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Planning and Analysis Capability Development for Asteroidal and
Planetary Occultation Observations, 15-9127

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Principal Investigator
Daniel D. Durda

Inclusive Dates: 03/11/99 - 07/11/99

Background - Occultations are a valuable and well-proven technique that planetary astronomers have developed to determine the size and shape of asteroids and pressure-temperature profiles through planetary atmospheres. Because occultation work has strong elements of fundamental research, practical applications, and planetary mission planning, SwRI researchers have worked to develop a low-cost, fast-response, airborne planetary occultation capability, putting the Institute at the forefront of man-in-the-loop, high-altitude astronomy, and allowing SwRI to go after high-priority occultation events that other observers may miss due to cloud cover or over-water ground paths.

Approach - This purpose of this project was to develop the tools and capabilities at SwRI for planning, conducting, and then analyzing occultation observations. With NASA support, SwRI researchers are developing the SWUIS-A (Southwest Ultraviolet Imaging System - Airborne), allowing the Institute to play a significant role in the occultation observation arena. The SWUIS-A flight hardware, centered on a sensitive image-intensified CCD (ICCD) camera, has been assembled and integrated into NASA F/A-18B aircraft and flown on a series of development, test, and observation missions. The success of these developmental flights and the high quality of the test data have allowed selection of a series of actual occultation events for observation in 1999 to establish a track record and to expand the SWUIS-A operational envelope. As successful as it has been, however, this observational aspect of the SWUIS-A program is only one of three elements necessary to have a self-contained program. SwRI must also develop the tools and capabilities in-house to 1) predict occultation events and map their ground paths, and 2) reduce and analyze occultation data gathered with the SwRI system.

Before the decision can be made to deploy SWUIS-A to gather scientifically useful occultation data on any given event, one must be able to predict the occurrence of those occultation events that the SWUIS-A system is capable of observing, and then examine the predicted ground paths for the events to decide upon a deployment strategy. After occultation data have been recorded, they must then be reduced and analyzed so that accurate determination of the immersion and emersion times and brightness variations can be obtained and converted into an estimate of the asteroid’s size or a temperature-pressure profile through the planet’s atmosphere.

Accomplishments - In the area of occultation predictions, the research team obtained from the International Occultation Timing Association (IOTA) DOS and Windows™ versions of the software package OCCULT, the most commonly used software package for event predictions and ground-path mapping among the occultation community. The team adapted the software and updated the relevant stellar and asteroidal astrometric data bases to its needs and has become proficient in using the package to predict events and generate ground path maps for events observable by SWUIS-A.

The second task included data reduction and analysis. The alteration of SwRI’s existing Interactive Data Language (IDL) software routines to analyze SWUIS-A image data was rightfully expected to be the most time-consuming task in this project. The existing IDL routine that co-registers individual SWUIS image frames to remove image drift and jitter was modified to allow hand-selection of stars on which images will be registered, thus avoiding problems previously encountered with the automated functions that often centered on bright, noisy pixels. The new IDL routine now also includes functions developed to measure the brightness variations in individual image frames at the pixel locations of specified target stars, thus allowing high time-resolution photometry of faint stars near the SWUIS-A detection limits. The new routines were tested on actual SWUIS-A imaging data gathered during developmental test flights and have proven to be very effective and versatile.

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