Alan S. Mazer

THE ON-ORBIT PERFORMANCE OF THE GALAXY EVOLUTION EXPLORER

We report the first years on-orbit performance results for the Galaxy Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter modified Ritchey-Chretien telescope with a 125 field of view, selectable imaging and objective-grism spectroscopic modes, and an innovative optical system with a thin-film multilayer dichroic beam splitter that enables simultaneous imaging by a pair of photon-counting, microchannel-plate, delay-line readout detectors. Initial measurements demonstrate that GALEX is performing well, meeting its requirements for resolution, efficiency, astrometry, bandpass definition, and survey sensitivity.

Image processing software for imaging spectrometry data analysis

Abstract The advent of a new generation of remote sensing instruments, called imaging spectrometers, promises to provide scientists a greatly enhanced capability for detailed observations of the earths surface. These instruments collect image data in literally hundreds of spectral channels simultaneously from the near ultraviolet through the short wavelength infrared, and are capable in many cases of providing direct surface materials identification in a manner similar to that used in laboratory reflectance spectroscopy. The volume and complexity of data produced by these instruments offers a significant challenge to traditional multispectral image analysis methods, and in fact requires the development of new approaches to efficiently manage and analyze these data sets. This paper describes a software system specifically designed to provide the science user with a powerful set of tools for carrying out exploratory analysis of imaging spectrometer data utilizing only modest computational resources.

The galaxy evolution explorer

The Galaxy Evolution Explorer (GALEX), a NASA Small Explorer Mission planned for launch in Fall 2002, will perform the first Space Ultraviolet sky survey. Five imaging surveys in each of two bands (1350-1750Å and 1750-2800Å) will range from an all-sky survey (limit mAB~20-21) to an ultra-deep survey of 4 square degrees (limit mAB~26). Three spectroscopic grism surveys (R=100-300) will be performed with various depths (mAB~20-25) and sky coverage (100 to 2 square degrees) over the 1350-2800Å band. The instrument includes a 50 cm modified Ritchey-Chrétien telescope, a dichroic beam splitter and astigmatism corrector, two large sealed tube microchannel plate detectors to simultaneously cover the two bands and the 1.2 degree field of view. A rotating wheel provides either imaging or grism spectroscopy with transmitting optics. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the red shift range zero to two. The GALEX mission will include an Associate Investigator program for additional observations and supporting data analysis. This will support a wide variety of investigations made possible by the first UV sky survey.

Image processing software for imaging spectrometry

Recent advances in remote sensing have enabled scientists to collect image data in literally hundreds of spectral channels simul-taneously, from the near ultraviolet through the short-wavelength infrared, using imaging spectrometers. In many cases this data is of sufficient resolution to provide a direct surface materials identification. Yet the volume and complexity of the data produced requires new algorithms and approaches beyond those traditionally used for multispectral image analysis, including algorithms for fast image segmentation, spectral identification and mixture analysis. This paper describes a software system specifically designed to provide the scientist with the tools necessary for exploratory analysis of imaging spectrometer data using only modest computa-tional resources.

Real-Time Atmospheric Correction of AVIRIS-NG Imagery

We demonstrate real-time model-based atmospheric correction onboard the Next Generation Airborne Visible/Infrared Imaging Spectrometer. We achieve a reduction in processing time from hours or days to seconds by modifying a standard physics-based atmospheric correction algorithm to support real-time execution. We achieved this reduction by modifying the physics-based ATmospheric REMoval algorithm to leverage a large lookup table of precomputed scattering and transmission coefficients, indexed by parameters specifying the aircraft operating conditions at capture time. Interpolation among the precomputed coefficients allows surface reflectance retrieval at the sensor acquisition rate of 500 Mb/s. Our system produced science-quality reflectance products during over 30 test flights and, to our knowledge, is the first reported demonstration of real-time model-driven visible shortwave infrared atmospheric correction onboard an aircraft.

Design and implementation of a concurrent image processing workstation based on the Mark III hypercube

Various image processing algorithms have been implemented on the hypercube architecture and many success stories have been reported. However, the traditional approach to programming the hypercube has been to write programs which perform ;I single operation or a fixed set of operations upon data items. This approach has several drawbacks when considered for use in an interactive computing environment. First, it is difficult to process data with a sequence of sim;ple programs in the Mark III Hypercube because the Mark III software does not support sharing of data between successive programs. This means that data must be reloaded into the cube for each individual program. It also implies that programs should be fairly large and complete, to minimize the repeated downloading of large data items for multiple programs. However, the entire program must be able to fit within the hypercube node memory, which limits what a program can do by putting a restriction on its size. Furtbermore, large programs limit the amount of memory available for data, which must also be present in memory if the communications overhead is to be effectively reduced. The development of an interactive image processing workstation based on the: Mark III Hypercube requires satisfactory solutions to these and other problems.

Automated Testing of Science Instrument Flight Software

This paper describes an approach to automated testing of science instrument flight software using scripted commanding, telemetry capture, and intelligent telemetry verification. The most difficult piece of this approach, telemetry verification, is accomplished with a JPL-developed software package, HKCheck, which takes user- defined packet formats, error conditions, and goals, and verifies that binary telemetry meets the specified conditions. Packet formats are defined using an ASCII file, easily created in any text editor. Test-specific and context-specific error conditions are expressed using a simple C-like notation, enabling large amounts of telemetry from normal flight operation or system testing to be rapidly scanned for anomalous telemetry. Goals, like waypoints, are similarly expressed in simple notation, and allow telemetry analysts or test conductors to look for specific events in the telemetry stream. The power and flexibility of the HKCheck error and goal specification language enables context-dependent analysis of telemetry, and rapid, repeatable analysis of test results, encouraging more frequent testing, and reducing the risk of human error in test analysis. Use of HKCheck on Mars Climate Sounder enabled full regression testing of flight software every month or two, rather than just on major deliveries, giving developers confidence that software was working correctly and uncovering potential bugs earlier in the development cycle than standard methods allow. It has since been used, with extensions, on other flight projects as well.