Since launch in 1999, the NASA EOS Terra MODIS has successfully operated for more than a decade. MODIS acquires
data in 36 spectral bands with wavelengths ranging from visible (VIS) to long-wave infrared (LWIR) and at three nadir
spatial resolutions: 250m for 2 bands, 500m for 5 bands, and 1km for 29 bands. In addition to its on-board calibrators
(OBC), designed for sensor radiometric calibration and characterization, MODIS was built with a unique device called
the spectro-radiometric calibration assembly (SRCA), which can be configured into three different modes: radiometric,
spatial, and spectral. When it is operated in the spectral mode, the SRCA can monitor changes in sensor spectral
performance for the VIS and near-infrared (NIR) spectral bands. For more than 10 years, the SRCA operations have
continued to provide valuable information for Terra MODIS on-orbit spectral performance. This paper briefly describes
Terra MODIS SRCA on-orbit operations and calibration activities and presents results derived from its decade-long
spectral characterization, including changes in the VIS and NIR spectral bands center wavelengths (CW) and bandwidths
(BW). It demonstrates that the SRCA on-orbit wavelength calibration capability remains satisfactory. For most spectral
bands, the changes in CW and BW are less than 0.5 nm and 1.0 nm, respectively. As expected, results and lessons from
Terra MODIS on-orbit spectral characterization have and will continue to benefit the operation and calibration of its
successor, Aqua MODIS, and the development of future missions and sensors, which have stringent requirements on
sensor spectral performance.
The near-simultaneous observations from Moderate Resolution Imaging Spectroradiometer (MODIS) and Atmospheric
Infrared Sounder (AIRS) sensors on-board the Aqua spacecraft provide a good opportunity to track the relative
calibration stability of both sensors over their entire mission. The AIRS is a hyper-spectral sensor with a spectral
resolution of around 5 nm that covers the spectral range of most MODIS thermal emissive bands (TEB) except for a few
small gaps. The simulated MODIS radiances can be derived by convolving AIRS spectral measurements with MODIS
relative spectral response functions. Using spatially collocated pixels, the differences between MODIS observed and
AIRS simulated brightness temperatures are computed for most MODIS TEB at various scan angles within ±49.5 degree
of the nadir position. Two regions of different scene temperatures are selected: the central Atlantic Ocean and the Dome
C at Antarctica. The trending of the MODIS - AIRS brightness temperature differences (BTD) is derived for years 2003-
2008. Results show that the magnitudes of the BTD are spectral band dependent. The values of BTD are generally less
than ± 0.5K for most TEB with a few exceptions. For band 27, the BTD are about -1.2 K over the Dome Concordia and
about -3.0 K over the northern Atlantic Ocean. Bands 35 and 36 have BTD about +1.0 K over the Atlantic Ocean. The
trending results show that the BTD changes over a six-year period are less than 0.3 K for all calculable MODIS bands,
and are slightly larger at large scan angles than those near nadir. Our results are consistent with previous studies.
MODerate resolution Imaging Spectroradiometer (MODIS), operated on both Terra and Aqua spacecrafts, measures
the Earth scenes with 36 spectral bands allocated into four Focal Plane Assemblies (FPAs). Mis-registration between
the spectral bands and FPAs was observed, which will lessen the data quality and reduce the accuracy of science
products generated with multiple spectral bands located on different FPAs. An approach using ground targets,
developed and validated in our previous work, is an alternative way for characterizing the MODIS Band-to-Band
Registration (BBR), by calculating the centroid location difference of same dark targets observed by each band in its
own field of view. The long term time series of spatial shift, not only for the band but also for the detector, are
presented over sensor's operation years (year 2000-2007 for Terra MODIS and year 2002-2007 for Aqua MODIS).
With this ground target approach, the spatial performances of both Terra and Aqua MODIS are evaluated. The results
show that spatial shifts are small except they are relatively large between bands on the warm FPA and cold FPA of
Aqua MODIS. The discrepancy between detectors is quite small and mainly attribute to the systematic error of the
approach. Moreover, the long term results reveal an annual variation for some high resolution bands.
The MODIS operated on both the Terra and Aqua spacecrafts has 36 bands located on four focal plane assembles
(FPAs). Although MODIS band-to-band registration (BBR) was carefully characterized pre-launch, mis-registration
between spectral bands and FPAs may still exist or occur during its on-orbit operation. Obviously the
mis-registration, or the BBR shift, could impact the quality of MODIS science data products that are produced using
multiple spectral bands. Because of the mis-registration, measurements over slightly different areas by different
spectral bands, when used together, will cause undesired effects, and consequently, lead to less accurate data
products. The spectro-radiometric calibration assembly (SRCA), a unique device within the MODIS instrument, is
used to track the sensor on-orbit BBR during its entire mission. On-orbit results show that the mis-registration is
typically very small for Terra MODIS but is relatively large for Aqua MODIS between the bands on the warm FPAs
and that on the cold FPAs. In this paper we present a theoretical sensitivity analysis of the BBR shift characterization
and its impact on MODIS data products. A real case is selected to estimate the impact on L1B data and science
products using the BBR shift derived from the SRCA. As expected the mis-registration of Aqua MODIS produces
small but potentially non-negligible impact on the science products, particularly at the mixed areas with various
surface cover types.
