  The circumgalactic medium (CGM) of the Milky Way is mostly obscured by nearby
gas in position-velocity space because we reside inside the Galaxy. Substantial
biases exist in most studies on the Milky Way's CGM that focus on
easier-to-detect high-velocity gas. With mock observations on a Milky-Way
analog from the FOGGIE simulation, we investigate four observational biases
related to the Milky Way's CGM. First, QSO absorption-line studies probe a
limited amount of the CGM mass: only 35% of the mass is at high Galactic
latitudes $|b|>20$ degrees, of which only half is moving at $|v_{\rm
LSR}|\gtrsim100$ km s$^{-1}$. Second, the inflow rate ($\dot{M}$) of the cold
gas observable in HI 21cm is reduced by a factor of $\sim10$ as we switch from
the local standard of rest to the galaxy's rest frame; meanwhile $\dot{M}$ of
the cool and warm gas does not change significantly. Third, OVI and NV are
promising ions to probe the Milky Way's outer CGM ($r\gtrsim$15 kpc), but CIV
may be less sensitive. Lastly, the scatter in ion column density is a factor of
2 higher if the CGM is observed from inside-out than from external views
because of the gas radial density profile. Our work highlights that
observations of the Milky Way's CGM, especially those using HI 21cm and QSO
absorption lines, are highly biased. We demonstrate that these biases can be
quantified and calibrated through synthetic observations with simulated
Milky-Way analogs.
