Grace Minitour

1
GRACE Follow-On Definition and Status M. M.
Watkins NASA Jet Propulsion Laboratory
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Pioneering Remote Sensing

• GRACE has pioneered a new type of remote sensing
  from space Global monitoring of the mass
  variations that comprise the water cycle.
• - How can we build on that for future missions?
• - What accuracy should we be looking for in the
  next decade?
• - What measurement technologies are most
  promising?

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Science Objective

• Mission Objective (Easton) Produce high spatial
  resolution (lt 1 cm equivalent water error at 50 -
  100 km wavelength) models for mean and time
  variable (lt15 days) components of global Earth
  gravity field accuracy for a period of at least 5
  years.
• Mapping of the Earths gravity field from space
  offers global, continuous and homogeneous high
  quality monitoring of the static and time
  variable components of the Earths gravity field.
  Areas of significant impact
• Oceanography - measurement of time varying
  ocean bottom pressure, deep currents, removal of
  mean geoid at cm level to 100 km or better (sub
  mm at longer wavelengths)
• Hydrology - monitoring groundwater, deep soil
  moisture, and aquifers at the cm level to 100 km
  or better (mm at longer wavelengths)
• Glaciology - monitor changes in polar ice caps
  at cm level to 100 km or better
• Solid Earth Sciences and Geodesy - measure
  lithospheric thickness, mantle viscosity, core
  modes (translational/rotational), etc

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Gradiometer or SST?
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What limits GRACE?

• GRACE is limited at low frequencies by the
  accelerometer and the ultrastable oscillator
  stability, and at high frequency by the microwave
  phase noise.
• To get improved spatial resolution from space is
  not easy
• Must decrease phase noise
• Should improve oscillator and accelerometer as
  well for best performance
• GRACE microwave system
• K band 1 cm wavelength
• Read to 1 part in 10000
• How to improve?
• Go to much shorter wavelength gt optical!

6
KBR Quality
1st 1000 s sigma 2.46 microns gt 5 sec av
sigma 0.18 microns 1st 5 Hrs sigma 3.04
microns gt 5 sec av sigma 0.22 microns
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EX-5 GRACE Follow-On

• Mission Overview
• Two coplanar spacecraft in circular, polar,
  orbits at 250 - 600 km altitude
• Distance between s/c measured with advanced laser
  interferometry. Periodic maneuvers to maintain
  separation loosely to 50-100 km. Ground track
  repeat controlled to within 5 km.
• Lifetime 5 year baseline, 10 yr goal
• Spacecraft
• Small sats (2), 150-200 kg, 100W including
  thermal control, ACS, etc.
• Optional low thrust drag compensation system
  (takes output of inertial sensor and thrusts
  accordingly) 40 kg, 25W
• Instrument
• Laser interferometer and Inertial sensor

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Technology Plan (Next 3 years)

• NASA/ESTO Funding for Instrument Development
• JPL partered with Ball/University of Colorado
  (R.S. Nerem)
• Measurement system performance requirements
  derived from science rqmt
• Science Rqmts (I. Velicogna)
• Mission and measurement system architecture
• Signal Analysis Algorithm Development
• Error analysis/Test plan for instrument
• Interface Definition
• Define rqmts on I/f of laser ranging system to
  disturbance reduction system, GPS rcvr, and other
  measurement system elements
• Readout Electronics
• Use part of GPS receiver electronics for laser
  ranging readout
• Similar to GRACE microwave readout
• Prototype (TRL 6) brassboard of system expected
  by FY06.

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Error Analysis

• Two types of Errors to be considered
• Instrument or Flight System induced
• Laser system
• Accelerometer/drag free system
• Attitude control
• Geophysical Noise/errors
• Atmosphere
• Tides
• Nontidal ocean mass variability

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Instrument Overview

• The instrument involves two major components
• Laser interferometry capable of precision phase
  extraction with lasers having frequency stability
  of
• lt 1 part in 1.E16 (rms over 1000 seconds)
• An inertial sensor (accelerometer) capable of
  measuring non-gravitational accelerations on the
  s/c of
• lt1.E-13 m/s2 (rms over 1000 seconds)
• It is important that this inertial sensor be
  fully integrated with the laser interferometer,
  ranging directly (or indirectly) to the proof
  mass, in order to reduce errors relating the
  interferometer reference point to the inertial
  reference point (including all geometric
  deformations of the bus, optical bench, etc)

