Launched in November 2017, the National Oceanic and Atmospheric Administration-20 (NOAA-20) Visible Infrared Imaging Radiometer Suite (VIIRS) has successfully operated over five years and produced high-quality sensor data records (SDRs), which have significantly contributed to the Earth’s environmental and climate change studies. The VIIRS instrument collects data in the reflective solar bands (RSBs) from bands M1 to M11 and I1 to I3 with two spatial resolutions of 375 m for imaging ($I$) bands and 750 m for moderate resolution ($M$) bands covering a wavelength range from 401 to 2284 nm. For the RSBs on-orbit radiometric calibration, VIIRS primarily uses solar diffuser (SD) observations along with alternative long-term lunar calibrations and deep convective cloud (DCC) trends. The NOAA VIIRS SDR team observed upward long-term trends after three years in the lunar calibration coefficients (called lunar F-factors) compared to the initial on-orbit SD F-factors. These long-term lunar trend changes were validated with the DCC observation results and the operational radiometric calibration coefficient (called F-PREDICTED) lookup table (LUT) was updated in November of 2021, which was proportional to observed radiance. After five years of on-orbit operations, the performance of the current operational F-PREDICTED LUT was evaluated in comparison with the long-term DCC trends. After application of the LUT, the results showed excellent on-orbit radiometric calibration stability providing confidence for the user communities of NOAA-20 VIIRS SDR products.

The Joint Polar Satellite System-2 (JPSS-2) was successfully launched on November 10, 2022 and renamed as NOAA-21 after it reached its final polar orbit. Twenty-five days after launch, on December 5, 2022, the NOAA-21 Visible Infrared Imaging Radiometer Suite (VIIRS) started collecting science data. After intensive analysis, calibration and validation of VIIRS science raw data record (RDR), telemetry RDR, and sensor data record (SDR) radiometric and geolocation data products, the NOAA-21 VIIRS SDRs reached Beta maturity on Feb. 23, 2023 and provisional maturity on March 30, 2023. This paper provides an update on the NOAA-21 VIIRS SDR availability, post-launch calibration and validation activities, and SDR quality assessments.

The Visible Infrared Imaging Radiometer Suite (VIIRS) on the National Oceanic and Atmospheric Administration-20 (NOAA-20) satellite performs on-orbit radiometric calibration based on regular solar diffuser (SD) observations illuminated by the Sun at the termination point near the South Pole. Due to exposure to the ultraviolet portion of the solar irradiance spectrum, the SD bidirectional reflectance distribution function (BRDF) has been degrading over time. The SD degradation (called H-factor) was measured by the on-board calibrator called the SD stability monitor (SDSM). Nevertheless, over two years of operation, there have been systematic on-orbit calibration differences between the SD-based and independent moon-based calibration results. In this study, the NOAA VIIRS team used a surface roughness Rayleigh scattering (SRRS) model as a baseline SD degradation, simulated on-orbit center wavelength-based approach of SD degradation, and a new SDSM relative spectral response (RSR)-dependent SD degradation estimation method to evaluate the degradation. There were time-dependent growing differences between the SDSM RSR-applied H-factors and center wavelength interpolated H-factors especially in the short wavelength detectors (SDSM detector 1–4). The NOAA-20 SD-based calibration coefficients (SD F-factors) were reprocessed using the RSR-applied H-factors, and the new SD F-factors show similar long-term trends compared with the independent monthly lunar F-factors. The newly processed SD F-factor suggested that the NOAA-20 VIIRS detectors in the reflective solar bands (M1–M11 and I1–I3) showed very stable responses within 0.5% level over the two years of on-orbit operation.

