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 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.
The Modulation Transfer Function (MTF) is a standard measure of the spatial quality of an imaging sensor. The MTF is
calculated by a normalized Fourier Transform of a Point Spread Function (PSF), which is a two-dimensional function.
For simplicity of calculation, a one-dimensional PSF, or Line Spread Function (LSF) is utilized in the along-scan
direction. The along-scan direction LSF model is sub-divided into the component level LSFs and the proper LSF is
constructed by the design specification information. To construct the modeled LSF, a spatial convolution is performed
for the three major components: optical, detector, and integration time LSFs. The optical LSF is calculated using Zemax.
The detector LSF is modeled as a rectangular function of the nominal detector size. Similar to the detector LSF, the
integration time LSF is modeled as a rectangular function by using the sampling frequency and the integration pulse
duration time. The modeling LSF is compared with the measured LSF from a prelaunch ground calibration device, the
Integrated Alignment Collimator (IAC). Because the IAC test slit has a width of 0.1 MODIS IFOV, an extra step pulse
convolution is added to the final LSF model. The comparison results show an excellent agreement between the modeled
and measured LSFs in the spatial domain and MTFs in the frequency domain for selected reflective solar bands (RSB).
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.
KEYWORDS: Modulation transfer functions, MODIS, Near infrared, Sensors, Staring arrays, Calibration, Signal to noise ratio, Collimators, Mirrors, Point spread functions
The Spectro-Radiometric Calibration Assembly (SRCA) is one of the on-board calibrators for the MODIS instrument.
The SRCA is operated in three modes: spectral, spatial, and radiometric. The spatial mode is used to track the changes
in band-to-band registration both along-scan (band and detector) and along-track (band) and the MTF in the along-scan
direction for all 36 MODIS bands over the MODIS lifetime. In the SRCA spatial mode, a rectangular knife-edge reticle,
located at the focus of the SRCA collimator, is imaged onto four MODIS Focal Plane Assemblies (FPA). The reticle is
illuminated by a spherical integration sphere and a glow-bar so that all bands can have an appropriate signal level. When
the MODIS scan mirror rotates, the illuminated knife-edge scans across the bands/detectors. In addition, there are five
electronic phase-delays so that the sampling spacing is reduced to 1/5 of the detector size, which results in dense data
points. After combining detector responses from all phase-delays, a combined bell-shaped response profile is formed.
The derivative of the detector response to the knife-edge is the Line Spread Function (LSF). In the frequency domain,
the Modulation Transfer Functions (MTF) are calculated from the normalized Fourier transform of the LSF. The MTF
results from the SRCA are validated by the pre-launch results from the Integrated Alignment Collimator (IAC) and a
SRCA collection performed in the Thermal Vacuum (TV). The six-year plus on-orbit MTF trending results show very
stable responses in the VIS and NIR FPAs, and meet the design specifications. Although there are noticeable MTF
degradations over the instrument lifetime in bands 1 and 2, they are negligible with the large specification margins. In
addition, a similar relationship is found between the band locations in the VIS and NIR FPAs versus MTF values.
Launched in May 2002, the Aqua MODIS has successfully operated on-orbit for more than five years and continuously
produced many high quality data products that have significantly contributed to studies of the Earth's climate and
environmental changes. The MODIS collects data in 36 spectral bands ranging from the visible (VIS) to the long-wave
infrared (LWIR) spectral region and at three (nadir) spatial resolutions: 250m (2 bands), 500m (5 bands), and 1km (29
bands). Bands 1-19 and 26 are the reflective solar bands (RSB) with wavelengths from 0.41 to 2.2μm and bands 20-25
and 27-36 are the thermal emissive bands (TEB) with wavelengths from 3.7 to 14.4μm. The MODIS on-board
calibrators, noticeably improved over those of its heritage sensors, include a solar diffuser (SD), a solar diffuser stability
monitor (SDSM), a blackbody (BB), a spectro-radiometric calibration assembly (SRCA), and a space view (SV) port.
This paper provides an overview of Aqua MODIS on-orbit operation and calibration activities with emphasis on the
performance of its on-board calibrators. Results discussed in this paper include TEB and RSB detector noise
characterization, short-term stability and long-term response change. The sensor's overall spectral (RSB) and spatial
(RSB and TEB) parameters are also presented in this paper.
