Simultaneous Nadir Overpass (SNO) Method

Overview

Inferring post-launch calibration-related Tb biases from microwave sounding instrument measurements made from multiple satellites are complicated by uniqueness and drifts in the orbital geometries of individual satellites (Price 1991). At the same time though, the fact that all polar-orbiting satellites revolve around the Earth at slightly different periods causes them to occasionally view the same nadir location at nearly the same time. Ideally, identical radiometers flown on different satellites that simultaneously view the same exact Earth target should produce redundant observations. Any deviation from these results would be attributable to relative calibration differences between the “identical” radiometers. Taking advantage of this concept, the Simultaneous Nadir Overpass (SNO) method was developed to estimate and track relative calibration-related measurement biases between visible/infrared radiometers flown on-board different polar-orbiting satellites (e.g., Cao et al. 2002, 2004, 2005). For a given pair of polar-orbiting satellites, the essence of the SNO method is to minimize Earth-scene-related differences found between instrument measurements by utilizing near-nadir observations close to satellite orbital intersections that have a relatively small time difference (~ 30 seconds). Application of the SNO method to microwave sounding instruments was performed by Zou et al. (2006) for the purpose of inter-calibration to create long-term temperature time series, and by Iacovazzi et. al. (2007, 2008) for the purpose of NOAA operational microwave sounding instrument calibration assessment.

Satellite SNO Predictions

Satellite SNO predictions are created by obtaining NORAD satellite two-line elements for a given pair of satellites, and inputting them into satellite prediction software such as the SGP4. The orbit predictions from the satellite pair can then be analyzed to find where the output orbital geographic locations match up in time. The following are links to polar satellite SNO predictions

• NOAA National Calibration Center SNO Predictions - LEO-LEO SNO events for NOAA, NASA and IJPS polar satellites, and GEO-LEO SNO events for NOAA GOES and NOAA, NASA, IJPS and FY-3 satellites.

References

Cao, C., C., H. Xu, J. Sullivan, L. Mcmillin, P. Ciren, and Y. Hou, 2005: Intersatellite radiance biases for the High Resolution Infrared Radiation Sounders (HIRS) on-board NOAA-15, -16, and -17 from simultaneous nadir observations. J. Atmos. and Ocn. Tech., 22, 381-395.

Cao, C., M. Weinreb, and H. Xu, 2004: Predicting simultaneous nadir overpasses among polar-orbiting meteorological satellites for the intersatellite calibration of radiometers. J. Atmos. and Ocn. Tech., 21, 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.

Iacovazzi Jr., R., and C. Cao, 2008: Reducing Uncertainties of SNO-Estimated Intersatellite AMSU-A Brightness Temperature Biases for Surface-Sensitive Channels. J. Atmos. Oceanic Technol., 25 , 1048-1054. doi:10.1175/2007JTECHA1020.1

Iacovazzi Jr., R., and C. Cao, 2007: Quantifying EOS-Aqua and NOAA POES AMSU-A brightness temperature biases for weather and climate applications utilizing the SNO method. J. Atmos. Oceanic Technol., 24 , 1895–1909. doi:10.1175/JTECH2095.1

Price, J. C., 1991: Timing of NOAA afternoon passes. Int. J. Remote Sensing, 12, 193-198.

Zou, C.-Z., M. D. Goldberg, Z. Cheng, N. C. Grody, J. T. Sullivan, C. Cao, and D. Tarpley, 2006: Recalibration of microwave sounding unit for climate studies using simultaneous nadir overpasses, J. Geophys. Res., 111, D19114, doi:10.1029/2005JD006798.

-- RobbiIacovazzi - 14 May 2020