Evaluation of Terra MODIS PC bands optical leak correction algorithm

MODIS has 36 spectral bands with wavelengths in the visible (VIS), near-infrared (NIR), short-wave infrared (SWIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR). It makes observations at three nadir spatial resolutions: 0.25km for bands 1-2 (40 detectors per band), 0.5km for bands 3-7 (20 detectors per band), and 1km for bands 8-36 (10 detectors per band). The VIS, NIR, and SWIR are the reflective solar bands (RSB), which are calibrated on-orbit by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM). The bi-directional reflectance factor (BRF) of the SD provides a RSB calibration reference and its on-orbit changes are tracked by the SDSM. In addition, MODIS lunar observations are regularly scheduled and used to track the RSB calibration stability. On-orbit observations show that the changes in detector response are wavelength and scan angle dependent. In this study, we focus on detector-to-detector calibration differences in the MODIS VIS/NIR spectral bands, which are determined using SD and lunar observations, while the calibration performance is evaluated using the Earth view (EV) level 1B (L1B) data products. For Aqua MODIS, the detector calibration differences and their impact are also characterized using standard ocean color data products. The current calibration approach for MODIS RSB carries a band-averaged response versus scan angle (RVS) correction. The results from this study suggest that a detector-based RVS correction should, due to changes in the scan mirror's optical properties, be implemented in order to maintain and improve the current RSB L1B data product quality, particularly, for several VIS bands in Terra MODIS.

Since launch, both Terra and Aqua MODIS have been making regular lunar observations with a primary objective of providing an independent stability monitoring for the reflective solar bands (RSB) calibration. To a large extent, this approach is based on the fact that the Moon has extremely stable surface reflectance properties. When combined with a lunar radiometric model, the applications of lunar observations can be significantly enhanced. Using MODIS as an example, this paper discusses various applications developed from its lunar observations. In addition to the RSB stability monitoring, MODIS lunar observations are regularly used to examine its calibration consistency between Terra and Aqua MODIS and to track the sensor's band-to-band registration (BBR) stability. Examples also presented in this paper include optical leak and electronic crosstalk characterization for MODIS thermal emissive bands (TEB) and shortwave infrared (SWIR) bands. Results from multi-year lunar observations show that the MODIS RSB calibration stability has been satisfactory when compared to its solar calibration, and that Terra and Aqua MODIS are calibrated consistently to within ±1% for most RSB. The spatial characterization results derived from MODIS lunar observations agree very well with that determined from its on-board calibrator. It is clear that the applications and results of MODIS lunar observations presented here will serve as good examples or references for other sensors that also make use of lunar surface observations.

Aqua MODIS, also known as the MODIS Flight Model 1 (FM1), was launched on May 4, 2002. It opened its nadir aperture door (NAD) on June 24, 2002, beginning its Earth observing mission. In this paper, we present early results from Aqua MODIS on-orbit calibration and characterization and assess the instrument's overall performance. MODIS has 36 spectral bands located on four focal plane assemblies (FPAs). Bands 1-19, and 26 with wavelengths from 0.412 to 2.1 microns are the reflective solar bands (RSB) that are calibrated on-orbit by a solar diffuser (SD). The degradation of the SD is tracked using a solar diffuser stability monitor (SDSM). The bands 20-25, and 27-36 with wavelengths from 3.75 to 14.5 microns are the thermal emissive bands (TEB) that are calibrated on-orbit by a blackbody (BB). Early results indicate that the on-orbit performance has been in good agreement with the predications determined from pre-launch measurements. Except for band 21, the low gain fire band, band 6, known to have some inoperable detectors from pre-launch characterization, and one noisy detector in band 36, all of the detectors' noise characterizations are within their specifications. Examples of the sensor's short-term and limited long-term responses in both TEB and RSB will be provided to illustrate the sensor's on-orbit stability. In addition, we will show some of the improvements that Aqua MODIS made over its predecessor, Terra MODIS (Protoflight Model - PFM), such as removal of the optical leak into the long-wave infrared (LWIR) photoconductive (PC) bands and reduction of electronic crosstalk and out-of-band (OOB) thermal leak into the short-wave infrared (SWIR) bands.

