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Published on January 17, 2008

Author: Melissa1

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A COTS-Based Attitude Dependent Contact Scheduling System:  A COTS-Based Attitude Dependent Contact Scheduling System Jonathan D. DeGumbia, Omitron, Inc. Shane T. Stezelberger, Goldbelt Orca, LLC Mark Woodard, Goddard Space Flight Center/NASA GLAST Mission Overview:  GLAST Mission Overview Gamma-ray Large Area Space Telescope (GLAST) NASA/DOE gamma-ray observatory space science mission including support from government agencies in France, Italy, Japan, and Sweden Launch: August 2007 Goldbelt Orca, LLC and Omitron, Inc. under the guidance of NASA/FDF are developing the Mission Operations Center at GSFC Scheduling System Needs:  Scheduling System Needs Principal Need To schedule science downlink contacts with TDRSS while considering: TDRSS & GLAST Orbital Positions GLAST’s attitude and the limited field-of-view of science downlink antenna Limited ability to store science data on-board and need for 100% data recovery TDRSS is a shared resource Principal Functions Predictively model TDRS and GLAST orbits Model GLAST attitude and determine TDRS Scheduling Windows Apply scheduling constraints and optimize contact schedule Interface with Space Network’s scheduling system Challenges:  Challenges Orbit prediction accuracy Limited TDRS contact time >17-24 day TDRS scheduling lead time Accurate attitude modeling Limited effective field of view of Ku antenna Complex and immovable attitude profile Complex scheduling problem Numerous scheduling constraints Resulting contact schedule must ensure 100% science data recovery Need for automation Reduce burden on flight operations staff Reduce risks associated with manually performing lengthy, complex procedures GLAST FDS Architecture:  GLAST FDS Architecture COTS Tools Built on modules from Satellite Toolkit product suite: Pro, Connect, Orbit Determination, Chains, Attitude & Scheduler Custom code 3 independent Perl Scripts Propagation, Event Reports & Scheduling Visual Basic GUI GLAST Flight Dynamics System consists of custom software built on the capabilities of COTS software Orbit Determination with STK OD:  Orbit Determination with STK OD Telemetry data from the GPS receiver is expected to have good position knowledge but relatively poor velocity knowledge. Velocity data is discarded before GPS telemetry is ingested by STK OD The OD Tool Kit provides filtering/smoothing of GPS point solutions and incorporates high fidelity force models Orbit propagation accuracy is ~150 km over 30 days. Requirement is 7.5 km over 3 days 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 Attitude Modeling :  Attitude Modeling A custom orbit frame-of-reference (similar to LVLH) is created within STK to match what is used by the spacecraft vender Perl scripts used to calculate orientation body z-axis as a function of vector information provided by STK Scripts use same bus pointing control logic as the satellite Scripts needed for each mode (Sky Survey and Inertial Point) Scripts “plug-in” to the core STK processing; the script executes once per STK update STK/Attitude supplied “Aligned and Constrained” attitude method used as a basis for GLAST’s custom attitude “Aligned” vector is set to align the body z-axis with the Perl script vector “Constrained” vector simply set to constrain the body x-axis to the Sun, keepin sun vector on body x-z plane on the +x-side Above method used to both of GLAST’s science gathering attitude modes Attitude Modeling :  Attitude Modeling 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 Attitude Modeling:  Attitude Modeling Segmented attitude profiles are then used to switch between the different science gathering modes The result is the ability to use satellite commands to create a predicted attitude profile that simulates the GLAST observatory 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 Line-of-sight access reports between the sensor and each schedulable TDRS provide attitude dependent view periods TDRSS Schedule Optimization:  TDRSS Schedule Optimization Attitude TDRS access and other event reports are ingested in STK/Scheduler Wherever possible, scheduling constraints are modeled using the tools provided by STK Scheduler Where not possible, constraints are modeled externally and re-ingested into STK Scheduler prior optimization Optimizing engine used to determine best contact schedule Resulting contact schedule used to request TDRS contact times from NCCDS GLAST Scheduling Constraints:  GLAST Scheduling Constraints Schedule only while Ku-band antenna has line-of-sight access with the available TDRS Schedule contacts with only one TDRSS at a time Do not schedule if the TDRS/GLAST RF link is within 5° of the Sun vector Do not schedule if the GLAST RF link is within 3.1° of the Earth limb Maximize duration of contacts, but do not exceed 15 minutes in duration Do not schedule contacts that are less than 5 minutes in duration Consecutive contacts must be at least 20 minutes apart Longer duration contacts are preferred over shorter duration ones Do not schedule while slewing Target a user-defined number of minutes of contact time spaced evenly throughout the scheduling week Optionally schedule only during TDRSS unused time Final Schedule Generation:  Final Schedule Generation Prior to upload to GLAST, planned contact times must be confirmed and adjusted Predictive ephemeris is now much more accurate Bus pointing commands may have changed Confirmed TDRS contact schedule from NCCDS may not include every contact that was requested Independent constraint validation routine within STK/Scheduler used to ensure all constraints are met Updated information again used to create attitude dependent timeslots in STK/Scheduler Confirmed TDRS contact schedule used to restrict contact times and TDRS Running optimization engine again will automatically adjust contact times Reports generated from Scheduler are sent out to external components and used to coordinate contacts FDS Screen Snap:  FDS Screen Snap Custom GUI used to initiate FDS processes Command prompt window provides feedback during processing STK/Scheduler displaying optimized TDRSS contact schedule STK product suite provides the majority of computational functions Applications to Future Missions:  Applications to Future Missions GLAST FDS designed to meet GLAST-specific needs However, methods used easily adaptable to other mission-specific needs The scheduling system is modular by nature, individual features easily swapped Attitude modeling scripts replaceable or removable May be used for scheduling contacts to any land, sea, air, or space –based stations Scheduling constraints easily tailored to meet specialized scheduling needs Schedule deconfliction and optimization routines used are universal and may be applied to any scheduling problem

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