Engineering topics

Kevin Twedt

Publications and source records attributed to Kevin Twedt.

21 records · Page 2

NASA NTRS2019

Improvements in the on-orbit response versus scan-angle characterization for the MODIS ocean color bands

On-orbit characterization of the response versus scan-angle (RVS) is one of the most challenging aspects of the reflective solar band (RSB) calibration for the MODIS instruments onboard the Terra and Aqua spacecraft. The degradation of the solar diffuser, together with the lack of onboard calibrators (OBCs) to cover additional scan angles, other than the one for the lunar observations, has resulted in the use of Earth view responses from the pseudo-invariant desert sites to track the on-orbit RVS changes. This approach has been implemented in Collection 6 (C6) and C6.1 for bands 1-4, 8 and 9 of both instruments and also band 10 of Terra MODIS. As the missions continue to operate over a decade beyond their designed life and the instrument optics continue to degrade, it is expected that the OBC based RVS currently applied to other bands, specifically the high-gain ocean bands, will be inadequate to maintain the long-term calibration stability. An interband calibration approach is formulated and implemented in this paper. The proposed approach relies on the use of a spectrally matching stable reference band to evaluate the long-term calibration stability of the high-gain ocean bands that typically saturate while viewing the selected calibration deserts. Results from this approach indicate a noticeable drift for Terra MODIS bands 11 and 12 whereas the Aqua bands continue to show excellent temporal stability. These results are consistent with the corrections derived by the NASA Ocean Biology Processing Group (OBPG) and are expected to have minimum impacts on the downstream science products.

MODIS↗

NASA NTRS2019

Terra MODIS: 20 years of on-orbit calibration and performance

For nearly 20 years, Terra MODIS observations have generated a broad range of data products, enabling the remote sensing community and users worldwide for their studies of many key geophysical parameters of the Earth’s system. MODIS collects data in 36 spectral bands, covering wavelengths from 0.41 to 14.4 μm, that are calibrated by a set of on-board calibrators (OBC). Also contributed to sensor on-orbit calibration and characterization are near monthly-scheduled lunar observations and long-term trends of sensor responses over select ground targets. The quality of MODIS data products relies strongly on the dedicated efforts to the operate instrument, derive and update calibration parameters, and improve calibration strategies and algorithms in order to address on-orbit changes of sensor characteristics and its OBC functions. This paper provides an overview of Terra MODIS on-orbit operation and calibration activities over the last 20 years, including changes made to extend and preserve instrument and OBC functions and their implementation strategies. It illustrates sensor on-orbit performance using data from its OBC, lunar observations, and select ground targets and discusses major changes in sensor characteristics and corrections applied to the L1B algorithms or updates of calibration look-up tables (LUTs). Also described in this paper are lessons learned from Terra MODIS and future efforts to further extend its long-term data records.

Terra↗

NASA NTRSDate not supplied

Using Scattered Light From Nadir Port to Aid the Terra and Aqua MODIS Reflective Solar Band Calibration

The MODIS instruments on the Terra and Aqua spacecraft employ a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) system to calibrate their reflective solar bands (RSBs), covering a spectral range from 0.4 to 2.1 µm. The UV exposure of the SD, from its sun-view port as well as the scattered light (sunlight reflected from top-of-atmosphere), has led to a wavelength dependent degradation of the SD, with larger degradation observed at shorter wavelengths. The scatter off the diffuser onto the scan mirror is in the forward direction, whereas the scatter off the diffuser onto the SDSM fold mirror is in the backward direction. Since the outgoing angles (viewed by MODIS detectors) are the same as the scheduled SD calibration, the gain derived from scattering light facilitates monitoring the dependence of the SD’s degradation on incident angles. In this paper, we present a method that uses multiple orbits over each mission to obtain a SD response to the nadir port illumination. The SD degradation estimated from the nadir port illumination is compared with the degradation derived from the sun-illuminated SD. As both Terra and Aqua spacecraft continue to drift from their nominal orbits, the SD calibration mechanism has been adapted to these drifts, especially in terms of characterizing the transmission screen function. This paper also presents the utility of this scattering light data to support the RSB calibration in the post-nominal orbit drift era of operations.

MODIS↗