Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-1156
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
20 Jan 2017
Review status
A revision of this discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
How long do satellites need to overlap? Evaluation of climate data stability from overlapping satellite records
Elizabeth C. Weatherhead1, Jerald Harder2, Eduardo A. Araujo-Pradere3, Jason M. English4, Lawrence E. Flynn5, Stacey Frith6, Jeffrey K. Lazo7, Peter Pilewskie2, Mark Weber8, and Thomas N. Woods2 1University of Colorado, Boulder, Colorado, USA
2Laboratory for Atmosphere and Space Physics, University of Colorado, Boulder, Colorado, USA
3School of Science, Miami Dade College, Miami, Florida, USA
4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
5NOAA, NESDIS, College Park, Maryland, USA
6NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
7National Center for Atmospheric Research, Boulder, Colorado, USA
8University of Bremen FB1, Bremen, Germany
Abstract. Sensors on satellites provide unprecedented understanding of the Earth’s climate system by measuring incoming solar radiation, as well as both passive and active observations of the entire Earth with outstanding spatial and temporal coverage that would be currently impossible without satellite technology. A common challenge with satellite observations is to quantify their ability to provide well-calibrated, long-term, stable records of the parameters they measure. Ground-based intercomparisons offer some insight, while reference observations and internal calibrations give further assistance for understanding long-term stability. A valuable tool for evaluating and developing long-term records from satellites is the examination of data from overlapping satellites. Prior papers have used overlap periods to identify the offset between data from two satellites and estimate the added uncertainty to long-term records. This paper addresses the length of overlap needed to identify an offset or a drift in the offsets of data between two sensors. The results are presented for the general case of sensor overlap by using the case of overlap of the SORCE SIM and SOLSTICE solar irradiance data as an example. To achieve a 1 % uncertainty in estimating the offset for these two instruments’ measurement of the Mg II core (280 nm) requires approximately 5 months of overlap. For relative drift to be identified within 0.1 % yr−1 uncertainty, the overlap for these two satellites would need to be 2.6 years. Additional overlap of satellite measurements is needed if, as is the case for solar monitoring, unexpected jumps may occur because these jumps add to the uncertainty of both offsets and drifts; the additional length of time needed to account for a single jump in the overlap data may be as large as 50 % of the original overlap period in order to achieve the same desired confidence in the stability of the merged dataset. Extension of the results presented here are directly applicable to satellite Earth observations. Approaches for Earth observations may be challenged by the complexity of those observations but may also benefit from ancillary observations taken from ground-based and in situ sources. Difficult choices need to be made when monitoring approaches are considered; we outline some attempts at optimizing networks based on economic principles. The careful evaluation of monitoring overlap is important to the appropriate application of observational resources and to the usefulness of current and future observations.

Citation: Weatherhead, E. C., Harder, J., Araujo-Pradere, E. A., English, J. M., Flynn, L. E., Frith, S., Lazo, J. K., Pilewskie, P., Weber, M., and Woods, T. N.: How long do satellites need to overlap? Evaluation of climate data stability from overlapping satellite records, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1156, in review, 2017.
Elizabeth C. Weatherhead et al.
Elizabeth C. Weatherhead et al.
Elizabeth C. Weatherhead et al.

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Short summary
Satellite overlap is often carried out as a check on the stability of the data collected. We looked at how length of overlap influences how much information can be derived from the overlap period. Several results surprised us: the confidence we could have in the match-up of two records was independent of the offset; understanding of the relative drift between the two satellite datasets improved significantly with 2–3 years of overlap. Sudden jumps could easily be confused with drift.
Satellite overlap is often carried out as a check on the stability of the data collected. We...
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