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Brown Bag Lunch Lecture ABI Calibration Kim Slack, ABI Lead MOST 11/19/2014.

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Presentation on theme: "Brown Bag Lunch Lecture ABI Calibration Kim Slack, ABI Lead MOST 11/19/2014."— Presentation transcript:

1 Brown Bag Lunch Lecture ABI Calibration Kim Slack, ABI Lead MOST 11/19/2014

2 Calibration of ABI ABI scans scenes with 16 spectral channels producing digital counts and telemetry The digital counts are calibrated into SI unit traceable radiance units using coefficients to remove instrument artifacts – Fixed: determined during prelaunch, not changing – Dynamic: changes during mission As the instrument changes over time, the coefficients are updates to correct for this changes – Diurnal or over the life of the instrument

3 SI Unit Traceability Scales – NIST 2000 Irradiance Scale – ITS-90 Scale Validation – VXR – TXR Instrument is calibrated directly with SI Unit Traceable Sources Prelaunch Instrument transfers that calibration to Internal Targets

4 Product Radiance Average space look counts (for each channel): Average scene counts (for each channel): 2 nd order coefficient determined at prelaunch 1 st order coefficient updated during operation Effective NS Mirror Radiance as a function of angle Mirror Reflectance as a function of angle Effective EW Mirror Radiance as a function of angle Mirror emissivity as a function of angle

5 Reflectivity/Emissivity Coefficients Corrects for scan mirror effects as the FOV is scanned across the FOR Initially determined prelaunch using reflectance measurements of scan mirror at different AOIs Polynomial coefficients are produced and remain fixed for life unless… PLT test show that the coefficients are not adequate – Spatial Uniformity Characterization Reserve PLT test allows for the calibration for the scanner effects on orbit – ABI scans space throughout a day and determines the difference from one side of the FOR to the other Range= 0° – 65 ° AOI

6 Mirror coefficients EW LOS offset from nadir -for each channel Scan mirror reflectivity equation coefficients Derived from witness sample reflectivity curves in ANGEN EW shaft angle EW LOS offset from nadir -for each channel NS shaft angle EW optical angleNS optical angle Mirror Reflectance as a function of angle Mirror emissivity as a function of angle

7 Internal Calibration for the IR Two measurements are used to calibrate the IR – Space (< 30 seconds) – Internal Calibration Target (ICT) (every 15 minutes) Space is near to zero flux – Provides offset ICT Radiance is known from contact thermal measurements & emissivity Delta counts produced from taking the difference between the ICT & Space measurements Slope (inverse responsivity) is produced per detector, no bit trim Heaters (red) PRT (yellow)

8 Effective Mirror Radiance Steinhart-Hart equation Weighted Sum for 1 mirror temp (3 per mirror) Counts to Resistance L NS and L EW are determined with Planck function with weighted sum temperature Effective mirror radiance at ICTEffective mirror radiance at space

9 ICT Temperature to Radiance Callendar-Van Dusen because temperature sensors are PRTs Vs. Steinhart-hart for thermistors like with mirrors Thermistor coefficients Weighted Temperature Average -primary plate weighted heavier than secondary plate Effective Radiance of ICT

10 Radiance/count slope for IR (m) Average space look counts (for each channel): Average ICT counts (for each channel): Effective self-emission radiance for EW and NS scan mirrors during space look Effective self-emission radiance for EW and NS scan mirrors during ICT measurement 2 nd order coefficient determined at prelaunch

11 2 nd order coefficient, Q Determined during prelaunch ECC testing ECT commanded to 7 different temperatures to provide adequate exercising of the dynamic range – ECT temperatures traceable to ITS-90 Temperature Scale, then validated by TXR in Vacuum Chamber Facility at Exelis Regression analysis determines both a linear and polynomial fit Statistical testing determines which is the best fit Determined per detector Remains the same throughout the life of the instrument External Calibration Target (ECT) Primary Plate Tertiary Plate Secondary Plate

12 Effective Radiance for Solar Calibration Target (SCT) Distance from Earth to sun (in AU) Sun-to-SCT angle of incidence Mirror reflectivity Solar in-band radiance at 1 AU (for each channel Real time distance between the sun and the earth in AU K is related to effective BRDF of the diffuser – correcting for vignetting Effective Radiance of SCT

13 Position of SCA

14 Z Y Glint shield Diffuser Solar Calibration Cover

15 Radiance/count slope for VNIR (m) is equal to the integration time factor for the SCT scene. is a correction factor from CDRL 79 adjusting for the fact that the SCT does not fill the ABI aperture Average space look counts (for each channel): Average SCT counts (for each channel): 2 nd order coefficient determined at prelaunch

16 Solar position effects Cos(57°)=0.54 Cos(64°)=0.44 Minimum incidence angle Maximum incidence angle Difference in radiance ~ 9-10%

17 2 nd order coefficient, Q Determined during prelaunch RCC testing Integrating sphere commanded to 10 albedo levels per channel (24 in all) to provide adequate exercising of the dynamic range – Calibration traceable to Detector-Based Spectral Irradiance Scale (2002) & validated by NIST VXR Regression analysis determines both a linear and polynomial fit Statistical testing determines which is the best fit Determined per detector Remains the same throughout the life of the instrument

18 Conclusion ABI flies with hardware to update response of VNIR and IR channels throughout life Expected lifetime coefficients – Q – determined prelaunch, no way of changing Determined during Reflective/Emissive Channel Calibration Prelaunch –  Mirror coefficients – to be tested during PLT. Update available on orbit Calculated with scan mirror reflectance Prelaunch (ANGEN) – K : BRDF coefficient for diffuser Determined during Irradiance Calibration Prelaunch Updated with each calibration – m – inverse responsivity


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