Vicarious Calibration of Earth Observing satellites using salt pans and Desert regions.

CASE collaborative project with National Physical Laboratories (KING_R19ARIES)

Vicarious Calibration of Earth Observing satellites using salt pans and Desert regions.

CASE collaborative project with National Physical Laboratories (KING_R19ARIES)

Project Description

Prof Martin King (Royal Holloway, University of London)

Dr Nigel Fox (National Physical Laboratory)

Scientific background

Earth Observation allows regular and synoptic observation of large areas and is critical for monitoring planetary change or providing data products such as soil moisture. Accurate calibration of space-borne Earth-observing satellites is needed for data products and inter-calibration is needed for time series data important to climate change.

Research methodology

Calibration of a flying satellite is by “vicarious calibration”[1], i.e. the observation of Earth surface targets which are reflective, spatially uniform over large areas and constant e.g. deserts[1] and sand dunes[2]. The directional reflectivity (BRDF) of natural surfaces is not isotropic and a function of the viewing and illumination angles. The BRDF of these surfaces must be characterised for the correction of Earth observed images. The reflectivity of sand and salt pans depends on the scattering and absorption of light in the surface layer and the surface roughness[4]. The minerals in the sand are small particles. Thus, the top few mm of a sand is a highly scattering domain with the absorption of photons by a few light absorbing particles (humic and coloured minerals). The aim is to measure and model the bidirectional reflectance, BRDF, of proxy sand and salt pan surfaces for the inter-calibration of optical space sensors on satellites used for long-term remote sensing data. The output will be a BRDF(bidirectional reflectance distribution function) of sand and salt pan at the bottom and top of atmosphere as a function of changing conditions for calibration and validation of EO satellites.[3]


  1. Measure the BRDF of artificially created sand and salt pans using the RHUL goniometer as function composition, light absorbing components and surface roughness.
  2. Reproduce laboratory and field BRDF with modelled BRDF from radiative transfer codes: PanarRad, and LayerTran, written by the project partner, and fully explore parameter space not possible in experiment.
  3. With reflectivity of real sand and salt pans[1] invert the absorption spectrum of the impurities by radiative transfer modelling.
  4. Use radiative transfer models with sand and salt pan BRDF to produce top of atmosphere BRDF for satellite calibration, with libRadtran or LayerTran.


You will be trained and gain skills in Earth observation, spectrometry, field optics, radiative-transfer and its inversion, metrology, computer control of laboratory rigs, laboratory rig construction and experimentation, programming, field based measurement science and chemical analysis of minerals. As well as producing academic papers and knowledge you will be trained in reporting to metrology standards for the space agencies and providing publically accessible standards.

Person specification

You will be a numerate person willing to conduct field, laboratory and modelling experiments with a numerate physical science or engineering degree.

[1] Thome et al. (1998) Metro. 35, 631. [2] Lacherade et al. (2013) IEEE TGRS. 51, 1098 . [3] Schaepman-Strub, et al (2006) Rem. Sens. Environ. 103, 27. [4] Abdou, et al., (2000) J. Geophys. Res.



  • Amelia Marks, Corrado Fragiacomo, Alasdair MacArthur, Giuseppe Zibordi, Nigel Fox, Martin King. “Characterisation of the BRDF (HDRF) of snow surfaces at Dome C, Antarctica, for the inter-calibration and validation of satellite remote sensing products” Remote sensing of Environment. 2015, 407-416, doi: 10.1016/j.rse.2014.11.013
  • Christopher Ball, Amelia Marks, Paul Green, Alasdair MacArthur, Marion Maturilli, Nigel Fox, Martin King. “Hemispherical-directional reflectance of windblown snow-covered Arctic tundra at large solar zenith angles” IEEE Transactions on Geoscience and Remote Sensing 53 (10) 5377 – 5387, 2015, doi: 10.1109/TGRS.2015.2421733
  • Lamare, M. L., Lee-Taylor, J., and King, M. D.: The impact of atmospheric mineral aerosol deposition on the albedo of snow & sea ice: are snow and sea ice optical properties more important than mineral aerosol optical properties?, Atmospheric Chemistry and Physics., 16, 843-860, doi:10.5194/acp-16-843-2016, 2016.
  • Nigel Fox, Andrea Kaisser-Weiss, Werner Schmutz, Kurtis Thome, Dave Young, Bruce Wielicki, Rainer Winkler, Emma Woolliams, “Accurate radiometry from space: an essential tool for climate studies” Phil. Trans. R. Soc. A 369, p4028-4063 (2011)
  • N.P.Fox, M.C. Greening, “Quality Assurance Framework for Earth Observation (QA4EO): The guide” CEOS portal and (2009)

Open for applications

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