Ginga's All-Sky Monitor (ASM) was designed to compliment the main science instrument. The ASM consisted of 2 identical proportional counter units, each filled with a mixture of xenon and carbon dioxide gasses.
The LAC was designed to make detailed, focused observations of a single source. Conversely, the ASM was designed to make less-detailed observations of a wide swath of the sky – it could see about 70% of the sky at a time. In this way, the ASM was able to act as an “alarm,” telling astronomers when a transient X-ray event was occurring. Without a wide-field telescope like the ASM, astronomers wouldn't know about such events unless a telescope just happened to be pointing toward the event.
