Visibility in low clouds and its impact on FSO links

Abstract

Fog and low clouds are the two atmospheric elements with the greatest impact on the performance of a free space optical (FSO) network. Predicting the effects of low clouds and ground based fog on FSO equipment performance is a challenging exercise. Usually, surface visibility records from airports in proximity to the deployment area are used to calculate the link availability. However, very little data are available on visibility within clouds, which have a larger impact on elevated links. To estimate the visibility in low clouds we have deployed visibility sensors at three different heights (33, 119, 188 meters above mean sea level) and a ceilometer in San Francisco from June to October of 2001.The data collected show substantial difference between the visibility reported at San Francisco International Airport (SF0) and the visibility recorded by our sensors in downtown San Francisco. More importantly, the data indicate a greater prevalence of low clouds downtown than at the airport.

Atmospheric Laser Communication

“…Improved understanding of optical extinction conditions in urban areas may lead into several areas that have been explored. These include fog typing (Byers 1959;Baars et al 2002a), fog trends , the nature of cloud bases and cloud vertical extent (Al-Habash et al 2002;Fischer et al 2003), the effects of scintillation on transmission, and even satellite-derived products to model local elevated fog and low-cloud occurrence (Ellrod 1995;Fischer et al 2001). Despite this work, much remains to be done.…”

“…Once above the polluted surface layer, a smaller number of pollution particles exist, and optical conditions improve. Some progress has been made in measuring the attenuation and vertical depth of low clouds; but, for most locations, assumptions must be made and then tested empirically (Al-Habash et al 2002).…”