LISA Mission System Engineering Status

1
LISA Mission System Engineering Status

• Marcello Sallusti (ESA)
• Colleen McGraw (GSFC)
• Jordan Evans (JPL)

2
Agenda

• Technical Organization
• Current Baseline Mission Architecture Parameters
• Latest Results for Sciencecraft Orbits
• Results and Baseline for Initial Acquisition
• Measurement Scheme and Performance
• Telescope Trade-Off and Far-Field Analysis (LOCS
  Status)
  
• DFACS During Science Mode
• LIMAS Status
• Spacecraft Configuration
• Technical Resource Budgets
• Near-Term Trade Studies and Activities

3
Technical Organization

• The three Mission Systems Engineering Managers
  (MSEMs) have the joint responsibility of leading
  and coordinating the overall engineering efforts
  across the LISA Project
• This involves Systems Engineers and Discipline
  Engineers from Astrium (ESAs Mission Formulation
  contractor) and NASA organizations (technical
  organizations and contractors)
• To date, the work has been a combination of
• Parallel work presented between the two agencies
  to converge on a baseline (e.g. baseline orbit
  definitions, end-to-end data architecture)
  
• Joint work developed via a consensus approach
  (e.g. the IMS architecture via the IMS ITAT)
  
• Technical Interchange Meetings (TIMs) for focused
  topics and Quarterly Meetings for general status
  are currently the primary vehicles for
  interchange

4
Baseline Mission Parameters (1/3)

• Following charts reflect a list of parameters
  being referred to as the Baseline Fact Sheet
  
• It is intended to be used to help track the
  current baseline for LISA
  
• It does not fully characterize a baseline
  architecture as it does not include items like
  block diagrams
  
• It provides a common source for some of the
  information needed to be shared across the team
  for consistency
  
• The current version is a work in progress
• Does not yet represent the baseline as it is
  still in review
  
• Many holes are identified by the Fact Sheet and
  represent areas of future work in defining the
  LISA baseline

5
Baseline Mission Parameters (2/3)
6
Baseline Mission Parameters (3/3)
7
Orbit Design Process

• NASA and ESA have independently optimized
  candidate operations orbits
  
• Different optimization processes were used
• ESA uses two phases and a simplified model (only
  Earth and that only an approximation)
  
• NASA included Earth and the other planets with
  standard ephemeredes
  
• Different independent variables were used
• NASA used position and velocity
• ESA used certain classical elements (eccentricity
  and argument of periaps) and then position and
  velocity
  
• Different cost functions were used
• NASA used average distance from Earth
• ESA used average of range rates between
  spacecraft
  
• Different constraints were assumed in the design
• NASA included an arm length requirement of 5 GM
  1, which is tighter than the separate
  requirement that the difference between any two
  arm lengths be less than 1 of their sum
• NASA included an allowance for delivery error (by
  linearly reducing the arm length variance allowed
  from 1 at the beginning of the mission to 0.8
  at the end of the nominal mission
• Although the 5 Gm requirement is in the latest
  Mission Requirements Document draft, it should
  probably be a requirement only on the initial
  state
• Final results were qualitatively the same and
  quantitatively similar
  

8
Orbit Elements Compared
NASA Baseline 2 Orbits
From Steven Hughes, Memorandum of 2005-05-02.
ESA / Astrium Orbits
9
Orbit Characteristics Compared
ESA / Astrium Orbits
NASA Baseline 2 Orbits
8.5 years 3100 days
From Steven Hughes, Memorandum of 2005-05-02.
10
LISA - Earth Range
NASA Baseline 2 Orbits
ESA / Astrium Orbits
20 deg 52 Gm 25 deg 65 Gm
8.5 years 3100 days
These results are documented in
LISA-ASU-TN-2001.doc (on PMIS)
LISANewConstraints.pdf (S. Hughes Memo, 5/2/05)
11
Acquisition Basics

• Constellation acquisition
• Defined as the process of bringing the three LISA
  spacecraft from zero to all six links phased
  locked and optimized (ready for science)
  
• Re-acquisition is a simpler (and faster) process
  with much smaller alignment and bias
  uncertainties
  
• Acquisition Process
• Spacecraft star tracker to telescope alignment
  calibrated with natural stars
  
• Command sequences and ephemeri uplinked to all
  three spacecraft
  
• Spacecraft attitude (by star tracker) and point
  ahead prescribed
  
• For each link.autonomous process, but monitored
  by ground
  
• Pointing acquisition (three strategies defocus,
  scan, super-CCD)
  
• Phase acquisition (scan frequency and lock to
  incoming)
  
• Tune up constellation
• Verify inter-SC clock, comm. and ranging
• Phase Gain tuning for laser and offset/arm-lock
  loops
  
• Pointing Brightness and flat-spot search and
  optimization
  
• Engineering Mode checkout, calibrations, and
  trial data runs

12
The Three Strategies

• Defocus
• Spoil beam to cover uncertainty cone
• 2 mrad beam spoiled 7.5 x (to 15 mrad), 56x
  weaker 2 pW
  
• Receiver fixed stares until integration time
  (SNR) good enough
  
• This sets time required for gyro-mode
  stability
  
• Scan
• Scan over uncertainty cone (continuous or
  step/stare)
  
• Receiver fixed stares until integration time
  (SNR) good enough
  
• This sets scan rate and time required for
  gyro-mode stability
  
• Thru-telescope Star Tracker
• Natural star fixes with visible CCD through LISA
  telescopes
  
• (mv) of 16, for 1 sec integration
  
• Issue of imaging quality FOV with telescope
  prescription

13
Issues

• Defocus
• Signal to noise ratio and fiber positioner
  mechanism complexity are driving issues
  
• Can be mitigated by both ends defocus and
  co-re-focusing in steps
  
• Scan
• Pointing stability over scan time is driving
  issue
  
• Can be mitigated with gyro-mode operation of
  GRSs
  
• Scan can always be done, even if another strategy
  is chosen as prime
  
• Thru-telescope star tracker
• FOV of telescope is driving issue
• Requires good alignment knowledge on bench or
  searching
  
• Eliminates requirement to have high performance
  SC star trackers

14
Acquisition Summary

• An observatory laser acquisition strategy and
  control has been developed
  
• Defocus and scan strategies
• Defocus should be considered the baseline with
  the scan strategy as backup
  
• Requires an order to the sequence
• Uses a Gyro mode for enhanced pointing stability
  with the drag-free loop following one of the test
  masses
  
• Star tracker bias estimation/calibration can be
  done a priori or with the acquisition algorithm
  
• A 57-dof Acquisition model has been developed
• SIMULINK for time-domain analysis
• STATEFLOW for switching logic
• Simulation results confirm the feasibility of
  both acquisition strategy
  
• Thru-telescope approach may not be feasible due
  to limited FOV and star availability needs
  further investigation
  
• For More Information
• Technical Note Acquisition with Scan and Defocus
  Methods (on PMIS, LISA-TN-GSFC-225, 10 Feb 2005)
  
• On VSDE The Feb 05 TN, presentations, papers,
  working files, animations, and the scan and
  defocus 57 DOF simulations (in Matlab/Simulink
  files)