Until the time of this reporting, when a space vehicle required a reference signal for inertial pointing, the choices were a signal beacon from an Earth location, the Earth radiance in the visible spectrum, or a star tracker. However, limitations can arise from using these techniques. For example, the signal beacon suffers from limited signal power (either in RF or optical) and will constrain the application to limited ranges, errors due to stray-light and centroiding limit the accuracy of a star tracker, and the spatial/temporal variability of the Earth’s albedo and its illumination by the Sun introduces limitations when used in the visible or near infrared light.
To bypass these limitations, the bright and near-uniform image of the Earth in the LWIR (long wavelength infrared) (8 to 13 micron) may be used to obtain a reference signal for inertial stabilization and pointing of a spacecraft platform.
The potential of using the bright and near-uniform Earth thermal infrared (or LWIR) signature as a stable reference for accurate (microrad or less) inertial pointing and tracking of an optical communication beacon onboard a space vehicle was investigated, including the determination of the fundamental limits of applicability of the proposed method for space missions in deep space. In the thermal infrared, Earth’s emission appears relatively uniform and is independent of the Sun’s illumination. Therefore, it can be used as a stable pointing reference.
Objective of the stabilization is to align the Earth’s image along the desired line of sight (LOS) in the face of existing system disturbances. Because of the submicroradian precision levels required by high-precision pointing, a large dynamic range is involved in the dynamic response of the pointing system. Hence, the pointing and tracking algorithms are based on a multistage approach that includes: (a) a coarse stage (outer loop) based on sequential loop closure of a vehicle attitude stabilization feedback control loop at a low control bandwidth (10–3 to l0–1 Hz) to compensate for typical attitude control disturbances; followed by (b) a fine stage (inner loop) based on an image boresight stabilization feedback loop at higher frequencies that makes use of an articulated (tip/tilt) fast steering mirror to track the Earth’s LWIR reference signal in the image plane.
A simulation package has been developed that generates spacecraft and gimbaled camera dynamic model, inputs realistic spacecraft disturbances, stabilizes the spacecraft attitude dynamics via a proportional-integral-derivative (PID) control loop at 0.01 Hz, propagates the azimuth and elevation states of the dynamics of the detector plane, adds the sensor noise model to the computed detector azimuth and elevation based on a detector model and centroiding algorithm, and stabilizes the detector line-ofsight via PID control at 100 Hz. A block diagram is shown in the figure.
A comparison of the performance of a quantum-well infrared photodetector sensor array using data from actual space-qualified components has been carried out. Preliminary results involving the effect of the sensor performance on the LOS system stabilization have been studied. Using realistic noise spectra from the Mars Reconnaissance Orbiter mission, simulations demonstrated that submicroradian stabilization accuracy, shown in the figure as time history of the detector line-of-sight azimuth and elevation angles with respect to a reference datum on-board-the spacecraft, is feasible using this approach.