INTRODUCTION

The StarNav I experiment is an advanced star tracker that flew on the STS-107/Columbia. The StarNav project was developed by the collaboration of the Spacecraft Technology Center (STC), the Texas A&M University Department of Aerospace Engineering , Jackson and Tull Inc., SPACEHAB Inc. and Society of Mexican American Engineers and Scientists(MAES).

The primary objective of the StarNav flight experiment was to validate a new algorithm developed by Dr.John Junkinsfor determining precise spacecraft attitude without prior knowledge of position. This is referred to as the 'Lost in Space Algorithm' (LISA). A successful demonstration of the LISA also provided an opportunity for students to become involved in understanding navigation with stars and general astronomy topics. For additional information on the algorithm please refer to:
http://jungfrau.tamu.edu/~html/StarNav/home.html

While developing an educational tool, the StarNav experiment also provided a validation of a new technology that would reduce the cost and mass of commercial star-trackers. The technology allowed real time star pattern recognition and attitude estimation in a self-calibrating unit that output the attitude quaternion and not image data. The StarNav experiment made attempts to demonstrate an attitude precision of 0.001 degree, an exposure time of 10 ms and a frame rate of greater than 1 Hz. To demonstrate the validity of the StarNav technology a requirement for a minimum of 20 one half hour unobstructed viewing opportunities was established.

The star tracker performed very well and provided excellent data during the course of the mission. A series of reports on the data collected can be found at http://starnav1.tamu.edu/after.html.

STARNAV FLIGHT EXPERIMENT DESCRIPTION

The StarNav flight experiment hardware was derived from engineering models used in ground tests and consisted of an electronics assembly, a lens housing, and a baffle assembly. The overall configuration was designed for simple mating to the QuEST platform. Some of these tests, were performed at the Spacecraft Technology Center's Space System Integration Lab (SSIL)

DEVELOPMENT HARDWARE

The StarNav flight experiment hardware was based on successful ground demonstration units that had been used to test attitude determination algorithms with the night sky. Figure 1 is a photograph of the CCD array and electronics development hardware.



One of the objectives of the development program was to develop a low cost star-tracker for small satellite applications. Therefore, the hardware used was commercial off the shelf (COTS) configured to support the unique algorithm development. The CCD array was housed in a small vacuum vessel and actively cooled by a thermoelectric cooler to -30 degrees Celsius. Figure 2 is a photograph of an early CCD array encapsulated in the windowed chamber.



The development cameras used simple customized lens assemblies and commercial lens units (e.g., Canon, Nikon). The frame transfer, analog to digital conversion, and power conditioning boards were supplied by the CCD vendor. Commercially available central processing units were successfully integrated to compute star angles, search the star catalogues, and determine attitude information. Validation of the StarNav prototypes were accomplished with night sky imaging. As a payload onboard the STS-107, we were able to confirm the operation of the hardware in the space environment and with unobstructed views of the stars.

FLIGHT CONFIGURATION

The primary objectives of the flight experiment were to develop educational interest in space technology and astronomy and validate unique algorithms for low cost star-tracker development. The flight hardware for the StarNav experiment consisted primarily of COTS components packaged in a custom configuration for mating to the QuEST. The program approach was to design for safety and conduct rigorous testing at prototypic environmental levels. Figure 3 depicts the StarNav located on the single QuEST (Q1) platform. The StarNav flight configuration consisted of the electronics assembly, the optical assembly, the baffle assembly, the flight enclosure and a mounting plate. Figure 4 provides an illustration of the entire assembly mounted to the Q1 plate.







The flight enclosure was been designed and fabricated to mitigate the potential hazards associated with detachment and collision within the Shuttle bay. Hence, the payload structure was comprised of two single components, the flight enclosure and the mounting plate each fabricated with aluminum. These were mated in such a way that there were no exposed fasteners other than the six bolts required to connect the mounting plate directly to the Q1. The electronics, optical assembly, and baffles were all contained within the flight enclosure. Figure 5 illustrates the components within enclosure.



After enclosure with the payload components was loaded, the box was fitted to the mounting plate and fasteners attached through from the bottom (Q1) side into helicoil inserts. Figure 6 provides a view of the mounting plate from the side of the enclosure and the from the interface with the Q1. The mounting plate was designed with an isogrid stiffening pattern to maximize the strength to mass ratio.



ELECTRONICS ASSEMBLY

The electronics assembly consisted of six circuit boards. The vacuum sealed CCD array, thermoelectric cooler and video frame transfer components were all on the video board located adjacent to and mounted with the optical assembly. The remaining boards included the timing board, ethernet communications board, thermal analog to digital (A/D) converter, PC-104 central processing unit card and the power regulation and distribution board. The video and timing printed circuit boards were supplied by the vendor (Patterson Electronics) as an integrated unit. The StarNav team worked with Patterson Electronics to identify and replace components of the video and timing boards to enhance the survivability of the cards in the radiation, vacuum, thermal, and vibration environments of the QuEST platform on the Orbiter. The PC104 CPU, communications board and A/D board were commercial off-the-shelf (COTS) products used primarily in terrestrial robotic applications. The power board was custom designed for StarNav by Jackson and Tull using space rated components. The COTS boards were baked-out and conformal coated to inhibit offgassing. Individual boards met stiffening requirements for launch vibration. The flight unit was tested to validate stability in the launch environment and vacuum conditions of on-orbit operation. Figure 8 provides labels for each of the electronic components.



OPTICAL ASSEMBLY

The optical assembly for the StarNav flight hardware was created by using a set of optics from a commercial 35mm lens (Canon) and designing a custom housing and support tube. The housing wasdesigned to tolerate thermal cycling without stressing the glass elements and yet remained at the precise focal length to enable star identification. Six glass lenses held within a Kovar barrel had the same thermal expansion coefficient as the glass. The individual elements were held in place by Kovar spacers which maintain the required axial displacement. Vent holes were located between each lens to allow trapped gases to escape to vacuum. The Kovar lens assembly was supported by an Invar tube which maintained a fixed distance from the CCD focal plane. Invar has a very low thermal expansion coefficient and the Kovar barrel was mounted to it at the aperture. Material properties of each glass piece was determined using refractometry.