Accurately identifying and bounding error sources in imaging spectro-polarimeters is a challenging task. Here we present an error evaluation methodology intended as an organizational tool for both itemizing and quantifying sources of error in polarimetric instruments. Associated with each source of error are both a metric and test by which these errors may be quantified. Using this procedure, we examine the accuracy and precision of a particular imaging Stokes vector hyper-spectral polarimeter. A subset of the identified error sources are selected and propagated through the system. These measured error quantities are then used to put absolute error bounds on the data acquired by our instrument. These measured error quantities are further documented and presented in the form of an error evaluation sheet.
Understanding the turbulence along a propagation path is required to evaluate new methods for tracking, pointing, and compensation of laser beams, studying image degradation, and interpreting remote sensing observations. This paper presents observations using a new balloon-ring platform equipped with multiple fine wire probes (1 μm diameter) at various separations for sensing both temperature and velocity fluctuations. These measurements provide profiles of the temperature structure parameter (<i>C<sub>T</sub></i><sup>2</sup>), refractive index structure parameter (<i>C<sub>n</sub></i><sup>2</sup>), eddy dissipation rate (ε), inner scale (<i>l<sub>o</sub></i>), and outer scale (<i>L<sub>o</sub></i>) values. Of particular interest is the path variability of <i>C<sub>n</sub></i><sup>2</sup>, ε, <i>l<sub>o</sub></i>, and <i>L<sub>o</sub></i>, and the relationship of these parameters to atmospheric stability. Since all “raw” data is archived, spectra are shown and discussed as to how often the atmosphere truly follows the “Kolmogorov-5/3,” or under what atmospheric conditions for which the structure function is represented by the r<sup>2/3</sup> law. Also the concerns of local isotropy and intermittency can be addressed. The interpretation of these results to propagation effects is discussed. Salient features of the new measuring system are presented as well as the rationale for its implementation, including the longstanding concern of wake contamination on conventional measuring systems tethered under a single balloon.
This paper present new methodology to address critical refractivity turbulence issues for laser propagation using a new measurement system-a portable balloon-ring platform with multiple fine wire sensors at several separations. All raw data is transmitted to a ground station-allowing spectra to be calculated. The new platform is discussed and preliminary examples of observations, including artifacts, are shown and discussed. This new platform provides capabilities during the daytime as well as nighttime-unlike conventional thermosondes that are used only at night. Such all time observations are important due to the pronounced diurnal variation in the planetary boundary layer where many laser systems are operated. Plans to address the longstanding concern of wake contamination on systems suspended below a balloon quantitatively will be presented. The objective of this effort is to develop the capability that can address several questions related to laser propagation such as: 1) Is the atmospheric isotropic for the scales of interest? 2) Is the turbulence Kolmogorov under various atmospheric conditions, or how often is the structure function represendted by the r<SUP>2/3</SUP> law? 3) what are the profiles of inner and outer scale? 4) To what degree does wake contamination affect conventional thermosonde measurements? 5) Does fine structure within the scattering volume sensed by radar affect refractive index structure parameter (C<SUB>n</SUB><SUP>2</SUP>) and eddy dissipation rate (epsilon) estimates? These questions and concerns will be addressed by making the appropriate observations using the balloon-ring platform. Many of the measurements will be taken at Vandenberg AFB since the Western Test Range operates a ground receiving station, balloon launch facility, VHF radar, boundary layer radars, sodars, and instrumented towers that will enhance this effort. This effort provides an observation platform that will ultimately lead to the development and validation of conceptual/statistical/physical models.
Optical turbulence measurements have been performed at North Oscura Peak, White Sands Missile Range using a variety of precision instruments. A commercially available sodar system is used to collect atmospheric profiles, which are calibrated with fine-wire probes to calculate the optical index of refraction structure parameter, C<SUB>n</SUB><SUP>2</SUP>, from a height of 15 meters to around 160 meters above the ground.
The Airborne Laser Concepts Testbed is located on White Sands Missile Range, NM and is used to explore and develop new methods for tracking, pointing, and compensation of laser beams. All of these efforts require a knowledge of the optical turbulence along the propagation path. The site utilizes a 52.6 km propagation path over a desert basin between two mountain peaks (North Oscuro Peak (NOP) and Salinas Peak). Characterization of the optical turbulence at ABL ACT is challenging due to the long path length int he atmospheric boundary layer and the complex terrain of the site. A suite of instrumentation is being used to approach the problem; a sodar, fine wire probes, a pupil plane imager, a differential image motion monitor, and a scintillometer. In addition, a weather station senses ambient temperature, humidity, pressure, wind speed and direction, and solar radiation-received both horizontally and parallel to the mountain west-facing slope at NOP.
Optical turbulence conditions at a mountain peak (North Oscura Peak, NM) have been calculated using two hot-wire anemometers. The anemometers (running in constant current mode, with a very low overheat ratio) measure temperature fluctuations. Combining the fluctuating temperature data with wind velocity data, local temperature and pressure, and invoking Taylor's hypothesis, the optical turbulence parameters can be calculated. These parameters include temperature structure parameter (C<SUP>2</SUP><SUB>t</SUB>) and the refractive index structure parameter (C<SUP>2</SUP><SUB>n</SUB>). The two probes are positioned at different elevations above the ground, thus the vertical optical turbulence gradient can be calculated. This relationship is used to calibrate an acoustic sounder. Optical turbulence data collected from the hot-wire anemometers as well as the acoustic sounder will be compared to meteorological events measured locally. Many days of data have bene collected and will be shown, of particular interest is the relationship between optical turbulence and solar radiation, as well as wind speed and direction. The diurnal relationship of the optical turbulence gradient will also be shown. As well as the effect of this parameter on the acoustic sounder calibration.
The optical turbulence conditions at a mountain ridge (North Oscura Peak, White Sands Missile Range, NM) were determined from observations of fine wire sensors and a sodar (sonic detection and ranging). Both instruments provided the temperature structure parameter (C<SUP>2</SUP><SUB>T</SUB>) from which the refractive index structure parameter (C<SUP>2</SUP><SUB>n</SUB>) was calculated using local measurements of temperature and pressure. The fine wire measurements were used to calibrate the sodar. Atmospheric measurements shown include wind speed and direction, temperature, and solar radiation sensed horizontally as well as parallel to the west-facing slope. Of particular emphasis is the relationship of the sodar observations to solar radiation and wind speed and direction. The results are explained in terms of the geometry of the site and the mountain-valley wind regime. Results are shown as average range profiles of C<SUP>2</SUP><SUB>n</SUB> sensed at various zenith angles at different times of the day and as contours of C<SUP>2</SUP><SUB>n</SUB> in a vertical plane oriented normal to the west-facing slope.
An Acoustic Sounder System has been installed on the side of the cliff at North Oscura Peak, WSMR to provide important refractive index structure parameter, Cn<SUP>2</SUP> data for laser propagation tests. The acoustic sounder system records echo information that is used to provide 3D wind and optical turbulence profiles. The received signal is the product of the interaction of the transmitted acoustic pulse with the small scale atmospheric temperature variations. This information is displayed as a time-height display of the signal intensity. The frequency of the received signals are processed and converted into time histories of the horizontal wind field. The data from the Acoustic Sounder is calibrated with the hot-wire anemometer temperature structure parameter (C<SUB>t</SUB><SUP>2</SUP>) data, and meteorological data measured locally to produce the C<SUB>n</SUB><SUP>2</SUP> profile. The design and location of the Acoustic Sounder System will be discussed along with the methodology of extracting the turbulence. Many days of data have been collected and representative data will be shown.