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

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

Instrument transfers that calibration to Internal Targets 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

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

Reflectivity/Emissivity Coefficients Range= 0° – 65 ° AOI 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

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

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)

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

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

Radiance/count slope for IR (m) 2nd order coefficient determined at prelaunch 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

2nd 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 Primary Plate External Calibration Target (ECT) Tertiary Plate Secondary Plate

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

Position of SCA

Glint shield Solar Calibration Cover Diffuser Z Y

Radiance/count slope for VNIR (m) 2nd order coefficient determined at prelaunch Average space look counts (for each channel): Average SCT counts (for each channel): Δ 𝑡 𝑆𝐶𝑇 Δ 𝑡 𝑜𝑝𝑠 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

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

2nd 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

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 r(q), e(q) 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