SPIN-STABILIZED SATELLITE WITH NUTATION CONTROL SUBSYSTEM

Abstract: A satellite communications system (100) incorporating a novel nutation control subsystem. A geosynchronous spin-stabilized satellite includes an antenna (113) for receiving rf signals from a ground station (101). An rf beacon pointing error sensor (115) associated with the antenna provides an error signal with a cyclic component at nutation frequency. A bandpass filter (117) is used to isolate this cyclic component. The cyclic component, which may be further phase shifted, activates a despin motor (119) of the satellite to affect spacecraft dynamics (at 121) to damp nutation.

Description: SPIN-STABILIZED- SATELLITE WITH NUTATION CONTROL SUBSYSTEM BACKGROUND OF THE INVENTION The present invention relates to a spin-stablized satellite incorporating a nutation control subsystem. "Spin-stabilized" charactertizes a stabilized device having a rotor or spun portion and a platform or despun portion- the two portions being coupled by a despin motor and bearing assembly. A spin-stabilized satellite may- exhibit certain types of troublesome motions called "wobble", "preces- sion" or "nutation". All such motions tend to result in a displacement of the satellite's geometric axis from its intended mission orientation or attitude. Nutation of a satellite, or the coning motion of the bearing or spin axis about the total angular momentum vector, may result from any of the following disturbances: (1) booster final stage angular motion, (2) operation of the separation equipment, (3) bombardment by micrometeorites, (4) operation of payload components with uncompensated momentum, and (5) operation of mass expulsion devices on the spin- stabilized device. In general, nutation may be reduced by energy absorbing or momentum transfer devices operable on either or both of the transverse axes to attenuate the nutation. Energy absorbing systems often add significant mass to the satellite. gome of these "passive" dampers must be "tuned" to nutation frequency, so that performance is sensitive to spin speed and mass property variations. Several systems use the despin motor to damp nutation. For example, in U.S. Patent No. 4,096,427 to Rosen et al., the outputs of an accelerometer-based nutation sensor and a relative rotation rate sensor are processed to provide despin motor control signals so that the appropriate torques are applied to the motor to damp nutation. Another nutation system, disclosed in U.S. Patent No. 4,272,045, uses a horizon sensor to detect nutation. Other nutation control systems are disclosed in U.S. Patent Nos. 3,695,554 and 3,830,447, and Slafer, L. I. and Marbach, H. D. , "Active Control of the Dynamics of a Dual-Spin Spacecraft" , Journal of Spacecraft and Rockets, Vol. 12, May 1975, pp. 287-293. One approach to nutation control, entitled "Despin Active Nutation Damping Electronics" (DANDE) is described by J. W. Sraay and L. I. Slafer in "Dual-Spin Spacecraft Stabilization Using Nutation Feedback and Inertia Coupling", Journal of Spacecraft and Rockets, Vol. 13, No. 11, November 1976, pp. 650-659. The DANDE system uses a separate nutation sensor which senses nutation angular rates directly to achieve greater nutation damping through the despin motor and to permit independent design of pointing control and nutation damping systems. Thus, the gain and phase nutation feedback can be adjusted independently of the pointing control mechanism. Satellites incorporating such nutation control systems have not solved the nutation problem well enough for all applications. For example, satellites designed for highly directional communications, must detect even small nutations and correct them quickly. In addition, the more effective nutation control systems are relatively complex. What is needed is a satellite system adapted for more precise and rapid nutation control than has been heretofore provided. SUMMARY OF THE INVENTION A satellite communications system includes a satellite with a directional rf receiver for receiving remote radio frequency transmissions from a ground station. The satellite is spin stabilized and has a nominally spun rotor, a nominally despun platform and a despin motor for controlling the relative rotational motion between the rotor and the platform. The satellite is adapted for orbit about a body, such as the earth, on which the ground station is located. The transmissions from the ground station are received by the satellite rf receiver. Means are provided for generating pointing error signals indicating a deviation between the transmission path and the alignment of the directional receiver. Cyclic components of the error signal are processed to provide a control signal to the despin motor to produce an oscillating torque to damp nutation. In one realization of the invention, the pointing error is divided into orthogonal North-South and East- West components; only one component, e.g. North-South, is required for nutation control. This allows decoupling of the nutation control subsystem from a pointing control subsystem. Accordingly, a precise and rapid nutation control is provided suitable for highly directional satellite communications systems. The decoupling from the pointing control subsystem simplifies implementation. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic of a satellite communi¬ cations system with a nutation control subsystem in accordance with the present invention. FIGURE 2 is a perspective view of a satellite communication system in accordance with the present invention at one point of a nutation cycle. FIGURE 3 is a perspective view of the satellite communications system of FIG. 2 at another point of a nutation cycle. FIGURE- 4 is an elevational view of a satellite in accordance with the present invention. FIGURE 5 is a plan view of the satellite of FIG. 4. FIGURE 6 is a schematic of the pointing and nutation control subsystems for the satellite of FIG. