Atmospheric turbulence parameters are measured routinely across the Chesapeake Bay. The 16.2km propagation path is slightly sloped, with the laser transmitter at a height of 30m while the receivers are at 5m above sea level. We measure angle-of-arrival variance to give Cn2 and irradiance variance over various apertured receivers to give σI2.The Navy Atmospheric Vertical Surface Layer Model (NAVSLaM) is used to predict Cn2 that is based upon prevailing meteorological parameters. Comparison with measured Cn2 over extended time periods shows reasonable correlation. A value of the inner scale of turbulence l0 is also predicted from NAVSLaM. This paper looks at the significance of using Cn2 and l0 from NAVSLaM in standard analytical models of scintillation.
Atmospheric boundary layer optical turbulence has a strong, and often dominant, impact on laser system performance. Optical turbulence is normally quantified by the refractive index structure parameter (C2n ). Numerical weather prediction (NWP) models do not produce direct forecasts of C2n , but gridded, multi-level forecasts of standard meteorological parameters, such as wind speed, air temperature, humidity, pressure and sea surface temperature, can be input to atmospheric optical turbulence models to indirectly forecast three-dimensional C2n conditions from the surface up through the troposphere. This paper describes methods and issues involved with integrating the Naval Postgraduate School’s Navy Atmospheric Vertical Surface Layer Model (NAVSLaM) and Tropospheric Optical Turbulence Ensemble of Models (TOTEM) with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data fields to produce C2n forecasts for laser performance predictions.
The performance of imaging and laser systems can be severely degraded by atmospheric turbulence, especially
for near-horizon propagation paths. Having the ability to predict turbulence effects from relatively easily obtained
measurements can be useful for system design and feasibility studies, and for real-time optimization of optical systems
for the current environment. For this reason, so-called 'bulk' models have been developed that can estimate turbulence
effects through the refractive index structure parameter (Cn2) from mean near-surface meteorological and sea surface
temperature measurements. Bulk Cn2 models are directly dependent upon empirically determined dimensionless
functions, known as the dimensionless structure parameter functions for temperature and humidity. In this paper we
attempt to improve bulk optical turbulence model performance by determining new over-ocean forms for the
dimensionless temperature structure parameter (T).
During 2005-2006 atmospheric propagation experiments were conducted in the Zuniga Shoals area near San
Diego to examine the impact of environmental conditions on low-altitude electro-optical propagation above the ocean
surface. As part of this experiment the Naval Postgraduate School (NPS) deployed its flux research buoy along the
propagation path. The measurements obtained on the NPS buoy enabled T values to be obtained and new functions to
be determined. These new functions differ greatly from those presented in the past, in that the new T values asymptote
towards very high values as the stability approaches neutrality. The dependence of the new T function on the stability
parameter in stable conditions was also different from that previously proposed. When these new functions were
inserted into the NPS bulk Cn2 model, the resulting values agreed much better with directly measured turbulent Cn2
values in unstable conditions, but in stable conditions the new function actually made the agreement worse.
Turbulence can be a dominant factor in image and laser beam degradation for optical systems operating in the near-surface maritime environment. A long-term propagation field experiment was conducted at Zuniga Shoal (near San Diego) to study the impact of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various points of the year, during which scintillometer measurements were obtained along a 7.2 km over-water path and a 'flux' research buoy deployed along the propagation path collected concurrent mean meteorological, atmospheric turbulence, and wave data.
We use the refractive index structure parameter (Cn2) as the critical parameter for quantifying the effects of atmospheric turbulence on laser system performance, including received power fluctuations, beam spread and beam wander. Bulk estimates of Cn2 were derived from the buoy mean meteorological measurements using the Navy Surface Layer Optical Turbulence (NSLOT) model. Cn2 was also determined from atmospheric turbulence measurements obtained from a sonic anemometer on the buoy. These independent Cn2 values derived from the buoy data are compared with Cn2 values computed from the infrared propagation measurements to determine how the NSLOT model performs under different environmental conditions. In addition, the optical measurements and bulk estimates of Cn2 are used to study the effects of the atmospheric turbulence on operational optical systems.
Infrared scintillation measurements were obtained along a 7.2 km path over San Diego Bay, concurrently with mean meteorological and turbulence measurements obtained from a buoy located along the path. Bulk estimates and turbulence measurements of Cn2 were computed from the buoy data and compared with the optical scintillation-derived Cn2 values. Similar to the results of previous experiments, the bulk Cn2 estimates agreed well with both the scintillation and turbulence measurements in unstable conditions, increasingly underestimated Cn2 as conditions approached neutral, and agreed less well with scintillation and turbulence Cn2 values in stable conditions. The mean differences between bulk Cn2 estimates and both the turbulence and scintillation measurements when conditions were not near-neutral exhibited an air-sea temperature difference and wind speed dependence, possibly indicating that the forms of the empirical stability functions used by the bulk model are incorrect. The turbulent Cn2 measurements from the buoy showed excellent agreement with the scintillation values in unstable conditions, but had surprisingly large differences in weakly stable conditions. This disagreement may be related to the fact that humidity fluctuations begin to increasingly influence refractive index fluctuations when the air-sea temperature difference is small and are not properly taken into account by the sonic temperature measurements. As the absolute air-sea temperature difference approaches zero the bulk Cn2 estimates decrease much more rapidly and to much smaller values than either the scintillation or turbulence measurements. Fortunately, in such near-neutral conditions scintillation is usually small enough to have little effect on many optical system applications.
