that come from non-adjacent beams but with similar time and frequency. When a signal comes from a point in space, it can be received by one beam and maybe an adjacent beam. However, terrestrial RFI signals are often picked up in multiple beams simultaneously. In this algorithm, each hit has a time and frequency box. If the box of one hit is in another hit’s box and hits are in non-adjacent beams, they are both marked as RFI.

Pipeline of RFI mitigation {#sec:4.3}
--------------------------

Section \[sec:4.1\] and \[sec:4.2\] demonstrate three kinds of RFI and their remove methods. In Nebula, we don’t remove these three kinds of RFI separately, because narrow-band RFI may probably be multi-beam RFI. We just mark the hit instead of removing it when the hit is selected by any one of three methods in Nebula. Finally, the hit with one or more RFI labels will be removed after going through all three kinds of methods. Figure \[fig:20\] shows the processing pipeline of Nebula. [**When the data was taken for this paper, real time pointing information was not available from the FAST telescope. For this early test data, telescope position information was calculated and merged with the SETI 