Kwo-Fu Chiang

VIIRS on‐orbit calibration methodology and performance

The Visible Infrared Imaging Radiometer Suite (VIIRS) sensor aboard the Suomi National Polar-orbiting Partnership spacecraft has successfully operated since its launch in October 2011. The VIIRS collects data in 22 spectral bands that are calibrated by a set of onboard calibrators (OBC). In addition, lunar observations are made to independently track VIIRS long-term calibration stability for the reflective solar bands (RSB). This paper provides an overview of VIIRS OBC functions as well as its on-orbit operation and calibration activities. It also describes sensor calibration methodologies and demonstrates VIIRS on-orbit performance from launch to present. Results reported in this paper include on-orbit changes in sensor spectral band responses, detector noise characterization, and key calibration parameters. Issues identified and their potential impacts on sensor calibration are also discussed. Since launch, the VIIRS instrument nominal operation temperature has been stable to within ±1.0 K. The cold focal plane temperatures have been well controlled, with variations of less than 20 mK over a period of 1.5 years. In general, changes in thermal emissive bands (TEB) detector responses have been less than 0.5%. Despite large response degradation in several near-infrared and short-wave infrared bands and large SD degradation at short visible wavelengths, the VIIRS sensor and OBC overall performance has been excellent postlaunch. The degradation caused by the telescope mirror coating contamination has been modeled and its impact addressed through the use of modulated relative spectral response in the improved calibration and the current sensor data record data production. Based on current instrument characteristics and performance, it is expected that the VIIRS calibration will continue to meet its design requirements, including RSB detector signal to noise ratio and TEB detector noise equivalent temperature difference, throughout its 7 year design lifetime.

Multiyear On-Orbit Calibration and Performance of Terra MODIS Thermal Emissive Bands

Since its launch in December 1999, Terra MODIS has been making continuous Earth observations for more than seven years. It has produced a broad range of land, ocean, and atmospheric science data products for improvements in studies of global climate and environmental change. Among its 36 spectral bands, there are 20 reflective solar bands and 16 thermal emissive bands (TEBs). MODIS TEBs cover the mid-wave infrared and long-wave infrared spectral regions with wavelengths from 3.7 to 14.4 . They are calibrated on-orbit using an onboard blackbody (BB) with its temperature measured by a set of thermistors on a scan-by-scan basis. This paper will provide a brief overview of MODIS TEB calibration and characterization methodologies and illustrate onboard BB functions and TEB performance over more than seven years of on-orbit operation and calibration. Discussions will be focused on TEB detector short-term stability and noise characterization and changes in long-term response (or system gain). Results show that Terra MODIS BB operation has been extremely stable since launch. When operated at its nominal controlled temperature of 290 K, the BB temperature variation is typically less than 0.30 mK on a scan-by-scan basis, and there has been no time-dependent temperature drift. In addition to excellent short-term stability, most TEB detectors continue to meet or exceed their specified noise characterization requirements, thus enabling calibration accuracy and science data product quality to be maintained. Excluding the noisy detectors identified prelaunch and those that occurred postlaunch, the changes in TEB responses have been less than 0.7% on an annual basis. The optical leak corrections applied to bands 32-36 have been effective and stable over the entire mission.

MODIS on-orbit characterization using the Moon

The MODIS Protoflight Model (PFM) on-board the Terra spacecraft has been in operation for more than two and half years since its launch on December 18, 1999. In addition to the on-board calibrators (OBCs), the observations of the moon have been planned monthly with carefully chosen viewing conditions. The data from these observations is used to support the instruments on-orbit calibration and characterization. In this paper, we describe the use of lunar observations for monitoring the MODIS reflective solar bands (RSB) radiometric stability and discuss related applications. For Terra MODIS, the lunar views have also been used to derive correction parameters for the optical leak among the photoconductive (PC) detectors (bands 31-36), to characterize the electronic crosstalk under different focal plane operational configurations, and to track on-orbit band-to-band registration (BBR). The same strategies are being applied to the Aqua MODIS (Flight Model 1 - FM1) launched on May 4, 2002. The lunar observation results from both instruments are compared.

