Three Years of Station Data for Free-Space Optical Link Design in Southern Iraq: Margin Saturation and Year-to-Year Reversal of the Dominant Outage Mechanism

Abstract

Free-space optical (FSO) links in the Arabian Gulf are usually designed against dust, and the region is often treated in the literature as a worst case. This paper tests that assumption against three complete years of hourly surface observations from Basra International Airport (ORMM, WMO 40689), covering 22,982 hours with a valid visibility report between 2023 and 2025. Applying the Kruse extinction model to a 2 km horizontal link at 813 nm, pooled availability at a 20 dB link margin is 99.13%. Three results follow. First, availability saturates with margin: raising the margin from 20 dB to 50 dB recovers less than one percentage point in every year (0.59, 0.20 and 0.75 points), because the visibility distribution is strongly bimodal. Second, the aggregate is stable while its cause is not — dust accounted for 0% of lost hours in 2023, 59% in 2024 and 45% in 2025, with fog supplying the remainder; a design justified by a single mechanism would have been misdirected in at least one of the three years. Third, the obscuration classes carry a repeatable wind signature: dust hours are windier than the annual mean in all three years (ratios 1.28, 1.72, 1.46) while haze hours are calmer in all three (0.66, 0.66, 0.76), which sets the fade correlation time by mechanism rather than by season. A negative result is also reported: the choice between 813 nm and 1550 nm is worth 2.3-3.4 dB at the median obscured hour under Kruse and exactly 0 dB in the hours that actually cause outage, where the Kruse exponent is zero by construction; and substituting a dust-appropriate exponent for Kruse's changes annual availability by less than 0.4 percentage points. The wavelength and size-distribution questions that dominate the regional FSO literature are therefore close to irrelevant to link availability at this range and site. Code: PolyForm Noncommercial 1.0.0. Commercial rights retained by the author.