The Terestrial Planet Finder (TPF) mission to search for exo-solar planets is extremely challenging both technically and from a performance modeling perspective. For the visible light coronagraph (the C) approach, the requirements for 1e10 rejection of star light to planet signal has not yet been achieved in laboratory testing and full-scale ground testing provides additional challenges to overcome. Therefore, end-to-end performance modeling will be relied upon to fully predict system performance. One of the key technologies developed for achieving the rejection ratios uses shaped pupil masks to selectively cancel starlight in planet search regions by taking advantage of the diffraction. Modeling results published to date have been based upon scalar wavefront propagation theory to compute the residual star and planet images. This ignores the 3D structure of the mask and the coupled EM fields resulting when light interacts with matter. Secondly it ignores a most important engineering question which is how well the proposed wavefront control system can correct any effects introduced by mask/ light interactions.
To address this problem we incorporate results from vector propagation through the masks. These fields, computed by the Finite Difference Time Domain (FDTD) method, are coupled into a TPF coronagraph integrated model and propagated end-to-end through the optical system. In this paper we build upon two recently published papers (refs 1,2) and evaluate this additional disturbance to the far field image, discuss the interface with surface-to-surface propagators and set up the formulism for polarization effects. A follow-on paper, part II, results will be presented with a surface-to-surface Fourier-based propagator coupled to the difference field models which include corrections from a wavefront control system.
The NASA Mars Exploration Rover mission will launch two scientific spacecraft to Mars in 2003. The primary goal of the mission is to obtain knowledge of ancient water and climate on the red planet. Each spacecraft will carry one rover with a mass of approximately 150 kg and a design lifetime of about 90 days to the surface of Mars. The rovers are intended to travel up to 100 meters per day. The scientific payloads of the rovers will include a stereo pair of Panoramic cameras and a Microscopic Imager. The Panoramic cameras also support the engineering functions of high gain antenna pointing and navigation by solar imaging. The rovers have six additional cameras that will be used, exclusively, for engineering. All nine cameras share a common design, except for their optics. The focal plane of each camera is a 1024 X 1024-pixel frame transfer CCD. A stereo pair of Navigation cameras is mounted on a gimbal with the Panoramic camera pair. The Navigation camera pair is used for traverse planning and general imaging. Finally, one stereo pair of wide-angle Hazard Avoidance cameras will be mounted on the front (and one pair on the back) of each rover to autonomously generate range maps of the surrounding area for obstacle detection and avoidance.
A new, photon-sensitive, imaging array, the active pixel sensor (APS) has emerged as a competitor to the CCD imager for use in star and target trackers. The Jet Propulsion Laboratory (JPL) has undertaken a program to develop a new generation, highly integrated, APS-based, multipurpose tracker: the Programmable Intelligent Microtracker (PIM). The supporting hardware used in the PIM has been carefully selected to enhance the inherent advantages of the APS. Adequate computation power is included to perform star identification, star tracking, attitude determination, space docking, feature tracking, descent imaging for landing control, and target tracking capabilities. Its first version uses a JPL developed 256 X 256-pixel APS and an advanced 32-bit RISC microcontroller. By taking advantage of the unique features of the APS/microcontroller combination, the microtracker will achieve about an order-of-magnitude reduction in mass and power consumption compared to present state-of-the-art star trackers. It will also add the advantage of programmability to enable it to perform a variety of star, other celestial body, and target tracking tasks. The PIM is already proving the usefulness of its design concept for space applications. It is demonstrating the effectiveness of taking such an integrated approach in building a new generation of high performance, general purpose, tracking instruments to be applied to a large variety of future space missions.
The most accurate instrument for spacecraft attitude determination is a star tracker. Generally, these are CCD- based instruments. Until recently, only first-generation units were available. However, these first-generation designs are limited to outputting positions of a few stars in sensor- referenced coordinates and require extensive external processing. Fortunately, advancing technology has enabled the development of a new second-generation class of star trackers. These designs are fully autonomous, solve the lost-in-space problem, have large internal star catalogs, use many stars for each data frame, have higher accuracy, smoother and more robust operation, potentially lower cost, and output attitudes which are referenced directly to inertial space without any further external data processing. Two currently available designs which are in production and meet these requirements are the AST-201 from Lockheed Martin Missile & Space and the ASC from the Technical University of Denmark. The first design is in the general size, power, mass, and reliability class of typical, conventional star trackers. The second one features reduced size, power, mass, and cost, with commercial off-the- shelf components. Second-generation star trackers have a promising future with a likely evolution to low cost, miniature, stock instruments with wide application to a growing variety of space missions.
