## 1 Introduction
9.4396\({}^{\circ}\)E), which operates in Continuous Wave (CW) mode at 410 MHz (P-band). The TRF is a 7 m dish parabolic antenna in primary focus configuration, with a maximum speed in Azimuth and Elevation of 3 degrees per second. The transmitter system is composed by a set of powerful amplifiers, installed in a cooled shelter, able to supply a maximum root mean square (RMS) power of 10 kW (continuous peak power of 14 kW) in the bandwidth 410-415 MHz, where the TRF has an antenna gain of about 27 dBi and an Equivalent Isotropic Radiated Power (EIRP) of more than 60 dB [12; 13]. The BIRALES system uses a portion of the Northern Cross radio telescope, located in Medicina near Bologna (with a baseline from the TRF of about 580 km), as a receiver and it is used for monitoring the space environment in survey mode. The receiver exploits a multi-beam configuration that provides, along with the available measurements, i.e., Doppler shift, illumination time and measured power intensity represented by signal-to-noise ratio (SNR), an estimate of the angular track of the transiting object in the receiver field of view obtained from the analysis of the beam illumination sequence. This innovative configuration allows the sensor to perform initial orbit determination [11]. The BIRALET system, on the other hand, makes use of the Sardinia Radio Telescope (SRT) as a receiver, with a baseline from the TRF of about 20 km, and it provides Doppler shift measurements and the received SNR in tracking mode, using the mono-beam P-band receiver [13]. Compared to the early years when the SRT was used for space debris measurement campaign, the system has been recently upgraded with a dedicated channel for space debris monitoring that comprises a dedicated backend with its acquisition chain [13; 14]. The data obtained from the measurement campaigns are used as input for orbit determination algorithms to estimate the orbital state of the detected object, and consequently update the database of known objects. Starting from an already available estimate of the state of the transiting object, the recorded measurements are then used in sequential or batch filters to obtain a refined estimate of the object state vector [15]. After a new estimate of the apparent space debris trajectory is obtained, the subsequent motion can be predicted for the observations. This prediction is unavoidably affected by errors, due to possible inaccuracies in the estimated state vector (i.e., position and velocity vector) caused by errors in the orbit determination process, such as approximations involved in the method of orbit improvement and in the mathematical model, errors in the observations and errors in the computational procedure used in the solution process. Consequently, the process of observation and estimation must be repeated continuously as the space debris motion evolves. Several orbit determination algorithms can be used, and different commercial software is available. Among these we can mention the AGI's Orbit Determination Tool Kit (ODTK) [16] and the ExoAnalytic's EigenOrbitFitter [17]. This paper proposes the application of an Unscented Kalman Filter for orbit determination refinement using BIRALET sensor. The paper is organized as follows. Section 2 illustrates the main characteristics of the SRT as a receiver in the BIRALET system, focusing on data produced for space debris dedicated measurement campaigns. Section 3 shows a preliminary analysis of the performance of the sensor in terms of both observation capabilities and orbit determination accuracy obtained with dedicated numerical simulations.