James W. Alexander

Stochastic Star Identification

An approach to star identification based on comparing observed pattern statistics with the precomputed star cataloged statistics is suggested. The identification criterion is based on evaluating a posteriori probabilities of designated star sequences obtained from observing different star fields. Numerical results based on a specific algorithm are presented. A number of references for other approaches are cited.

Optical Tracking Using Charge-Coupled Devices

Techniques and instruments developed for extracting precise positional information from CCD images of point-source and extended optical targets are described. With the use of thinned, backside-illuminated devices, performance levels close to those expected from straightforward geometric considerations are obtained. For ideal point sources (ie., stars outside of the earths atmosphere), centerfinding accuracy of 1/100 pixel and measurement jitter of less than 1/250 pixel have been measured. Detailed tests showed that the main factor limiting tracker accuracy is small variation in the optical image shape rather than CCD noise or response nonuniformity. Methods of searching the entire field for the desired targets are described, along with windowing techniques for tracking. Also discussed are three specific examples of flight instruments previously developed or now being developed at the Jet Propulsion Laboratory.

Field Testing of the Mars Exploration Rovers Descent Image Motion Estimation System

The Mars Exploration Rover (MER) Descent Image Motion Estimation System (DIMES) is the first autonomous machine vision system used to safely land a robotics payload on another planet. DIMES consists of a descent camera and an algorithm for estimating horizontal velocity using image, inertial and altitude measurements. Before DIMES was accepted by MER for inclusion in the mission, its performance was validated through field testing using a manned helicopter to image three Mars analog test sites. Statistical analysis of the resulting 1900+ test cases showed that DIMES met its velocity estimation requirement. This paper describes the DIMES field test approach and associated results.

Astros: A Sub-Arcsec CCD Star Tracker

In early March 1986 the first Astro** mission will be launched to observe Halleys comet and a variety of other astronomical targets. Operating from the Shuttle bay, this payload consists of three large ultraviolet telescopes and a smaller wide field camera. An important part of the payload will be the first of a new generation of star trackers using CCDs as the star sensing element. The ASTROS tracker provides extremely precise measurements of star image coordinates as inputs to the Image Motion Compensation (IMC) system used to stabilize the science instrument focal planes. These coordinates, which reflect true star image motion to an accuracy of 0.2 arcsec, are required over a field of view of 2.2 x 2.5 degrees. This paper describes the design and application of ASTROS, with emphasis on performance test results acquired with a prototype system. Algorithms and software will be described in a later paper. Performance tests on real and simulated stars have consistently demonstrated 1/100 pixel accuracy and a noise equivalent angle of 1/300 pixel. These performance levels provide dramatic improvements, both in tracking accuracy and stability, relative to image dissector designs with comparable fields-of-view. These improvements, combined with other advantages inherent in a CCD-based approach are expected to lead to widespread application of this technology on future missions.

A high frame rate CCD camera with region-of-interest capability

This paper presents the design and preliminary results of a custom high-speed CCD camera utilizing a Texas Instruments TC237 CCD imager chip with sub-frame window read out. The camera interfaces to a C40 digital signal processor (DSP), which is used to issue commands and read images from the camera. The camera design consists of a two-card set including the CCD imager card and the focal plane array (FPA) interface card. The CCD imager card contains the level translator and buffer circuitry for the CCD strobe lines, the TC237 CCD imager chip and a pair of analog signal processor chips, each with a 10-bit analog-to-digital converter. The analog signal processor is a TLV987 with correlated double sampling (CDS) and serial programming capability to set amplifier gain, pixel bias level and background level illumination to name a few. The second card contains a pair of field programmable gate arrays (FPGA) used to interface the CCD imager card to the C40. The goal of this camera development is to provide a high-quality, high-speed camera as part of the tracking apparatus for a free-space optical communications terminal. Preliminary data suggests frame rates of 6 kHz for 8/spl times/8 subwindows in the current testbed with 7-bit pixel resolution. Refinements in camera and testbed operation target performance goals of 17 kHz for 8/spl times/8 sub-windows with 10-bit pixel resolution.

A Terrain Relative Navigation sensor enabled by multi-core processing

Terrain Relative Navigation (TRN) provides accurate position estimates to spacecraft for precision planetary landing and autonomous primitive body exploration. A bolt-on instrument that provides the sensing and computing required for TRN will result in more accurate and robust position estimates and will simplify TRN validation. Multi-core processors provide the significant computational capability required for TRN, are straightforward to program and are being developed for space applications. We have implemented two versions of TRN on a multi-core processor and tested them in a laboratory setting. For primitive-body navigation we have demonstrated 4 second TRN updates with accuracies on order 1% of altitude. For a Mars landing application we have shown two second updates while taking out kilometer scale position uncertainties.

Electron-Induced Displacement Damage Effects in CCD s

We compare differences in parametric degradation for CCDs irradiated to the same displacement damage dose with 2-MeV, 10-MeV, and 50-MeV electrons. Charge transfer efficiency degradation was observed to not scale well with non-ionizing energy loss (NIEL) for small signals. Short term annealing of mean dark current in a CCD sample irradiated with 2-MeV electrons at -85C is discussed, as well as additional annealing achieved by warming to temperatures up to and including room temperature. In contrast, charge transfer inefficiency was not observed to anneal following room temperature cycling for the sample irradiated at -85C

Accelerometer-assisted tracking and pointing for deep space optical communications

NASA/JPL has been developing acquisition, tracking and pointing (ATP) technologies for deep space tracking and pointing of an optical communication beam using linear accelerometers to enhance pointing. Linear accelerometers provide excellent accuracy in sensing the vehicles acceleration with the advantage of small size, low power, low cost, and a broad range of well developed products. We present the concept of accelerometer-assisted tracking, error analysis, and progress made on its implementation.

Sun Imaging though the Martian Atmosphere

This paper discusses the sun images acquired with the panoramic cameras (pancam) on the Mars exploration rovers. The mission goals, the camera design, the data processing and the results are discussed. Each of the two rovers has two cameras, one with a bandpass filter centered at 880 nm and one centered at 440 nm. It is observed that some of the pancams show ghost images. Based on analysis of the images, the transparency of the martian atmosphere (tau) is estimated to be ~0.9. A seasonal change in tau is observed.

Field Testing of Lunar Access and Navigation Device (LAND)

A laser radar system has been constructed. It is based on a commercial PC with digitizer, pulse delay instrument, National Instruments IO card and an optical head from a previous laser radar program. The laser radar was mounted on a gyro stabilized gimbal on the nose of a helicopter and flown in the Mojave Desert in September 2006. The collected data will be used to test algorithms for future precision lunar landers, which may be utilizing a laser radar as the primary landing sensor. This paper will describe the laser radar and PC based acquisition system used for the data collection, and provide an overview of the supporting test sensors and architecture. Preliminary data collected during the helicopter field testing will also be presented.