GEDI Precision Attitude Determination

Abstract:

Launched to the International Space Station (ISS) in December of 2018, the Global Ecosystem Dynamics Investigation (GEDI) instrument began collecting scientific data in operational mode on April 17^th, 2019. GEDI is currently scheduled as a two-year mission and is producing high resolution laser ranging observations of the 3D structure of the Earth. The fundamental geodetic measurement used to achieve these mission science objectives is the geolocation of individual laser waveform returns. Geolocation is computed as a function of three complex measurements: (1) the position of the instrument in inertial space, (2) the pointing of each individual laser beam in inertial space, and (3) the laser waveform round trip travel time observation. GEDI Precision Pointing Determination (PPD) is responsible for computing the precise pointing of each laser. PPD is comprised of the Precision Attitude Determination (PAD) of the GEDI Optical Bench Frame (OBF) with respect to inertial space, and the calibration of the laser pointing with respect to the OBF. Here we focus on the algorithms, methodology and performance of the GEDI PAD.

GEDI PAD, along with real-time pointing control, are accomplished with 3 DTU Star Tracker heads. Each head comes with an integrated MEMS micro inertial reference unit (MIRU). Due to a restricted field of view, as well as the harsh glinting and blockage environment on the ISS, all trackers are often blinded and present challenges to achieving an optimal PAD solution. The ground post-processed PAD implements a Multiplicative Extended Kalman Filter (MEKF) and Rauch-Tung-Striebel (RTS) filter. The filter states include attitude corrections to the attitude reference frame, angular rate biases, and relative alignment estimates to each tracker. The GEDI mission’s challenging geolocation budget requires attitude solutions to be better than 4.5 arcseconds 1σ, with an overall geolocation knowledge budget of 10 m. Of particular note is the novel use of ISS attitude data to constrain the GEDI PAD solutions during significant GEDI tracker blinding and poor performance periods. We present the performance of the PAD solutions during ideal data collection, and also during significant blinding and glinting periods. We discuss the current PAD performance and its impact on the overall precise geolocation error budget, as well as future improvements and expected performance.