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 MODIS scan mirror reflectance is a function of angle of incidence (AOI). For the MODIS solar reflective bands
(RSB), it is specified that the calibrated response variation versus scan angle (RVS) should be less than 2% and the
uncertainty of the RVS characterization should be less than 0.5% within the scan angle range of -45° ~ +45°. During
MODIS pre-launch RVS calibration and characterization, a series of laboratory tests were performed to assess the
relative response versus scans angle for all MODIS bands. Utilizing a Spherical Integrating Source, SIS, as an
illumination source, the test data was collected at various angles of incidence. The characterization of the RVS included
a measurement uncertainty assessment, repeatability analysis, RVS modeling and determination. The results show good
repeatability on the order of less than 0.5% for all the near infrared (NIR) bands and the visible (VIS) bands. The
detector response variation across scan angles for the majority of the NIR and VIS bands meets the instrument
specification. The derived RVS model enabled appropriate implementation of on orbit calibration. This paper
summarizes the methodologies and the algorithms used in the MODIS pre-launch RVS calibration for the RSB bands,
illustrates detector response variation with scan mirror angle of incidence, and demonstrates instrument specification
compliance within the scan angle coverage of ±55 degree. As a result, the RVS model and the correction coefficients
developed in the pre-launch calibration have been adopted during the MODIS on-orbit calibration.
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
combined.
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 β and θoff. 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.
MODIS is a major instrument for NASA's EOS missions, currently operating aboard the EOS Terra and Aqua spacecraft
launched in December 1999 and May 2002, respectively. It was designed to extend heritage sensor measurements and
data records and to enable new research studies of the Earth's land, oceans, and atmosphere. MODIS has 36 spectral
bands (0.41 - 14.4μm) located on four focal plane assemblies (FPA). It makes measurements at three nadir spatial
resolutions: 0.25km, 0.5km, and 1km. Because of instrument design complexity and more stringent calibration
requirements, extensive calibration and characterization activities were conducted pre-launch by the sensor vendor for
both Terra and Aqua MODIS. For the 20 reflective solar bands (RSB) with wavelengths below 2.2μm, these activities
include detector noise characterization, radiometric response at different instrument temperatures and at different scan
angles, and relative spectral response. On-orbit RSB calibration is performed using a solar diffuser (SD) and a solar
diffuser stability monitor (SDSM). In addition, regular lunar observations are made to track RSB radiometric stability.
This paper provides a summary of Terra and Aqua MODIS RSB pre-launch and on-orbit calibration and characterization
activities, methodologies, data analysis results, and lessons learned. It focuses on major issues that could impact MODIS
RSB calibration and data product quality. Results presented in this paper include RSB detector noise characterization,
response versus scan angle and instrument temperature, SD bi-directional reflectance factors characterization, and on orbit
calibration stability. Similar discussions on MODIS thermal emissive bands (TEB) are presented in a separate paper in these proceedings (Xiong et. al).
The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the five Earth-observing instruments onboard the NASA EOS Terra spacecraft launched in December 1999. It makes frequent global observations over a broad spectral range (0.41 to 14.4μ) and at three spatial resolutions (0.25km, 0.5km, and 1km at nadir). The MODIS was designed with a set of on-board calibrators (OBCs) that include a solar diffuser (SD), a blackbody (BB), and spectro-radiometric calibration assembly (SRCA). One SRCA function is to provide on-orbit spectral characterization of the MODIS reflective solar bands (RSB) with wavelengths from 0.41 to 2.2μ. This paper provides an overview of the MODIS SRCA on-orbit spectral characterization approach and summarizes the results derived from five years of Terra MODIS on-orbit observations. In general, the on-orbit characterization of the Terra MODIS RSB relative spectral responses (RSR) has been satisfactory. The measured center wavelength (CW) shifts are less than 0.6nm for the 412nm spectral band, 0.5nm for the 443nm band, and 0.4nm for the remaining reflective solar bands (short-wave infrared bands excluded). The bandwidth (BW) changes are typically less than 1nm. Excluding the differences between pre-launch and initial on-orbit results, the CW shifts and BW changes are very stable. For a given band, the detector-to-detector spectral characterization differences are typically less than 0.2nm.
