Calibration of the OLFAR space-based radio telescope using a weighted alternating least squares approach

Radio astronomy below 30 MHz has never been properly performed because the ionosphere inhibits this on Earth. The Orbiting Low-Frequency Antennas for Radio Astronomy (OLFAR) project is aimed at building a radio telescope consisting of 50 or more nano-satellites in space, forming a swarm-like array. The observational antenna systems need to be calibrated to successfully detect astronomical signals. However, a satellite swarm presents a unique calibration challenge, which is outlined in this paper. An approach is proposed for the calibration of several important system parameters, based on (weighted) alternating least squares (ALS/WALS). In the proposed method, polarization is taken into account without the common simplifications, such as only considering fully linear polarization. For validation, Monte Carlo simulations were performed to verify that the calibration works. The results are compared with the Cramer-Rao bound, which is reached by WALS if the receiver noise is low. Otherwise ALS is more accurate. The effect of the quality of the initial guess is analyzed, and it is found to impose a limit on the calibration accuracy. The severity of the two effects mentioned above depend, among others, on the specific parameter under calibration.