The Passive A-Band Wind Sounder (PAWS) was funded through NASA's Instrument
Incubator Program (IIP) to determine the feasibility of measuring tropospheric wind speed profiles
from Doppler shifts in absorption O<sub>2</sub> A-band. It is being pursued as a low-cost and low-risk alternative
capable of providing better wind data than is currently available. The instrument concept is adapted
from the Wind Imaging Interferometer (WINDII) sensor on the Upper Atmosphere Research Satellite.
The operational concept for PAWS is to view an atmospheric limb over an altitude range from the
surface to 20 km with a Doppler interferometer in a sun-synchronous low-earth orbit. Two orthogonal
views of the same sampling volume will be used to resolve horizontal winds from measured line-of-sight
A breadboard instrument was developed to demonstrate the measurement approach and to
optimize the design parameters for the subsequent engineering unit and future flight sensor. The
breadboard instrument consists of a telescope, collimator, filter assembly, and Michelson
interferometer. The instrument design is guided by a retrieval model, which helps to optimize key
parameters, spectral filter and optical path difference in particular.
3D imaging LADARs have emerged as the key technology for producing high-resolution imagery of targets in 3-dimensions (X and Y spatial, and Z in the range/depth dimension). Ball Aerospace & Technologies Corp. continues to make significant investments in this technology to enable critical NASA, Department of Defense, and national security missions. As a consequence of rapid technology developments, two issues have emerged that need resolution. First, the terminology used to rate LADAR performance (e.g., range resolution) is inconsistently defined, is improperly used, and thus has become misleading. Second, the terminology does not include a metric of the system’s ability to resolve the 3D depth features of targets. These two issues create confusion when translating customer requirements into hardware. This paper presents a candidate framework for addressing these issues. To address the consistency issue, the framework utilizes only those terminologies proposed and tested by leading LADAR research and standards institutions. We also provide suggestions for strengthening these definitions by linking them to the well-known Rayleigh criterion extended into the range dimension. To address the inadequate 3D image quality metrics, the framework introduces the concept of a Range/Depth Modulation Transfer Function (RMTF). The RMTF measures the impact of the spatial frequencies of a 3D target on its measured modulation in range/depth. It is determined using a new, Range-Based, Slanted Knife-Edge test. We present simulated results for two LADAR pulse detection techniques and compare them to a baseline centroid technique. Consistency in terminology plus a 3D image quality metric enable improved system standardization.
This paper details experimental and numerical studies of the effect of astigmatism on the performance of an unstable ring resonator with eight mirrors, causing a round trip 180 deg beam rotation, or seven mirrors, producing a beam flip. The resonators were studied with integer and integer plus one half equivalent Fresnel numbers and with and without intracavity spatial filtering. The experiment was performed on a CW electric discharge, fast flow CO2 unstable ring resonator with two removable and orthogonal focal line apertures (FLAs). The astigmatism was produced by rotating one cylindrical focusing mirror with respect to the other, producing astigmatism oriented at 45 deg to the focal line axes. Intracavity power, near field intensity distribution, and far field power and beam quality measurements were taken. The far field beam quality behavior with astigmatism was not very sensitive to the Fresnel number, but was dramatically dependent on the number of mirrors.
During a series of experiments to test the concept of using an intracavity spatial filter to improve far field resonator performance, some interesting anomalies with established theory was discovered. A brief description of the gain medium and resonator geometries will be followed by a detailed discussion of the behavior.
Analytical and numerical results for guardband criteria for unstable-resonator mode calculations are discussed. The energy-loss guardband derivation is outlined, guardband criteria based on the point-by-point accuracy of the computed amplitude of the complex field are determined, and point-accuracy guardband calculations are presented. Using the criteria established, the new guardband calculated for the propagation step, N(c) = 9.5 and N(eq) = 3.56, is found to be much larger than previous calculations. It is pointed out that the power calculations are nearly the same for these guardband requirements and those based on a percent energy loss.