Description: A compact, lightweight, high accuracy star tracker is a key enabler for future NASA space missions, from precise pointing of the large space telescopes to autonomous navigation and formation flying. For high precision attitude determination system, one of the identified needs is to provide milli-arcsecond class pointing for large space telescopes. Optical Physics Company (OPC) has developed a novel interferometric star tracker than can provide milli-arcsecond class accuracy for future NASA space missions. The interferometric is similar to a standard star tracker, but with the addition of the interferometer subassembly in front of the imaging lens. This subassembly converts incoming starlight into a set of quadrature signals with sinusoidal dependence on the starlight incident angle. These quadrature signals are used to determine star position, rather than the centroiding used in standard start trackers which enables the high accuracy. OPC has designed and is presently manufacturing a compact (0.35U), lightweight (0.35 kg) and low power (<4W) interferometric star tracker for the Air Force.

Benefits: Application most relevant to this project is the use of star trackers for spacecraft. OPC is building a cubesat star tracker at this time to deliver to a DOD customer. The performance is power dependent - ranging from sub arcsecond to 2 arcsecond accuracy. The package is small - 250 cc in volume. Another visible band star tracker application was proposed to the Navy recently under the SBIR program. This proposal has been selected for award. The application involves space situational awareness, specifically to develop, build and deliver a version of the interferometric tracker capable of detecting and tracking dim objects. Another application is GPS denied navigation which has increasingly become a more acute need in the past few years. Customers are GPS denied navigation systems include virtually the whole armed forces. Our approach is primarily tailored to airborne platforms such as fighter aircraft, ground attack aircraft, next generation bomber, Tier II and Tier II UAVs, airborne early warning and control aircraft, ICBMs, and missiles including hypersonic missiles. OPC is currently funded under Navy RIF to build and flight test a stellar inertial navigation system prototype.

This high accuracy tracker can be used for precision pointing of the large telescope. Its field of view is much wider than the fine guidance sensors (FGS) currently employed by HST and JWST. The JWST FGS, for example, has a field of view of only 0.04 x 0.04 degrees. With such a small field, fainter guide stars need to be used which then drives the sensor sensitivity and noise requirements. For the interferometric star tracker proposed, the angular sensitivity is defined by the angular period of the grating LSI, which can be independent of the field of view. This allows a wide field sensor to achieve similar level (milliarcsecond) of pointing performance, which then allows the use of brighter guide stars reduce the demand on sensor sensitivity. Another application is as reference sensor for laser beam pointing: A celestial reference based beam pointing eliminates the need for having a ground based beacon for return beam pointing which significantly reduces the operational complexity (power, pointing, regulatory compliance) and improves link performance (no round trip delay). OPC has also been funded recently for a study by JPL to investigate if its interferometric star tracker can be used for a spinning spacecraft. The study findings have shown the feasibility of adapting the interferometric tracker to provide accurate attitude information for a rapidly spinning spacecraft.