Launched to the International Space Station (ISS) in December of 2018, the Global Dynamics Investigation (GEDI) instrument began collecting scientific data in operational mode on April 17th, 2019. GEDI is deployed on the Japanese Experiment Module – Exposed Facility (JEM-EF) of the ISS. It is currently scheduled as a two-year mission and it is producing high resolution ranging observation 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 computed as a function of three complex measurement:
The position of the instrument in inertial space
The pointing of each individual laser beam in inertial space
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) calibration of the laser pointing with respect to the 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.
The GEDI mission’s challenging geolocation budget requires attitude solutions to be better than 4.5 arcseconds (1-sigma), with an overall geolocation knowledge budget of 10 meters.
GEDI PAD, along with real-time pointing control, is accomplished using 3 camera head units (CHU) made by the Denmark Technical University (DTU).
A real-time onboard platform or OBF attitude solution is computed by optimally fusing the attitude solution from each head. Using the onboard position and attitude allows GEDI to actively point the instrument (within 6 degrees of nadir) to target specific ground-tracks and achieve optimal global sampling coverage.
GEDI PAD utilizes all CHUs on GEDI. The primary inputs used to include the optical solutions from each star tracker, and the MIRU fused rate data.
With multiple sensor systems, an attitude determination frame (ADF) is commonly formulated which serves as the basis frame for processing measurements and computing filter state estimates. CHU-B is designating the ADF, given it is the one most closely aligned (opposite direction) with the OBF boresight vector. This closely follows past and current NASA missions with extensive heritage (e.g. ICESat-2).
A multiplicative Extended Kalman Filter (MEKF) is used as an inertial forward pass on the data, and a backwards smoothing pass is followed (Rauch-Tung-Striebel). The trackers optical measurements are used as the filter’s observations and the fused angular rates from MIRU-B are used to propagate the estimated ADF attitude forward. The filter estimates include: inertial to ADF quaternion, relative alignment corrections, and rate bias states.
Key Project Innovations
A limited field of view (FOV) onboard the JEM has constrained tracker placement and resulted in regular blinding of all CHUs simultaneously. In addition to the expected blinding caused from the Sun in Low Earth Orbit (LEO), excessive glint is also received off the ISS solar arrays and other reflective surfaces.
To meet these challenges to data availability and quality, the novel use of the ISS attitude solution was introduced.
In order to make these solutions useful, the relative alignment dynamics between the ISS and GEDI must be modeled and predicted accurately. Inclusion of the ISS attitude and complimentary alignment model has significantly improved the ground-processed attitude product during significant blinding periods.
Future work will mainly focus on improving rate data quality, performance blinding periods, and LFE compensation. These tasks include: optimally combining the rates from all three MIRUs enhanced attitude propagation, dynamic relative alignment modeling improvements between GEDI and ISS attitude solution frames, temperature compensation of relative alignment variation between GEDI trackers, and systematic periodic error estimation using improved angular rate data. Much of the above work has been successfully demonstrated in simulations. The next step is to carry the improvements to the mission with mission data.
Rebold, T. W., Luthcke, S. B., Pennington, T. A., Thomas, T. C., “GEDI Precision Attitude Determination”, AGU Fall Meeting, Online Everywhere, December 2020.