The Moderate Resolution Imaging Spectroradiometer (MODIS) has 36 spectral bands that are distributed, according to their wavelengths, on four focal plane assemblies (FPAs): visible (VIS), near infrared (NIR), short- and mid-wave infrared (SMIR), and long-wave infrared (LWIR). One of the MODIS on-board calibrators, the spectro-radiometric calibration assembly (SRCA), is used to track the sensor's on-orbit spatial characterization. It is also capable of performing instrument radiometric stability monitoring and spectral characterization (measurements of the center wavelengths and bandwidths). This paper focuses on the SRCA's spatial characterization function and presents the results (on-orbit performance) derived from the observations made by the Terra MODIS since its launch in December 1999. The SRCA spectral characterization results of Terra MODIS over the same five-year period are covered in another paper in these proceedings (Xiong et. al.). The MODIS on-orbit spatial characterization discussed in this paper includes measurements of the detector-to-detector registration (DDR) in the along-scan direction, the band-to-band registration (BBR) in both along-scan and along track directions, and the focal plane-to-focal plane registration (FFR) in both directions. These measurements are typically performed bi-monthly. The results will show that the overall along-scan BBR performance of the Terra MODIS has been satisfactory (less than 0.16km), meeting the specification of 0.20km. Except for a few bands of slightly over 0.20km, the along-track BBR values are also within the specification.
KEYWORDS: MODIS, Calibration, Sensors, Mirrors, Reflectivity, Space operations, Detection and tracking algorithms, Electronics, Diffusers, Signal to noise ratio
The MODIS Flight Model 1 (FM1) has been in operation for more than two years since its launch onboard the NASA's Earth Observing System (EOS) Aqua spacecraft on May 4, 2002. The MODIS has 36 spectral bands: 20 reflective solar bands (RSB) with center wavelengths from 0.41 to 2.2mm and 16 thermal emissive bands (TEB) from 3.7 to 14.5mm. It provides the science community observations (data products) of the Earth's land, oceans, and atmosphere for a board range of applications. Its primary on-orbit calibration and characterization activities are performed using a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) system for the RSB and a blackbody for the TEB. Another on-board calibrator (OBC) known as the spectro-radiometric calibration assembly (SRCA) is used for the instrument's spatial (TEB and RSB) and spectral (RSB only) characterization. We present in this paper the status of Aqua MODIS calibration and characterization during its first two years of on-orbit operation. Discussions will be focused on the calibration activities executed on-orbit in order to maintain and enhance the instrument's performance and the quality of its Level 1B (L1B) data products. We also provide comparisons between Aqua MODIS and Terra MODIS (launched in December, 1999), including their similarity and difference in response trending and optics degradation. Existing data and results show that Aqua MODIS bands 8 (0.412mm) and 9 (0.443mm) have much smaller degradation than Terra MODIS bands 8 and 9. The most noticeable feature shown in the RSB trending is that the mirror side differences in Aqua MODIS are extremely small and stable (<0.1%) while the Terra MODIS RSB trending has shown significant mirror side difference and wavelength dependent degradation. The overall stability of the Aqua MODIS TEB is also better than that of the Terra MODIS during their first two years of on-orbit operation.
MODIS, one of the key instruments for the NASA's Earth Observing System (EOS), is currently operating on both the Terra and Aqua spacecraft making continuous observations in 36 spectral bands from 0.4 to 14.5μm. A complete suite of on-board calibrators (OBC) have been designed for the instruments on-orbit calibration and characterization, including a solar diffuser (SD) and solar diffuser stability monitor (SDSM) system for the radiometric calibration of the 20 reflective solar bands (RSB), a blackbody (BB) for the radiometric calibration of the 16 thermal emissive bands (TEB), and a spectro-radiometric calibration assembly (SRCA) for the spatial (all bands) and spectral (RSB only) characterization. The task of continuously performing high quality on-orbit calibration and characterization of all 36 spectral bands with a total of 490 detectors located on four focal plane assemblies is extremely challenging. The use of a large two-sided paddle wheel scan mirror with a ±55° scan angle range and a retractable pinhole attenuation screen in front of the SD panel for calibrating the high gain bands have resulted in additional unanticipated complexity. In this paper, we describe some of the key issues in the Terra and Aqua MODIS on-orbit calibration and characterization, and discuss the methods developed to solve these problems or to reduce their impact on the Level 1B calibration algorithms. Instrument performance and current issues are also presented.
