Microwave Scan Bias Status Report Bjorn Lambrigtsen February 25, - - PowerPoint PPT Presentation

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Microwave Scan Bias Status Report Bjorn Lambrigtsen February 25, - - PowerPoint PPT Presentation

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MICROWAVE SCAN BIAS Microwave Scan Bias Status Report Bjorn Lambrigtsen February 25, 2003 AIRS Science Team Meeting, Camp Springs, MD Lambrigtsen-1 MICROWAVE SCAN BIAS Background Observations Substantial scan bias Scan bias is

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SLIDE 1

February 25, 2003 Lambrigtsen-1

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Microwave Scan Bias

Status Report

Bjorn Lambrigtsen

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SLIDE 2

February 25, 2003 Lambrigtsen-2

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Background

Observations

  • Substantial scan bias
  • Scan bias is asymmetric
  • Magnitude and asymmetry is location dependent

Status until now

  • No sidelobe corrections applied in L1b so far
  • L1b data slots exist for Ta and Tb (= Ta + sidelobe correction)
  • Interim solution: Microwave tuning applied at L2 (pre-processing)

Ongoing effort

  • Characterize the scan bias
  • Develop sidelobe corrections to be applied at L1b
  • Remove scan bias
  • Allow estimates of local scene Tb from measured Ta
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SLIDE 3

February 25, 2003 Lambrigtsen-3

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

NOAA Method

  • 1. Compute antenna efficiencies

Integral of antenna func. over solid angles:

  • Earth
  • Cold Space
  • Spacecraft
  • 2. Estimate effective measured antenna temperature
  • Ta ≈ fe • Te + fc • Tc + fs • Ts
  • 3. Solve for scene brightness temperature
  • Tb ≈ Te = (Ta - fc • Tc - fs • Ts) / fe

Assumptions

  • a. All regions have azimuthal symmetry
  • b. Spacecraft covers entire backside half-sphere
  • c. fs is negligibly small -> See next slide!
  • d. Te is uniform over entire Earth view
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SLIDE 4

February 25, 2003 Lambrigtsen-4

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

NOAA Method - cont.

Assumption of negligible contribution from spacecraft

  • Based on computations by Aerojet:
  • Source in antenna near field reduces effective antenna efficiency by more than 10
  • Spacecraft is in near field
  • Radiation emitted and reflected by Spacecraft can be ignored (<< 0.1 K)

Results

  • Assumed radiometric field is azimuthally symmetric
  • Therefore, any computed scan asymmetry is entirely due to

asymmetric antenna function

  • Computed asymmetry is then very small (fraction of 1 K)
  • This may not account for observed asymmetries
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SLIDE 5

February 25, 2003 Lambrigtsen-5

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Our Approach

  • Take into account actual S/C configuration
  • Azimuthal-symmetry assumption is invalid
  • Spacecraft does not cover entire half-sphere
  • Space solid angle is larger than assumed - and asymmetric
  • Predicted effect: Negative bias on “space” side of scan
  • Re-examine Aerojet’s model of spacecraft radiation
  • Reduced contribution may not apply to reflected radiation
  • Predicted effect: Variable bias on “spacecraft” side of scan
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SLIDE 6

February 25, 2003 Lambrigtsen-6

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Spacecraft Configuration

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SLIDE 7

February 25, 2003 Lambrigtsen-7

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Effects of S/C Configuration

AMSU-A1

  • Positioned at +y edge of S/C - corresponds to right side of scan
  • Sees space in ~1/4-1/2 of backside half-sphere
  • Leads to cold bias at right swath edge
  • Sees S/C in other 3/4-1/2 of half-sphere (causes bias if Aerojet is wrong)
  • Leads to cold bias where cold space is reflected
  • Leads to variable bias where off-boresight Earth radiation is reflected

AMSU-A2

  • Positioned at -y edge of S/C - corresponds to left side of scan
  • Sees space in ~1/2-1/8 of backside half-sphere
  • Modulated by Solar Array
  • Leads to variable cold bias at left swath edge
  • Sees S/C in other part of half-sphere
  • Leads to scene dependent and latitude dependent bias at right swath edge

HSB

  • Positioned near -y edge, but sees mostly S/C (not space)
  • Leads to variable bias from SA reflections (left side of scan)
  • Leads to smaller cold bias from structural reflections (right side of scan)
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SLIDE 8

