Peter Zimmer, J.T. McGraw and Associates, LLC (JTMA); John McGraw, J. T. McGraw and Associates; Mark Ackermann, J.T. McGraw and Associates, LLC
Keywords: SSA, daylight, optical, telescope, LEO, NIR
Abstract:
Low Earth orbit (LEO) is getting busy. As more and more satellites are launched, the demand for more and better data will grow. We’ve previously demonstrated how small optical systems can find, measure and monitor LEO objects, even very small and faint ones. But the limitations of terminator viewing mean that maintaining custody of these objects with only these observations is not practical.
Other work presented at this conference in the last few years has shown the potential for daylight measurements of satellites. Much of that work has focused on short-wave infrared (SWIR) band methods. The far lower sky brightness in the SWIR regime has considerable benefits. However, the fundamental light source, the Sun, puts out dramatically more energy in the visible regime and thus the advantages of SWIR are not unequivocal. And while SWIR systems can be made cost effective, visible and near-infrared telescopes and detectors are commercial products, benefiting from the full effects of productions scale and availability.
We’ve presented extensively in the past on how to optimize twilight and nighttime SSA detection systems, but daylight operations posed a significantly different set of challenges. We start from a radiative transfer model (MODTRAN) of the daylight observing for LEO objects. We derive models for the signal and background levels that can be expected over a range of observing conditions. This becomes the basis of modeling throughput and detectivity as a function of different system configurations and we make recommendations for various combinations of aperture size, focal length, pixel scale, exposure time, etc. We then compare this model to observational data obtained with a proof-of-concept system.
But detecting LEO objects in the daytime is only useful if those measurements can be calibrated to known astrometric reference stars. We discuss the challenges of doing so for a daylight system where the solar thermal load can appreciably change opto-mechanical alignment and make some recommendations for minimizing these effects.
Date of Conference: September 15-18, 2020
Track: Optical Systems Instrumentation