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EX-5 and GRACE Geoid Errors
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Swenson and Wahr, 2000)
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So Suppose all this H/W works

• Then we have the problems caused by the Earth
  itself
• Short Spatial scale atmosphere
• Errors may already be 1 mbar or less down to 100
  km
• Ocean Tides
• Current tide models definitely not good enough
  (cf. M2 signals exceed Grace Follow-On errors to
  l gt100, errors to l gt 80)
• May need tide models computed with baroclinic
  ocean model.
• Good thing Aliasing from ocean tides tends to be
  at defined (aliased frequencies). Thus posteriori
  solution from gravity data may be possible.
• General long period aliasing
• Current atmosphere and ocean models not good
  enough - will affect spatial resolution
• Further study solution methodology for minimizing
  
• Hope for future improvements in global data
  assimilation models

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Aliasing Problems
GRACE baseline calculated from a monthly gravity
simulation that incorporated only measurement
errors (no temporal gravity) Aliasing effect
from temporal variable model error limits
accuracy of recovered solution
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Spatial/Temporal Aliasing

• GRACE - orbit groundtrack uncontrolled, drifts
  across geoid, allowing changes in harmonics due
  to both time variations in geoid and failure to
  overfly same geoid features
• GRACE Follow-On - use of micronewton FEEP-type
  thrusters allows both drag free control and exact
  ground track repeat to a few km.

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Proposed Mission Timeline

• Master Mission Schedule
• Technology Development FY03 - FY05
• Mission Start 2/05
• Phase A 2/05 - 4/05
• Phase B 5/05 - 10/05
• Phace C/D 11/05- 10/08
• Launch 11/08
• Phase E 11/08 - 11/13 (nominal)
• 11/18 (goal)
• Note GRACE Mission nominal Phase E 4/02 - 4/07.
  Overlap possible with extended GRACE Mission or
  slightly accelerated Follow-On schedule.

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Partnerships and Leverage

• Potential Partnerships
• Technology Development - leverage Code S and ESA
  development for LISA
• Disturbance Reduction System selected for ST-7
• LISA probably headed for 2012-15 launch
• Mission Partners - exploring NASA/national
  partnerships for implementation. Grace Follow-On
  and GOCE Follow-On the same mission?
• Most important - wont happen without strong
  science support and participation in requirement
  definition

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Summary

• It is possible to develop and fly a mission with
  significantly better spatial resolution than
  GRACE (and we are working on it).
• It is a technical challenge from an engineering
  point of view
• It is a challenge from a geophysical modelling
  point of view to remove noise at a range of
  spatial and temporal frequencies
• We welcome engagement from the science community
  to make this mission happen

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Grace/Grace Follow-On MissionDevelopment at JPL

• Early 1990s JPL begins studies of the mission
  that will become GRACE, looking at system
  requirements, available technology from the DSN
  and GPS programs, required technology
  development, and science applications
• 1996 ESSP Step 1 and Step 2 proposals written at
  JPL. Laser interferometry evaluated for possible
  use but K/Ka microwave selected due to technology
  maturity
• 1997 GRACE selected1997-2002 GRACE built and
  launched under JPL management. JPL also
  responsible for sat-sat ranging system.
• 1998 M. Watkins proposed laser interferometer
  GRACE Follow-On to the NASA Post-2002 Eos RFI. W.
  Folkner is technologist.
• 1999 Easton Conference rates the mission (dubbed
  EX-5) highly in oceans and solid earth panels.
  Concept presented to Code Y, ESTO, etc.
• 1999 W. Klipstein/JPL win ATI proposal on laser
  stabilization for future gravity missions
• 1999-2002 LI gravity mission system analysis
  conducted at JPL to refine science rqmts and
  mission architecture. International interested
  expresses in partnering with NASA.
• 2001 W. Folkner proposes Disturbance Reduction
  System demo as ST-7 and is selected. Provides key
  leverage funding in tech development for future
  gravity missions.
• 2002 JPL (Watkins and Folkner) propose to IIP,
  JPL selected to manage CU/Ball effort and provide
  system engineering and mission architecture