Variations of apparent target position due to refraction

Seiffer, Dirk; Stein, Karin

doi:10.1117/12.629574

Cited by 4 publications

(8 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The air temperature and humidity probe was 5.2 m MSL and the sea temperature was measured about 1 m below the surface. Wind speed and direction were measured about 5.5 m MSL near the camera system [see Heemskerk (2005) and Seiffer and Stein (2005) for more information on VAMPIRA].…”

Section: A the Eopace And Vampira Trialsmentioning

confidence: 99%

An Empirical Analysis of Bulk Cn2 Models over Water

Potvin

Dion

Claverie³

et al. 2008

Journal of Applied Meteorology and Climatology

View full text Add to dashboard Cite

This article examines some of the difficulties associated with the determination of C 2 n over water in a coastal region using a bulk model. The analysis shows the need to supplement bulk models with elements that do not belong to traditional Monin-Obukhov surface-layer theory. A reexamination of the scintillation measurements collected during the Electro-Optical Propagation Assessment in a Coastal Environment (EOPACE) campaign leads the authors to include 1) the nonuniformity of sensible heat and humidity fluxes and 2) a deficit of the scintillation that seems to depend only on the characteristic virtual potential temperature. It is suggested that the anomalous scintillation deficit represents an alteration of a characteristic length related to the optical turbulence, likely caused by the interaction of the surface layer with the sea surface. The new parameters are estimated using Bayesian regression methods applied to the EOPACE data. Predictions obtained with this new model are then compared with scintillation measurements obtained during the recent Validation Measurement for Propagation in the Infrared and Radar (VAMPIRA) campaign. Better agreement is obtained with the new model than with a conventional bulk model. The implications of the modifications made to the calculation of C 2 n are discussed.

show abstract

Section: A the Eopace And Vampira Trialsmentioning

confidence: 99%

An Empirical Analysis of Bulk Cn2 Models over Water

Potvin

Dion

Claverie³

et al. 2008

Journal of Applied Meteorology and Climatology

View full text Add to dashboard Cite

show abstract

“…During the POLLEX trial, carried out in Livorno, Italy, May 2001 [7], super-refraction did occur, leading to distortions for an array of lamps located at a distance of 33 km [5] and over the horizon target observation for a point source on a boat on 10, 11 and 17 May. Similar effects did occur during the VAMPIRA trials [8], although the fixed sources were located at a rather short distance (about 8 km). Denis Dion did analyse data from previous trials ( [9] and [10]), while fine tuning the stability functions in the bulk model in order to fit the predicted observation angles to the measured data.…”

Section: Introductionmentioning

confidence: 63%

Refraction measurements and modeling over the Chesapeake Bay during the NATO (TG-51) SAPPHIRE trials, June 2006

Jong

Fritz

2007

SPIE Proceedings

View full text Add to dashboard Cite

Optical refraction tends to occur frequently in the atmospheric boundary layer. Due to a gradient in the temperature as function of height, rays are bending down towards the earth (super-refraction), or up towards to the sky (sub-refraction). As a consequence, images of targets at long range may be distorted and mirages may occur, while the maximum detection range may be affected. In addition the irradiance, received from a point target at a sensor pupil, may increase or decrease due to atmospheric focusing effects. Sub-refraction tends to occur more frequently over sea-paths, as the air is normally cooler than the water. Currently available propagation models, like IRBLEM [1] and EOSTAR [2] adequately describe the mentioned effects for this condition. However sparse data are available for model validation in super-refractive conditions, when the air is warmer than the water. For this reason, the SAPPHIRE trial, (Ship and Atmospheric Propagation Phenomena Infrared Experiment), was organized in the Chesapeake Bay near Washington DC by NATO Task Group TG51 in June 2006. At this location and in this time of the year, the probability for having positive ASTD (Air to Sea Temperature Difference) conditions is high. In the bay a buoy was positioned, on which a set of precision temperature sensors was mounted. They provided as well ASTD values as temperature gradients at a height of 3.7 m. By means of a geodetic theodolite, absolute AOA's (Angle Of Arrival) were measured for a set of lights at various altitudes, located on the other side of the bay at a distance of 16.2 km. The buoy was located at about mid-path position. Positive ASTD conditions did occur on a number of days allowing validation of the values of the AOA, predicted by the models, based upon the meteorological data, simultaneously collected at the buoy. It was found, that the temperature profile, generated by the bulk model for the marine surface layer [3] is incorrect, resulting in deviations in the AOA, compared to the data from the theodolite. A set of empirical temperature profiles has been created, using the ASTD and the gradient in the temperature as function of height as input. One of them provides of predicted AOA values, which fit well with the measurements during the trials period. Errors in the prediction are resulting from the variability of the meteorological conditions, causing inhomogeneities along the path. Furthermore the measurement accuracy was limited due to image blur by atmospheric turbulence in a number of occasions.

show abstract

“…This trial was organized by the NATO Technical Group (TG) 32 [8] (References [9], [1], [5], and [6] are also related to some of the results of this measurement campaign).…”

Section: The Vampira Experimental Datamentioning

confidence: 99%

“…The main effects of interest are the operational range limitation (caused by thermally unstable atmospheric conditions), the mirage effect, the possibility of beyond-thehorizon propagation and the effect of ray focusing/defocusing on radiation intensity. As straight line propagation could only be possible in non refractive atmospheres, the position of a maritime target appears lower or higher than it actually is, depending on atmospheric stability [1]. Propagation geometry can be efficiently calculated by using ray-tracing, provided the variation of the air refractivity is reliably known.…”

Section: Introductionmentioning

confidence: 99%

Refraction effects under atmospheric stable conditions in coastal environments

Claverie¹,

Dion²,

Stein

2007

SPIE Proceedings

Self Cite

View full text Add to dashboard Cite

The performances of Electro-Optical (EO) systems such as visible or infrared cameras, lasers, operating within the Marine Surface Boundary Layer (MSBL), i.e. at heights up to a few tens of meters above the sea surface, are disturbed by various propagation mechanisms: molecular attenuation, aerosol extinction, refraction and turbulence. Refraction is responsible for focusing and defocusing of rays, detection range limitations, mirage formation and angular deviation. These refraction phenomena can be efficiently described using ray-tracing in conjunction with bulk estimations of the refractivity profiles based on the Monin-Obukhov (MO) theory. For stable atmospheric conditions (i.e. air temperature greater than sea temperature), the accuracy of the model predictions has been strongly discussed in the recent years. By using measurements of apparent target elevations recorded during the VAMPIRA trial, this paper aims at clarifying this discussion.

show abstract

Variations of apparent target position due to refraction

Cited by 4 publications

References 0 publications

An Empirical Analysis of Bulk Cn2 Models over Water

An Empirical Analysis of Bulk Cn2 Models over Water

Refraction measurements and modeling over the Chesapeake Bay during the NATO (TG-51) SAPPHIRE trials, June 2006

Refraction effects under atmospheric stable conditions in coastal environments

Contact Info

Product

Resources

About