Microwave AMSU Calibration Update Bjorn Lambrigtsen Frank Sun Thomas Hearty

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Microwave (AMSU) Calibration UpdateBjorn
LambrigtsenFrank SunThomas Hearty
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Topics

• Radiometric calibration upgrade
• Moon in cold-cal FOV
• Pointing validation
• Radiometric validation

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Radiometric Calibration Upgrade

• The issue
• Cold-cal Tbc is assumed invariant
• Sum of cosmic background and Earth radiation into
  sidelobes
• Baseline approach (from NOAA) use climatology to
  pre-compute Tbc(Earth)
• In reality Tbc (Earth) varies
• Strong function of latitude moderate func. of
  lon weak func. of season
• The effect The computed cold-cal Tbc is
  erroneous
• Prominent orbital cycle gt Varying calibration
  error
• The solution
• Take into account variability of Tbc(Earth)
• Time/location-dependent climatology from prior
  observations
• Tables are in place and can be populated for V4.0
• Will improve absolute radiometric accuracy by up
  to 1 K

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Cold-cal observations Ch. 1-4
45 ltgt 1.35 K
Ch. 1 1.35 K
Ch. 3 0.3 K
Ch. 4 0.6 K
Ch. 2 0.5 K
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Cold-cal observations Ch. 5-8
Ch. 5 0.8 K
Ch. 7 1.9 K
Ch. 8 0.3 K
Ch. 6 0.7 K
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Cold-cal observations Ch. 9-12
Ch. 9 0.4 K
Ch. 11 1.0 K
Ch. 12 0.8 K
Ch. 10 0.3 K
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Cold-cal observations Ch. 13-15
Ch. 13 0.9 K
Ch. 15 0.4 K

• Will derive lat/lon climatology
• Based on one 16-day cycle
• Populate existing tables
• Expect residual errors to be small
• lt 0.2 K
• Should improve retrievals
• May help ease sidelobe analysis

Ch. 14 0.8 K
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Moon in Cold-Cal FOV

• The issue
• Moon gets into cold-cal FOV several times a year
• Can cause 3-4 K effect in AMSU (up to 20 K in
  HSB)
• Baseline approach
• Reject cold-cal if moon-in-FOV is predicted
  (computed moon angle lt threshold)
• Then use last previously computed cal.
  coefficients
• But not across granule boundaries
• The result
• If moon is in FOV, it may happen for several
  minutes
• Because AMSU cold-cal view is snapshot in one
  single direction
• Large gaps in calibration therefore occur
  periodically
• This can span across granules - in that case,
  large data gaps can occur
• The solution
• Account for moons radiometric effect
• Estimate ?Tb from predicted moon angle - add to
  Tbc
• Continue updating cal. coefficients gt no
  calibration gaps gt no data gaps
• Can be implemented for V4.0

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Moon observations Ch. 1-4
Ch. 1 3.5 K
Ch. 3 2.6 K
Ch. 4 2.5 K
Ch. 2 2.9 K
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Moon observations Ch. 5-8
Ch. 5 2.6 K
Ch. 7 4.4 K
Ch. 8 2.6 K
Ch. 6 3.0 K
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Moon observations Ch. 9-12
Ch. 9 3.1 K
Ch. 11 3.5 K
Ch. 12 3.5 K
Ch. 10 2.9 K
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Moon observations Ch. 13-15
Ch. 13 3.6 K
Ch. 15 2.9 K

• Results are close to expectations
• Tb 200 25cos(?-40) _at_ 30 GHz
• ?(Aqua) 270 (1/2-waxing) 8 (inclin)
• Tb(moon/Aqua) 185 K _at_ 30 GHz
• ?(moon) 0.52 5
• Use simple functional approximation
• Error lt 1K
• Use flag to alert data users
• Can be improved with better pointing knowledge

Ch. 14 3.7 K
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AMSU Pointing Analysis Method

• Method as described in 2002
• Instrument in nadir stare mode
• Analyze coastal crossings
• Perpendicular gt Pitch error
• Oblique gt Roll error
• Determine obs-calc time lag
• Obs from Tb
• Calc from landfrac
• ?t t(calc)-t(obs)
• ?pitch ?t(6.65 km/sec)/(705 km)
• Works well for window channels
• Results shown here for pitch analysis
• Several dozen crossings
• 5 quasi window channels
• Moon analysis will also be used
• Results to be presented later

Example HSB perpendicular crossing (sampled every
0.02 sec for 1.7 sec, followed by 1-sec gap)
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AMSU Pointing Analysis Results
Summary for 11 selected perpendicular crossings
Example AMSU perpendicular crossing (sampled
every 0.2 sec for 6 sec, followed by 2-sec gap)
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AMSU Pointing Analysis Summary

• Pitch errors are within requirements (10 of
  FOV)
• Can be largely corrected for in L1A geolocation
  processing
• Relative tightly clustered
• Small sample, large scatter analysis statistics
  can be improved
• Roll analysis to be reported on later
• Yaw analysis must use different method using
  full-scan data

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AMSU Pointing Analysis Using Moon

• Branches should be equal
• Approach Recede
• At least near closest approach
• Difference due to pointing error
• MoonAng from assumed boresight centroid
• Actual boresight differs
• Complex geometry
• Lunar path not symmetric
• Varying mix of pitch/roll/yaw
• Precise analysis needs work
• Example shown
• Offset 0.35
• Implies pointing error of 0.175
• Combined pitch/roll/yaw

Single lunar encounter - Ch. 1
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Lunar Pointing Analysis Example
Ch. 4
Ch. 15
Small pointing error indicated for ch. 4
Large pointing error indicated for ch. 15
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AMSU Moon Analysis Extra

• Curious behavior of Ch. 14
• Should be identical
• Shared front end
• Ch. 9-13 look very similar

Ch. 10
Ch. 14
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AMSU Radiometric Validation

• Objectives
• Absolute radiometric validation
• Definitive sidelobe analysis
• So far
• Assess effect of clouds
• Assess effect of wind
• Comparison with ECMWF
• Analysis is on-going
• Comparison with radiosondes

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AMSU Obs-Calc vs. Wind speed
Ch. 1
Ch. 2
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AMSU Obs-Calc vs. Wind speed
Ch. 3
Ch. 15