Modeling the effects of aerosols on transmission measurements at Zuniga Shoal, California

An infrared (IR) signal propagating along a 'line-of-sight' horizontal or slant path within the marine surface layer can encounter substantial perturbations. These perturbations include signal extinction due to molecules or aerosol particles; refractive modulations that can amplify or reduce a signal; and scintillation, which is a higher frequency fluctuation in signal intensity. In an effort to elucidate these issues an infrared transmission link was included in a long term propagation field experiment conducted at Zuniga Shoal to study the effects of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various times of the year. The transmission path was 7.2 km long connecting an IR broadbeam transmitter (at about 6.5 m ASL) at the Naval Amphibious Base at Coronado, and an IR telescope receiver at Zuniga Shoal (at about 11.5 m ASL). Both locations are in the general San Diego area. The transmission measurements were made at two wavelengths: near-IR, centered at 1.061 μm and short-wave IR, centered at 1.622 μm. In this paper we discuss prominent features of the long-term measurements including diurnal variations and the effects of the marine layer. We also compare the field measurements with the extinction predictions generated by the Advanced Navy Aerosol Model (ANAM), and we discuss how long-term field measurements can be used to tune and correct the ANAM.