A simplified laser communications (lasercom) system architecture, primarily for a deep-space flight transceivers, can be realized by decoupling the lasercom optical components from the host spacecraft using a disturbance-free platform (DFP) developed by Lockheed Martin Space System Company. Unlike conventional lasercom system architectures where a high bandwidth control loop is used to stabilize the optical line-of-sight in the presence of platform disturbance, the DFP package isolates the optical train from the high frequency platform jitter produced by the host. By preventing the vibration from coupling into the optics train, the need for a high bandwidth beam stabilization control loop (including fast steering mirror, detectors, controls and the associated relay optics) is eliminated with possible mass savings. Effective isolation of the platform jitter also enables the optical focal plane array to operate at a much longer integration time, thus enabling the use of either faint stars or a weak beacon as a pointing reference. This feature can allow the same lasercom system architecture to be employed for deep space and some near Earth applications, and can potentially enable deep space return signal pointing without the need of an uplink beacon.
The Terrestrial Planet Finder Coronagraph is a visible-light coronagraph to detect planets that are orbiting within the Habitable Zone of stars. The coronagraph instrument must achieve a contrast ratio stability of 2e-11 in order to achieve planet detection. This places stringent requirements on several spacecraft subsystems, such as pointing stability and structural vibration of the instrument in the presence of mechanical disturbance: for example, telescope pointing must be accurate to within 4 milli-arcseconds, and the jitter of optics must be less than 5 nm. This paper communicates the architecture and predicted performance of a precision pointing and vibration isolation approach for TPF-C called Disturbance Free Payload (DFP)* . In this architecture, the spacecraft and payload fly in close-proximity, and interact with forces and torques through a set of non-contact interface sensors and actuators. In contrast to other active vibration isolation approaches, this architecture allows for isolation down to zero frequency, and the performance of the isolation system is not limited by sensor characteristics. This paper describes the DFP architecture, interface hardware and technical maturity of the technology. In addition, an integrated model of TPF-C Flight Baseline 1 (FB1) is described that allows for explicit computation of performance metrics from system disturbance sources. Using this model, it is shown that the DFP pointing and isolation architecture meets all pointing and jitter stability requirements with substantial margin. This performance relative to requirements is presented, and several fruitful avenues for utilizing performance margin for system design simplification are identified.
The control system architecture and vibration mitigation approach for a Terrestrial Planet Finder (TPF) mission based on structurally connected interferometers are defined. The spacecraft configurations investigated and associated control and operational requirements are presented. Disturbance sources are identified and their relevance assessed. Results of dynamics analysis are presented, as well as a description of the dynamic models and simulations used to predict on-orbit performance. An assessment of the maturity of the technologies for control and vibration mitigation is provided. Analysis results indicate that pointing and path-length control requirements for a TPF mission based on structurally connected interferometers with baselines from 9 to 80 meters can be achieved using a conventional spacecraft attitude control system combined with active pointing and path-length control and a vibration mitigation approach that does not rely on structural damping.
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