The second Visible Infrared Imaging Radiometer Suite (VIIRS) instrument aboard the National Oceanic and Atmospheric Administration (NOAA) 20 satellite has been successfully operating since its launch on November 18, 2017. Since VIIRS does not include onboard calibrators to perform the on-orbit geometric data quality characterization for parameters such as the modulation transfer function (MTF) and band-to-band registration (BBR), the monthly scheduled lunar observations are used in this study. The radiometric property of the moon surface has demonstrated its long-term stability and it is also a suitable spatial target for the on-orbit calibration of remote sensing instruments. Using the commonly practiced methodologies, the VIIRS BBR results are derived in the scan and track directions. The initial trends show that the VIIRS BBR is very stable on-orbit within ±0.1 pixels in both scan and track directions meeting the highest requirements of within 0.2 pixels. Using the sharp edge of the moon, the scan-direction MTF at the Nyquist frequency was approximately 0.23. The MTF values are well above the specification of 0.3 in imaging (I)-bands and they are very stable over the study period, whereas the moderate-resolution (M)-bands results were slightly below the specification line as suggested by prelaunch test results. The scan-direction MTF estimations were consistently near 0.2 over four years of operations. Track-direction MTF values showed oscillations because of the annual cycle of lunar shadow angle and spatial features in the moon side. However, the track-direction MTF values met the specification with large margin.

As a panchromatic band, the Day-Night Band (DNB) on National Oceanic Atmospheric Administration (NOAA)-20 Visible Infrared Imaging Radiometer Suite (VIIRS) has a unique capability of observing very low radiances down to Nano-watt [cm-1sr-1] level, because of its three different focal planes with gain stages. The DNB radiometric calibration is based on the Solar Diffuser observations near the night to day time termination points. Besides the primary calibration source of SD, the moon provides alternative source of calibration with a specific lunar roll maneuvers. This study provides detailed methodology and results of lunar calibration for NOAA-20 VIIRS sensor and initial two year DNB radiometric calibration results from SD and scheduled lunar calibration. Over two years of NOAA-20 VIIRS operation, the Low Gain Stage (LGS) slope (inverse gain) trend showed a decreasing trend, whereas the calculated monthly lunar F-factor showed a stable response (inverse gain normalized to the lunar irradiance model) using a lunar irradiance model. These differences are monitored, compared, and applied for the best quality of NOAA-20 VIIRS DNB product.

This paper presents NOAA-20 and S-NPP Visible Infrared Imaging Radiometer Suite (VIIRS) Reflective Solar Bands (RSB) and Thermal Emissive Bands (TEB) Sensor Data Records (SDR) performance and recent improvements to support user communities. Results for NOAA-20 VIIRS and in 2019 are emphasized. NOAA-20 VIIRS geolocation errors are within ±100 m, comparable to S-NPP; RSBs have been stable after achieved validated maturity status, except that small upward trends were observed; TEBs agree with co-located Cross-track Infrared Sounder (CrIS) observations within ~0.1 K at nadir, more stable (relative to CrIS) compared to S-NPP TEBs in 2019. NOAA-20 RSBs continue bias ~2-4.5% lower than S-NPP. Three major improvements to VIIRS SDRs, including the new operational M6 saturation rollover flagging method, the operational TEB warm-up/cool-down bias correction, and the latest results for correcting NOAA-20 TEB scan angle/scene temperature dependent biases, are also presented to address users' concerns.

The NOAA-20 (N-20) VIIRS has successfully operated for more than two years since its launch in November 2017. Shortly after completing its initial instrument check-outs and post-launch testing (PLT) activities, the N-20 VIIRS sensor data records (SDR) achieved the beta, provisional, and validated maturity status in January, February, and April 2018, respectively. In this paper, we briefly describe the instrument on-orbit operation and calibration activities, provide an overall assessment of its on-orbit performance, and discuss the methodologies developed to maintain and improve sensor calibration and data quality. As illustrated in this paper, the N-20 VIIRS continues to perform with excellent stability, allowing high-quality environmental data records (EDR) to be generated from its well-calibrated SDR.