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in FIGS. 1, 2 and 3, a nutation control subsystem 103 for a satellite communications system 100 includes a ground station 101 capable of rf transmissions and a satellite 111 with a receive antenna 113 capable of receiving remote transmissions from the ground station, an rf beacon pointing error sensor 115, a filter 117, and a despin motor 119. The despin motor 119 can be operated to control satellite dynamics, indicated at 121 in FIG. 1. The ground station 101 is located on the earth 99, or other orbitable body, as shown in FIGS. 2 and 3. The mass distribution of the satellite 111 is arranged so that a product of inertia, shown as a dumbbell 124, of the platform 125 exists between at least one lateral axis Y and the pitch axis Z. (The other lateral axis of FIGS. 2 and 3 is perpendicular to the page.) Thus, the pitch axis Z is not colinear with any of the principal axes of the satellite, it being understood that the principal axes define axes about which no products of inertia exist. The magnitude and polarity of the respective products of inertia are selected so that changes in torque and thus speed of the rotor 123 in response to a signal representing the nutation induce counter torques on the non-spinning axis to attenuate motions which cause the nutation of the pitch axis Z. The pitch axis Z is colinear with the axis of rotation of the rotor 123. The platform 125 provides support for the receive antenna 113 and a transmit antenna 127, as well as an omni-directional antenna 129, as illustrated in FIGS. 4 and 5. In the illustrated embodiment, all three antennas, 113, 127 and 129, operate at C band. The receive antenna 113 includes a highly directional reflector 131 and a feed horn 133. Likewise, the transmit antenna includes a reflector 135 and a feed horn 137. The diameter of the illustrated reflector 131 of the transmit antenna is 2 meters, and the diameter of the reflector 135 of the transmit antenna is 3.2 meters. This highly directional system 100 is adapted for high-efficiency and high-security rf communications. Even small nutational motions can significantly degrade the performance of such a directional satellite communications system. Fortunately, and in accordance with the present invention, the components that provide for the high directionality, can also provide precise and rapid nutation control. The error sensor 115 measures the extent to which the receive antenna 113, at a given moment, is off alignment for receiving the signal from the ground station transmitter 101. In the illustrated embodiment, the error sensor 115 is a two-axis rf sensor which can isolate the North-South component of any deviation from the East-West component. A similar sensor, not shown, is used for pointing control for the transmit antenna 127. Subsequent nutation control utilizes the North-South error signal alone. However, a combination of the North-South and East-West error signals, or the East-West error signal alone could be used. When the satellite 111 is spinning properly in orbit, one principal axis Z is along the spin axis H, indicated in FIGS. 2 and 3. However, perhaps due to some perturbation, that principal axis Z might undergo an angular displacement from the spin axis. In such a case, the principal axis Z will precess about the spin axis H at a predictable nutation rate. As the receive antenna 113 receives transmissions from the ground station 101, the rf beacon pointing error sensor 115 produces an error signal with a cyclic component corresponding to the nutation frequency. By passing this error signal through the bandpass filter 117 centered at a precalculated "expected" nutation frequency, a control signal is produced which can activate the despin motor 119 in alternating directions to affect the satellite dynamics so as to damp nutation. In the illustrated embodiment, the novel nutation control subsystem 103 is integrated with a pointing control subsystem 105, as shown in FIG. 6. The receive antenna 113 receives the rf transmission from the ground station 101, and the pointing error signal is resolved into orthogonal components by the East-West axis 141 and the North-South axis 143, respectively of the pointing error sensor 115. The East-West and North-South error signals are used for correcting the pointing along the respective directions. The East-West axis 141 of the error sensor 115 is used as a reference to control the platform East-West pointing through a relatively high bandwidth continuous loop. A low pass filter 145 is used to eliminate cyclic components, and the filtered error signal activates the despin motor 119 until the East-West error signal is eliminated. The North-South pointing of the receive antenna 113 is corrected by moving the receive reflector 131 relative to the platform 125. This movement is driven by a stepper motor 149 in response to the North-South error signal from the North-South axis 143 of the sensor 115. A low pass filter 147 is used to eliminate periodic nutation components of the North-South error signal from affecting the stepper motor operation. The output of the low pass filter 147 controls a threshold switch 139, which in turn operates the stepper motor 149. A component of the North-South pointing error signal is preserved for the nutation control subsystem 103. The North-South error signal represents an angle from the ideal North-South coordinate for reception of the ground station transmission. When nutation occurs, the signal has a cyclic component, which corresponds to the nutation frequency. A nutation compensation bandpass filter 117 isolates the nutation frequency component. The bandpass filter 117 is AC coupled to block coupling of constant pointing offset from North-South to East-West. The bandpass filter 117 is reduced in magnitude as rapidly as practical above nutation frequency to decouple undesirable oscillatory and noise signals. A phase shifter 151 may be included to compensate for known systematic errors in phase or delays introduced by the components of the nutation control system. Platform nutation frequency is typically approximately 0.25 Hz so that implementation of a phase shift does not compromise the self-tuning property of the nutation control loop. Proper calibration of the phase shifter 151 can be determined empirically or by known algorithms. The cyclic output of the bandpass filter 117 and the phase shifter 151 is summed with the East-West output signal at hybrid 153. The summed signal is then the control signal for the despin motor 119. The despin motor 119 then can affect satellite