Terra and Aqua MODIS calibration algorithms and uncertainty analysis

NASAs Earth Observing System (EOS) Terra spacecraft was launched in December 1999 and the Aqua spacecraft in May 2002. The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key instruments for NASAs EOS missions, currently operated on both the Terra and Aqua spacecrafts. Together they have made continuous global observations for more than 8 years and led to many applications and studies for the Earths system of land, oceans, and atmosphere. Compared to its heritage sensors, the MODIS was designed with more stringent requirements on the sensors calibration accuracy and data product quality. Because of this it is equipped with a set of on-board calibrators (OBCs), including a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) for the reflective solar bands (RSB) calibration and a blackbody (BB) for the thermal emissive bands (TEB) calibration. In addition to the sensors intrinsic design characteristics, the quality of MODIS data products depends on the quality of its on-orbit calibration and characterization and on its on-orbit performance. The primary objective of this paper is to provide an overview of MODIS on-orbit radiometric calibration approaches and a summary of the calibration uncertainties for both RSB and TEB (Terra and Aqua). This paper provides an update to our previous reports with considerations based on each sensors characteristics identified pre-launch, measured and validated on-orbit. It also serves as a useful reference for the users of MODIS data products.

MODIS thermal emissive bands calibration algorithm and on-orbit performance

The MODerate Resolution Imaging Spectroradiometer (MODIS)is one of the key instruments for the NASA s Earth Observing System (EOS).MODIS ProtoFlight Model (PFM)was launched on-board the EOS Terra spacecraft on December 18,1999 and the MODIS Flight Model (FM-1)was launched on-board the EOS Aqua spacecraft on May 4, 2002.MODIS has 36 spectral bands with wavelengths ranging from 0.41 to 14.5 μm and nadir spatial resolutions of 250m (2 bands),500m (5 bands),and 1km (29 bands). The sensor s 20 reflective solar bands (RSB)from 0.41 to 2.1 μm are calibrated on-orbit by a solar diffuser (SD)and a solar diffuser stability monitor (SDSM).The other 16 thermal emissive bands (TEB)with wavelengths above 3.7 μm are calibrated by a blackbody.This paper follows the discussions on the RSB calibration and instrument performance presented in a separate paper (Xiong et.al.)in these proceedings, and focuses on the 16 thermal emissive bands (TEB).

On-orbit Characterization of RVS for MODIS Thermal Emissive Bands

Response versus scan angle (RVS) is a key calibration parameter for remote sensing radiometers that make observations using a scanning optical system, such as a doubled sided scan mirror (MODIS and GLI) or a rotating telescope (SeaWiFS and VIIRS). This is because the calibration is typically performed at a fixed viewing angle whereas the Earth scene observations are made over a range of viewing angles and the system’s response is a function of the scan angle. The NASA EOS Terra MODIS has been in operation for more than four years since its launch in December 1999. It has 36 spectral bands covering wavelengths from visible (VIS) to long-wave infrared (LWIR). It is a cross-track scanning radiometer with a two-sided paddle wheel scan mirror, making observations over a wide field of view (FOV) of ±55° from nadir thereby enabling frequent global coverage. Due to pre-launch measurement limitations, the Terra MODIS thermal emissive bands (TEB) RVS characterization did not produce valid data sets that could be used to derive a reliable system level RVS. Because of this, a RVS was developed for use at launch and subsequent efforts have been made to characterize the RVS using on-orbit observations. This paper describes the Terra MODIS on-orbit characterization of TEB RVS, including the data from scanning the instrument’s closed nadir aperture door (CNAD) and the use of Earth view data collected during spacecraft deep space maneuvers (DSM). Comparisons of pre-launch analysis and early on-orbit measurements are also provided. Noticeable improvements have been made for several thermal emissive bands for observations at large angles of incidence (AOI). Using the correct RVS improves the image quality and the radiometric calibration accuracy. For bands 34-36, an adjustment of as much as 0.5-1.5K can be made at the end of scan (worst case) for mirror side 2. The impacts at smaller AOI and from mirror side 1 are much smaller. Based on RVS comparison studies and science test results, the on-orbit derived DSM RVS has been chosen for the ongoing L1B data processing and future reprocessing.

MODIS correction algorithm for out-of-band response in the short-wave IR bands

The MODerate Resolution Imaging Spectroradiometer (MODIS) has 36 spectral bands with wavelengths from 0.41 to 14.5 micrometers. The 36 spectral bands, with a total of 490 detectors, are distributed on four focal plane assemblies (FPAs): visible (VIS), near infrared (NIR), short- mid-wave infrared (SMIR), and long wave infrared (LWIR). Nearly identical copies of the MODIS are currently operating onboard the NASA EOS Terra (launched on December 18,1999) and Aqua spacecraft (launched on May 4, 2002). Prelaunch and on-orbit characterizations of both Terra and Aqua MODIS have shown small but non-negligible out-of-band (OOB) response in the sensors short-wave infrared bands (SWIR): bands 5-7, and band 26. To minimize the impact due to OOB response on the MODIS SWIR bands calibration and the Earth scene product retrieval, an algorithm has been developed and implemented in the Level 1B (L1B) software for both Terra and Aqua MODIS. In this paper, we describe the algorithm and its applications to the MODIS L1B calibration algorithms. We illustrate how the correction coefficients are derived from on-orbit observations and discuss the test procedures involved before the final implementation in the L1B code. Performance is evaluated for both Terra and Aqua MODIS and the two results are compared.