Five cases using NASA ER-2 aircraft based SHIS and MAS radiances have been used to assess the L1B radiometric performance of Terra and Aqua MODIS Collection 5 radiances for LWIR bands 31-36. The composite results of these cases show that the split window bands 31 (11 μm) and 32 (12 μm) have performed well within the 0.5% radiometric specification over their lifetime. This is in agreement with results from other ground based and satellite based comparisons that are discussed in the paper. However, the LWIR CO2-sensitive bands 34-36 appear to be performing outside of their 1% accuracy specification, especially for Terra MODIS. This is also observed in global Aqua AIRS-MODIS comparisons. Possible causes for this behavior are under investigation, with the most likely contributors being spectral characterization error, OOB influences due to spectral filter leaks, or possibly scan mirror characterization. It seems that an optical leak from Terra MODIS band 31 into bands 32-36 is probably not a significant contributor to the large residuals of bands 34-36, owing to an effective radiometric correction. Calibration coefficient error is probably only a small contributor since, after adjustments in 2002, the on-orbit calibration now closely follows that of the pre-launch calibration.

The short-wave infrared (SWIR) bands (5-7, 26) of Terra MODIS, co-located with the mid-wave infrared (MWIR) bands (20-25) on the short and mid-wave infrared (SMIR) Focal Plane Assembly (FPA) have a known issue related to 5.3 μm out-of-band (OOB) thermal leak and electronic crosstalk that was identified prelaunch. Intensive efforts were undertaken shortly after launch to mitigate its impacts on the on-orbit calibration and in turn the level 1B (L1B) products. In order to the isolate the OOB contribution among the SWIR bands, special night time day mode (NTDM) operations have been regularly scheduled to collect Earth scene reflective solar bands (RSB) data during spacecraft night time. As MODIS does not have a spectral band centered at 5.3 μm, measurements from the MODIS Airborne Simulator (MAS) spectrometer field campaigns in the early months after Terra launch were used to help identify band 28 (7.325 μm) as the best surrogate to simulate the radiances at 5.3 μm. As a result, band 28 is used as the sending band for the SWIR crosstalk correction for Terra MODIS. In the case of Aqua MODIS, band 25 (4.52 μm) was found to be more effective as the sending band for the SWIR crosstalk correction. In recent years, the Terra MODIS PV LWIR electronic crosstalk (including band 28), has gradually increased, more significantly after the safe-mode event occurred in Feb, 2016. This accentuated degradation in the PV LWIR performance has also impacted the performance of on-orbit SWIR crosstalk correction algorithm and thus the L1B products. In this paper, we examine the use of band 25 as the sending band for Terra MODIS SWIR crosstalk correction and compare its performance with that based on band 28 as the sending band. Results indicate an improvement in the stability of the on-orbit gain for the SWIR bands and a reduced detector-detector and subframe striping in the L1B products, especially during the period when the PV LWIR electronic crosstalk is more severe.

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 instrument's 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.