February 25, 2003 Lambrigtsen-8

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Proposed Algorithm

Measured antenna temperature is Ta = fe • Tb + fc • Tc + η • fs • Ts where

The first term represents Earth radiation

fe is computed from antenna patterns Tb is the (unknown) scene brightness temp. - assumed uniform across Earth

The second term represents direct space radiation

fc is computed from antenna patterns over actual space solid angles Tc is space brightness (3.9 K for AMSU ch. 8)

The third term represents Earth and space radiation reflected from the S/C

fs is computed from antenna patterns over actual S/C solid angles Ts is the effective reflected radiation - initially, Te = Tb or Tc η is the effective S/C reflectivity - assumed the same for all channels

Solve equation for Tb We will use channel-8 Obs-Calc to determine best value for η

η η η

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SLIDE 9

February 25, 2003 Lambrigtsen-9

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Scan Bias Estimate

Approximation:

Ta = Obs; Tb = Calc; ∆T = Ta - Tb (bias) ∆T = Ta - (Ta - fc • Tc - η • fs • Ts)/fe ∆T ≈ - fc•Ta - fsc•Ta - fse•(Ta - ηTe) (note: fe+fc+fs=1) where

the first term represents bias due to direct space radiation the second term represents bias due to space radiation reflected from the S/C the third term represents bias due to reflected off- boresight Earth radiation

Approximation is based on fe ≈ 1; fc,fs << 1; Tc << Ta Positive bias can occur only if η η η ηTe > Ta e.g., in window channels) In the polar regions it may be possible to have reflected solar radiation contribute to the third term, resulting in a positive bias

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SLIDE 10

February 25, 2003 Lambrigtsen-10

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Efficiencies: Ch. 1-2

1 2 Dotted line: fc Green line: fc+fsc Red line: fss

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SLIDE 11

February 25, 2003 Lambrigtsen-11

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Efficiencies: Ch. 3-6

3 5 6 4

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SLIDE 12

February 25, 2003 Lambrigtsen-12

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Efficiencies: Ch. 7-15

7 9-14 15 8

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SLIDE 13

February 25, 2003 Lambrigtsen-13

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Reference: AMSU Ch. 8

Why channel 8?

  • No surface effects
  • Relatively low variability in radiometric field
  • “Truth” is relatively well known
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SLIDE 14

February 25, 2003 Lambrigtsen-14

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Focus Day 3, Granule 002 (9°S Desc) Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb
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SLIDE 15

February 25, 2003 Lambrigtsen-15

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 004 (52°S Desc)

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SLIDE 16

February 25, 2003 Lambrigtsen-16

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 006 (80°S Asc)

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SLIDE 17

February 25, 2003 Lambrigtsen-17

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 008 (40°S Asc)

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SLIDE 18

February 25, 2003 Lambrigtsen-18

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 010 (4°N Asc)

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SLIDE 19

February 25, 2003 Lambrigtsen-19

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 012 (47°N Asc)

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SLIDE 20

February 25, 2003 Lambrigtsen-20

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 014 (82°N A/Desc)

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SLIDE 21

February 25, 2003 Lambrigtsen-21

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 016 (45°N Desc)

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SLIDE 22

February 25, 2003 Lambrigtsen-22

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Bias Comparisons - Ch. 8

4 Red: Obs-Calc Black:

  • fc•Tb

Green:

  • (fc+fsc)•Tb

Focus Day 3, Granule 018 (1°N Desc)

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SLIDE 23

February 25, 2003 Lambrigtsen-23

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

Implementation Strategy

For L1b

  • Implement baseline algorithm
  • Determine f-tables
  • Determine optimal S/C effective reflectivity (η)
  • Test against model data

For L2

  • Install switch to select Ta or Tb
  • Match with tuning selection

Tuning

  • If sidelobe correction is good: skip MW tuning
  • If only fairly good: recompute MW tuning coefficients
  • If poor: use current MW tuning
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SLIDE 24

February 25, 2003 Lambrigtsen-24

AIRS Science Team Meeting, Camp Springs, MD MICROWAVE SCAN BIAS

What’s Next?

Radiometric benchmark

  • Identify best “truth”
  • Use to baseline correction method

Spacecraft environment

  • Determine exact space-view solid angles
  • Classify reflection angles & determine solid angles
  • Space
  • Earth
  • Sun

Baseline bias corrections

  • Static approach (no scene dependence)

Future improvements

  • Dynamic corrections
  • Include possible solar reflection