The Radiometric Calibration Network (RadCalNet, www.radcalnet.org) routinely provides top-of-atmosphere (TOA)
reflectance data from instrumented ground sites. The data represents the nadir view of the ground for different sites that
cover areas ranging from 50 m × 50 m to 1 km x 1 km. The smaller sites can only be used with high resolution sensors
(≤ 30 m), but the larger sites, such as Railroad Valley (RRV) in Nevada can also be used for the validation or vicarious
calibration of medium resolution sensors (> 250 m spatial resolution). Prior to utilising RadCalNet data in this manner,
this paper describes the application of a high and a medium resolution sensor to assess potential biases between the
RadCalNet data and satellite data at two different spatial resolutions. Results are shown for initial comparisons over
RRV for the high resolution Sentinel-2 MultiSpectral Instrument (S2-MSI) and the medium resolution Sentinel-3 Ocean
and Land Colour Instrument (S3-OLCI), and indicate the potential for RadCalNet to validate and vicariously calibrate
sensors with differing spatial resolutions. The comparison analysis includes taking into account the temporal differences
between the Sentinel-2 and Sentinel-3 overpasses and the time of RadCalNet data collection, as well as the spectral
response functions (SRF) of the bands for both instruments. The comparison against the RRV site has shown there are
significant biases between the RadCalNet data and S2-MSI and S3-OLCI for non-nadir viewing geometries that may be
due to directional viewing and illumination effects and the non-Lambertian character of the RadCalNet RRV site.
The Radiometric Calibration Network (RadCalNet, www.radcalnet.org) routinely brings together data from several instrumented ground sites to provide users with top-of-atmosphere (TOA) reflectance data. These data are provided on cloud free days between 09:00 and 15:00 for the spectral range 400 to 1000 nm (and up to 2500 nm depending on available instrumentation) at a 10 nm spectral resolution. The data represents the nadir view of the ground. A key aspect to RadCalNet is a strict adherence to SI-traceability leading to well-understood and defensible uncertainty analysis to ensure that the different sites operating within RadCalNet are consistent with one another. This process includes the requirement to validate uncertainty analyses. One way in which this can be achieved is through field-based comparisons between independently measured reflectance of the ground and the RadCalNet data product for that date / time. To test the potential of such comparisons for uncertainty validation, a comparison campaign has been un- dertaken by the UK’s National Physical Laboratory (NPL) with the University of Arizona (UA) in March 2017 at the Railroad Valley radiometric test site in Nevada, USA using instruments developed for the purpose by UA and the Czech Metrology Institute (CMI). The measurements taken at the site with a new instrument, the Multispectral Transfer Radiometer (MuSTR) have been compared against the RadCalNet bottom-of-atmosphere (BOA) dataset to determine the equivalence of the reflectance. Radiances from MuSTR have also been compared against radiance measurements from the in-situ instrumentation at the site using a 48 % reflectance tarpaulin as a target. The comparisons presented here have demonstrated the utility of field-based comparisons for RadCalNet. In addition, a potential methodology for these comparisons has been developed and potential areas for improvement, including the systematic development of field-based uncertainty analyses, have been identified.