The National Oceanic and Atmospheric Administration (NOAA)-20 Visible Infrared Imaging Radiometer Suite (VIIRS) was launched on November 18, 2017, and it has been operational for more than two years and follows the first Joint Polar Satellite System (JPSS) series of the Suomi National Polar-orbiting Partnership (S-NPP) mission. VIIRS has 14 reflective solar bands (RSBs) covering a spectral range of 0.41-2.3 μm. The primary source of RSB calibration is the solar diffuser (SD), and the time-dependent SD degradation is monitored by the SD stability monitor (SDSM). The initial instability of the SD degradation (H-factor) was resolved by updating SDSM sun screen transmittance function combining yaw maneuver data and on-orbit SDSM data sets. After the H-factor improvements, the VIIRS RSB calibration coefficients (F-factors) are updated and applied to the operational Sensor Data Record (SDR) product generation. To validate the SD F-factors, the lunar F-factors are calculated by using a lunar irradiance model and comparing the trend differences between them. Over the two years of operation, decreasing trends have been calculated with the SD F-factors, whereas constant lunar F-factors were observed in bands M1-M4. With these discrepancies, the operational F-factors remained unchanged since April 2018 because the deep convective cloud (DCC) and cross-calibration comparison results did not show any further degradations in these bands. All the possible radiometric calibration sources, such as SD and lunar F-factors, DCC trends, and cross-calibration results, are monitored, compared, and applied by the NOAA VIIRS SDR science team for the best quality of the VIIRS SDR product.

The National Oceanic and Atmospheric Administration (NOAA)-20 Visible Infrared Imaging Radiometer Suite (VIIRS) was launched on November 18, 2017 and has been in operation for about one year. It has 14 Reflective Solar Bands (RSBs) covering a spectral range of 412nm to 2.25 um at two spatial resolutions at nadir 750m for Moderate resolution (M) bands and 350m for Imaging (I) bands. The spatial characterization of the VIIRS instrument was performed during the prelaunch calibration with parameters such as Band-to-Band Registration (BBR). Accurate estimation of BBR is a key spatial parameter when the inter-band calculation is required. Because there is no on-board spatial calibration source like Spectro Radiometric Calibration Assembly (SRCA) with Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), VIIRS instrument does not have on-board capability of measuring on-orbit BBR. As an alternative method, scheduled moon collections can be used to estimate on-orbit BBR characterization within the RSBs. In this paper, NOAA-20 VIIRS on-orbit BBR estimation methodology and initial results are presented.

On February 15 and 16, 2012, a series of yaw maneuvers was performed on the Suomi National Polar-Orbiting Partnership (S-NPP) spacecraft for Visible Infrared Imaging Radiometer Suite (VIIRS) to characterize screen transmission functions and Bidirectional Reflectance Distribution Functions (BRDFs). For accurate time-dependent calibration coefficient (or F-factor) calculation in the Reflective Solar Bands (RSBs), the Solar Diffuser (SD) and SD Stability Monitor (SDSM) screen transmission functions are measured using the on-orbit yaw maneuver data with three screen transmission functions. The SD screen transmittance function coupled with the SD BRDF (τSD BRDF) is updated and then the SDSM Sun and SDSM SD BRDF functions are updated. The SDSM solar and SD view functions are used in the SD degradation estimation, which affects the VIIRS view of the SD BRDF through the rotating telescope over time. The prelaunch measurements of these transmission or BRDF functions were too coarse to capture the small features of the transmission functions. There have been several efforts to derive better models but the annual oscillations in the F-factors remained to be a major issue. To mitigate these F-factor oscillations, the yaw maneuver data are reanalyzed to derive the transmittance and BRDF functions. This paper shows that the annual oscillations are caused by the imperfect SDSM Sun screen transmittance and SDSM SD BRDF functions. When the newly calculated SDSM SD BRDF and Sun transmittance lookup tables (LUTs) are applied, the annual oscillations in the H-and F-factors are significantly reduced, especially in the bands M1-M4.

