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Satellite sensors can measure the solar energy reflected from the Earth's atmosphere and surface, and provide the only feasible method of obtaining global scale data on the state of the planet. The accuracy and reliability of data from satellite sensors is therefore of vital importance, and much effort goes into characterizing such instruments before launch. However, the stress of launch, and the extreme environment of space is known to affect the calibration of sensors, introducing uncertainty into the data they provide, especially the long time-series data necessary to investigate and monitor climate change. It is therefore vitally important that methods for the post-launch calibration and validation of data from satellite sensors are developed and tested. In order to do this it is necessary to have highly accurate data on the angular reflectance of natural surfaces, and this project aims to develop and test a new instrument to achieve this. To improve the accuracy of field measurements, a new instrument is being designed, in conjunction with the Natural Environmental Research Council Field Spectroscopy Facility (NERC FSF). The Gonio RAdiometric Spectrometer System (GRASS) is being developed at the National Physical Laboratory (NPL), in its Optical Radiation Measurement Team. It is intended to provide quasi-simultaneous, multi-angle, multi- spectral measurements of Earth surface reflected sunlight to support vicarious calibration of satellite sensors operating in the optical region and the validation of their products. The instrument is required to be easily and quickly assembled in remote situations, be robust and transportable to remote locations and its measurements fully traceable to SI units. The GRASS instrument is currently being assembled at the NPL and will be deployed in summer 2006 as part of an experiment to validate data from a range of airborne and satellite sensors. The paper will review existing methods of measuring BRDF in the field, highlight the improvements that GRASS will provide to users, and describe the experiments performed to test its performance.