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Frames of Reference and Data Definitions

The primary data return from the S3S is a four-element quaternion vector representing the rotation from the inertial reference frame into the frame of the sensor. This section presents a short summary of these reference frame definitions and some helpful mathematical relationships to help make best use of the sensor output.

The inertial frame used by the sensor is the Earth-Centred Inertial J2000 system. Star positions are not corrected for parallax, sacrificing some precision for simpler clock-free operation.

The sensor reference frame is defined by mechanical features on the housing. Two primary vectors define the sensor reference frame. We have included both informal and formal definitions of these vectors.

· The +Z axis is informally the outward boresight of the sensor (i.e., direction the lens is facing). Formally, this direction is normal to the mounting bosses on the back of the sensor, pointing away from host spacecraft.

· The +X axis is informally in the direction of the power/data connector. More formally, the line of this vector is oriented 26.997° counterclockwise (facing the lens), from the datum formed by the two alignment pins.

· The +Y is chosen to make the coordinate system right-handed.

The quaternion returned by the sensor is a vector of four double-precision numbers. The scalar part of the quaternion is the first element; the vector part, the last three. I.e.:

The rotation matrix from the inertial frame (I) to the sensor body frame (B) can be found from:

The attitude error covariance returned from the sensor follows the classic definition by Shuster, calculated from the body-frame star vectors.

Note: This covariance is expressed in the sensor frame.

The epoch time measurement is expressed in seconds from the reception of the command that initiated the cycle (see section 5.8). If time correction is turned off, then this represents the moment in time at which the returned quaternion is valid. If time correction is turned on, the star tracker uses its attitude and rate knowledge to extrapolate the quaternion to the moment the command was received. In this case the quaternion telemetry can be used in the satellite ACS without requiring epoch telemetry.


Date: 2015-12-17; view: 667


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