  Since autumn 2008 a new L-band 7-Feed-Array receiver is used for an HI 21-cm
line survey performed with the 100-m Effelsberg telescope. The survey will
cover the whole northern hemisphere comprising both, the galactic and
extragalactic sky in parallel. Using state-of-the-art FPGA based digital Fast
Fourier Transform spectrometers, superior in dynamic range and temporal
resolution, allows to apply sophisticated radio frequency interferences (RFI)
mitigation schemes to the survey data.
  The EBHIS data reduction software includes the RFI mitigation, gain-curve
correction, intensity calibration, stray-radiation correction, gridding, and
source detection. We discuss the severe degradation of radio astronomical HI
data by RFI signals and the gain in scientific yield when applying modern RFI
mitigation schemes. For this aim simulations of the galaxy distribution within
the local volume (z<0.07) with and without RFI degradation were performed.
These simulations, allow us to investigate potential biases and selection
effects introduced by the data reduction software and the applied source
parametrization methods.

Instrumentation and Methods for Astrophysics;Astrophysics of Galaxies