Dr. Cheng-Hsuan Lyu Profile

Dr. Cheng-Hsuan Lyu

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Proceedings Article | 27 September 2006 Paper

Multiyear lunar observations from TRMM/VIRS, Terra/MODIS, and Aqua/MODIS

C. Lyu, J. Sun, X. Xiong, W. Barnes

Proceedings Volume 6298, 629811 (2006) https://doi.org/10.1117/12.679084

KEYWORDS: MODIS, Calibration, Sensors, Reflectivity, Diffusers, Sun, Mirrors, Satellites, Data modeling, Phase measurement

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This work demonstrates how lunar data are used by three different imaging radiometers: the Visible and InfraRed Scanner (VIRS) onboard the Tropical Rainfall Measuring Mission (TRMM) and the MODerate resolution Imaging Spectroradiometers (MODIS) onboard the EOS Terra and Aqua satellites. Using the measured lunar data, radiometric models have been developed for the on-orbit calibration of the remote sensing systems' reflected solar bands. For the VIRS with spectral bands nearly identical to several of the MODIS bands, the integrated lunar reflectance data were measured, from Jan 1998 to Jun 2006, at phase angles ranging from 1.2° to 120.2°. For the two nearly identical MODIS instruments, the lunar irradiance was measured at phase angles from 54° to 56°. We present stability trending of the lunar data at selected phase angles over periods of four to eight years and use these observations to examine instrument radiometric stability. Moreover, we discuss implications of these results.

Proceedings Article | 27 September 2006 Paper

Tracking TRMM/VIRS on-orbit calibration with MODIS

A. Wu, C. Lyu, X. Xiong, W. Barnes

Proceedings Volume 6298, 62980Y (2006) https://doi.org/10.1117/12.681138

KEYWORDS: MODIS, Calibration, Reflectivity, Bidirectional reflectance transmission function, Sensors, Thermography, Infrared radiation, Diffusers, Black bodies, Space operations

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The Visible and Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission (TRMM), launched on 28 November 1997, has two reflected solar bands and three thermal infrared bands. The solar bands are calibrated using an onboard solar diffuser (SD) and the thermal bands are calibrated using an onboard blackbody (BB). Since launch, VIRS has provided more than eight years of on-orbit observations. The five VIRS bands have a close spectral match with corresponding Moderate Resolution Imaging Spectroradiometer (MODIS) bands. MODIS was launched on 18 December 1999 and 4 May 2002 aboard the NASA EOS Terra and Aqua spacecrafts, respectively. In this study, six years of VIRS and MODIS overlapping data are used to examine VIRS long-term calibration stability and consistency. This is particularly useful for the VIRS solar band calibration due to a lack of capability to track the on-orbit SD degradation. To reduce impacts due to scene variations, measurements from simultaneous nadir overpasses (SNOs) for VIRS and MODIS are co-located and aggregated to 30-by-30km areas for inter-comparison. Results show that the VIRS thermal bands maintain a stable calibration. For the two VIRS solar bands at 0.62μm and 1.62μm, the calibrated reflectance values gradually drift higher over the six-year period. The 0.62μm band increases at a rate of 1.1%/yr over the period, compared to an increase of 0.4%/yr for the 1.62μm band.

Proceedings Article | 28 December 1999 Paper

On-orbit measurement of response vs. scan angle for the infrared bands on TRMM/VIRS

William Barnes, Cheng-Hsuan Lyu, Robert Barnes

Proceedings Volume 3870, (1999) https://doi.org/10.1117/12.373188

KEYWORDS: Mirrors, MODIS, Reflectivity, Sensors, Signal to noise ratio, Infrared radiation, Infrared imaging, Calibration, Data modeling, Silver

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The Visible and Infrared Scanner on the Tropical Rainfall Measuring Mission (TRMM/VIRS) is whiskbroom imaging radiometer with two reflected solar bands and three emissive infrared bands. All five detectors are on a single cooled focal plane. This configuration necessitated the use of a paddlewheel scan mirror to avoid the effects of focal plane rotation that arise when using a scan mirror that is inclined to its axis of rotation. System radiometric requirements led to the need for protected silver as the mirror surface. Unfortunately, the SiOx coatings currently used to protect silver from oxidation introduce a change in reflectance with angle of incidence (AOI). This AOI dependence results in a modulation of system level response with scan angle. Measurement of system response vs. scan angle (RVS) was not difficult for the VIRS reflected solar bands, but attaining the required accuracy for the IR bands in the laboratory was not possible without a large vacuum chamber and a considerable amount of custom designed testing apparatus. Therefore, the decision was made to conduct the measurement on-orbit. On three separate occasions, the TRMM spacecraft was rotated about its pitch axis and, after the nadir view passed over the Earth's limb, the VIRS performed several thousand scans while viewing deep space. The resulting data has been analyzed and the RVS curves generated for the three IR bands are being used in the VIRS radiometric calibration algorithm. This, to our knowledge, the first time this measurement has been made on-orbit. Similar measurements are planned for the EOS-AM and EOS-PM MODIS sensors and are being considered for several systems under development. The VIRS on-orbit results will be compared to VIRS and MODIS system level laboratory measurements, MODIS scan mirror witness sample measurements and modeled data.

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