RFI mitigation of GNSS signals for radio astronomy: problems and current techniques

Abstract

Mitigation of man-made radio frequency interference (RFI) is a problem of increasing importance for radio astronomy. Indeed, numerous terrestrial and satellite services of all kinds create lots of electromagnetic signals in a large part of the radio spectrum that are likely to disturb radio astronomical observations. Moreover, the advent of large, highly sensitive radio telescopes such as SKA, LOFAR, etc., will permit the observation of extremely faint and distant radio sources which exhibit large amounts of red shift, hence overlapping with unprotected bands in the radio spectrum. Among various possible causes of RFI, GNSSs have specific characteristics which can be handled appropriately to design efficient mitigation techniques. GNSS signals are generally well documented and the ephemerides of satellites are well known. Therefore, various partially or fully informed methods can be used, which exploit knowledge of the characteristics of the modulations. Adaptive cancellation techniques can also be used that use auxiliary observations coming from additional antennas. Besides, knowing the spatial direction of the disturbing source, spatial filtering and related techniques that can be implemented in antenna arrays are likely to mitigate RFI with minimum knowledge of the GNSS. After recalling some radio astronomy basics, the paper presents a few examples of RFI caused by satellite systems on radio-astronomical observations. Then, we give a brief overview of state-of-the-art RFI mitigation techniques for radio astronomy and we present in some details the principles and results pertaining to some methods that seem particularly appropriate to mitigate RFI arising from GNSS signals. The importance of cooperation between GNSS designers and astronomers is finally pointed out.