intercomparison of the modis instruments using radcalnetthe two modis instruments (wu et.al, 2004,...

Intercomparison of the MODIS

instruments using RadCalNet

Amit Angala, Brian Wennya,

Kurt Thomeb and Jack Xiongb

a Science Systems and Applications Inc,

10210 Greenbelt Rd, Lanham, MD 20706bNASA Goddard Space Flight Center

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Outline

• MODIS Instrument Overview• Intercomparison Methodologies• RadCalNet-based comparison• Results

• At-nadir comparison• Off-nadir comparison and BRF discussion• Intercomparison results

• Summary and Future Applications

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MODIS Instrument Overview

Terra Launch: December 18, 1999 Aqua Launch: May 04, 2002

• Each MODIS instrument has 36 spectral bands covering the wavelength range from 0.4 to 2.1 µm

• Equipped with on-board calibrators: Solar Diffuser (SD) and Solar Diffuser Stability Monitor (SDSM) for reflective bands, a Blackbody for thermal bands, and spectro-radiometric calibration assembly (SRCA) for spatial and spectral characterization

• Calibration requirements: 2% in reflectance and 5% in radiance for reflective bands and 1% in radiance for most thermal bands

• Focus of this talk is to assess the calibration consistency of the two MODIS instruments in the reflective bands

10:30 am Equator Crossing Time

1:30 pm Equator Crossing Time

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20 Reflective Solar Bands at 3 nadir spatial resolutions (250m, 500m and 1km)

250m and 500m bands

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SDSM

Scan Mirror

SolarDiffuser

SRCA

Blackbody

SpaceView

EV Reflectance:

SDSD

Sun

dc

dc

SD: SD degradation factor

GSD: SD screen vignetting function

d: Earth-Sun distance

dn*: Corrected digital number

dc: Digital count of SDSM 5

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Motivation

• The RSB of two MODIS instruments are calibrated independently using the

on-board SD, SDSM and lunar measurements, supplemented by long-term

response trends from the pseudo-invariant desert sites from North Africa

• Given the broad usage of MODIS measurements in science products

monitoring the calibration consistency between the two instruments is vital

• Lack of simultaneous observations warrants a need for a transfer mechanism to

compare the two instruments

✓ Use of simultaneous nadir overpasses with a third-sensor (AVHRR) to compare

the two MODIS instruments (Wu et.al, 2004, Xiong et.al, 2004)

✓ Using ROLO-model normalized irradiances from the Moon to compare the two

instruments (Xiong et.al, 2008)

✓ Vicarious measurements from instrumented ground sites (Thome et.al, Czapla

Myers)

o At-sensor reflectance produced from RadCalnet as a common transfer

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RadCalNet-based comparison

o RadCalnet provides nadir-looking surface reflectance from fourinstrumented sites: RailRoad Valley Playa, Baotou, La Crau and the recentlyadded Gobabeb siteo Data provided at 30 minute intervalo From 9 am to 3 pm local standard timeo 10 nm intervals from 400 nm to 2500 nmo Ancillary information such as pressure, temperature, aerosol, water-vapor and

ozone are also provided.

o Using radiative transfer simulation, at-sensor reflectance (nadir look) at allthe wavelengths.o After integrating the reflectance to the MODIS band relative spectral response,

the at-sensor Radcalnet predicted reflectance is calculated for each RSB (bothinstruments)

o Finally, double ratio technique used to estimate the difference between thedifference between Terra and Aqua MODISo Separate nadir and off-nadir overpasses

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1 km x 1km region enclosing the 4 Radcalnet Ground Viewing Radiometers (GVR) chosen from the Terra and Aqua MODIS overpasses

At-sensor reflectance on a per-band basis is computed from the MODIS L1B C6.1 product (latest calibration version)

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At-nadir difference between Radcalnet and Terra MODIS

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Near-nadir overpasses from 2015-2017 (summer months only). Differences in the view-

angle (~5°) between Terra and Aqua a possible source for the disagreement.

Noisy and inoperable detectors at SWIR wavelengths contribute to the greater

uncertainties

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Double difference between the two MODIS instruments using Radcalnet as a common

transfer. At-nadir summer months measurements from 2015-2017 considered.

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MODIS/Radcalnet reflectance ratios binned as a function of Sensor Zenith Angles. All cloud-free scene pairs between Radcalnet and MODIS from 2015-2017 binned together as a function of sensor zenith angles

Similar trend observed for all the bands (i.e. no obvious wavelength dependence feature)

The nadir to off-nadir bias observed is likely due to the surface directional characteristics

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RSG developed CCD-array based Bidirectional Reflectance Factor Camera for ground based characterization of the RRV test sites.

Spectral measurements at four wavelengths 470 nm, 575 nm, 660 nm, and 835 nm were obtained

The relative BRF derived from these measurements is used to get an estimate of the site’s directional characteristics

Simulation(view geometry over RRV) of Terra MODIS overpasses from June to December.

Nandy, P., K. Thome, and S. Biggar, “Characterization and field use of a CCD camera system for retrieval of bidirectional reflectance distribution function” (JGR 2001)

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RRV BRF model applied to real Terra MODIS

observations to estimate the directional impacts

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RadCalNet-based Inter-comparison

Version 1. (presented JACIE 2017) Version 2. (presented JACIE 2018)

Same-day overpass between Terra MODIS and Landsat 7 ETM+. The large deviation at SWIR wavelengths has been significantly minimized in the latest version of the Radcalnet data.

Future Work: Reprocess the time-series of same-day overpasses between Terra MODIS, Terra MISR, Terra ASTER, Landsat 7 ETM+ Aqua MODIS, Landsat 8 OLI, SNPP & NOAA 20 VIIRS.

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Summary

o MODIS instruments have been operating successfully for over a decadebeyond their design lifeo Facilitate the generation of number of science productso The RSB of the two MODIS instruments are calibrated using the on-board

calibrators, supplemented by some Earth scene (desert) measurementso Lack of simultaneous overpasses

o Radcalnet used as a transfer mechanism to evaluate the calibration consistencybetween the two MODIS instrumentso Overpasses from 2015-2017 over the RVUS site used to trend the predicted and

measured at-sensor reflectance ratioso A 5% agreement observed between the two instruments at nadiro Greater differences due to site’s BRDF characteristics observed in off-nadir

observationso Extend this intercomparison technique to other EOS instruments

intercomparison of the modis instruments using radcalnetthe two modis instruments (wu et.al, 2004,...

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