The line of sight (LOS) jitter on an imaging sensor of the observation satellite is defined as the time varying motion on the focal plane during image acquisition, which is caused by internal and external disturbances acting on the optical payloads. In general, the term of jitter is referred to high frequency random motion of the satellite structure that seriously affect the image quality, and it is regarded as an important factor to determine the quality of the image. This means that, to comply with the strict mission requirement for the acquisition of high-quality images, it should be performed as a principal task to strictly predict the jitter level and verify whether the level of jitter is in compliance with given requirements or not through incorporating numerical and experimental approaches. In case of dissatisfaction with the desired jitter requirement, a particular method or plan should be prepared to attenuate the jitter.
The internal jitter contributors of the satellite are commonly having mechanical moving parts such as reaction wheel (RWA), control moment gyro (CMG) for attitude control, gimbal antenna for transmitting the massive data to the desired ground station, spaceborne cryocooler to cool down the focal plane of an infrared (IR) sensor to low cryogenic temperatures. In case of spaceborne cryocooler, pulse-tube type cryocoolers are widely used on accounts of their various advantages of simplicity, low cost, and high reliability. However such cryocooler also produces undesirable micro-vibration, which is the term generally used in spacecraft community to describe low amplitude vibrations and can introduce jitter problems into an optical payload system. This micro-vibration induced by the spaceborne cooler is the one of main degradation source of the image quality. Therefore, to obtain high quality images, micro-vibration disturbances need to be isolated.
In general, micro-vibration from the cooler can be easily isolated by mounting the cooler on a vibration isolator with low stiffness to attenuate the vibration transmitted to the satellite structure. Better isolation performance can be obtained by placing the eigen frequency of the isolation system lower than the cooler operation frequency as much as possible. However, the structural safety of the cooler supported by an isolator with weak stiffness cannot be guaranteed under the much more severe vibration condition in launch environments. If the cooler assembly supported by a low-stiffness isolator is rigidly fixed by a holding-and-release mechanism during launch and released in orbit, the above problem can be easily solved. However, this approach increases the system complexity, lower the reliability, and propagate the mass of the total system. In addition, if some problem occurred in activating holding-and-release mechanism on-orbit, the micro-vibration cannot be isolated anymore.
In this study, we proposed a spaceborne cryocooler micro-vibration isolator employing a pseudoelastic shape memory alloy (SMA) mesh washer, which guarantees vibration isolation performance in a severe launch vibration environments without applying an additional holding-and-release mechanism while effectively isolating the micro-vibrations from the cooler on-orbit. To investigate the basic characteristics of the isolator, we performed static tests at the isolator level and vibration isolator assembly level combined with the cryocooler. The dynamic characteristics at the cooler assembly level were also investigated through free vibration tests. Based on the results of basic characteristic tests at isolator level and geometry of the isolator, we proposed a simple equivalent mathematical model of the isolator where the stiffness and damping mainly vary according to the compressive displacements of the SMA mesh washer. In addition, based on the static and dynamic tests at vibration isolator assembly level, mathematical model of the vibration isolator assembly was proposed and used to predict the micro-vibration isolation capability. The effectiveness of the isolator design in launch and on-orbit micro-vibration environments was demonstrated through launch vibration tests and micro-vibration measurement tests.