MODIS has 20 reflective solar bands (RSB) in the visible (VIS), near infrared (NIR), and short-wave infrared (SWIR) spectral regions. In addition to instrument on-board calibrators (OBC), lunar observations have been used by both Terra and Aqua MODIS to track their reflective solar bands (RSB) on-orbit calibration stability. On a near monthly basis, lunar observations are scheduled and implemented for each instrument at nearly the same lunar phase angles. A time series of normalized detector responses to the Moon is used to monitor its on-orbit calibration stability. The normalization is applied to correct the differences of lunar viewing geometries and the Sun-Moon-Sensor distances among different lunar observations. Initially, the lunar calibration stability monitoring was only applied to MODIS bands (1-4 and 8-12) that do not saturate while viewing the Moon. As the mission continued, we extended the lunar calibration stability monitoring to other RSB bands (bands 13-16) that contain saturated pixels. For these bands, the calibration stability is monitored by referencing their non-saturated pixels to the matched pixels in a non-saturation band. In this paper, we describe this relative approach and apply it to MODIS regularly scheduled lunar observations. We present lunar trending results for both Terra and Aqua MODIS over their entire missions. Also discussed in the paper are the advantages and limitations of this approach and its potential applications to other earth-observing sensors.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua platforms has 20 reflective solar bands (0.4 to 2.1 μm), for which the on-orbit detector gain changes are tracked using the on-board solar diffuser (SD) and SD stability monitor. SD and lunar observation data, supplemented by Earth-view (EV) observations for some bands, are used to characterize the scan-mirror response versus scan angle (RVS). The short-wave infrared (SWIR) bands (1.2 to 2.1 μm) have issues, such as electronic crosstalk and an out-of-band optical leak from the thermal bands, which complicate the use of the Moon for reliably characterizing the on-orbit RVS. Before Collection 7 (C7), the pre-launch RVS was deemed sufficient for both Aqua and Terra MODIS SWIR bands throughout the missions. However, recent studies have found that the RVS behavior does change with time in some SWIR bands, up to 2% and 1% for Terra MODIS bands 5 and 26, respectively. We assess the residual RVS effects present in the current MODIS calibration at desert sites and deep convective clouds (DCC). Due to the large uncertainties caused by the water vapor absorption effects from desert sites, especially for band 26 (1.375 μm), EV observations from DCCs are chosen to characterize the on-orbit RVS behavior for the SWIR bands. A time-dependent RVS characterization method for the SWIR bands is described and assessed by comparing the results with Collection 6.1 (C6.1) Level 1B (L1B) reflectance products. This new algorithm will be used in the upcoming C7 L1B reprocessing for Terra MODIS, providing more stable and accurate SWIR band reflectance data over time and across the scan range compared with the C6.1 results. For Aqua MODIS SWIR bands, performance evaluations with desert and DCC data indicate stable performance, so the pre-launch RVS will continue to be used in C7.

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.

Both MODIS and VIIRS use a solar diffuser (SD) to calibrate their reflective solar bands (RSB), covering wavelengths from 0.41 to 2.3 μm. On-orbit changes of the SD bi-directional reflectance factor (BRF) are tracked by an on-board solar diffuser stability monitor (SDSM). The current SDSM design only covers the spectral range from 0.41 to 0.93 μm. In general, the SD degradation is strongly wavelength-dependent with larger degradation occurring at shorter wavelengths, and the degradation in the SWIR region is expected to be extremely small. As each mission continues, however, the impact due to SD degradation at SWIR needs to be carefully examined and the correction if necessary should be applied in order to maintain the calibration quality. For Terra MODIS, alternative approaches have been developed and used to estimate the SD degradation for its band 5 at 1.24 μm and a time-dependent correction has already been applied to the current level 1B (L1B) collection 6 (C6). In this paper, we present different methodologies that can be used to examine the SD degradation and their applications for both Terra and Aqua MODIS and S-NPP VIIRS SWIR calibration. These methodologies include but not limited to the use of lunar observations, Pseudo Invariant Calibration Sites (PICS), and deep convective clouds (DCC). A brief description of relative approaches and their use is also provided in this paper.