MODIS thermal emissive bands calibration uncertainty analysis

A key instrument for the NASA EOS mission, the Moderate Resolution Imaging Spectroradiometer (MODIS) is currently operating on-board the Terra and Aqua spacecrafts. The MODIS has 16 Thermal Emissive Bands (TEB), with each having 10 detectors, covering the wavelengths from 3.7 to 14.4 mm. On-orbit each detector is calibrated by an on-board calibrator (OBC) blackbody (BB). Except for the low gain band used for fire detection, the thermal emissive bands use a quadratic algorithm in the Level 1B (L1B) code for calibration and for retrieval of top of the atmosphere (TOA) scene radiance. The specified calibration uncertainty of 1% applies to most of the TEB at their typical scene radiance levels and for scene-viewing angles inside a ±45° range (relative to instrument nadir). The requirements for two Sea Surface Temperature (SST) bands at 11 mm and 12 mm and for a low gain fire band are 0.5% and 10% respectively. The uncertainty requirements are twice as large at other non-typical radiance levels or at viewing angles outside the ±45º range. This paper reviews the MODIS TEB calibration algorithms and presents the calibration uncertainty analysis, including the methodology and results. Discussions will be focused on the key contributors to the uncertainty computation in the L1B. Results of the estimated uncertainties with the specifications at typical radiance level and at instrument nadir will be provided. A separate paper in this proceeding gives similar analysis for the MODIS Reflective Solar Bands (RSB).

Assessment of SNPP VIIRS VIS/NIR Radiometric Calibration Stability Using Aqua MODIS and Invariant Surface Targets

The first Visible Infrared Imaging Radiometer Suite (VIIRS) is onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite. As a primary sensor, it collects imagery and radiometric measurements of the land, atmosphere, cryosphere, and oceans in the spectral regions from visible (VIS) to long-wave infrared. NASAs National Polar-orbiting Partnership (NPP) VIIRS Characterization Support Team has been actively involved in the VIIRS radiometric and geometric calibration to support its Science Team Principal Investigators for their independent quality assessment of VIIRS Environmental Data Records. This paper presents the performance assessment of the radiometric calibration stability of the VIIRS VIS and NIR spectral bands using measurements from SNPP VIIRS and Aqua MODIS simultaneous nadir overpasses and over the invariant surface targets at the Libya-4 desert and Antarctic Dome Concordia snow sites. The VIIRS sensor data records (SDRs) used in this paper are reprocessed by the NASA SNPP Land Product Evaluation and Analysis Tool Element. This paper shows that the reprocessed VIIRS SDRs have been consistently calibrated from the beginning of the mission, and the calibration stability is similar to or better than MODIS. Results from different approaches indicate that the calibrations of the VIIRS VIS and NIR spectral bands are maintained to be stable to within 1% over the first three-year mission. The absolute calibration differences between VIIRS and MODIS are within 2%, with an exception for the 0.865- μm band, after correction of their spectral response differences.

MODIS TEB calibration approach in collection 6

The MODerate Resolution Imaging Spectroradiometer (MODIS) is a heritage sensor operating on both the Terra and Aqua platforms, and has collected remotely sensed data for a combined mission time of twenty plus years. The instrument robustness and performance over their lifetimes has been very satisfactory and is well calibrated using the onboard calibrators (OBC). The radiometric fidelity of the MODIS instruments has ensured the high quality of science products derived from the Level 1B (L1B) imagery. MODIS Thermal Emissive Bands (TEB) are calibrated on-orbit using an on-board blackbody (BB) and through the space-view (SV) port. The MODIS BB is nominally controlled at 290K for Terra and at 285K for Aqua. Periodically, a BB warm-up and cool-down (WUCD) process is implemented, during which the BB temperatures vary from instrument ambient (approximately 272K) to 315K. The calibration coefficients for the 16 TEB bands are characterized using the above mentioned on-board BB operations (i.e. using nominal and WUCD operations). This paper will focus on the calibration algorithms of the TEB developed for collection 6 (C6) processing, its impact on the Level 1B (L1B) product in comparison to collection 5 (C5), and the methodology for issuing a Look Up Table (LUT) update for L1B processing.