Abstract

There is a great need for atmospheric trace constituent measurements with higher resolution than attainable with passive radiometers. Infrared (IR) differential absorption lidar (DIAL), which depends on Mie scattering from aerosols, has special advantages for tropospheric and lower stratospheric applications and has great potential importance for measurements from Shuttle and aircraft (refs. 1,2,3, and 4). DIAL data reduction involves comparing large amplitude signals which have small differences. The commonly used SNR expression (signal divided by noise in the absence of signal) is not adequate to describe DIAL-measurement accuracy and must be replaced by an expression which includes the random coherent (speckle) noise within the signal (refs. 5 and 6). A comprehensive DIAL computer algorithm for heterodyne and direct detection including speckle noise is used to calculate DIAL measurement sensitivities. Results are presented for measurement of ground-level plumes using direct and heterodyne detection and distributed ambient trace gases using heterodyne detection.