The long-wave infrared (LWIR) band holds critical value in applications such as industrial monitoring, biomedical imaging, and spectral analysis. However, frequency conversion for wavelengths above 5 μm, which covers the core LWIR spectral range, remains challenging due to the limited effective nonlinear coefficients of most optical crystals and severe absorption losses in the mid-infrared to LWIR region, restricting the efficiency of upconversion processes. Meanwhile, mainstream commercial mercury cadmium telluride (HgCdTe) detectors typically require cryogenic operation to achieve high signal-to-noise ratios (SNRs), coupled with high fabrication costs, which severely hinders their widespread deployment under room-temperature conditions. This paper presents a room-temperature LWIR detection device based on frequency upconversion technology, which enables high-sensitivity detection across a broad spectral range (7.5-9 μm) without the need for cooling. Experimental results demonstrate that the system achieves a minimum detectable energy at the fJ level under nanosecond-pulse operation, along with excellent wavelength stability and high responsivity. Compared with typical commercial HgCdTe detectors, the proposed device exhibits a sensitivity improvement of up to approximately 2.4 orders of magnitude at room temperature, significantly enhancing detection accuracy and signal-to-noise ratio for LWIR signals under room temperature conditions. This work provides an effective technical pathway toward the development of room-temperature, low-cost, and high-performance LWIR detection systems, showing promising potential for applications in real-time monitoring, portable sensing, and spectroscopic analysis.

The first Visible Infrared Imager Radiometer Suite (VIIRS) instrument was successfully launched on-board the Suomi National Polar-orbiting Partnership (SNPP) spacecraft on October 28, 2011. Suomi NPP VIIRS observations are made in 22 spectral bands, from the visible (VIS) to the long-wave infrared (LWIR), and are used to produce 22 Environmental Data Records (EDRs) with a broad range of scientific applications. The quality of these VIIRS EDRs strongly depends on the quality of its calibrated and geo-located Sensor Date Records (SDRs). Built with a strong heritage to the NASA's EOS MODerate resolution Imaging Spectroradiometer (MODIS) instrument, the VIIRS is calibrated on-orbit using a similar set of on-board calibrators (OBC), including a solar diffuser (SD) and solar diffuser stability monitor (SDSM) system for the reflective solar bands (RSB) and a blackbody (BB) for the thermal emissive bands (TEB). On-orbit maneuvers of the SNPP spacecraft provide additional calibration and characterization data from the VIIRS instrument which cannot be obtained pre-launch and are required to produce the highest quality SDRs. These include multi-orbit yaw maneuvers for the characterization of SD and SDSM screen transmission, quasi-monthly roll maneuvers to acquire lunar observations to track sensor degradation in the visible through shortwave infrared, and a driven pitch-over maneuver to acquire multiple scans of deep space to determine TEB response versus scan angle (RVS). This paper pro-vides an overview of these three SNPP calibration maneuvers. Discussions are focused on their potential calibration and science benefits, pre-launch planning activities, and on-orbit scheduling and implementation strategies. Results from calibration maneuvers performed during the Intensive Calibration and Validation (ICV) period for the VIIRS sensor are illustrated. Also presented in this paper are lessons learned regarding the implementation of calibration spacecraft maneuvers on follow-on missions.

Since launch in May 2002, Aqua MODIS has successfully operated for nearly 10 years, continuously collecting global datasets for scientific studies of key parameters of the earth's land, ocean, and atmospheric properties and their changes over time. The quality of these geophysical parameters relies on the input quality of sensor calibrated radiances. MODIS observations are made in 36 spectral bands with wavelengths ranging from visible (VIS) to longwave infrared (LWIR). Its reflective solar bands (RSB) are calibrated using data collected from its on-board solar diffuser and regularly scheduled lunar views. The thermal emissive bands (TEB) are calibrated using an on-board blackbody (BB). The changes in the sensor's spectral and spatial characteristics are monitored by an on-board spectroradiometric calibration assembly (SRCA). This paper presents an overview of Aqua MODIS 10-year on-orbit operation and calibration activities, from launch to present, and summarizes its on-orbit radiometric, spectral, and spatial calibration and characterization performance. In addition, it will illustrate and discuss on-orbit changes in sensor characteristics and corrections applied to continuously maintain the sensor level 1B (L1B) data quality, as well as lessons learned that could benefit future calibration efforts.

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.

The Moderate Resolution Imaging Spectroradiometer (MODIS) is a major instrument for NASA's 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 14.4&#956;m. 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 <i>et. al</i>).