A COTSBased Attitude Dependent Contact Scheduling System

1
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

2
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

3
Scheduling System Needs

• Principal Need
• To schedule science downlink contacts with TDRSS
  while considering
• TDRSS GLAST Orbital Positions
• GLASTs 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 Networks scheduling system

4
Challenges

• Orbit prediction accuracy
• Limited TDRS contact time
• gt17-24 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

5
GLAST FDS Architecture
GLAST Flight Dynamics System consists of custom
software built on the capabilities of COTS
software

• 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

6
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

7
(No Transcript)
8
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

9
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 GLASTs
  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 GLASTs science
  gathering attitude modes

10
Attitude Modeling
11
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

12
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

13
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

14
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

15
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

16
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

17
FDS Screen Snap
STK/Scheduler displaying optimized TDRSS contact
schedule
STK product suite provides the majority of
computational functions
Command prompt window provides feedback during
processing
Custom GUI used to initiate FDS processes
18
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