The Danish geomagnetic microsatellite, Orsted, is an autonomous sciencecraft which is scheduled for a May 1997 launch into polar orbit. It is produced by a consortium of universities, industry and government and is Denmark's first national spacecraft. NASA support includes JPL real sky evaluation of its star tracker, the advanced stellar compass (ASC). The ASC features low cost, low mass, low power, low magnetic disturbance, autonomous operation, a high level of functionality and the high precision. These features are enabled by the use of advanced optical and electronic design which permit the direct integration of the ASC and the science payload. The ASC provides the required attitude information for its associated vector magnetometer and the sciencecraft. It consists of two units, a CCD based camera head and a data processing unit with a powerful microcomputer. The microcomputer contains two large star data bases which enable the computer to recognize star patterns in the field-of-view, to quickly solve the lost-in- space acquisition problem and to derive the attitude of the ASC camera head. The flight model of the camera head has a mass and a power consumption of 127 grams (without baffle) and 0.5 W, respectively. Typical, beginning-of-life, relative measurement precision in pitch and yaw are in the order of two arcseconds (1 sigma) or better have been achieved in the tests and are substantiated.
The Orsted Star Imager, comprises the functionality of an advanced stellar compass (ASC), i.e. it is able to autonomously solve 'the lost in space' attitude problem, as well as determine the attitude with high precision in the matter of seconds. The autonomy makes for a high capability for error rejection and faulty recovery, as well as graceful degradation at radiation, false object or thermal loads. The instrument was developed from concept to flight model within 3 years. The instrument surpasses the initial specifications for all parameters. For precision, computational speed and fault detection and recovery by orders of magnitude. This was accomplished, by the use of advanced high level integrated chips in the design, along with a design philosophy of maximum autonomy at all levels. This approach necessitates a prototyping facility, capable of extensive component screening. This screening addresses topics such as chip technology and thermo-mechanical propeties, as well as radiation sensitivity. The purpose of this facility is to reduce costs, by generating early information concerning whether specific components have the ability to survive space environs. This paper describes the development philosophy and the development process. Starting with the system specifications and its derived design drivers. Via the design process, iteration levels and the specifications and capability of the prototyping facility. Ending with the final system design. During this, actual choices of IC- levels and system flexibility are addressed.
The Orsted Star Imager comprises the functionality of an advanced stellar compass (ASC), i.e. it is able to autonomously solve 'the lost in space' attitude problem, as well as determine the attitude with high precision in the matter of seconds. The autonomy makes for a high capability for error rejection and fault recovery, as well as 'graceful degradation' at radiation, false object or thermal loads. The instrument was developed from Concept to flight model within 3 years. The instrument surpasses the initial specifications for all parameters, for precision, computational speed and fault detection and recovery by orders of magnitude. This was accomplished by the use of advanced high level integrated chips in the design, along with a design philosophy of maximum autonomy at all levels. The instrument tracks all stars in the field of view, which enables a variety of applications not normally associated with conventional star trackers. Initially, this paper gives a general description of the ASC, including its primary specifications and performance levels. Some of the more promising of the advanced applications are then discussed, along with test-results and methodologies. The diversity of the advanced applications are vast, as depicted by the topics addressed, namely: (1) Detection and tracking of distant non-stellar objects (e.g. meteors). (2) Delta-V correction, for encounter phases. (3) Tracking of selected objects (e.g. guidance for other instruments). (4) Mass estimation via pellet ejection. (5) Complex object surface tracking (e.g. space docking, planetary terrain tracking). All the above topics have been realized in the past, either by open loop, or by man-in-the-loop systems. By implementing these methods or functions in the onboard autonomy, a superior system performance could be achieved by means of the minimal loop delay. But also reduced operations cost should be expected.
New developments in image sensors and optical materials have opened the door to dramatic mass and power reductions in celestial tracker design. The rapid development of active pixel sensors (APS) has provided a new detector choice offering high on-chip integration of support circuitry at reduced power consumption. Silicon-carbide optics are one of the new developments in low-mass optical components. We describe the celestial tracker needs of an Autonomous Feature and Star Tracking (AFAST) system designed for autonomous spacecraft control. Details of a low-mass celestial tracker based on a low-power APS array and optimized for an AFAST system are discussed.
A star tracker for a mission to the planet Pluto is described. Low mass, low power and high levels of performance are required at the lowest possible cost. These goals are achieved for the FY94 baseline Pluto mission by a modified, commercially available star tracker, the HD- 1003, from Hughes Danbury Optical Systems, The Pluto mission, the spacecraft design, the star tracker and its performance are discussed.
Charge-coupled devices (CCDs) have been used extensively in the past in star trackers and fine guidance systems. A new technology, the active pixel sensor, is a possible successor to CCDs. This technology potentially features the same sensitivity and performance of the CCD with additional improvements. These improvements include random access capability, easy window-of- interest readout, non-destructive readout for signal-to-noise improvement, high radiation tolerance, simplified clocking voltages, and easy integration with other on-chip signal processing circuitry. The state-of-the-art of this emerging technology and its potential application to guidance and navigation systems is discussed.