The Rough Evaporation Duct Experiment (RED) assessed the effects of the air-sea boundary layer on microwave and infrared (IR) signal propagation near the sea surface. The experiment was designed around the Floating Instrument Platform (FLIP) research platform, which was moored 10 kilometers off the northeast shore of Oahu, Hawaii. A 10-kilometer infrared propagation path was created from FLIP to a shore-based receiver and both scintillation and transmission measurements were made around the clock for a two-week period.
An accurate model for the propagation of infrared signals in the marine atmospheric surface layer remains an elusive goal. Within the first tens of meters of elevation above the sea surface there are substantial vertical gradients of mass and temperature, and this has a strong effect on the prediction of extinction of the infrared signal. The effectiveness of the propagation models will be investigated and the results from the infrared signal propagation study during RED will be shown.
Infrared scintillation measurements were obtained along a 7 km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter Cn2, were computed from the buoy data and compared with scintillation-derived Cn2 values. The bulk Cn2 estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk Cn2 estimates were higher than the scintillation data, by up to an order of magnitude on average. This disagreement may be due to the effects of ocean waves in decreasing the vertical temperature and humidity gradients in stable conditions from the assumed Monin-Obukhov similarity theory forms, resulting in bulk Cn2 values that are too high. The bulk Cn2 estimates decrease rapidly when the absolute air-sea temperature difference approaches small positive values. These predicted decreases in Cn2 were not observed in the path-averaged scintillation measurements or in single-point turbulence measurements, demonstrating that bulk models which estimate structure parameters based on mean air-sea differences are not valid when the mean air-sea difference approaches zero. It is believed that obtaining a better understanding of surface wave modification of near- surface atmospheric gradients represents the most promising means toward improving the bulk model.
Following scintillation experiments during EOPACE measurement campaigns in the spring of 1996, a new set of experiments was designed in November 1996 and August/September 1997 over San Diego Bay. The main purpose was to further investigate the discrepancy between measured scintillation data and values predicted from turbulence models. One simple way to measure scintillation was the decrease of the signal variations with increasing integration time in the IR transmissometer. In addition near IR and midwave IR imagery was used of static and modulated sources. One of the static sources was mounted on a small boat, sailing out to horizon ranges, as was done earlier in the LAPTEX experiments at Crete. For the detection of the signals of the modulated source a set of receivers was used, providing 2 lines of sight at different altitudes above the water. The results show small differences in scintillation at 2 altitudes. The midwave and near-IR scintillation values show little correlation with Cn2 values measured at the midway buoy, confirming the earlier experiments. The impact of atmospheric scintillation of IRST performance is again demonstrated.
During the Electro-Optical Propagation Assessment in a Coastal Environment (EOPACE) experiment of May-June 1998, IR scintillation measurements were obtained along a 7 km path over San Diego Bay. Simultaneous meteorological measurement were obtained from a buoy located at the midpoint of the transmission path. Bulk estimates of the refractive index structure parameter, Cn2 were computed from the buoy data and compared with scintillation-derived Cn2 values. The bulk Cn2 estimates agreed well with the scintillation measurements in unstable conditions. The agreement between the two methods was poor for near-neutral and stable conditions. In particular, when the air-sea temperature difference has small positive values the bulk model predicts the vertical refractive index gradient approaches zero, resulting in rapid decrease in bulk Cn2 estimates. These predicted decreases in Cn2 were not observed in the path-averaged scintillation measurements.
An analysis is presented showing the effects of molecules and aerosols on atmospheric transmission data obtained during the
Electro-Optical Propagation Assessment in Coastal Environments (EOPACE) campaign carried out in San Diego during
March and April, 1996. Mid wave infrared transmission was measured over San Diego Bay along a 14.9 km path and a 7.0
km path at heights less than 4 meters above the water. Simultaneous meteorological measurements were obtained from two
buoys placed at the mid-points of each path. An aerosol spectrometer was used to measure the aerosol size distribution over
each transmission path. Data were analyzed with MODTRAN and Mie theory. The conclusion of this and the next two
papers is that low altitude infrared transmission is a complex phenomenon whose mean value may be controlled either by
molecular absorption, aerosol scattering, or refractive focusing, and whose fluctuating value is controlled by scintillation.
Turbulent wind and temperature fluctuations were measured with an ultrasonic anemometer deployed on a buoy near the Meetpost Nordwijk platform during MAPTIP. These measurements were designed to provide MAPTIP investigators with knowledge of over-water turbulence properties and the suitability of existing bulk models for describing them. The friction velocity, u*, and the temperature structure function parameter, Ct2, were determined using both bulk and inertial-dissipation methods. Inertial-dissipation measured CT2 values agreed well with bulk estimates for values greater than about 0.02. Below this value the inertial-dissipation CT2 measurements were larger than the bulk estimates, probably due to noise contamination of the weak temperature fluctuation signal in the temperature variance spectrum. Inertial-dissipation u* values were larger than the bulk determinations for higher wind speeds. The bulk formulation used in this study was determined for large fetches with west winds at the MPN location, while only east winds with shorter fetches occurred during MAPTIP. Since fetch limited seas are rougher for a given wind speed, this would cause the inertial- dissipation measured u* values to be higher than the bulk estimates.
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