Abstract: A spacecraft uses monopropellant arcjets for velocity change such as for north-south stationkeeping. It has been discovered that, while an arcjet cannot be modulated by pulsing the fuel supply, the amount of thrust can be varied by modulating the power applied to the arc, without extinguishing the arc. While the specific impulse (I.sub.SP) of the arcjet is thereby reduced from the maximum I.sub.SP of which the arcjet is capable, the resulting I.sub.SP may still be larger than the combined I.sub.SP of an unmodulated arcjet in conjunction with a modulated chemical thruster in a typical scenario. According to the invention, attitude control is provided in conjunction with north-south stationkeeping or other velocity change by, in response to an error signal generated by an attitude control system, modulating the arc power(s) of an arcjet thruster(s), which provides the velocity change. The arc is not extinguished during the stationkeeping maneuver, but is varied in magnitude.

Abstract: A pitch momentum stabilized spacecraft includes an earth sensor for generating roll signals. The roll signals are applied to torquers for proportional (P) or proportional-integral PI control of roll and yaw. According to the invention, the sensed roll signals are bandpass filtered at the orbit rate, phase adjusted and applied to the roll and yaw torquers for closing a high gain degenerative feedback loop at the orbit rate, which reduces orbit-rate components of roll and yaw. A second bandpass filter at the second harmonic of the orbit rate is also connected to the earth sensor, for producing second harmonic signals, which are phase controlled and summed with the fundamental orbit rate signals, for degeneration of attitude perturbations occurring at twice the orbit frequency.

Abstract: An attitude control system for a spacecraft includes sensors for generating attitude control signals and logic for producing torque demand signals in the form of a T.sub.d matrix. The thrusters are oriented in complementary pairs about the center of mass for, when energized, producing mutually opposite torques. Information about the location of thrusters and their thrusts may be represented by a pulse-width-to-torque transformation matrix C. A weighted pseudo inverse of C is precalculated asQ=WC'(CWC').sup.- (13)and is stored in memory associated with the flight computer, thereby avoiding the need for the flight computer to perform the intensive calculations of Q. Thruster pulsewidth for attitude control is easily calculated by the flight computer as.DELTA..sub.p =.tau..sub.p QT.sub.d (14)where .tau..sub.p is the control cycle time.

Abstract: An attitude control system using magnetic torquing is enhanced using a momentum wheel which is pivoted if the sensed error signal about the roll axis exceeds a given threshold. The momentum wheel is pivoted under control of a stepping motor through a first angle proportional to the detected average roll error, and one-half nutation period later through an equal angle in the same direction. Alternatively, the momentum wheel is pivoted over first and second angles of different values wherein the difference is related to nutation.