Solar diffusers (SDs) have often been used as the onboard calibrators for the radiometric calibration of reflective solar band imaging sensors. After being spaceborne, the reflectance of SDs is observed to degrade with spectral dependence due to exposure to solar UV and energetic particle radiation. Long-term spectral reflectance data of SDs onboard multiple LEO imaging sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua and the Visible Infrared Imaging Radiometer Suite (VIIRS) on SNPP, are analyzed. The reflectance of SDs on these three instruments degrades faster for the shorter wavelength (0.4-0.6 μm) bands than the longer wavelength bands. The Surface Roughness-induced Rayleigh Scattering (SRRS) model is applied to simulate the SD degradation on these instruments, and the growth of the surface roughness parameter of the SDs is derived. It is determined that the change of surface roughness scale length is ~tens of nanometers. To show the consistency of roughness growth rates among the SDs on Terra/Aqua MODIS and SNPP VIIRS instruments, the functional dependences of the growth rates are characterized according to the SD exposure time and the stage of surface roughness. It is also found that the flattening or reverse in the growth trend of the surface roughness for these three SDs occurred around the same interval between October 2013 and October 2015. The confirmation of the applicability of SRRS model with the long-term spectral reflectance data from three independent spaceborne SDs facilitates a better understanding of the origin and physical processes of the SD degradation.

Since launch in November 2018, the VIIRS on-board the NOAA-20 (or JPSS-1) satellite has completed its initial intensive on-orbit check-outs and several key calibration and validation activities scheduled to help evaluate sensor at launch performance. This paper provides a brief overview of NOAA-20 VIIRS on-orbit operation and calibration activities, presents early results derived from its on-board calibrators and lunar observations, and discusses potential improvements and future effort to assure sensor data product quality.

This study describes the improved calibration applied for VIIRS SDR reprocessing. At reflective solar bands (RSBs), the earth-view F-factor is adjusted with the lunar observations and changes about 2% in band M2-M4 in past five years. An empirical method is applied to remove the radiance spikes in VIIRS thermal emissive bands (TEBs) during the blackbody Warm-Up-Cool-Down (WUCD). VIIRS DNB calibration is improved by applying continuous spectral response changes, new bias tables, updated gain ratio tables, stray light tables, and terrain corrections.

Spatial quality of an imaging sensor can be estimated by evaluating its modulation transfer function (MTF) from many different sources such as a sharp edge, a pulse target, or bar patterns with different spatial frequencies. These well-defined targets are frequently used for prelaunch laboratory tests, providing very reliable and accurate MTF measurements. A laboratory-quality edge input source was included in the spatial-mode operation of the Spectroradiometric Calibration Assembly (SRCA), which is one of the onboard calibrators of the Moderate Resolution Imaging Spectroradiometer (MODIS). Since not all imaging satellites have such an instrument, SRCA MTF estimations can be used as a reference for an on-orbit lunar MTF algorithm and results. In this paper, the prelaunch spatial quality characterization process from the Integrated Alignment Collimator and SRCA is briefly discussed. Based on prelaunch MTF calibration using the SRCA, a lunar MTF algorithm is developed and applied to the lifetime on-orbit Terra and Aqua MODIS lunar collections. In each lunar collection, multiple scan-direction Moon-to-background transition profiles are aligned by the subpixel edge locations from a parametric Fermi function fit. Corresponding accumulated edge profiles are filtered and interpolated to obtain the edge spread function (ESF). The MTF is calculated by applying a Fourier transformation on the line spread function through a simple differentiation of the ESF. The lifetime lunar MTF results are analyzed and filtered by a relationship with the Sun–Earth–MODIS angle. Finally, the filtered lunar MTF values are compared to the SRCA MTF results. This comparison provides the level of accuracy for on-orbit MTF estimations validated through prelaunch SRCA measurements. The lunar MTF values had larger uncertainty than the SRCA MTF results; however, the ratio mean of lunar MTF fit and SRCA MTF values is within 2% in the 250- and 500-m bands. Based on the MTF measurement uncertainty range, the suggested lunar MTF algorithm can be applied to any on-orbit imaging sensor with lunar calibration capability.

