X. Xiong

VIIRS thermal emissive bands calibration algorithm and on-orbit performance

The Visible-Infrared Imaging Radiometer Suite (VIIRS) was launched October 28, 2011 on-board the Suomi National Polar-orbiting Partnership (NPP) spacecraft as a primary sensor. It has 22 bands: 14 reflective solar bands (RSBs), 7 thermal emissive bands (TEBs) and a Day Night Band (DNB). There are 2 TEBs with a resolution of 371 m and 5 with 742 m which cover the spectral wavelengths between 3.7 to 12 μm. In addition to sea surface temperature (SST), a VIIRS Key Performance Parameter (KPP), the TEB detector dynamic range and spectral placement allow cloud, atmospheric and surface temperatures as well as water vapor to be measured. VIIRS TEB on-orbit calibration uses a quadratic algorithm with its calibration coefficients derived from pre-launch measurements and an on-board calibration blackbody (OBC BB) to provide scan-to-scan gain drift compensation. This paper will discuss the calibration methodology, OBC BB performance and stability, detector noise equivalent delta temperature and radiometric performance.

Terra and Aqua MODIS inter-comparison of three reflective solar bands using AVHRR onboard the NOAA-KLM satellites

Cross‐sensor inter‐comparison is important to assess calibration quality and consistency and ensure continuity of observational datasets. This study conducts an inter‐comparison of Terra and Aqua MODIS (the MODerate Resolution Imaging Spectroradiometer) to examine the overall calibration consistency of the reflective solar bands. Observations obtained from AVHRR (the Advanced Very High Resolution Radiometer) onboard the NOAA‐KLM series of satellites are used as a transfer radiometer to examine three MODIS bands at 0.65 (visible), 0.85 (near‐IR) and 1.64 µm (far near‐IR) that match spectrally with AVHRR channels. Coincident events are sampled at a frequency of about once per month with each containing at least 3000 pixel‐by‐pixel matched data points. Multiple AVHRR sensors on‐board NOAA‐15 to 18 satellites are used to check the repeatability of the Terra/Aqua MODIS inter‐comparison results. The same approach applied in previous studies is used with defined criteria to generate coincident and co‐located near nadir MODIS and AVHRR pixel pairs matched in footprint. Terra and Aqua MODIS to AVHRR reflectance ratios are derived from matched pixel pairs with the same AVHRR used as a transfer radiometer. The ratio differences between Terra and Aqua MODIS/AVHRR give an indication of the calibration biases between the two MODIS instruments. Effects due to pixel footprint mismatch, band spectral differences and surface and atmospheric bi‐directional reflectance distributions (BRDFs) are discussed. Trending results from 2002 to 2006 show that Terra and Aqua MODIS reflectances agree with each other within 2% for the three reflective solar bands.

NPP VIIRS On-Orbit Calibration and Characterization Using the Moon

The Visible Infrared Imager Radiometer Suite (VIIRS) is one of five instruments on-board the Suomi National Polarorbiting Partnership (NPP) satellite that launched from Vandenberg Air Force Base, Calif., on Oct. 28, 2011. VIIRS has been scheduled to view the Moon approximately monthly with a spacecraft roll maneuver after its NADIR door open on November 21, 2012. To reduce the uncertainty of the radiometric calibration due to the view geometry, the lunar phase angles of the scheduled lunar observations were confined in the range from -56° to -55° in the first three scheduled lunar observations and then changed to the range from -51.5° to -50.5°, where the negative sign for the phase angles indicates that the VIIRS views a waxing moon. Unlike the MODIS lunar observations, most scheduled VIIRS lunar views occur on the day side of the Earth. For the safety of the instrument, the roll angles of the scheduled VIIRS lunar observations are required to be within [-14°, 0°] and the aforementioned change of the phase angle range was aimed to further minimize the roll angle required for each lunar observation while keeping the number of months in which the moon can be viewed by the VIIRS instrument each year unchanged. The lunar observations can be used to identify if there is crosstalk in VIIRS bands and to track on-orbit changes in VIIRS Reflective Solar Bands (RSB) detector gains. In this paper, we report our results using the lunar observations to examine the on-orbit crosstalk effects among NPP VIIRS bands, to track the VIIRS RSB gain changes in first few months on-orbit, and to compare the gain changes derived from lunar and SD/SDSM calibration.

