BACIMO ’05 Conference Session 8: EM/EO Propagation Monterey, CA, 14 October 2005 Results on the Bulk Modeling of Cn2 Over the Ocean from an Ongoing Experiment Paul Frederickson and Kenneth Davidson Naval Postgraduate School Monterey, CA Stephen Hammel and Dimitris Tsintikidis Space and Naval Warfare System Center San Diego, CA
Cn2 – statistical measure of refractive-index fluctuations caused by atmospheric turbulence (optical turbulence). The critical parameter in quantifying the effects of optical turbulence on EM/EO propagation: Scintillation: high-frequency signal intensity variations Image degradation: image blurring, image “dancing” Laser beam distortion: beam spread, beam wander Above effects can degrade target detection/identification, intelligence collection, laser target designation, potential HEL use for ship-defense & other military applications. What is Cn2 & why is it important?
Typical Optical Scintillation Measurements Cn2 = 1 10–14 Cn2 = 1 10–16
“Bulk” models estimate Cn2 from mean meteorological measurements (U, Tair, Tsea, RH, Pr). Mean met measurements much easier and cheaper to obtain than single-point turbulence or optical propagation measurements of Cn2. Bulk models can be used to construct Cn2 climatologies from historical meteorological data sets. Can be used in numerical weather prediction models to produce forecasts of Cn2 conditions. Can be used in operational Tactical Decision Aids (TDA’s) with real-time, in situ measurements to optimize optical system performance for current environment (e.g. EOSTAR). Importance of Bulk Cn2 Models
To determine how near-surface scintillation over the ocean varies with mean atmospheric and sea surface conditions. To determine the accuracy of the current NPS bulk Cn2 model under different environmental conditions. To develop improvements in the current bulk Cn2 model. Objectives of the Study
The Experiment • Ongoing experiment conducted off San Diego, began late 2004. • SSC-SD measured IR scintillation along 7 km propagation path over San Diego Bay. • NPS buoy measured mean meteorological data, turbulence, sea surface temperature and wave conditions along the propagation path. • Examined data only from Intensive Observation Period in May 2005.
Experiment Area & Measurement Points IR Detector 7.2 km path NPS Buoy IR Source
Bulk: Estimated Cn2 from mean meteorological measurements (U, Tair, Tsea, RH, Pr) on NPS buoy using NPS bulk model. Turbulent: Estimated Cn2 from single-point sonic temperature fluctuation measurements (10 Hz) on NPS buoy. Scintillation: SSC-SD determined Cn2 from measured amplitude fluctuations in near-IR (1.06 m) signal transmitted along 7.2 km path over San Diego Bay. Cn2 Determined by Three Methods
Scintillation Cn2 vs T: Wind Speed Variations
Turbulent CT2 EOPACE Scintillation Scintillation and Bulk Cn2 vs T: Wind Speed Variations Scintillation Cn2
Scintillation and Bulk Cn2 vs T: Wind Speed Variations Scintillation Cn2 Bulk Cn2
Scintillation-derived Cn2 values vary systematically with T, and wind speed; little dependence upon relative humidity. Bulk model Cn2 estimates agree well with scintillation measurements in unstable conditions, underestimate scintillation measurements in near-neutral conditions and do not agree as well in stable conditions. Air-sea temp difference & wind speed dependence in bulk-scintillation Cn2 comparisons indicate empirical stability functions used in bulk model need improvement, especially in stable conditions. Will have several more months of data to examine in future to determine better comparison statistics and hopefully improvements to bulk model. Conclusions
“Bulk models” developed by finding empirical relationships between mean atmospheric parameters and turbulent fluxes from past experiments and assuming these relationships hold in all cases. Bulk Cn2 Estimates Possible Wave Influence? Empirical Relationships Mean Atmospheric Parameters