The MODerate Resolution Imaging Spectroradiometer (MODIS) is a key instrument for the NASA Earth Observing System (EOS) mission. It was successfully launched onboard the Terra satellite in December 1999 and Aqua satellite in May 2002. MODIS senses the Earth's surface in thirty-six spectral bands which are distributed on four Focal Plane Assemblies (FPAs): Visible (VIS), Near-Infrared (NIR), Short-and Middle-wavelength IR (SMIR), and Long-wavelength IR (LWIR). It was found from sensor pre-launch measurements that Aqua MODIS SMIR/LWIR FPAs had a large misalignment or misregistration relative to the VIS/NIR FPAs in both along-scan and along-track directions. The misregistration of the two FPA groups has remained nearly the same during its on-orbit operation. Consequently this has been a major concern for Aqua MODIS performance since it could affect the quality of MODIS products which utilize bands from both the VIS/NIR and SMIR/LWIR FPAs, for example, the snow index. This paper focuses on investigating the impact of Aqua MODIS FPA-to-FPA or band-to-band misregistration on its snow index (NDSI) derived from measurements made by VIS band 4 and SWIR band 7. Preliminary results show that shifting one pixel (500 m) forward in the along-track direction of band 7 can improve the band-to-band registration between bands 4 and 7 and, therefore, the quality of Aqua MODIS snow mapping. This study will help MODIS data users to understand the potential impact of band-to-band misregistration on MODIS science products, and also be useful for the future sensor design.
NASA's EOS Aqua spacecraft was launched on May 04, 2002. The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the six Earth-observing sensors aboard the EOS Aqua spacecraft. MODIS is the highest spatial resolution instrument on the Aqua platform with data products generated in 250m, 500m, and 1000m resolutions (nadir). It has 36 spectral bands, a total of 490 detectors, located on four focal plane assemblies (FPAs) with two of them controlled during operation at 83K by a passive radiative cooler. In addition to radiometric calibration and spectral characterization, MODIS spatial performance was extensively characterized pre-launch, including measurements of band-to-band registration (BBR), FPA to FPA registration (FFR), line spread function (LSF), modulation transfer function (MTF), and instantaneous field-of-view (IFOV). The sensor's spatial characterization is monitored by an on-board calibrator, the spectro-radiometric calibration assembly (SRCA). In this paper, we will briefly describe MODIS SRCA spatial characterization methodologies and operational activities. We will focus on the sensor's spatial performance using four years of on-orbit observations and, consequently, evaluate the SRCA's performance. On-orbit results of key spatial characterization parameters (BBR, FFR, and MTF) will be examined and compared to pre-launch measurements and design requirements.
The Moderate Resolution Imaging Spectroradiometer (MODIS) flight model 1 (FM-1) was launched on-board NASA's EOS Aqua spacecraft on May 04, 2002. MODIS has 20 reflective solar bands (RSB) with wavelengths from 0.41 to 2.2μm and 16 thermal emissive bands (TEB) with wavelengths from 3.7 to 14.4μm. Typical sensor spectral characterization includes measurements of in-band (IB) and out-of-band (OOB) relative spectral responses (RSR) or spectral response functions (SRF), center wavelengths (CW) and bandwidths (BW). During MODIS instrument pre-launch calibration and characterization, these parameters were measured using a spectral measurement assembly (SpMA) by the instrument vendor. In addition to its on-orbit radiometric calibration capability, MODIS has a unique on-board calibrator, spectro-radiometric calibration assembly (SRCA) that can be used to monitor RSB on-orbit spectral performance. This paper presents an overview of MODIS spectral characterization methodologies, from pre-launch to on-orbit. It describes Aqua MODIS SRCA operational activities in spectral mode, summarizes the results from its four-years of on-orbit spectral measurements, and discusses lessons learned for future sensor design and development. The results show that on-orbit changes of Aqua MODIS RSB center wavelengths and bandwidths have been very small, typically less than 0.5nm for the CW and less than 1nm for the BW.
