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Moon glint in AMSU Nigel Atkinson and Roger Saunders Met Office (UK) Moon glint in AMSU Moon passes through space calibration view in AMSU-A and AMSU-B at certain seasons of year Typically affects ~10 minutes per orbit for ~10 consecutive orbits

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Moon glint in AMSU

Nigel Atkinson and Roger Saunders

Met Office (UK)


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Moon glint in AMSU

  • Moon passes through space calibration view in AMSU-A and AMSU-B at certain seasons of year

  • Typically affects ~10 minutes per orbit for ~10 consecutive orbits

  • Tends to affect tropics (inclination of moon's orbit is low)

  • Magnitude: 2K for AMSU-A; 30K for AMSU-B

  • Errors maximum at cold scenes; zero for ambient scenes.

  • Various strategies have been devised to correct for this.


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Impact on Earth view radiances

Moon in space view

AMSU-A channel 8

No data

bias

degK

Insert cardiogram here

sdev

Global O-B stats for 2001


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AMSU-B

Hot Target View

AMSU-B has 4 space view samples, each separated by 1º. Normally only 1 or 2 views are contaminated (occasionally 3)

Platform

4 space views

(within SV3)

Moon

Edge of Earth’s disk

90 Earth views


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AMSU-B moon-glint corrections

  • NESDIS 1B files: A moon glint correction was introduced in December 2000 (following Met Office recommendation). If the space views fail the normal consistency checks, the 2 or 3 samples with the lowest space view counts are used.

  • AAPP local 1B files: Contaminated data fail the consistency checks and are flagged 'do not use'

  • Code is available for AAPP to implement the NESDIS correction. However, at mid-latitudes the problem is rare.


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AMSU-A

  • AMSU-A has only one space view. Therefore normal quality checks are passed.

  • Moon glint events are detectable as a degradation in NWP model statistics.

  • Met Office has implemented prototype 'patch' to AAPP:

    • Implemented at 1B-1C converter, thus works on global and local datasets, and also on historical data.

    • Position of moon is calculated using standard astronomical formula, and hence angle between moon and space view

    • Space view data rejected if angle <3.5º. Gain interpolated between end points. Use offset from hot black body.

    • Zero error at ambient scenes; <0.1K at 200K.


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Corrected

Raw

Met Office interpolation scheme

Space view

Hot target

290K

3K

Plots show brightness temperature for fixed raw count,

i.e. calibration variation around an orbit


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NESDIS scheme

  • NESDIS are working on a correction scheme at 1B level

  • Involves correcting the measured space view readings on the basis of pre-launch antenna patterns and known moon surface temperature

  • No interpolation, so consistent with the architecture of the NESDIS pre-processor

  • NESDIS are using the method to derive post-launch antenna patterns


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Results

  • Correction Algorithm Works Well on A1-2 using pre-launch antenna pattern. Ripples on A1-1 and A2

A1-2

A2


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Post-launch antenna pattern measurement

NESDIS interpolation using Smooth Curve


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Preliminary NESDIS Results

NOAA-16 AMSU-A

Pre-launch

Post-launch


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Conclusions

  • The moon can affect AMSU calibration and hence Earth view radiances for NWP assimilation

  • For AMSU-B correction code is ready to be included in AAPP. Used at Met Office.

  • For AMSU-A Met Office interpolation scheme implemented operationally and tested with several events. Could easily be included in AAPP.

  • The NESDIS scheme could in principle be implemented in AAPP also, but would require more effort.


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