IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 56, NO. 3, MARCH 2018 1507 Intercalibration of Polar-Orbiting Spectral Radiometers Without Simultaneous Observations Terhikki Manninen , Aku Riihelä, Andrew Heidinger, Crystal Schaaf, Alessio Lattanzio, and Jeffrey Key Abstract—A new intercalibration method for two polar- orbiting satellite instruments or two instrument constellations’ Fundamental Climate Data Records (FCDRs) is presented. It is based on statistical fitting of reflectance data from the two instruments covering the same area during the same period, but not simultaneously. A Deming regression with iterative weights is used. The accuracy of the intercalibration method itself was better than 0.5% for the Moderate Resolution Imaging Spectroradiometer (MODIS) versus MODIS and Advanced Very High Resolution Radiometer (AVHRR) versus AVHRR test data sets. The intercalibration of an AVHRR FCDR generated by NOAA versus a combined MODIS Terra and Aqua data set of red and near-infrared (NIR) channels was carried out and showed a difference in the reflectance values of about 2% (red) and 6% (NIR). The presented intercalibration method can be used for checking the calibration of two instruments or FCDRs in all viewing angles used separately. Index Terms— Calibration, remote sensing, statistics. I. I NTRODUCTION A NECESSARY condition for high-quality long-term Climate Data Records (CDRs) derived from satellite remote sensing is accurate intercalibration of instruments over long timescales [1]. For CDRs, this most often involves the same type of sensor on a series of satellites, e.g., the Advanced Very High Resolution Radiometer (AVHRR) on the series of NOAA polar-orbiting satellites. It may also be useful, however, to combine observations from different sensors and satellites. For example, surface albedo is one of the essential climate variables and a key parameter for the energy balance of the earth [2]. Albedo retrievals are usually performed utilizing only a single instrument or an instrument “family” (i.e., AVHRR on multiple satellites). This limits the total number of available observations per time period per terrestrial scene. Combining multiplatform observations can yield improve- Manuscript received March 7, 2017; revised June 14, 2017 and August 29, 2017; accepted September 25, 2017. Date of publication November 7, 2017; date of current version February 27, 2018. This work was supported in part by EUMETSAT through the Project CM SAF and in part by the National Aeronautics and Space Administration for support of the MODIS work under Grant NNX14AI73G. (Corresponding author: Terhikki Manninen.) T. Manninen and A. Riihelä are with the Finnish Meteorological Institute, FI-00101 Helsinki, Finland (e-mail: terhikki.manninen@fmi.fi; aku.riihela@fmi.fi). A. Heidinger and J. Key are with NOAA/NESDIS, Madison, WI 53706 USA (e-mail: andrew.heidinger@noaa.gov; jeff.key@noaa.gov). C. Schaaf is with the University of Massachusetts Boston, Boston, MA 02125-3393 USA (e-mail: crystal.schaaf@umb.edu). A. Lattanzio is with EUMETSAT, 64295 Darmstadt, Germany (e-mail: alessio.lattanzio@eumetsat.int). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2017.2764627 ments in both the accuracy and temporal resolutions of surface albedo retrievals. This is especially important for optical remote sensing in frequently cloudy areas, such as the Arctic. Various spaceborne optical imager families, such as AVHRR or Moderate Resolution Imaging Spectroradiome- ter (MODIS), have different imaging channel wavebands. Also, the individual imagers within instrument families have different spectral responses. The intrafamily differences can and recently have been compensated for by intercalibration, leading to the creation of Fundamental CDRs (FCDRs) spe- cific to a single instrument family [7]. When intercalibrating observations from two different instrument families, even though both are FCDR quality, due attention must still be paid to the differences in spectral coverage between the instrument imaging channels. In this paper, we seek to perform such an intercalibration between MODIS and AVHRR data sets. In this case, the intercalibration is extremely important for the near-infrared (NIR) channel, the spectral width of which is markedly larger for AVHRR than for MODIS. Although the motivation of this paper is to compensate for the spectral difference of MODIS and AVHRR, the presented intercalibra- tion method can also be applied to normal intercalibration of different individual instruments of the same family. The basic premise of intercalibration is that two similar instruments should produce the same reflectance value when they view the same target simultaneously with identical view- ing geometry. In reality, this requirement cannot be rigorously fulfilled in the calibration of earth-observing instruments. An established intercalibration method for two satellite instru- ments is the use of simultaneous nadir observations (SNOs), which has been proved to be very effective over a wide spectral range [1]. Another approach is statistical intercomparison, which is the most frequently used for low-resolution data. Pseudoinvariant calibration sites (PICSs), such as the Libyan Desert, have been utilized for both the SNO-based absolute calibration and the statistical intercomparison of two or more satellites [2]–[4]. Several approaches have been investigated for the intercalibration of the AVHRR instrument series with MODIS data over the PICS [5]–[8]. These methods require reliable atmospheric correction and either SNOs or reliable bidirectional reflectance distribution factor estimates. The method presented here does not need any information about the surface or atmosphere properties concerning the images used. The goal of this paper is to derive a general concept of intercalibration applicable to any collection of optical polar-orbiting satellite imagers. Hence, one must consider the 0196-2892 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.