Attitude angle calculation apparatus for a geostationary satellite

Claim 2

Attitude angle calculation apparatus to be mounted on a satellite for calculating an attitude angle of the satellite, characterised by an earth sensor (3) for detecting deviation of an angle (6(t)) defined between a predetermined axis (Z B ) of the satellite and a straight line (PO) connecting the satellite and the geocentre of the earth (A); a fixed-star sensor (5) for detecting a physical intermatrix of a fixed star (2); and yaw angle determination means (4,6,7) responsive to orbital position information (D) of the satellite sent from a ground station and to outputs of the earth sensor and the fixed-star sensor to determine the yaw angle (Ψ(t)) of the satellite independently of roll angle (ϕ(t)) and/or pitch angle (6(t)) thereof.

Claim 3

Apparatus as claimed in claim 1, characterised in that the fixed-star sensor (5) provides coordinates of the fixed star in a field of sight thereof; and in that the yaw angle determination means comprises orbital coordinate calculating means (4) responsive to the orbital position information to calculate orbital coordinates of the satellite, fixed-star vector calculating means (6) responsive to an output of the fixed-star sensor for calculating a unit vector in a direction to the fixed star with respect to coordinates of the satellite axis, and yaw angle calculating means (7) responsive to the outputs of the earth sensor, the orbital coordinate calculating means and the fixed-star vector calculating means to calculate the yaw angle of the coordinates of the satellite axis with respect to the orbital coordinates.

Claim 4

Apparatus as claimed in claim-1, characterised in that the fixed star is the sun; and in that the yaw angle determination means comprises orbital coordinate calculating means (4) responsive to an output of the earth sensor (3) for calculating orbital coordinates of the satellite, a sun sensor (11) for determining azimuth angle and angle of incidence of the sun with respect to coordinates of the sun sensor, sun vector determining means (6) responsive to an output of the sun sensor for calculating a unit vector in a direction to the sun with respect to coordinates of the satellite axis, sun ephemeris calculating means (9) for calculating the right ascension and the declination of the sun with respect to geocentric inertial coordinates, and a yaw angle calculator (7) responsive to outputs of the earth sensor, the orbital coordinate calculating means, the sun vector calculating means, and the sun ephemeris calculating means for calculating the yaw angle of the satellite axis coordinates with respect to the orbital coordinates.

Claim 5

Apparatus as claimed in claim 1, characterised in that the fixed star is the sun; and in that the yaw angle determination means comprises a register (10) for storing positional information of the satellite and its orbit received from the ground station as a command signal, initial correction calculating means (12) responsive to the position information stored in the register and an output of the earth sensor at a time instant at which the command signal is transmitted for calculating an initial value of correction, a timer (14), orbital position calculating means (4) responsive to the satellite position stored in the register and an output of the timer for calculating an angular relation between the satellite and the sun, a sun sensor (5) for measuring a relative variation of an incident angle of the sun, correction amount calculating means (16) responsive to outputs of the earth sensor, the orbital position calculating means and the initial correction calculating means for calculating a correction amount sequentially, and yaw angle deviation calculating means (17) responsive to an output of the sun sensor and the correction amount calculating means for calculating deviation of yaw attitude angle of the coordinates of the satellite axis with respect to the orbital coordinates.