the image shown in Fig. \[fig:image\]. The large scale features have been removed and the fluctuation is reduced by one order of magnitude. []{data-label="fig:FGRM"}](./Fig_paper/MWA_image_removed.eps){width="7cm"}

![[The squared error of the polynomial foreground removal with $n=7$. In the center of EoR0 field, the polynomial fitting works properly. ]{}[]{data-label="fig:chi2"}](./Fig_paper/MAP_chic7.eps){width="7cm"}

Addressing Systematics {#S:system}
======================

[ 21cm observations with interferometers suffer from systematics such as refraction due to electrons in the Earth’s ionosphere, instrumental systematics, and intrinsic foreground spectra. Thus, a better understanding about the systematics of the data is worthwhile for future further analysis. In this section, we study the effect of ionosphere, pointing of observation and frequency dependence of the CPS by comparing with that of the APS. ]{}

Pointing Dependence
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The MWA observation is 3 hour continuous observation around zenith, and the signal can be calculated as a function of the Local Sidereal Time (LST). Furthermore, the observation strategy is the drift and shift observation, and the direction of pointing changes 5 times toward the center of the EoR0 field. Fig. \[fig:APSLST\] shows the LST dependence of the APS calculated from 90 visibility data sets. The colors represent the group of pointing. The right panel shows that the APS at 168 