Grace Mission Planning

1
Grace Mission Planning

• Application of the Linear Programing Method to
  On-Board Memory Management
• Armin Braun (DLR)
• Spaceops 2002

2
Grace Space Segment
see http//www.csr.utexas.edu/GRACE/
3
Grace Mission Objectives

• Primary Mapping of Earth Gravitation
• orbit repeating in 30 days
• uninterrupted data recording
• Secondary Climate Experiment
• Occultation of GPS Satellites

4
Tasks for The MPS

• Mass Memory Management(under operational
  margin constraint)
• station schedule
• Satellite-Station assignments
• H/K to Science distribution
• Maximum Rates
• Event Driven Procedures
• solely dependent from orbit events

5
Complications

• Baseline station network
• 1 Site Up- and Downlink
• 1 Site Downlink only
• vs. two satellites in need for commanding
• Climate experiment
• Second in priority
• Large additional data load
• not to disturb continuity of prime science

6
Operational Margin

• Definition It shall be possible to re-transmit
  a lost dump, bevor it is overwritten by
  pre-planned recording.

memory size
35000
30000
25000
20000
Write Pointer
Read Pointer
15000
10000
5000
DATA LOSS!
0
0
0.2
0.4
0.6
0.8
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Mathematical Formulation
or
8
Formulation of the Grace problem (ctd. 1)

• Resource limit
• dump limit

9
Formulation of the Grace problem (ctd. 2)
Operational margin for a few passes after a
dump failure, before the pointers could be reset
by TC, the spacecraft continues to operate as
scheduled.
There must be
10
Applied Method
good
var1
bounding plane f(var1,var2,var3)lt const

• The problem is linear in all parameters
• (in other words, solution space is bounded by
  planes.
• Optimum must be on a edge at least one corner
  is optimal)
• Constitutes a Linear Program
• if an appropriate objective function is added
• Can be optimzed by simplex method (or variants
  hereof)

bad
var2
var3
11
Convergence and Ambiguity
Performance of Two-Phase Algorithm
Degenerated Optima
Sensitivity to objective Function
12
Simplex Variants
Dual Simplex

• Interior Point methods

13
Activities correlated to orbit events

• Solved by state engine
• Completely configurable by means of miniature
  programming language

14
Experience

• Status of Space Segment
• The Grace Mission is in transition to the
  operational phase. (since May)
• Using just 14 days of data, a preliminary Grace
  gravity field map has already been produced and
  is proving to be substantially more accurate than
  the combined results of more than three decades
  of satellite and surface instrument gravity
  measurements collected before Grace. B. Tapley,
  PI (from NASA newsletter Sep 12)

15
Summary

• Minimizing operational risk
• by optimizing on-board data generation budget
• in compliance with operational margin
  constraint
• mathematical formulation
• builds bridge to well established solving methods
• Applicable to similar budgeting problems in Space
  System design