They can be used from the ground (in a vehicle or on a tripod) to scan a plume
, or can be mounted to an aircraft and flown traverses underneath the plume at right angles to the direction of plume travel. DOAS
instruments are also mounted on certain satellites orbiting the Earth. These instruments provide global coverage but can only detect relatively large plumes. Today, DOAS
instruments have replaced the reliable but dated COSPEC (Correlation Spectrometer) technology at most observatories. At many active volcanoes, permanent scanning DOAS instruments are mounted downwind of the vent
and measure the SO2 emission
rate in near-real time.