The M21 RSCAAL is a two-man portable, automatic scanning passive infrared sensor which detects nerve and blister agent vapor clouds based on changes in the infrared energy emitted from remote objects, or from a cloud formed by the agent. The M21 RSCAAL is a stand-alone tripod mounted, chemical agent overwatch system. It consists of a detector, tripod, - transit case, power cable assembly, and standard military power source, capable of scanning horizontally 60 degrees. It will be used for reconnaissance and surveillance missions to monitor avenues of approach and egress, bridges, road junctions, and other point targets to search areas between friendly and enemy forces for chemical agent vapors, and to provide advanced detection and warning of chemical hazards. Where possible, the M21 RSCAAL will be employed in pairs (2 reconnaissance teams) so that one M21 RSCAAL can be used in an overwatch position when the other reconnaissance team is moving.