The high-contrast edge of the Moon has been used for the on-orbit measurement of the modulation transfer function (MTF) of remote sensing instruments with a lunar observation capability. With the lunar edge as a target, the classical edge method is applied to the National Aeronautics and Space Administration's MODerate resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites. One of the major difficulties encountered during the calculation is that the spatial resolution of MODIS is too coarse to capture the fine structure of the edge spread function (ESF), which is required to calculate the MTF. To produce the MODIS ESF in high resolution, lunar images of a selected edge acquired by multiple instrument scans need to be superposed by aligning the edge positions accurately. In this paper, an algorithm is developed to perform the alignment, based on the lunar position data generated by the MODIS geolocation algorithm and recorded scan by scan. The positions of the lunar edges at the focal plane are calculated scan by scan, allowing the construction of a high-quality ESF for MTF derivation. The algorithm is applied to all MODIS bands with 250-m, 500-m, and 1-km spatial resolutions in both along-scan and along-track directions. The along-track MTF results are particularly valuable because the onboard SpectroRadiometric Calibration Assembly can only monitor the along-scan MTF. The trending results show that the along-track MTF of MODIS has been stable throughout the MODIS lifetime and is well above the design specification for all bands. The limitation of the algorithm is analyzed. The algorithm developed in this paper can be applied to other instruments with similar design features.

In recent years, there has been a significant increase in the use of remotely sensed data to address global issues. With the open data policy, the data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Enhanced Thematic Mapper Plus (ETM+) sensors have become a critical component of numerous applications. These two sensors have been operational for more than a decade, providing a rich archive of multispectral imagery for analysis of mutitemporal remote sensing data. This paper focuses on evaluating the radiometric calibration agreement between MODIS and ETM+ using the near-simultaneous and cloud-free image pairs over an African pseudo-invariant calibration site, Libya 4. To account for the combined uncertainties in the top-of-atmosphere (TOA) reflectance due to surface and atmospheric bidirectional reflectance distribution function (BRDF), a semiempirical BRDF model was adopted to normalize the TOA reflectance to the same illumination and viewing geometry. In addition, the spectra from the Earth Observing-1 (EO-1) Hyperion were used to compute spectral corrections between the corresponding MODIS and ETM+ spectral bands. As EO-1 Hyperion scenes were not available for all MODIS and ETM+ data pairs, MODerate resolution atmospheric TRANsmission (MODTRAN) 5.0 simulations were also used to adjust for differences due to the presence or lack of absorption features in some of the bands. A MODIS split-window algorithm provides the atmospheric water vapor column abundance during the overpasses for the MODTRAN simulations. Additionally, the column atmospheric water vapor content during the overpass was retrieved using the MODIS precipitable water vapor product. After performing these adjustments, the radiometric cross-calibration of the two sensors was consistent to within 7%. Some drifts in the response of the bands are evident, with MODIS band 3 being the largest of about 6% over 10 years, a change that will be corrected in Collection 6 MODIS processing.

