The Italian biralet radar system to perform range and range rate measurements in the eusst European space surveillance and tracking program

Abstract

Space debris is a term for all human-made objects orbiting the Earth reentering the atmosphere. These objects have variable sizes and shapes and they can include non-functional satellites and spacecraft, abandoned launch vehicle stages and fragmentation debris. The population of space debris is continuously growing and it represents a potential issue for active satellites and spacecraft. New collisions and fragmentation could exponentially increase the amount of debris and so the level of risk represented by these objects. The monitoring of space environment is necessary to prevent new collisions and consequently to offer collision avoidance services. For this reason, radar measurements are relevant, in particular to observe objects in Low Earth Orbit, between 200 and 2000 km of altitude, the most crowded orbit with more than 24,000 trackable objects with sizes greater than 10 cm. In this paper, we present the design of the Italian BIRALET system, an acronym which stands for BIstatic RAdar for LEo Tracking. It is a bi-static radar used for space debris detection and tracking in Low Earth Orbit. The transmitter is the so called Radio Frequency Transmitter (TRF), a fully steerable parabolic antenna with a 7 m diameter dish, with a primary focus configuration, located inside the Military Italian Joint Test Range in the region of “Salto di Quirra” (Cagliari, Sardinia, Italy). The TRF has a set of powerful amplifiers able to supply a transmitting RMS power between 1 and 10 kW in the bandwidth of 410-415 MHz. The receiver system of the BIRALET is the Sardinia Radio Telescope (SRT), located in San Basilio, around 35 km from Cagliari (Sardinia, Italy), and with a baseline from the TRF of about 20 km. It is a flexible instrument designed for radio astronomy studies and space science, which recently was also utilized for space debris monitoring. The antenna is a multi-reflector system with a quasi-Gregorian configuration, with a 64-meter parabolic primary mirror and a 7.9-meter elliptical secondary mirror. The telescope has three other mirrors (two with a diameter of 2.9 m and one with a diameter of 3.9 m), which provide the Beam Wave-Guide (BWG) system. The telescope is able to host up to 20 remotely controllable receivers and to observe the sky with high efficiency in the frequency range between 0.3-116 GHz. The front-end used for space debris monitoring is the P-band receiver of the SRT, which is installed in the primary focus of the telescope. In fact, the receiver is a cryogenically cooled dual-band coaxial feed that simultaneously covers the L-band (1300 - 1800 MHz) and the P band (305 - 418 MHz) frequency range. The back-end of the BIRALET system, funded and developed under the European Space Surveillance and Tracking program, is based on the National Instrument USRP-2954R board. The electronic board in the TRF is configured to transmit a mixed signal composed from a chirp plus a continuous wave tone, with an overall bandwidth of 5 MHz. In this way, it is possible to perform range, with a spatial resolution of few tens of meters, and range rate measurements, with a bandwidth resolution of about 10 Hz. The overall performances of the radar are also determined by the synchronization accuracy between the TRF and the SRT. The receiver and the transmitter are synchronized in time and frequency using the GPS 1PPS (pulse per second) and the 10 MHz reference signals. An integrated GPS receiver in the USRP-2954R guarantees a maximum error of 10-7 seconds. In this paper, we present a preliminary measurement campaign of detection of known objects, for which it is possible to estimate range rate and range, in order to establish the performance of the system and in particular of the dedicated back-end. Thanks to range and range rate data collected by the BIRALET system in the measurement campaign (using a dedicated orbit determination software), it is possible to improve the knowledge of the orbit of the observed space debris and consequently to update our database of known objects.