<title>Atmospheric effects on low-elevation transmission measurements at EOPACE</title>

Abstract

An analysis is presented showing the effects of refraction, aerosol extinction, and molecular extinction on transmission measurements obtained during the EO Propagation Assessment in Coastal Environments (EOPACE) campaign carried out in San Diego during March and April 1996. Infrared transmission measurements were made over both a 7 km path (mid IR) and a 15 km path (mid JR and far IR) at heights below 10 m above sea level. The average difference between all the measured transmissions and aerosol transmittances over the two paths with results obtained using the JR Boundary Layer Effects Model (IRBLEM) were found to be relatively small, even though the difference for individual measurements can be significant. The effect of molecular transmittance, as calculated using MODTRAN, is found to reduce the transmission by about 35% forthe 7 km path, 72% for the mid JR over the 15 km path, and between 70% and 90% for the far JR over the 15 km path.The effect of aerosol transmittance, as calculated using a variation of the Navy Aerosol Model (NAM), is found to reduce the transmission from 10% to 90% for the mid JR over both the 7 and 15 km paths, and from 10% to 60% for the far JR over the 15 km path. The effect of refractance, the focussing and defocussing of radiation due to atmospheric refraction, on the predicted transmissions is found to account for gains and losses up to 20% for the 7 km path, and gains and losses up to 100% for the 15 km path. Consequently, any JR transmission model for the marine boundary layer (MBL) must properly take into account the effects on the transmission due to molecular extinction, aerosol extinction, and refractance.