Hamed Nosrati, CSIRO Space & Astronomy; Stephanie Smith ,CSIRO Space & Astronomy; Douglas B. Hayman, CSIRO Space and Astronomy; Shinji Horiuchi, CSIRO Space & Astronomy; Andrew Hellicar, CSIRO Data61; Ken Smart, CSIRO Space & Astronomy
Keywords: Space Situational Awareness, Direction of Arrival Estimation, Radio Interferometry, Space Debris
Abstract:
Space debris comprises artificial objects left in the Earth’s orbit across more than six decades of space activities. The increasing growth of the orbital debris population exhibits a critical risk for existing and upcoming space missions. As a result, there has been an ever-growing interest in expanding the network of available sensors to detect, localize and characterize the space debris in orbit about the Earth to increase and maintain Space Situational Awareness (SSA). Currently, optical, laser ranging and radar sensors constitute the backbone of SSA networks. Complementary to the existing sensors, radio astronomy interferometers featuring extremely high sensitivity, narrow beamwidth, and wide bandwidth offer an excellent opportunity for SSA activities.
Considering radio observations for orbit determination, a dedicated mono-static or bi-static radar can potentially provide the range, velocity (Doppler), and direction information. On the other hand, a radio interferometer can only provide direction and rough range estimates. An example of such a dedicated radio interferometer is the now retired US Navy Space Surveillance System (NAVSPASUR), colloquially known as space fence, which employed radio interferometry using three transmitters and six receivers. While SSA-dedicated radio interferometers have not been commonly used to provide SSA data products, repurposing existing assets such as radio telescopes allows access to additional SSA data. Radio telescopes have already been employed to provide SSA data products in single dish mono-static radar mode or multi-element passive bi-static radar mode, mainly using processed raw voltages from each antenna.
In this work, we study the performance of a correlator radio telescope array for providing SSA data products. We develop a system model for interferometry-based localization and present the results based on a series of experiments for Resident Space Object (RSO) location refinement in a bi-static interferometry configuration using NASA’s Deep Space Network (DSN) facility located at Tidbinbilla in the Australian Capital Territory (ACT) Australia, and Australia Telescope Compact Array (ATCA) at the Paul Wild Observatory near Narrabri in New South Wales (NSW), Australia.
Two data products of interest from interferometric radio telescopes are 1) raw voltages or baseband signal at each antenna and 2) visibilities or spatial correlation lags at the interferometer’s output. In order to devise a generic method applicable to interferometry systems, we use visibilities as they are the standard data output of most radio astronomy interferometers. We develop a signal and system model for estimating spatial spectrum, i.e., Direction of Arrival (DOA) and Range from the visibilities. In particular, we use the interferometry equations and apply offline re-focusing to focus the visibilities, delay-tracked for a solid angle in the far-field, on a grid of ranges near the location of interest. Employing re-focusing, we establish a measurement vector with right ascension (RA), declination (DEC), and range (Range) being the unknown variables. We then use the measurement vector in a Multiple Signal Classification (MUSIC) estimator to estimate RA, DEC, and range offsets of the source of interest.
We carried out a measurement campaign in June 2021 to demonstrate the capability of the proposed system model. Various pieces of orbital debris in Geostationary orbit (GEO) and Medium Earth Orbit (MEO) were illuminated by a 36-meter DSN transmitter, and the corresponding reflected signal’s spatial correlation was recorded by ATCA. We employ the proposed technique on the recorded data and provide preliminary estimates for direction and range. Using the estimated location datapoints, we investigate the measurement system’s performance by analyzing the stability of the phase of the visibilities and show the phase precision, which is an indicator of the accuracy of the interferometer’s expected coordinate, improves significantly after using the estimated datapoints.
Date of Conference: September 27-20, 2022
Track: Astrodynamics