the data stream of the photon event recorder obtained during the measurement process in close to real-time is analyzed (Figure 7). The absence or negligible number of photon events \(E_{n}\) at the expected time \(t_{n}\), which is  
Figure 6: Block diagram of the active control correction algorithm based on ephemeris data and image analysis.  
determined by the distance \(R_{x}\) to the orbital object, forces the procedure of searching for the object to be performed.  
In the case of uncooperative space debris, the quantitative measures of acquired photon events are completely different from the cooperative ones. Due to the lack of a retroreflector element, the number of returning photons is incomparably low. This does not change the fact that each return echo from space debris objects provides information about their orbit and of the behavior of these objects (e.g., spin). Furthermore, the notion of a normal point in the case of uncooperative space debris becomes less relevant and even negligible. Space debris mainly consists of unstable objects, so any information is valuable. Therefore, the algorithm provides two alternative paths for assessing the presence of photon events: by the comparison of the similarity of successive photon events in the time domain or by evaluating the return rate. Both paths depends on the end capabilities of the laser-detector pair in terms of the maximal detection range and given Radar Cross-Section (RCS) of the population of the orbital objects.  
The advantage of this solution is the ability to correct mount motion parameters even when the object is invisible (daylight) or the object is at higher orbits (MEO, GTO), which makes passive optical acquisition with a short exposure time difficult to perform. The use of such an algorithm leads to the automation of the measurement process, which in the case of 24/7 operation relieves the staff of the SLR station. There is no need for the operator to manually search for an object, even for laser ephemerides (CPF), which always have some TimeBias. Another convenience compared to the manual search for an object by the operator is that the algorithm works based on the distance parameter \(R_{x}\) and the beam divergence \(\theta_{y}\) for a given moment of time \(t_{n}\). In manual mode, the operator sees the emission spot of the laser beam on the preview screen, but does not know its real cross-section at the orbital altitude. From this, it follows that its manual X/Y or manual spiral search can create search gaps. Another aspect is that the operator is unable to calculate for a given moment of time the exact time interval that is needed to obtain an echo from the object (long time intervals for the MEO and GEO regimes). Thus, the operator may wait too short or too long to receive an echo, which reduces the effectiveness of the measurement. The operation of the algorithm takes into account all these components; therefore, it leads to faster and more effective measurements. The main disadvantage of the process is the time it takes to complete the procedure. The most common approach is the spiral search method,  
Figure 7: Block diagram of the active control correction algorithm based on ephemeris data and photon events.  
which iteratively increments the \(Az/El\) axes' deflection. With reference to Section 2.3.1, a spiral radius \(r\) calculation is implemented depending on the laser beam divergence used. Hereby, an optimal search of the area is ensured, which leads to a quick search for an object, eliminating the possibility of omitting the object.