RFI in Nebula).

### Zone RFI {#sec:4.1.1}

Zone RFI is narrow-band RFI exhibiting a stable frequency which persists throughout most or all of the total data set. These “zones” become global exclusion filters. There are many sources of such interference, including television, radio broadcasts, cell phone, and satellite signals. As shown in Figure \[fig:3\], compared with the distribution of the hits we expected, the zone RFI in the middle is more concentrated, forming a vertical narrow band. The existence of such RFI seriously affects the extraction of candidate targets.

For zone RFI, we examine each frequency bin and calculate the number of hits in this frequency bin. If the number reaches above the threshold, all the hits in this bin can be marked as zone RFI and removed. The threshold is set by the Poisson cumulative probability function. The Poisson cumulative distribution is used to model the number of events occurring within a given time interval. Poisson cumulative distribution probability function can be expressed as: $$F(x;\lambda) = \sum_{i=0}^{x}\frac{e^{-\lambda }\lambda ^{i}}{i!},$$ where $\lambda$ is the mean number of hits in a bin, and x is the number of the hits. This function can give the probability of the number of hits 