The MODerate Resolution Imaging Spectroradiometer (MODIS) Flight Model 1 (FM1) was launched on May 4, 2002 onboard the NASA Earth Observing System (EOS) Aqua spacecraft. It has provided more than a year of global data for studies of the Earth’s land, oceans, and atmosphere in support of the science community and public users. To assure the quality of the data and science products, extensive efforts have been made to collect and analyze data on the instrument’s on-orbit performance using its on-board calibrators (OBCs). MODIS has 36 spectral bands: 20 reflective solar bands (RSBs) with wavelengths from 0.41 micrometer to 2.2 micrometers and 16 thermal emissive bands (TEBs) from 3.7 to 14.2 micrometers. For radiometry, the RSBs are calibrated by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) system and the TEBs by a blackbody (BB). An on-board Spectroradiometric Calibration Assembly (SRCA) is used for the instrument’s spectral (RSBs only) and spatial (all 36 bands) characterization. Using the first year’s calibration data sets, this paper presents Aqua MODIS on-orbit performance in three areas: radiometric, spatial, and spectral. Comparisons with the sensor’s specifications and with the performance of its predecessor, Terra MODIS (launched in December 1999), are discussed. Excluding a few problems identified pre-launch, such as non-functional detectors in the 1.6 micrometers band and the out of specification performance for the band to band registration (BBR), the on-orbit observations and analyses show that Aqua MODIS has been performing according to its design characteristics.
The MODIS Protoflight Model (PFM), launched onboard the NASA s Earth Observing System (EOS) Terra spacecraft, has been in operation for more than three years. In addition to constant radiometric calibration activities, the sensor s on-orbit spectral bandpasses of the reflective solar bands (RSBs) with wavelengths from 0.41 to 2.2 micrometers have been measured (every three months) using the on-board Spectral Radiometric Calibration Assembly (SRCA). The spectral characteristics of the SRCA were calibrated pre-launch using the Spectral Measurement Assembly (SpMA). The MODIS on-orbit spectral characterization using the SRCA has been performing as designed and the key spectral parameters, with few exceptions, are well within the specification limits. This paper provides a brief review of the MODIS prelaunch spectral characterization. It focuses on the Terra MODIS instrument s on-orbit spectral characterization activities, trending results, and comparisons with pre-launch characterizations and the specifications.
KEYWORDS: Sensors, Modulation transfer functions, Staring arrays, MODIS, Long wavelength infrared, Near infrared, Signal detection, Calibration, Reticles, Remote sensing
MODerate Resolution Imaging Spectroradiometer (MODIS) Proto-Flight Model (PFM) on-orbit spatial characterization includes Band to Band registration (BBR) and the Modulation Transfer Function (MTF) of 36 bands located on four focal plane assemblies (VIS, NIR, SMWIR, and LWIR). These parameters were also measured prelaunch using ground calibration equipment. The on-board Spectro-Radiometric Calibration Assembly (SRCA) was used both prelaunch and on-orbit to monitor the BBR and MTF changes. In this paper, we report the MODIS on-orbit spatial characterization results derived from the SRCA and their comparisons with pre-launch values. Results from SRCA measurements show that the BBR stabilized on-orbit after about 100 days. Currently, the averaged FPA positions in the along-scan direction, relative to band 1 (NIR), have changed from prelaunch values by -2m for VIS, 17m for SMWIR, and -20m for LWIR; along-track changes are 43m for VIS, -36m for SMWIR, and -22m for LWIR. The MTF in the along-scan direction shows a small improvement over prelaunch. Also in this paper, we discuss a methodology that uses the sensor's on-orbit views of the Moon for the BBR characterization. Comparison of the results from the Moon and those from the SRCA provides an evaluation of the methodology and its applicability for other remote sensing instruments without an on-board spatial characterization calibrator.
The MODIS Protoflight Model (PFM), on-board the NASA EOS Terra spacecraft, has been in operation for more than two years. Its 20 reflective solar bands (RSB) from 0.412μ to 2.13μ are calibrated on-orbit by a solar diffuser (SD) with its degradation tracked by a solar diffuser stability monitor (SDSM). The results derived from the SD/SDSM calibration data have shown that SD degradation is wavelength dependent. After nearly 2.5 years, the SD has degraded about 7.0% at 0.412μ, 4.0% at 0.466μ, 2.1% at 0.530μ, and the degradation is smaller at other longer wavelengths. The MODIS optical system includes a rotating scan mirror and other fixed aft optics. Overall system response in the visible spectral range has also shown wavelength dependent degradation over time. This degradation varies with the angle of incidence (AOI) to the scan mirror and the degradation rate is different between two sides of the scan mirror. During the first 20 months of instrument on-orbit operation, the system degradation (mirror side 1) at SD calibration AOI (50.2β) is about 11% at 0.412μ (MODIS Band 8), 6.5% at 0.443μ (Band 9), 5.0% at 0.469μ (Band 3), and 4.0% at 0.488μ (Band 10). Again the degradation is smaller for other bands with longer wavelengths. At other smaller AOIs, our results show that the degradation rate is higher. Since Oct./Nov. 2001, the system response degradation has essentially stopped. In this paper, we present MODIS RSB degradation analyses and the associated trending results including degradation at different AOIs to the scan mirror. We also address their impact on and application to the RSB on-orbit calibration.