Discovery and characterization of on-orbit degradation of the VisibleInfrared Imaging Radiometer Suite (VIIRS) Rotating TelescopeAssembly (RTA)

The Suomi National Polar-orbiting Partnership (NPP) satellite was launched on Oct. 28, 2011, and began the commissioning phase of several of its instruments shortly thereafter. One of these instruments, VIIRS, was found to exhibit a gradual but persistent decrease in the optical throughput of several bands, with the near-infrared bands being more affected than those in the visible. The rate of degradation quickly increased upon opening of the nadir door that permits the VIIRS telescope to view the earth. Simultaneously, a second instrument on NPP, the Solar Diffuser Stability Monitor (SDSM), was experiencing a similar decrease in response, leading the investigation team to suspect that the cause must be the result of some common contamination process. This paper will discuss a series of experiments that were performed to demonstrate that the VIIRS and SDSM response changes were due to separate causes, and which enabled the team to conclude that the VIIRS sensor degradation was the result of ultraviolet light exposure of the rotating telescope assembly. The root cause investigation of the telescope degradation will be addressed in a separate paper.

Terra MODIS Band 2 Electronic Crosstalk: Cause, Impact, and Mitigation

The MODerate-resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the Earth Observing System (EOS). The first MODIS instrument was launched in December, 1999 on-board the NASA EOS Terra spacecraft. MODIS has 36 bands, covering a wavelength range from 0.4 μm to 14.4 μm. MODIS collects data at three spatial resolutions: 0.25 km (2 bands), 0.5 km (5 bands), and 1 km (29 bands). In the Earth scene images of Terra MODIS band 2 (0.85μm), two sets of regularly distributed anomalous pixels are observed in each scan, of which one is brighter and the other is darker than surrounding pixels. MODIS band 2 is a 0.25 km resolution band, having 40 detectors and 4 subframes for each detector. The brighter dots correspond to the subframe 1 pixels of detector 30 and the darker dots correspond to the same subframe of detector 29. In this manuscript, it is demonstrated that the anomaly is due to electronic crosstalk. The sending bands and detectors for the crosstalk are identified using lunar images and are confirmed using the Spectroradiometric Calibration Assembly (SRCA) observations. A linear algorithm is developed to describe the crosstalk, and crosstalk coefficients are derived using lunar observations. With the derived coefficients, the dotted features in Earth view images of Terra band 2 can be significantly reduced.

On-orbit calibration and inter-comparison of Terra and Aqua MODIS surface temperature spectral bands

Two moderate resolution imaging spectroradiometer (MODIS) instruments have been operating on board the NASA EOS Terra and Aqua spacecraft for 7 and 4.5 years, respectively. The MODIS is a cross‐track scanning radiometer that collects data in 36 spectral bands with wavelengths from visible (VIS) to long‐wave infrared (LWIR). MODIS thermal emissive bands (TEB) on‐orbit calibration is performed using an on‐board blackbody (BB). Bands 31 and 32, with centre wavelengths at 11 and 12 µm, are primarily used for surface temperature (ST) retrieval. This paper provides a brief description of MODIS TEB calibration methodology and presents bands 31 and 32 on‐orbit performance in terms of their calibration stability and detector noise characterization. It also describes an approach to examine the ST spectral band calibration consistency between two MODIS instruments using similar spectral channels of an advanced very high resolution radiometer (AVHRR) instrument currently operated on board NOAA‐16 and NOAA‐17. Inter‐comparison data sets are carefully selected from near‐simultaneous and near‐nadir observations made by each pair of MODIS and AVHRR instruments. Results show that ST spectral bands 31 and 32 in both the Terra and Aqua MODIS are performing better than their design requirements, with excellent calibration stability and satisfactory cross‐sensor calibration consistency. The on‐orbit noise equivalent temperature differences (NEdTs) of all detectors have been continuously below the design specifications of 0.05 K. The calibration coefficients are extremely stable with scan‐by‐scan variations of less than 0.10%. In addition, calibration differences between two MODIS instruments are less than 0.10 K for both bands 31 and 32.

Evaluation of Terra and Aqua MODIS thermal emissive band calibration consistency

Terra and Aqua MODIS have operated continuously for more than 12 and 10 years respectively and are key instruments for NASA’s Earth Observing System missions. The 16 thermal emissive bands (TEB), covering wavelengths from 3.5 to 14.4 μm with a nadir spatial resolution of 1 km are used to regularly generate a variety of atmosphere, ocean and land science products. As the sensors age well past their prime design life of 6 years, understanding the instrument on-orbit performance is necessary to maintain consistency between sensors in the long-term data records. Recurrent observations of Dome C, Antarctica by both Terra and Aqua MODIS over mission lifetime are used to track the calibration consistency and stability of the two sensors. A ground temperature sensor provides a proxy reference measurement useful for determining the relative bias between the two instruments. This technique is most useful for the land surface sensing bands, such as bands 29, 31 and 32, but can be applied to all other TEB to provide a metric to assess long-term trends. A change in the TEB calibration approach for the MODIS Collection 6 reprocessing mitigate a cold scene bias previously observed for retrievals of brightness temperatures well below the on-board blackbody calibrator temperature range (270-315 K). The impact of the Collection 6 calibration changes are illustrated using the Dome C observations.

