in a bin. For the FAST data, we set the probability to be $1\times 10^{-7}$, so we can get a threshold value $x$ from the function.

### Drifting RFI {#sec:4.1.2}

Drifting RFI is narrow-band RFI that drifts in frequency, mainly coming from mobile devices. We can not find them with zone RFI algorithm, because the frequency of signals is changing over time. Figure \[fig:4\] is a typical drift RFI from FAST data.

Due to its changing frequency, we cannot simply remove it by frequency channel. In Nebula, we make two symmetrical triangles for each hit. As shown in Figure \[fig:9\], the shape of the triangle is determined by drift rate and time, which are set empirically. For FAST data, we set drifting rate to 20 Hz/s and $\Delta$t to 600 s. Then we can separate the triangle into 21 bins. If the number of signals in each bin and its opposite three bins is above the threshold, we mark all the signals in the bins as drifting RFI. The threshold is set in the same way as Section 4.1.

Multi-beam RFI {#sec:4.2}
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When we use the multi-beams receiver such as the 19 Beams from FAST, we can identify signals 