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Proceedings Paper

Measurement of optical refraction, transmission, and turbulence effects in False Bay, South Africa: June 2007
Author(s): Arie N. de Jong; Piet B. W. Schwering; Peter J. Fritz
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Paper Abstract

Complementary to a measurement campaign of small surface targets in the False Bay, South Africa [1], a set-up could be arranged of atmospheric propagation experiments. This opportunity allowed us to collect another set of transmission data in a coastal area, where the environmental conditions are generally non-homogeneous and rapidly changing. It was found before, that the validity of models, predicting the aerosol size distribution, the vertical temperature profile or the structure constant for the refractive index Cn 2 tends to be questionable in this type of areas [2,3]. Proper knowledge of the relation between the range performance of electro-optical and infrared sensors and in-situ weather parameters is however of key importance for operational use of this type of sensors, so the collection of additional propagation data was very relevant. Refraction data were collected continuously by using a geodetic theodolite with camera system over a 15.7 km path in the False Bay. Transmission- and scintillation data were collected over a 9.6 km path by means of our MSRT (Multi- Spectral Radiometer Transmissometer) and a Celestron telescope (with camera) with a focal length of 1.25 m. Weather parameters were measured at a shore station and on a rock in the bay. The weather was greatly variable with many showers, while the visibility, cloudiness and ASTD (Air-Sea Temperature Difference) conditions were continuously changing. Analysis of the theodolite data delivered absolute AOA (Angle of Arrival) data, which have been compared with predictions from the bulk model for marine boundary layers and from two empirical two-parameter temperature profiles. Transmission data, collected in three spectral bands (around 0.6, 0.9 and 1.5 µm), provided information on the particle size distribution, assumed to be of a Junge type. Knowledge of this information allows the prediction of the atmospheric transmission in other spectral bands, including the IR. The transmission data were compared with the data from a visibility meter on the roof of the IMT building. Both data sets correlated reasonably well. From the high speed MSRT transmission data (integration time 10 ms, sampling rate 30 Hz) the scintillation index (SI) was calculated, which showed a reduction in SI value when it starts to rain, while the SI came back to normal shortly after the shower. The measured SI data were transformed into Cn 2 values (the atmospheric refractive index structure function) and compared with predictions from the bulk model with different type of stability functions for a selected set of measurement periods. The model predictions show deficiences for conditions with small ASTD. The SI data from the MSRT were compared with the scintillation data, collected with the Celestron imaging system, which showed interesting correspondences and differences, which are discussed in the paper. From the Celestron data also the beam wander was determined, providing, similar to the SI, a source of information on Cn 2. It was shown, that the beam wander (blur) also correlates with ASTD.

Paper Details

Date Published: 1 October 2008
PDF: 12 pages
Proc. SPIE 7108, Optics in Atmospheric Propagation and Adaptive Systems XI, 710803 (1 October 2008); doi: 10.1117/12.797460
Show Author Affiliations
Arie N. de Jong, TNO-Defence, Security and Safety (Netherlands)
Piet B. W. Schwering, TNO-Defence, Security and Safety (Netherlands)
Peter J. Fritz, TNO-Defence, Security and Safety (Netherlands)

Published in SPIE Proceedings Vol. 7108:
Optics in Atmospheric Propagation and Adaptive Systems XI
Anton Kohnle; Karin Stein; John D. Gonglewski, Editor(s)

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