Peter Zimmer, J.T. McGraw and Associates, LLC ; John T. McGraw, J.T. McGraw & Associates, LLC; Mark R. Ackermann, J.T. McGraw and Associates, LLC;
Keywords: daylight, earthshine, SSA, SDA, LEO, telescope, astrometry
Abstract:
Over the coming decade, the number of satellites launched into low Earth orbit (LEO) will experience unprecedented growth, a level unimaginable even a few years ago. SpaceX launches hundreds Starlink satellites per month, and that is just the tip of the iceberg: Over 50,000 LEO satellites are planned to launch in the next ten years. Our current space domain awareness (SDA) infrastructure is aging and already stressed. While new systems like the Space Fence are coming online soon, and commercial radar systems are growing, the number of objects in LEO is increasing far faster than the resources dedicated to monitor them.
Small optical telescopes, such as those deployed worldwide by a number of commercial firms, have proven to be a cost-effective augmentation to Department of Defense SDA systems for geostationary satellites. Because of observational geometry, LEO satellites are more challenging for optical telescopes. The satellites are only viewable for a few hours each day, in twilight just after dusk and just before dawn, and any particular satellite is only above the horizon of a given site for a few minutes per day if ever and may only be observable in twilight every few weeks. So, while each telescope is itself cost effective, the number of telescopes required to provide adequate twilight coverage of LEO undercuts that advantage.
That problem can be greatly reduced if LEO satellites can be observed during the day. Instead of an eighth of a day of usable time, the value increases to more than half. Moreover, if daylight LEO tracking systems can be produced with largely COTS-based components, then the cost effectiveness remains. Weve shown that earthshine provides a natural light source to enable just that.
However, significant technical challenges remain. The most obvious challenges revolve around measuring these objects against a very intense and changing background sky the daytime sky background is a strong function of both angle above horizon and distance from the Sun. Aerosols and cloud particles complicate this further. Daytime convection drives intense optical turbulence blurring the images received in the day. And particulate matter in the atmosphere above the observing site appear as out-of-focus objects moving with the mean wind, sometimes looking like observing through a snowstorm, making for a complicated image processing problem.
Additionally, relatively few stars are bright enough to be visible in the daytime, only a few per square degree, rather than hundreds or thousands at night. That means that astrometric measurements must be made indirectly, relying on the stability of the telescope mount and optomechanical system. COTS-based mount frequently have open loop rms pointing repeatability in the 5-10 arcsecond range. It is unclear that these values will hold for daytime observations when thermal changes will assuredly affect performance, as will a far different atmospheric refraction environment.
Well address the progress weve made over the last year on overcoming these various technical challenges and report results from a new prototype system designed to help quantify and eventually mitigate these issues.
Date of Conference: September 14-17, 2021
Track: Optical Systems & Instrumentation