Since launch in December 1999, Terra MODIS has successfully operated for nearly 15 years, making continuous observations. Data products derived from MODIS observations have significantly contributed to a wide range of studies of key geophysical parameters of the earth’s eco-system of land, ocean, and atmosphere, and their changes over time. The quality of MODIS data products relies on the dedicated effort to monitor and sustain instrument health and operation, to calibrate and update sensor parameters and properties, and to improve calibration algorithms. MODIS observations are made in 36 spectral bands, covering wavelengths from visible to long-wave infrared. The reflective solar bands (1-19 and 26) are primarily calibrated by a solar diffuser (SD) panel and regularly scheduled lunar observations. The thermal emissive bands (20-25 and 27- 36) calibration is referenced to an on-board blackbody (BB) source. On-orbit changes in the sensor spectral and spatial characteristics are monitored by a spectroradiometric calibration assembly (SRCA). This paper provides an overview of Terra MODIS on-orbit operation and calibration activities and implementation strategies. It presents and summarizes sensor on-orbit performance using nearly 15 years of data from its telemetry, on-board calibrators, and lunar observations. Also discussed in this paper are changes in sensor characteristics, corrections applied to maintain MODIS level 1B (L1B) data quality, and efforts for future improvements.
The MODIS instrument on the Terra and Aqua spacecrafts is a 12 bit sensor with an analog-to-digital (A/D) range of 0 to 4095 DN. Each sensor system is limited by a range at the low and high ends of the dynamic scale. At the low end, quantization noise is the limiting factor whereas at the high end the maximum value is limited by the capability
of the amplifier, 4095 in the case of MODIS. However, in both Terra and Aqua MODIS certain detectors in the Reflective Solar Bands (RSB) tend to pre-saturate at a value lower than 4095. This paper serves as a comprehensive report on the algorithms developed to characterize the pre-saturation limit in the RSB. The paper also provides the digital and pre-saturation (analog saturation) limits for the RSB that are currently being used in the Level 1B (L1B) products. The digital and analog saturation limits are well characterized using the Level 1A (L1A) raw Earth-View (EV) data and through the on-board Electronic Calibration (E-CAL). Also, in this paper an analysis is done to study
the sensors dynamic range due to the long term changes in the instrument response. In summary, the algorithms and results reported in this paper are important as the radiometric accuracy / uncertainty for instruments such as MODIS, VIIRS (NPP) tends to be coupled to pre-saturation.
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.
Launched in May 2002, the NASA EOS Aqua MODIS has successfully operated for more than 8 years. Observations
from Aqua MODIS and its predecessor, Terra MODIS, have generated an unprecedented amount of data products and
made significant contributions to studies of changes in the Earth's system of land, oceans, and atmosphere. MODIS
collects data in 36 spectral bands: 20 reflective solar bands (RSB) and 16 thermal emissive bands (TEB). It has a set of
on-board calibrators (OBC), providing sensor on-orbit radiometric, spectral, and spatial calibration and characterization.
This paper briefly summarizes Aqua MODIS on-orbit operation and calibration activities and illustrates instrument onorbit
performance from launch to present. Discussions are focused on OBC functions and changes in detector
radiometric gains, spectral responses, and spatial registrations. With ongoing calibration effort, Aqua MODIS will
continue serving the science community with high quality data products.
The MODerate resolution Imaging Spectroradiometer flies on board the Earth Observing System (EOS) satellites Terra
and Aqua in a sun-synchronous orbit that crosses the equator at 10:30 AM and 2:30 PM, respectively, at a low earth orbit
(LEO) altitude of 705 km. Terra was launched on December 18,1999 and Aqua was launched on May 4, 2002. As the
MODIS instruments on board these satellites continue to operate beyond the design lifetime of six years, the cumulative
effect of the space environment on MODIS and its calibration is of increasing importance. There are several aspects of
the space environment that impact both the top of atmosphere (TOA) calibration and, therefore, the final science
products of MODIS. The south Atlantic anomaly (SAA), spacecraft drag, extreme radiative and thermal environment,
and the presence of orbital debris have the potential to significantly impact both MODIS and the spacecraft, either
directly or indirectly, possibly resulting in data loss. Efforts from the Terra and Aqua Flight Operations Teams (FOT),
the MODIS Instrument Operations Team (IOT), and the MODIS Characterization Support Team (MCST) prevent or
minimize external impact on the TOA calibrated data. This paper discusses specific effects of the space environment on
MODIS and how they are minimized.
The 16 MODIS Thermal Emissive Bands (TEB), with wavelengths covering from 3.7μm to 14.4μm, are calibrated using
scan-by-scan observations of an on-orbit blackbody (BB). Select Earth surface targets can be used to track the long-term
consistency, stability and relative bias between the two MODIS instruments currently in orbit. Measurements at Dome C,
Antarctica have shown a relative bias of less than 0.01K over a 5 year period between Terra and Aqua MODIS Band 31
(11μm). Dome C surface temperatures are typically outside the MODIS BB calibration range. Sea surface temperature
(SST) measurements from data buoys provide a useful reference at higher scene temperatures. This paper extends the
techniques previously applied only to Band 31 to the remaining TEB using both Dome C and SST sites. The long-term
calibration stability and relative bias between Terra and Aqua MODIS is discussed.