  One of the Survey Science Projects that the Australian Square Kilometre Array
Pathfinder (ASKAP) telescope will do in its first few years of operation is a
study of the 21-cm line of HI and the 18-cm lines of OH in the Galactic Plane
and the Magellanic Clouds and Stream. The wide-field ASKAP can survey a large
area with very high sensitivity much faster than a conventional telescope
because of its focal plane array of receiver elements. The brightness
sensitivity for the widespread spectral line emission of the interstellar
medium depends on the beam size and the survey speed. In the GASKAP survey,
maps with different resolutions will be synthesized simultaneously; these will
be matched to different scientific applications such as diffuse HI and OH
emission, OH masers, and HI absorption toward background continuum sources. A
great many scientific questions will be answered by the GASKAP survey results;
a central topic is the exchange of matter and energy between the Milky Way disk
and halo. The survey will show how neutral gas at high altitude (z) above the
disk, like the Magellanic Stream, makes its way down through the halo, what
changes it experiences along the way, and how much is left behind.