dynamics to correct East-West pointing or to damp nutation, or both con¬ currently, as needed. The orthogonality of the component signal inputs to the hybrid 153 provides for simplicity in their combination. Since platform nutation frequency is the same as that of the filter output, spin axis torque appropriately phased at this frequency and applied to the dynamically imbalanced platform 125 generates a transverse reaction torque that reduces the transverse angular rate associated with nutation. Thus, the filter output, properly phase adjusted by the phase shifter 115, is used to control the despin motor 119 to damp nutation. The operation of the nutation control subsystem of the present invention is illustrated in FIGS. 2 and 3. These figures illustrate. an embodiment of the invention in which the spin-stabilized satellite 111 is in orbit around the earth 99. It is desired that the satellite 111 point the antenna toward the ground station transmitter 101. As indicated above, the platform 125 has a dynamic imbalance. In the presence of nutation, the spin axis Z cones around the angular momentum vector H at inertial nutation frequency, which is equal to the platform nutation frequency since the platform 125 is despun. The spin axis Z and angular momentum vectors each pass through the center of mass of the satellite 111. In FIG. 2, where the antenna leans backward., a platform spinup torque 21 causes the platform product of inertia to react against the motion by generating a transverse torque 22 along a line away from the earth. Since the transverse angular rate 23 is earth oriented at this time, such reaction tends to reduce the nutation angle. When the antenna 113 leans forward 180° later in the nutation cycle as shown in FIG. 3, a despin torque 31 is required. In this case the transverse angular rate 33 is oriented away from the earth, and the platform despin torque 31 causes the product of inertia to react against the motion by generating a transverse torque 32 toward the earth 99. When the antenna 113 leans East or West, the reaction torque is always orthogonal to the transverse rate and no torque is desirable at this time. Thus, to damp nutation, the sinusoidal spin torque at platform nutation frequency must be phased such that peak spinup torque occurs when the transverse rate is along the boresight of the antenna 113, for the example shown. The above system is simpler and more direct than the DANDE system, discussed above, since both sensor and actuator are based in the platform coordinate system. Furthermore, the North-South error signal provides a very sensitive and noise free measurement of nutation. Another advantage to the present system is that the pointing control subsystem 105 based on the East-West error signal can be largely decoupled from the nutation control system, based on the North-South error signal. At least in the case of a rigid platform, the effect of spacecraft wobble (due to rotor imbalance) coupled through the nutation damping path is negligible. To assure a large nutational stability margin despite imperfections, sensor cross coupling, and flexible structure effects, the receive antenna North- South signal is compensated and used to command despin torques for nutation damping. Damping of the transverse plane nutation rates is achieved through cross coupling provided by the despun product of inertia. in accordance with the above, spin-stabilized satellite system incorporates improved nutation control. Many variations upon and modifications to the illustrative embodiment are within the scope of the present invention as defined by the following claims.

Claims: CLAIMS What is Claimed is: 1. A satellite system including a satellite for orbiting a body comprising: a dual-spin stabilized satellite having a nominally spun portion, a nominally despun portion, and a despin motor for controlling the relative rotational motion between said spun and despun portions; a transmitter for transmitting a radio frequency signal toward said satellite; a directional radio frequency receiver mounted on said despun portion for receiving said signal and for generating pointing error signals in response thereto; and means for processing said error signal to provide a control signal to said despin motor to cause said motor to apply torque to reduce nutation. 2. The satellite system of Claim 1 further characterized in that said transmitter is located on said body. 3. The -satellite system of Claim 2 further characterized in that said body is earth. 4. The satellite system of Claim 1 further characterized in that said receiver is adapted for generating orthogonal pointing signals, said control signal being a function of exactly one of said orthogonal pointing error signals. 5. The satellite system of Claim 4 further characterized in that said control signal is a function of a North-South pointing error signal. 6. A satellite system including a satellite for orbiting a body comprising: a dual-spin stabilized satellite having a nominally spun portion, a nominally despun portion, and a despin motor for controlling the relative rotational motion between said spun and despun portions; a remote transmitter for transmitting a radio frequency signal toward said satellite; a directional radio frequency receiver mounted on said despun portion for receiving said signal and for generating first and second orthogonal pointing error signals in response thereto, the pointing of said directional receiver being subject to change relative to said platform; a pointing control subsystem for controlling the pointing along one orthogonal dimension of said directional receiver by operation of said despin motor as a function of said first pointing error signal, and for controlling the pointing along a second orthogonal dimension of said directional receiver by adjusting the pointing of said directional receiver relative to said platform as a function of said second pointing error signal; and a nutation control subsystem for damping nutation by operation of said despin motor as a function of said second pointing error signal.

Inventors: ROSEN HAROLD A, SMAY JOHN W, ROSEN, HAROLD, A., SMAY, JOHN, W.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1985
Main classification: B64G1
IPC codes: B64G1, G05D1