FLOCS, a Folded-Optic CubSat Sensor for Littoral Observations

Lead PI: David Landis, The Charles Stark Draper Laboratory Inc
Start Year: 2018 | Duration: 2 years
Partners: NASA Ames Research Center

A strong candidate for solving the operational requirements of high temporal cadence, high spatial resolution and large area coverage is a constellation of CubeSats, which individually and collectively offer several key benefits. First, a typical CubeSat bus and payload can be manufactured for significantly lower cost than a traditional space-based asset, due in part to lower component cost and significantly smaller size. Radiation requirements for low-earth orbit (LEO) are lower than for satellites in high or geostationary orbits, allowing for the use of industrial or military- grade components. Additionally, manufacturing multiple quantities of identical satellites provides and economy-of-scale in the fabrication and test process. Second, CubeSats are often deployed in at a lower orbital altitude, providing a closer view of the earth which reduces the size of the optics and detectors necessary to make observations. Traditional space-based assets are placed in much higher orbits, partially to increase their operational life by reducing atmospheric drag. Third, multiple CubeSats are commonly deployed from a single launch vehicle, often as secondary payloads.
India’s Polar Satellite Launch Vehicle (PSLV) recently deployed 104 CubeSats on a single launch. Both of these contribute strongly to an overall reduction in launch cost for a single or multiple CubeSats.
The goal of the proposed research is to develop an optical system that can monitor sea surface temperature and perform cloud characterization by acquiring images of littoral regions from low- earth orbit. There are two primary challenges in the proposed work: first, designing an optical system that fits within the volume of a 6U CubeSat and operates efficient in the short-wave infrared (SWIR), mid-wave infrared (MWIR) and long-wave infrared (LWIR); second, designing an optical system behind the foreoptics to efficiently detect multiple spectral bands within the SWIR, MWIR and LWIR.
Compared to a conventional telephoto lens, multi-fold reflective optics offer significant size and weight savings while still maintaining comparable imaging performance. These benefits are realized by compressing the length of the optic and by reducing the number of elements needed to focus the light. Using mirrors to fold the path has the added benefit of greatly extending the spectral range of the optic by eliminating the chromatic aberration inherent in a refractive system. A mirror based folded design can work from the SWIR to the LWIR, making it a viable option for multispectral imaging applications.
We are choosing to utilize a custom image sensor such that each segment collects a separate waveband. This operates in much the same manner as a consumer camera. Red, blue and green filters are placed over each pixel in the image sensor. The individual pixels are combined to produce a single color pixel with red, blue and green components. The filters on consumer image sensors are colored gels placed directly on the pixels of the image sensor.