This paper addresses the methodology and algorithm for using the Spectro-Radiometric Calibration Assembly (SRCA) lamp electrical parameters to track its output band radiance change. This allows the use of the SRCA to track MODurate resolution Imaging Spectroradiometer (MODIS) detector gain variation at different orbit positions because the MODIS dn change is attributed to its gain change after subtracting the SRCA output radiance change itself. Pre-launch test data show that using lamp current as a parameter is valid. Orbit data prove that the approach is valid for longer periods of time for all Solar Reflective Bands (SRBs). Data indicate that the MODIS has undetectable gain change at different orbit positions.
The Spectro-Radiometric Calibration Assembly (SRCA) was calibrated prelaunch in thermal vacuum against a 100-cm spherical integration source for all 20 MODIS solar reflective bands. Two methods of tracking the SRCA radiance change from ground to orbit are addressed. The broadband radiance of the SRCA will be held constant by a temperature-controlled SiPD feedback signal. While in spectral mode a reference SiPD located in the optical path of the SRCA measures the spectral profile of the radiance. The ratio of the reference SiPD signals from two different calibrations indicates a spectral profile change of the SRCA source and, therefore, predicts the SRCA band radiance changes. The second method is based upon monitoring the SRCA lamp resistance, which is non-linearly related to the lamp temperature. This provides a prediction of the SRCA spectral component so that the SRCA radiance variation from the previous calibration will be known. This paper emphasizes prediction of band radiance change. It demonstrates that both approaches track the SIS(100) calibration to within 2% and the inter-comparison of the two methods is within 2%. The success of the tracking will provide a physical transfer from pre-launch to on-orbit.
The launch-related spectral band radiance change of the Spectro-Radiometric Calibration Assembly (SRCA) of MODurate Resolution Imaging Spectrometer (MODIS) is mainly attributable to its lamp temperature variations. Comparison of the spectral profiles, measured by the SRCA at different times, provides a way of tracking change in the band radiance while the integrated SRCA radiance remains constant. Prelaunch the SRCA was calibrated against a ground Spherical Integration Source (SIS100). Meanwhile the SRCA calibrations were run in spectral and radiometric modes. Comparison of the on-orbit data from the SRCA spectral and radiometric modes to prelaunch data will transfer SIS100 ground calibration to orbit. For validation, data form TV at nominal temperature plateau will define the radiometric transfer; this transfer will be applied to SRCA data measured at other temperature plateaus and compared with the measured SIS100 radiance values.
The Spectro-Radiometric Calibration Assembly (SRCA) is briefly described. This paper discusses one of the SRCA functions -- spatial calibration to determine the channel position shift along-scan. The band centroid shift along- track will be discussed in Part II. Discussion in this part mainly focuses on the algorithm, the methodology, and the stability of the calibration signals. The algorithm and the methodology includes: a model of the image motion as a function of data sample and the centroid calculation to determine the detector/band positions. The SRCA stability is examined by checking the data inconsistencies between redundant samples, sub-samples, and the two mirror sides. Also described is the handling of the dark-reading for the thermal bands, which is affected by background energy for the opaque part of the reticle. A series of results are presented, which compare the relative position shift and co- registration between channels, bands, and Focal Plane Assemblies. Test results are illustrated for calibration under different environments and by different instruments. These results indicate that the SRCA is a stable an sensitive calibrator which provides consistent results. The overall uncertainty analysis will be presented in Part II.
This paper discusses the SRCA spatial calibration for the band centroid shift along-track. It is the second part of spatial calibration, which is for along-track. Next is a discussion of the linear range of weighted Y, which is the moment arm in the centroid computation, and the effect of MODIS/SRCA magnification change on the Y value. Also described is the relative calibration between channels under different environments and by different instruments. An overall summary of uncertainty sources and spatial calibration accuracy are presented.
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