To monitor land surface processes over a wide range of temporal and spatial scales, it is critical to have coordinated observations of the Earth's surface acquired from multiple spaceborne imaging sensors. However, an integrated global observation framework requires an understanding of how land surface processes are seen differently by various sensors. This is particularly true for sensors acquiring data in spectral bands whose relative spectral responses (RSRs) are not similar and thus may produce different results while observing the same target. The intrinsic offsets between two sensors caused by RSR mismatches can be compensated by using a spectral band adjustment factor (SBAF), which takes into account the spectral profile of the target and the RSR of the two sensors. The motivation of this work comes from the need to compensate the spectral response differences of multispectral sensors in order to provide a more accurate cross-calibration between the sensors. In this paper, radiometric cross-calibration of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors was performed using near-simultaneous observations over the Libya 4 pseudoinvariant calibration site in the visible and near-infrared spectral range. The RSR differences of the analogous ETM+ and MODIS spectral bands provide the opportunity to explore, understand, quantify, and compensate for the measurement differences between these two sensors. The cross-calibration was initially performed by comparing the top-of-atmosphere (TOA) reflectances between the two sensors over their lifetimes. The average percent differences in the long-term trends ranged from -5% to +6%. The RSR compensated ETM+ TOA reflectance (ETM+*) measurements were then found to agree with MODIS TOA reflectance to within 5% for all bands when Earth Observing-1 Hyperion hyperspectral data were used to produce the SBAFs. These differences were later reduced to within 1% for all bands (except band 2) by using Environmental Satellite Scanning Imaging Absorption Spectrometer for Atmospheric Cartography hyperspectral data to produce the SBAFs.

Pseudo invariant calibration sites (PICS) have been used for on-orbit radiometric trending of optical satellite systems for more than 15 years. This approach to vicarious calibration has demonstrated a high degree of reliability and repeatability at the level of 1-3% depending on the site, spectral channel, and imaging geometries. A variety of sensors have used this approach for trending because it is broadly applicable and easy to implement. Models to describe the surface reflectance properties, as well as the intervening atmosphere have also been developed to improve the precision of the method. However, one limiting factor of using PICS is that an absolute calibration capability has not yet been fully developed. Because of this, PICS are primarily limited to providing only long term trending information for individual sensors or cross-calibration opportunities between two sensors. This paper builds an argument that PICS can be used more extensively for absolute calibration. To illustrate this, a simple empirical model is developed for the well-known Libya 4 PICS based on observations by Terra MODIS and EO-1 Hyperion. The model is validated by comparing model predicted top-of-atmosphere reflectance values to actual measurements made by the Landsat ETM+ sensor reflective bands. Following this, an outline is presented to develop a more comprehensive and accurate PICS absolute calibration model that can be Système international d'unités (SI) traceable. These initial concepts suggest that absolute calibration using PICS is possible on a broad scale and can lead to improved on-orbit calibration capabilities for optical satellite sensors.

The Sonoran Desert is a large, flat, pseudo-invariant site near the United States-Mexico border. It is one of the largest and hottest deserts in North America, with an area of 311,000 square km. This site is particularly suitable for calibration purposes because of its high spatial and spectral uniformity and reasonable temporal stability. This study uses measurements from four different sensors, Terra Moderate Resolution Imaging Spectroradiometer (MODIS), Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+), Aqua MODIS, and Landsat 5 (L5) Thematic Mapper (TM), to assess the suitability of this site for long-term stability monitoring and to evaluate the “radiometric calibration differences” between spectrally matching bands of all four sensors. In general, the drift in the top-of-atmosphere (TOA) reflectance of each sensor over a span of nine years is within the specified calibration uncertainties. Monthly precipitation measurements of the Sonoran Desert region were obtained from the Global Historical Climatology Network (GHCN), and their effects on the retrieved TOA reflectances were evaluated. To account for the combined uncertainties in the TOA reflectance due to the surface and atmospheric Bi-directional Reflectance Distribution Function (BRDF), a semi-empirical BRDF model has been adopted to monitor and reduce the impact of illumination geometry differences on the retrieved TOA reflectances. To evaluate calibration differences between the MODIS and Landsat sensors, correction for spectral response differences using a hyperspectral sensor is also demonstrated.

Different applications and technology developments in Earth observations necessarily require different spectral coverage. Thus, even for the spectral bands designed to look at the same region of the electromagnetic spectrum, the relative spectral responses (RSR) of different sensors may be different. In this study, spectral band adjustment factors (SBAF) are derived using hyperspectral Earth Observing-1 (EO-1) Hyperion measurements to adjust for the spectral band differences between the Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) and the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) top-of-atmosphere (TOA) reflectance measurements from 2000 to 2009 over the pseudo-invariant Libya 4 reference standard test site.

