Three axis thruster modulation

A spacecraft having three mutually orthogonal control axes comprising:

• a plurality of thrusters capable of providing torque impulses in both directions about each of said axes;
• means for generating acceleration commands for each axis for performing spacecraft attitude control and stationkeeping functions;
• modulation means for generating thruster selections and thruster on-times for execution during an upcoming sample period based on said acceleration commands, said modulation means including means for generating a rate change command for each axis for execution during an upcoming sample period by multiplying said acceleration commands by a time length of said sample period and adding thereto any residual unfired rate change from a preceding sample period;
• said modulation means including thruster-select and timing logic means for determining simultaneously from said rate change command the thruster selection and the thruster on-times for the upcoming sample period, said logic means also forwarding the thruster selection and thruster on-times to thruster output processing means for firing of the thrusters during the upcoming sample period;
• and
• said modulation means further including means for computing an actual rate change estimate for the upcoming sample period based on the thruster selection and thruster on-times and means for generating a residual unfired rate change for each axis for use in generating a rate change command for a subsequent sample period by comparing the rate change command for the upcoming sample period with the computed actual rate change estimate for the upcoming sample period.

A modulation method for selecting thrusters and thruster on-times from angular acceleration commands to accomplish attitude control and stationkeeping operations in a spacecraft having thrusters producing full torquing capability about three spacecraft control axes, said method comprising the steps of:

• receiving acceleration commands for each axis;
• generating a rate change command for each axis for execution during an upcoming sample period by multiplying said acceleration commands by a time length of said upcoming sample period and adding thereto any residual unfired rate change from a preceding sample period;
• selecting thrusters and determining thruster on-times from the generated rate change command using spacecraft mass properties, thruster placement and alignment relative to the spacecraft center of mass and thruster minimum on-times and impulse verses time characteristics for the thrusters;
• forwarding the thruster selection and thruster on-times to thruster output processing means for firing of the thrusters during the upcoming sample period;
• determining an estimate of the actual rate change for the upcoming sample period based on a thruster selection and thruster on-times;
• and
• generating a residual unfired rate change for each axes from the upcoming sample period for use in generating a rate change command for a subsequent sample period following said upcoming sample period.

A modulation method for selecting thrusters and thruster on-times for a spacecraft having thrusters for producing torques in three spacecraft control axes, said method comprising the steps of:

• generating a rate change command for each axis for execution during an upcoming sample period from predetermined acceleration commands and from any residual unfired rate change from a preceding sample period;
• determining simultaneously, from said rate change command for each axis, the thruster selection and the thruster on-times for the upcoming sample period;
• forwarding the thruster selection and thruster on-times to thruster output processing means for firing of the thrusters during the upcoming sample period;
• computing an actual rate change estimate for the upcoming sample period based on the thruster selection and thruster on-times;
• and
• retaining a residual unfired rate change for each axis from the upcoming sample period for use in generating a rate change command for a subsequent sample period following said upcoming sample period.