On-board three-axes attitude determination and control system

An on-board method of autonomously determining and controlling the attitude of an unbalanced spacecraft during its transfer orbit, comprising the steps of:

• stabilizing said spacecraft after separation from a launch vehicle;
• determining the spin axis orientation of said spacecraft relative to a first planetary reference;
• determining the three-axis attitude of said spacecraft once a second planetary reference is in view;
• bias calibrating a set of three-axis gyros;
• repeatedly integrating the outputs from said bias calibrated gyros in a state estimator model of said spacecraft for continously updating an estimate of the three-axis attitude of the spacecraft, body rate of said spacecraft, and acceleration states of said spacecraft during each phase of said transfer orbit;
• and
• generating at least one command signal for controlling a thruster motor from a linear combination of said estimated attitude, rate and acceleration states of said spacecraft.

In a spacecraft having at least one planetary sensor, at least one horizon crossing indicator and a set of three-axis gyros, an on-board three-axes attitude determination and control system, comprising:

• means for initially determining the spin axis of said spacecraft from said planetary sensor relative to a first planetary reference;
• means for determining the three-axis attitude of said spacecraft for said horizon crossing indicator when a second plantery reference comes into view;
• means for bias calibrating at least one of said three-axis gyros;
• means for repeatedly determining the current attitude of said spacecraft by integrating said bias calibrated gyro outputs in a state estimator model wherein estimated attitude states of the spacecraft, body rate states of said spacecraft, and acceleration states of said spacecraft are linearly combined to generate at least one thruster command signal;
• and
• means for actuating at least one thruster to control said attitude of said spacecraft in response to said determination of the current attitude.