Intersatellite Calibration of Polar-Orbiting Radiometers with Simultaneous Nadir Overpass (SNO) Observations

There is a need to inter-calibrate polar orbiting radiometers across satellites to achieve the consistency and traceability required for long term climate studies from the more than  20 years of NOAA satellite data.  In addition, the calibration of current operational radiometers should be linked to those of the next generation meteorological satellites such as NPOESS (National Polar-orbiting Operational Environmental Satellite System).  In this study, we focus on inter-satellite calibration of polar orbiting radiometers using Simultaneous Nadir Observations.  At each Simultaneous Nadir Overpass (SNO), radiometers from each pair of satellites view the same place at the same time at nadir, thus eliminating uncertainties associated with the atmospheric path, view geometry, and time differences.  As a result, uncertainties in the inter-satellite calibration are greatly reduced.  This method is useful for the on-orbit verification of instrument performance for newly launched radiometers, calibration transfer from instruments on one satellite to those on another, as well as retrospective analyses of historical data in constructing time series for climate studies.  The SNOs typically occur in the Arctic and Antarctic regions (70N to 80N and 70S to 80S) for sun synchronous polar-orbiting satellites. In addition to the intersatellite calibration of radiometers such as AVHRR, HIRS, MSU/AMSU, MODIS, AIRS and future systems, the long-term time series of intersatellite radiance biases at the SNOs may also be very useful for the study of polar climate.

Objectives

• Accurately and automatically predict the SNOs for operational polar-orbiting meteorological satellite pairs with SGP4.
• Develop a system for validating radiance and brightness temperature using coincidental/collocated data from NOAA instruments at the SNOs.
• Use the system for the on-orbit verification of new polar instruments, as well as the long term monitoring of the performance of existing instruments.
• Accurately and objectively evaluate and characterize the calibration consistency and continuity of the AVHRR, HIRS, and MSU/AMSU across succeeding satellites from 1980 to present. 

Our approach takes advantage of inter-satellite calibration using observations at SNOs between satellite pairs e.g.,  NOAA-17/AVHRR vs. NOAA-16/AVHRR, or NOAA-17/AVHRR vs. AQUA/MODIS.  Because the observations are collocated and coincidental at the pixel level, it greatly reduces calibration uncertainties, which is especially important for the infrared radiometers due to the transient nature of brightness temperature.

The basic techniques are as follows:

• Predict SNOs using satellite orbit perturbation models with appropriate two-line-elements.
• Acquire satellite observation data at the SNOs.
• Perform pixel-by -pixel match of the data from satellite pairs, with minimized navigation errors.
• Analyze the correlation radiance/temperature between the data pairs, and find the causes for the biases in the context of sensor physics.
• Generate a longterm time series of the biases and RMS for monitoring instrument performance and potentially for climate trending in the polar regions.

Products

• Intersatellite radiance biases from simultaneous nadir overpass (SNO) observations in the Arctic and Antarctic regions for HIRS, AMSU, and AVHRR on NOAA-17 and -16 (updated monthly)
  
  
• Predicted Simultaneous Nadir Overpasses (SNOs) between Meteorological Satellite Pairs (updated weekly for the next two weeks)
  
  
• Inter-satellite calibration of HIRS from 1980 to 2002 (in progress)

References

Cao, C., M. Weinreb, and H. Xu, 2004, Predicting Simultaneous Nadir Overpasses among Polar-orbiting Meteorological Satellites for the Intersatellite Calibration of Radiometers, Journal of Atmospheric and Oceanic Technology, Vol. 21, pp. 537-542.

Cao, C. and A. K. Heidinger. (2002). Intercomparison of the longwave infrared channels of MODIS and AVHRR/NOAA-16 using simultaneous nadir observations at orbit intersections. In Earth Observing Systems VII, William L. Barnes (editor), Proceedings of SPIE,  4814:306-316.

Heidinger, A. K., C. Cao, and J. Sullivan. (2002). Using Moderate Resolutin Imaging Spectrometer (MODIS) to calibrate advanced very high resolution radiometer reflectance channels. Journal of Geophysical Research, 107 (D23), 4702, doi:10.1029/2001JD002035, 2002.

Gunshor, M. M., T. J. Schmit, and W. P. Menzel. (2001). Intercalibration of Geostationary and Polar-Orbiting Infrared Window and Water Vapor Radiances. In Proceedings of the 11th AMS Conference on Satellite Meteorology and Oceanography, held at Madison, WI, Oct. 15-18, 2001.

Minnis, P, L. Nguyen, D. R. Doelling, D. F. Young, W. F. Miller, D. P. Kratz. (2002). Rapid Calibration of Operational and Research Meteorological Satellite Imagers. Part I: Evaluation of Research Satellite Visible Channels as References. Journal of Atmospheric and Oceanic Technology. 19(9):1233-1249.

Minnis, P., L. Nguyen,  D. R. Doelling, D. F. Young, W. F. Miller, D. P. Kratz. (2002). Rapid Calibration of Operational and Research Meteorological Satellite Imagers. Part II: Comparison of Infrared Channels. Journal of Atmospheric and Oceanic Technology. 19(9):1250-1266.

• Link to the High Resolution Infrared Radiation Sounder (HIRS) page
• Link to Celestial Trak - an orbital tracking site

For questions or comments, please contact the webmaster.  Revision date: 4/29/2004