Keywords: Space Debris, Characterization, LiDAR, wide FOV
Abstract:
Millions of debris pieces are currently in Earth orbit. The debris objects depending on their sizes can pose either mission termination risks (larger than ~5 mm) or mission degradation risks (smaller than 1 mm). However, the current sub-mm debris environment has not been defined well because measurements are quite limited in terms of the orbital regimes and that not continuously available yet. We have designed a special LiDAR as a novel approach to the detection and characterization of space sub-mm debris objects, in situ, and without tactile contact. The design of novel LiDAR is constrained in size and power to be flown aboard a 6U CubeSat and would provide a detection area that is one order of magnitude larger than the current state-of-the-art which is a one square meter impact detector. The debris-sensor incorporates a pulsed laser that generates an optical fan-shaped beam. Given the nature of the pulse, and its operational duty cycle, it is designed to detect small debris passing through the laser net. The design would be based on commercial-off-the-shelf lasers, detectors, and filters and meets the technical specifications and mission requirements for a small debris detection system that can operate in low earth orbit (LEO). The debris-sensor design was modeled to ensure an acceptable signal-to-noise ratio (SNR) based on calculated density of sub-mm debris. The results of the model were used to create an optical link budget which is further used to adjust and optimize the receiver optical system. The transmit fan beam is achieved by spreading the laser energy using a pair of Powell lenses which convert the Gaussian distribution of laser energy into a nearly flat-top energy profile. The combination of lenses then forms a rectangular cross section optical window which serves to illuminate all debris passing through. In this paper, we present the design of the receiver optical system taking into consideration that the orbital debris velocity relative to the LiDAR will be at maximum,15 km/sec (a head-on with respect to the satellite motion). The receiver optics will accept and image scattered laser light from a debris particle located within the illuminated volume. This consists of a region bounded by one wide angle (50 degrees (wide portion of the fan beam)), a narrow angle (6 degrees) and a range spanning to 20 meters. Our design uses Photo-Multiplier Tubes (PMT) to acquire both high gain (e.g., 106) and fast response (e.g., ns) in comparison to CCD or CMOS detectors which acquire photons and store charge to realize gain. Thus, PMTs can respond to extremely fast events due to high relative speed of sensor and debris particles as they do not store charge. Three PMTs, each with an effective area of 14x14mm, are used as the detector system. The collecting lens, constrained by the 6U CubeSat structure, is designed with 90 mm diameter aperture and 100 mm focal length. While PMTs have advantages over solid state detectors, one disadvantage is that the PMT photo cathode sensitivity is rarely uniform. A photon flux hitting different parts of the cathode surface will cause signal fluctuations. In stellar photometry, a Fabry lens system is commonly used that spreads out the incoming light over the whole cathode area. Meanwhile, the PMT module housing prevents the collinear placement of the detection areas. So, Fabry lens combination has been integrated into the optical path to reduce signal fluctuations derived from variations in the optical sensitivity of the photo cathode as well as to fill in the gaps between the PMT effective areas to some extent. The Fabry lenses of 25mmx16mm are located close behind the focal plane of objective lens. The active area of PMTs covering distinct portions of angular field of view (FOV) are somewhat compromised for a benefit of increased SNR by minimizing the background noise. The Fabry lenses and the associated PMTs conform to the curved focal surface of the objective lens to maximize the FOV. The available space in a 6U CubeSat enclosure does not allow a group of lenses as collective lens that would produce an aberration-free image plane over a wide FOV. The collecting lens group is roughly f/1.2, which is extremely fast. Placing the Fabry lenses and PMTs on a curved focal plane trades imaging resolution, within the fan beam, for reduction of off axis aberration. Moreover, due to the short focal length of the required Fabry lenses, they are designed as achromats using two glasses of high index of refraction, the use of which reduces the thickness and curvature of the lenses. The ZEMAX optical design program has been used to model and design the lens combination system based on the trade studies of multiple available options and variation.
Date of Conference: September 17-20, 2024
Track: Space-Based Assets