Mitigating the Impact of GNSS Signals in the Radio Astronomy Band 1610.6-1613.8 MHz

The band 1610.6-1613.8 MHz is used by the radio astronomy (RA) community in order to study the spectral line of the Hydroxyl (OH) radical transitions. This radical is considered to be one of the most important lines for the observation of physical phenomena associated with the formation of protostars and the initial stages of star formation. The current ITU recommendation to protect this RA band is to ensure an Equivalent Power-Flux Density (EPFD) lower than -258 dBW/m/20kHz. Radio Frequency Interference (RFI) has, however, been reported in this band that significantly disturbs the exploitation of OH spectral lines. Examples of these interference are GLONASS L1 signal and Iridium [Monstein and Meyer, 2007; ERC, 1997; Galt, 1991]. In order to be immunized from these sources of disturbance, different antenna or signal processing techniques have been investigated and used by the RA community. Cooperation with GLONASS was also used in order to reach an agreement satisfying both parties. The main reason for GLONASS to provide unwanted interference in the RA band is its close proximity with this band. Although further away from the RA band than GLONASS, Galileo is still relatively close to it (15 MHz only). In particular, the ITU recommendation on the RA band implies a fairly steep slope to the Galileo E1 payload filter, which represents a high constraint for the realization of the payload. Consequently, it might result in imperfect filtering of the signals, potentially leaking in the RA band. It is then interesting to develop a method that could mitigate the effect of GNSS signal in the RA band, in order to ensure that after using this method, the level of the interference PFD is below the ITU recommendation. This is the goal of this paper, which introduces a general method to mitigate GNSS signals (using Code Division Multiple Access (CDMA) technique) from the RA band. In particular, the method is adapted to signals that have an open signal and a restricted signal, with is generally the case for GNSS signals. The method is based on the estimation of the GNSS signal, followed by its subtraction from the incoming signal in order to just leave the RA signal of interest. The paper particularly emphasis the adaptation of this method to the Galileo E1 signal, showing a reduction of approximately 10 dB of the maximum level of the expected Galileo PFD within the RA band.