Overview

NCAVEO comprises a network of scientists, engineers and applications specialists interested in the calibration and validation (cal/val) of remotely sensed data. Both of these activities are vital if remotely sensed data from aircraft and satellite sensors are to be trustworthy and reliable components of the systems to monitor and manage the Earth's environment. Inaccurate or unvalidated data severely compromise the value of Earth Observation (EO) data for monitoring habitat change, biodiversity assessment and resource mapping. Long time series of EO data are vital in studying the process affecting climate and the nature and rate of change of carbon fluxes in the oceans and on land. It is essential that researchers and policy makers have confidence in the data provided by EO, and that requires the data to be carefully validated against conventional, ground-based observations.

The main aim of NCAVEO is to provide a co-ordinated resource for users from industry and academia and also to facilitate access to benchmark methods and algorithms as well as identifying areas where additional research and improved methods are required. This website is one of the main ways in which NCAVEO disseminates information to the community. The network also holds workshops and technical meetings and these will be advertised on the website.

Baseline funding for NCAVEO is provided by the UK Natural Environment Research Council and additional funds for specific activities are sought from a range of sources including government and industry.

The need for calibration

Making scientific measurements of the Earth from space relies upon data from many different satellites, operated by a number of organisations, so it is important that these are all calibrated to the same physical standards. Without this, we would have no means of knowing whether the changes we observe from space are real, or are due to sensor error. Furthermore, data from calibrated sensors can be interchanged, which is essential if one system has technical problems, or if an area is cloud-covered, for example.

Laboratory calibration of sensors before launch is a complex, but well-established procedure, and organisations like the UK National Physical Laboratory have an international reputation for this type of work. The best way to check and maintain that calibration after launch is still the subject of active research. Some sensors have small panels that are deployed in space to check on the calibration; others are periodically pointed at the Sun or moon. Another way to check is to use ‘vicarious calibration’ (VC) targets. These are large, uniform areas on the Earth’s surface, which have reflectance properties that are very stable. Typical VC sites include dry lake beds or uniform desert areas. The best VC sites are at high altitude so that the atmospheric effect upon the signal is reduced.

The need for validation

‘Validation’ refers to the comparison between satellite sensor measurements and those made by an independent method. Both the fundamental physical measurements made by the sensor (e.g. radiance) and the derived geophysical variables (e.g. biomass) must be properly validated. The process of validation therefore involves geographical sampling and interpretation, as well as consideration of the physical measurements and the algorithms used.

The algorithms used for atmospheric correction are one of the main issues in validation, as their quality affects our ability to make accurate measurements of the reflectance of the Earth’s surface from space.