Orbit Determination Attitude Omitron NASA

Flight Dynamics System: 

Flight Dynamics System For GLAST Spacecraft Operations Jonathan DeGumbia/Omitron Mark Woodard/NASA

Mission Overview: 

Mission Overview Gamma-ray Large Area Space Telescope (GLAST) NASA/DOE gamma-ray orbiting space science observatory Launches August, 2007 Goldbelt Orca, LLC and Omitron, Inc. are teamed to proved the Mission Operations Center at GSFC Under the guidance of NASA/FDF, the Flight Dynamics System (FDS) is under development as a component of the MOC

Flight Dynamics System: 

Flight Dynamics System Principal Functions of the FDS Orbit determination Orbit events product generation TDRSS Scheduling Attitude dependent TDRSS scheduling Schedule validation

Challenges for the FDS (1 of 2): 

Challenges for the FDS (1 of 2) Need for accurate orbit determination GPS Inaccuracies Limited TDRS contact time 17-24 day TDRS scheduling lead time Need for attitude dependent contact scheduling Limited effective field of view of Ku antenna Limited TDRS contact time Complex and immovable attitude profile

Challenges for the FDS (2 of 2): 

Challenges for the FDS (2 of 2) Need to optimize TDRS contact schedule Complex and numerous scheduling constraints Must quickly and accurately deconflict constraints Need to be highly automated Reduce burden on flight operations staff Reduce risk of manually performing lengthy complex procedures

GPS Improvement with OD Tool Kit: 

GPS Improvement with OD Tool Kit Telemetry data from the GPS receiver is expected to have good position knowledge but relatively poor velocity knowledge. Velocity knowledge must be improved before propagating the orbit The OD Tool Kit provide filtering/smoothing of GPS point solutions to incorporate high fidelity force models Orbit propagation accuracy is expected to improve by roughly 2 orders of magnitude

Orbit Determination Results: 

Orbit Determination Results Raw GPS telemetry was simulated with 20 cm/sec RSS velocity uncertainty OD ToolKit reduced RSS velocity uncertainty to 0.29 cm/sec at the center of the solution interval OD Toolkit results compare favorably to Goddard Trajectory Determination System (GTDS) results, 0.25 cm/sec RSS

Attitude Modeling Problem Definition: 

Attitude Modeling Problem Definition Need for attitude dependent scheduling Gimbaled, narrow beam antenna used to downlink science data through TDRS Unfortunate placement of the antenna Complex, immovable, non-repeating attitude profile Must predictively model GLAST attitude using weekly bus pointing commands issued by science center to determine when TDRS contacts are possible STK/Pro & STK/Attitude “canned” attitude profiles were not adequate

Attitude Modeling : 

Attitude Modeling Vector Geometry Tool used to create a custom orbit frame-of-reference (similar to LVLH) to match what is used by the spacecraft vender Perl plug-in scripts used to create a custom vectors within the custom frame-of-reference One plug-in script needed for each mode (Sky Survey and Inertial Point) Scripts simulate the bus pointing control logic provided by the spacecraft vender STK/Attitude “Aligned and Constrained” attitude profile used as a baseline for custom attitudes “Aligned” vector is set to align the body z-axis with the custom vector “Constrained” vector simply set to constrain the body x-z plane on the Sun, always keeping the solar arrays normal to the Sun

Attitude Modeling: 

Attitude Modeling Segmented attitude profiles are then used to switch between the different science gathering modes A simple, well-defined sensor fixed to the spacecraft body simulates the effective field-of-view of the combined science downlink antenna and its gimbal A line-of-sight access report between the sensor and each TDRS provide attitude dependent view periods

Attitude Modeling Results: 

Attitude Modeling Results Sky Survey mode Zenith orientation with a timewise-varying rocking angle about the velocity vector Yaw-steering performed to maintain Sun vector normal to the body y-axis Sun must always be on +x body side of bus causing high-rate yaw flips twice per orbit (as ±zbody-axis approaches sun vector.) Complex sun avoidance maneuver reduces body rates during yaw flips

Attitude Modeling Results: 

Attitude Modeling Results Inertial Point mode ±zbody-axis inertially fixed on target Yaw-steering performed to maintain Sun vector normal to the body y-axis on +x body side of bus Earth limb-tracing when target is occulted by Earth Time varying additional radial offset during Earth limb trace, offset a function of the angle of the target off of the orbit plane

TDRSS Contact Scheduling: 

TDRSS Contact Scheduling Highly constrained scheduling problem mandated the need for STK/Scheduler Attitude TDRS access and other event reports are ingested in STK/Scheduler Each constraint modeled as time windows in STK/Scheduler Optimizing engine used to determine best contact schedule Resulting contact schedule used to request TDRS resource time from NCCDS

TDRS Schedule Validation: 

TDRS Schedule Validation Just prior to upload to GLAST, planned contact times must be validated Predictive ephemeris is now much more accurate Bus pointing commands may have changed Independent constraint validation routine within STK/Scheduler used to ensure all constraints are met Updated info used to create attitude dependent timeslots in STK/Scheduler as normal Confirmed TDRS contact schedule from NCCDS used to manually assign contacts to timeslots (STK/Scheduler’s optimizer not used) Independent STK/Scheduler validation is run Conflict report provides all necessary information about any constraints that are violated

Conclusions: 

Conclusions GLAST mission provided challenging & unique requirements for the MOC FDS Were able to select STK components to satisfy the majority of the requirements Wherever STK tools could not perform the necessary functions we were able to integrate custom code and modify work flow to meet GLAST needs STK tools allowed for automation

Back-up Slides: 

Back-up Slides

Why Sun Avoidance Maneuvers?: 

Why Sun Avoidance Maneuvers? Sun Avoidance maneuvers bound the body yaw rates and prevent the rates from becoming infinite as Sun vector approaches body z-axis