Event Location: NSH 1507
Bio: Dr. John Christian is an assistant professor in the Department of Mechanical & Aerospace Engineering at West Virginia University (WVU). He is an expert in spacecraft navigation and space systems. Prior to joining WVU, Dr. Christian was an aerospace engineer in the GNC Autonomous Flight Systems Branch at the NASA Johnson Space Center. During his time at NASA and in academia, he has acquired extensive experience in the areas of navigation system design, hardware test programs, and conceptual space system design. He was one of the lead analysts for the Sensor Test for Orion RelNav Risk Mitigation (STORRM) flight experiment that flew aboard the Space Shuttle Endeavour on STS-134 and tested the Flash LIDAR and docking camera being designed for the Orion Muitl-Purpose Crew Vehicle (MPCV). He was also involved with the design and implementation of the navigation system for NASA’s Morpheus Vertical Test Bed. Dr. Christian holds a Ph.D. in aerospace engineering from the University of Texas at Austin, and a B.S. and M.S. in aerospace engineering from the Georgia Institute of Technology.

Abstract: Attitude estimation is a critical task for most modern spacecraft. Whether
it’s pointing a telescope, an engine nozzle, or a communications antenna,
accurate knowledge of the vehicle’s attitude in real-time is often
necessary for a spacecraft to accomplish its mission. To obtain the
information required to estimate its attitude, a typical spacecraft in Low
Earth Orbit (LEO) will contain an Inertial Measurement Unit (IMU) along
with one or more of the following reference sensors: star trackers, sun
sensors, or magnetometers. In practice, all of these data are typically
combined in a Multiplicative Extended Kalman Filter (MEKF) to improve the
quality of the resulting attitude estimate. The MEKF a particular
modification of the regular EKF that both (1) attempts to address the
multiplicative nature of attitude rotations/errors and (2) maintains a
reference attitude quaternion to avoid the singularities associated with
all three-parameter attitude representations (important for spacecraft that
can freely tumble in space and take on any attitude).

This talk begins by exploring the fundamentals of spacecraft attitude
estimation. We will discuss the present state-of-the-art in techniques for
combining multiple known line-of-sight measurements to compute the globally
optimal attitude – as is done on star trackers. This will also include a
careful treatment of measurement statistics that will lead to an accurate
analytic approximation of the attitude estimate covariance. With this as a
foundation, we will continue by discussing current research at WVU that is
exploring novel techniques for nonlinear attitude estimation aboard
spacecraft. These new techniques are straightforward to implement, can be
run in real-time, and significantly out perform their MEKF counterparts in
situations with large angular errors.