Snow cover is one of the sensitive indicators of global climate change. Numerous studies have shown the importance of accurate measurements of snow cover. The Moderate Resolution Imaging Spectroradiometer (MODIS) is well suited to the measurement of snow cover because snow characteristically has high reflectance in the MODIS Visible (VIS) and low reflectance in the MODIS Shortwave Infrared (SWIR) wavelengths, a characteristic that allows for snow detection by a normalized ratio of VIS and SWIR bands. The automated MODIS snow-mapping algorithm uses at-satellite reflectance in MODIS VIS band 4 (0.545-0.565 μm) and SWIR band 6 (1.628-1.652 μm) to calculate the Normalized Difference Snow Index (NDSI). Aqua MODIS band 7 (2.105-2.155 μm) instead of band 6 has been used to calculate NDSI, in response to band 6 striping problem caused by non-functional or noisy detectors. In our early study, a feasible algorithm to map Aqua MODIS band 6 based on the relationship between Terra MODIS bands 6 and 7 has been developed and validated. This algorithm has been used to retrieve Aqua MODIS band 6. Aqua MODIS NDSI values computed from Aqua MODIS observed band 6, simulated band 6, and observed band 7 are used to map snow based on current MODIS snow algorithm, respectively. Snow coverage mapped using NDSI computed from observed band 6 is regarded as a standard snow product, comparison and analysis are performed between snow mapping using NDSI computed from simulated band 6 and observed band 7. This paper will investigate the measurement continuity between Terra and Aqua MODIS snow coverage products, and propose another alternative for Aqua MODIS NDSI retrieval. Our approach for monitoring snow coverage is valuable to keep the continuity and consistency for MODIS snow products.
MODerate resolution Imaging Spectro-radiometer (MODIS), as part of NASA's Earth Observe System (EOS) mission, is
widely utilized in diversified scientific research areas. Both Terra and Aqua MODIS observe the earth in
sun-synchronous orbit at three nadir spatial resolutions. MODIS has thirty-six bands that are located in four Focal Plane
Assembles (FPAs) by wavelength: Visible (VIS), Near-Infrared (NIR), Short-and Middle-wavelength IR (SMIR), and
Long wavelength IR (LWIR). MODIS Band-to-Band Registration (BBR) was measured pre-launch at the instrument
vendor. Mis-registration exists between bands and FPAs. The spatial characterization could change in storage, at launch,
and years on-orbit. In this study, a special ground scene with unique features has been selected as our study area to
calculate the spatial registration in both along-scan and along-track for bands 2 - 7 relative to band 1. The results from
the earth scene targets have been compared with on-board calibrator, the Spectro-Radiometric Calibration Assembly
(SRCA), with good agreement. The measured differences between the SRCA and our ground scene approach are less
than 20m on average for VIS/NIR bands both along-scan and along-track. The differences for SMIR bands are 20m
along-scan and 0.1 - 0.18 km for along track. The SMIR FPA crosstalk could be a contributor to the difference. For Aqua MODIS instruments, the spatial deviation is very small between the bands located on the same FPA or between
VIS and NIR FPAs but is relatively large between warm (VIS and NIR) and cold (SMIR and LWIR) FPAs. The spatial
deviation for MODIS/Terra can be ignorable but not for MODIS/Aqua. The results from this study show that the spatial deviation of Aqua MODIS may impact on the science data when multi-band data from both warm and cold FPAs is
MODerate resolution Imaging Spectro-radiometer (MODIS) has been operated on-board the Terra spacecraft since December 18, 1999 and Aqua MODIS since May 4, 2002. Both MODIS Relative Solar Bands (RSBs) and Thermal Emissive Bands (TEBs) are calibrated on-orbit by a set of on-board calibrations (OBCs) in radiometric, spatial and spectral modes, providing accurate measurements for scientific researches. The Spectro-Radiometric Calibration Assembly (SRCA) is one of the key OBCs which can be operated at all three calibration modes. When operating in spectral mode, the SRCA is utilized for MODIS On-Orbit Spectral Characterization (MOOSC), monitoring and measuring the center wavelength (CW) shift of each RSB throughout the entire mission. However, some uncertainties in the SRCA measurement may affect the precision of the results due to possible system degradation, mechanical/optical backlash, deformation, and optical performance change.
In this study, the instrument background and the algorithm for calculating the CW shift of RSBs using the SRCA measurements are briefly introduced. We analyze or estimate the impact on the final CW value caused by the uncertainties on the Terra MODIS on-orbit spectral characterizations, including cavity temperature variation, limited number of sample points, noise of background, and the variation of <i>β</i> and <i>θ<sub>off</sub></i>. The results show that the influence is small and the maximum uncertainty is less than 1nm.
The lessons we learned in this study provide helpful information and experiences for the sensors which have no on-orbit spectral characterization capability and the useful guidance for the next generation satellite remote sensors.