Patent ID: 6076774
Filing Date: 2000-06-20
Classification: B64G

Abstract:
A method of acquiring the Earth by a three-axis stabilized spacecraft, the spacecraft having an Earth sensor with a boresight direction and a Sun sensor with a boresight direction, wherein a first vector from the spacecraft to the Sun defines a Sun vector, a second vector from the spacecraft to the Earth defines a nadir vector, and wherein the angle between the Sun vector and the nadir vector defines a Sun-Earth separation angle, the Sun-Earth separation angle changing over time due to the relative movements of the Earth, the Sun and the spacecraft, the method comprising:obtaining an Earth cone described by rotating the nadir vector about the Sun vector;determining a slewing axis by taking the cross product of the Earth sensor boresight with the Sun sensor boresight;determining which edge of the Earth cone will be used to stop the slew based on the desired final attitude of the spacecraft;slewing the spacecraft about the slewing axis until the Earth sensor boresight touches the determined edge of the Earth cone;updating the Earth cone due to the changing Sun-Earth separation angle;performing a spiral coning maneuver about the updated Earth cone until the Earth sensor detects the Earth;locking onto the Earth so as to hold the Earth sensor boresight coincident with the nadir vector; androtating the spacecraft about the nadir vector so as to bring the spacecraft into a desired final orientation.