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 (SFO) 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.
Free-space optical (FSO) links for high-speed communications between buildings must consider detrimental environmental effects including interference from sunlight in the receiver's instantaneous field of view (IFOV). Sunlight can degrade receive sensitivity resulting in link disruptions, even with significant optical filtering. Thus it is important to characterize this environmental effect for designing and testing optical transceivers. Background light levels are highly dependent on the geometry and environmental conditions of a specific link making general statements difficult. However, we have characterized the likelihood and frequency of direct or reflected sunlight passing into or near a terminal's IFOV. We have also measured detector solar power levels under sunny and partly cloudy conditions, and measured detector sensitivity degradation as a function of background light levels. This paper presents a summary of our results.
Free-space optical links for high-speed network communications between buildings must consider the detrimental environmental effects of terminal base disturbances. Terminal base motion results in tracking and pointing losses, which cause link outages if the base motion is sufficiently large (beyond terminal field of regard) or fast (too fast for the tracking system to reject). Thus it is important to characterize this environmental effect for design and test of optical terminals. Base motion is highly dependent on the installation environment of a specific link making general statements difficult. We have characterized terminal base disturbance levels through a combination of vibration measurements in numerous buildings, data gathered from operating links, and review of building stiffness and wind statistics in various cities. This paper presents a summary of our results.
Meteorological visibility data are the most commonly used data to estimate terrestrial Free Space Optics (FSO) availability in a given city. Visibility data can be used to estimate transmission efficiency at desired IR wavelengths using a semi-empirical equation, and are often archived over many years allowing the calculation of long-term averages of availability. However, these data are taken at near-surface levels (historically within a few meters of the surface) and are therefore only appropriate for estimating FSO availability near the surface. Examination of long term cloud observations, including percent frequency of cloud ceilings occurring at various heights above the ground, show the importance of including low clouds into the consideration of FSO availability for any situation above about 30-m above ground level (AGL). In most locations, low clouds occurring very near the surface are relatively common -- more so than surface-based fog (which is measured in terms of visibility). Thus, FSO availability will decrease with height, sometimes dramatically, in most cities. Cloud data is also archived over long periods of record and can thus be used to calculate long-term averages of availability.