The Committee on Earth Observation Satellites (CEOS) Infrared and Visible Optical Sensors (IVOS) subgroup members established a set of CEOS-endorsed globally distributed reference standard test sites for the postlaunch calibration of space-based optical imaging sensors. This paper discusses the top five African pseudo-invariant sites (Libya 4, Mauritania 1/2, Algeria 3, Libya 1, and Algeria 5) that were identified by the IVOS subgroup. This paper focuses on monitoring the long-term radiometric stability of the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and the Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) sensors using near-simultaneous and cloud-free image pairs acquired from launch to December 2008 over the five African desert sites. Residual errors and coefficients of determination were also generated to support the quality assessment of the calibration differences between the two sensors. An effort was also made to evaluate the relative stability of these sites for long-term monitoring of the optical sensors.

Short-term radiometric stability was evaluated using continuous ETM+ scenes within a single orbit (contact period) and the corresponding MODIS scenes for the four matching solar reflective visible and near-infrared (VNIR) band pairs between the two sensors. The near-simultaneous earth observations were limited by the smaller swath size of ETM+ (183 km) compared to MODIS (2330 km). Two sets of continuous granules for Terra MODIS and Landsat 7 ETM+ were selected and mosaicked based on pixel geolocation information for noncloudy pixels over the African continent. The matching pixel pairs were resampled from a fine to a coarse pixel resolution, and the at-sensor spectral radiance values for a wide dynamic range of the sensors were compared and analyzed, covering various surface types. The following study focuses on radiometric stability analysis from the VNIR band-pairs of ETM+ and MODIS. The Libya-4 desert target was included in the path of this continuous orbit, which served as a verification point between the short-term and the long-term trending results from previous studies. MODTRAN at-sensor spectral radiance simulation is included for a representative desert surface type to evaluate the consistency of the results.

The Moderate Resolution Imaging Spectroradiometer (MODIS) Proto-Flight Model (PFM), launched on December 18, 1999, aboard NASA's Earth Observing System (EOS) Terra satellite has 20 reflective solar bands (RSB) with wavelengths ranging from 0.41 to 2.1 mum over a wide field of view (plusmn55deg). The Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) sensor was launched on April 15, 1999, and has six spectral bands located in the visible and shortwave infrared (SWIR) part of the electromagnetic spectrum (0.4 - 2.5 mum). The ETM+ belongs to the family of Thematic Mapper sensors flown on previous Landsat missions. In this study, over 75 cloud-free nadir near-simultaneous images over Railroad Valley Playa, Nevada (RVPN) were chosen covering entire missions of both sensors. RVPN (38.5degN and 115.7degW), located between the cities of Ely and Tonopah, Nevada, USA, is a high reflectance site with very high spatial, spectral, and temporal uniformity. It is referenced to the Worldwide Reference System-2 (WRS-2) with path 40 and row 33. Homogeneous regions of interest (ROI) were chosen and cross-calibration was performed using an image statistics approach to monitor the long-term stability of the two sensors.

MODIS reflective solar bands (RSB) cover the VIS, NIR, and SWIR spectral regions. They are calibrated by a set of on-board calibrators (OBC). The 7 VIS spectral bands are located on the same focal plane assembly (FPA), with wavelengths from 0.41 to 0.56 mum. On-orbit observations show that the changes in the VIS response have been relatively large compared to the other spectral bands. This paper provides a comprehensive study of changes in the VIS response for both Terra and Aqua MODIS. It examines the differences among individual detectors within the same spectral band and between the two mirror sides. Results derived from measurements made by the instrument OBC and regularly scheduled lunar observations show that Aqua MODIS has been more stable than Terra MODIS in the VIS spectral region, with small response changes and mirror side differences over time.