Analysis of MODIS solar diffuser screen vignetting function

Twenty of the 36 MODIS spectral bands are reflective solar bands (RSB). They are calibrated on-orbit by an onboard solar diffuser (SD). For the high-gain ocean color bands (8-16), an attenuating solar diffuser screen (SDS) is used in front of the SD panel to avoid detector saturation caused by direct solar illumination of the SD. The use of the SDS, a metal plate with uniformly distributed pinholes, introduces an additional factor to the radiometric calibration uncertainty. Since a system level characterization of the SDS transmission versus SD viewing geometry was not performed pre-launch, the vignetting function (VF) for both Terra and Aqua MODIS had to be characterized on-orbit. The VF can be derived either from SD observations made with and without the SDS in place during specially planned spacecraft yaw maneuvers or by using routine SD calibration pairs (with and without the SDS) accumulated over a long period in order to cover all possible viewing geometries. In this paper we present details of the methods used to characterize the MODIS SDS VFs and examine the results derived from both spacecraft yaw maneuvers and long-term SD calibration pairs. The VF results obtained for Terra and Aqua MODIS are discussed and compared. In addition, an estimate of the calibration uncertainties introduced by the SDS is provided.

An Overview of Inter-comparison Methodologies for Terra and Aqua MODIS Calibration

With increasing efforts on data fusion and long-term climate data records (CDR) using observations made by multiple sensors on the same or different platforms, sensor cross-calibration has become increasingly important. It is known that the uncertainty of climate models or science data records depends not only on the calibration quality of individual sensors but also on their calibration consistency. This paper provides an overview of inter-comparison methodologies applied by the MODIS Characterization Support Team (MCST) at NASA/GSFC for the studies of Terra and Aqua MODIS on-orbit calibration consistency. Improved over heritage sensors, MODIS was built with a set of on-board calibrators (OBC) that include a blackbody (BB), a space view (SV) port, a solar diffuser (SD), and a solar diffuser stability monitor (SDSM). The BB is primarily used for the thermal emissive bands (TEB) calibration and the SD/SDSM system for the reflective solar bands (RSB) calibration. Detector responses to the SV provide measurements for the instrument background. Although instrument design requirements and calibration approaches are nearly identical for both Terra and Aqua MODIS and they all went through extensive and similar pre-launch calibration and characterization activities, their on-orbit calibration consistency still has to be carefully examined and validated as many science products are generated from observations made by both instruments. Methodologies discussed in this paper include inter-comparison studies using the Moon, a third sensor, and ground targets. Our results show that Terra and Aqua reflective solar bands and thermal emissive bands have been calibrated consistently to within their combined uncertainty requirements. For the 11mm and 12mm bands used for surface temperature measurements, the calibration differences between Terra and Aqua MODIS are less than ±0.15K at scene temperatures from 240-280K and less than ±0.50K at cold scene temperatures from 190 to 230K (before corrections). For most reflective solar bands, their reflectance calibration differences are typically less than ±2%.

Results and lessons from MODIS thermal emissive bands calibration : Pre-launch to on-orbit

The Moderate Resolution Imaging Spectroradiometer (MODIS) is a major instrument for NASAs Earth Observing System (EOS), currently operating on-board the EOS Terra spacecraft, launched in December 1999, and Aqua spacecraft, launched in May 2002. MODIS is a whiskbroom scanning radiometer using a double-sided paddle wheel scan mirror. It makes measurements in 36 spectral bands with wavelengths from visible (VIS) to long-wave infrared (LWIR). Bands 20-25 and 27-36 are the thermal emissive bands (TEB) covering wavelengths from 3.5 to 14.4μm. During pre-launch thermal vacuum measurements, a laboratory blackbody calibration source (BCS) was used as the primary calibration source for the TEB. For on-orbit operation, an on-board blackbody (BB) source and a space view (SV) port are used together for the TEB calibration on a scan-by-scan basis. This paper provides an overview of Terra and Aqua MODIS pre-launch and on-orbit calibration and characterization activities, methodologies, data analysis results, and lessons learned for the thermal emissive bands. It focuses on major issues that could impact MODIS TEB calibration and data quality. Results presented in this paper include detector noise characterization, response versus scan angle (RVS), and response versus instrument and focal plane temperatures. Similar discussions for the MODIS reflective solar bands (RSB) are presented in a separate paper in these proceedings (Xiong et. al).