Joshua Garretson, U. S. Space Force
Keywords: Centroiding, Centroid Anisoplanatism, Tilts, Tracking Error, Jitter, Adaptive Optics
Abstract:
The strong correlation between the estimated tilt given by centroiding, sometimes referred to as the centroid tilt, and the gradient tilt decreases as certain conditions change. It can be demonstrated in some scenarios that there is higher correlation between centroid estimated tilt and Zernike tilt when a threshold is applied or in the presence of strong fluctuations of atmosphere. When an extended target or beam is being tracked there may also be lower tilt-centroid correlation over all, but this error may also have less dependency phase fluctuation strength. Wave optics and numerical simulations have been performed to show various dependencies of tilt-centroid correlation and centroid anisoplanatism, or tilt-centroid residual error. Additionally, the correlation between centroid tilt and Zernike coma and the effects of thresholding are discussed. Lastly, the correlation between G-tilt, a combined tilt-coma function, and various centroid types are explored. There is a need to better understand centroids and their effects in both tracking and adaptive optics technology. Developing this understanding will lead to improved jitter mitigation techniques, wavefront estimation, and less space object state covariance. Image centroiding is done routinely in many optical technologies and space situational awareness techniques. Devices such as Shack-Hartmann wavefront sensors and position sensitive devices use centroids to estimate wavefront tilts. Moreover, centroids have a long history of being used in resident space object tracking, specifically for geostationary objects, and satellite state estimation. In lay terms, the centroided image is used to estimate an angle. This angle could be wavefront tilt, or it may be the angle off boresight or optical mechanical axis. It’s observed that the value of the estimated tilt from centroiding varies from that of the actual tilt. This error, termed centroid anisoplanatism, changes strength under varying conditions and is stochastic. During most scenarios centroid anisoplanatism can be ignored because it is quite minor compared other sources of error such as jitter or higher order aberrations. In moderate to strong atmospheric fluctuations though centroid anisoplanatism can be significant, inducing Strehl ratios as low as 0.5. It is possible that this is a primary source of performance degradation for traditional Shack-Hartmann wavefront sensors in stronger fluctuations. Centroid anisoplanatism strength is also impacted by source and receiver characteristics.
Date of Conference: September 17-20, 2024
Track: Satellite Characterization