7.2.2 Attitude Determination

The hardware comprising the attitude sensing equipment consists of several components. A star tracker is employed for primary stellar attitude determination. Two digital sun sensors supplement the star tracker for attitude determination. Inertial orientation is resolved with solid state rate gyros.

Spacecraft stellar position and attitude determination will be achieved primarily through the use of optical sensors. The ACS subsystem design for Asterius employs a star tracker as the primary attitude sensor. The star tracker will be of the wide field of view (WFOV) variety. The Ball Aerospace & Technologies Corp. CT-631 Star Tracker is the preferred choice. This conclusion was made after researching and comparing the available technology. The decision of WFOV tracker (15 -40 FOV) over a higher accuracy narrow field of view (NFOV) tracker (<10 FOV), was based on several factors.

Aside from the dependable history of Ball Aerospace, several technical countenance of the CT-631 Star Tracker were considered. A WFOV and a NFOV tracker were studied and compared. The initial aspects compared were the cost, power consumption, and weight. In all three of these considerations the WFOV tracker surpasses the NFOV tracker. Table 4 summarizes this comparison.

  

Table 4: Comparison of Narrow Field and Wide Field Star Trackers from Bell Aerospace (adapted from Reference [4])

Star Tracker	FOV	Power	Weight	Accuracy
Type		Consumption
	(deg)	(W)	(kg)	(arcsec)
NFOV	< 10	> 20	> 20	$\sim 1$
WFOV	15-40	< 5	< 4	$\sim 20$

The advantages of a WFOV tracker over the NFOV are substantial. These benefits can be largely attributed to the decreased database size of the WFOV tracker. The CT-631 has a FOV of $20\ensuremath{^\circ}\times 20\ensuremath{^\circ}$, which has a database about 80% smaller than an average NFOV tracker with a FOV of 8  [4]. The FOV does not significantly affect the percent error of an incorrect attitude estimate when all stars in the FOV are true and correct. However, when two false stars or images are introduced, a FOV representative of the CT-631 has a gain of approximately 30% over the NFOV tracker in estimating the correct attitude.

The published specs of the CT-631 give an attainable accuracy of 20''. However the Near Earth Asteroid Rendezvous (NEAR) spacecraft, which carries a CT-631 star tracker, has attained accuracies of 3''-7'' [5]. This is comparable to the published specs of the Ball Aerospace CT-601/602 NFOV trackers of 3''. It is expected that the integration of the CT-631 star tracker into the ACS design will require certain modifications to tailor the sensor to the specific needs of Asterius which may slightly alter, hopefully improving, the CT-631 performance.

Although the CT-631 star tracker is capable of initial attitude acquisition, it is not fully autonomous. Therefore, two digital sun sensors are also included in the ACS design. They will be able to aide the star tracker in determining spacecraft attitude both initially and from `lost' conditions, and to supplement the attitude estimate accuracy. The two sun sensors comprise a two-for-one system of redundancy. Therefore, only one need be active at any time. Such an arrangement will reduce power consumption and is repeated throughout the ACS design. The implementation of the digital sun sensors will also proxy for the star tracker when it is in sight of the sun or other circumstances where it may be inoperable.

The angular motion of the spacecraft about its inertial reference frame is determined by solid state rate gyros. They will consist of three units constituting three-for-two redundancy. Thus only two units need be operable at any given time while the third will serve as a backup in the event of equipment failure.