The only feasible method of obtaining global-scale data on the state of the planet is through the use of satellite sensors that can measure the solar energy reflected from the Earth's atmosphere and surface. The accuracy and reliability of satellite sensors' data is of great importance, and much effort goes into characterising such instruments before launch. However, the stress of launch and the extreme environment of space affect the calibration of sensors, introducing additional uncertainty into the data, especially the long time series data necessary to investigate and monitor climate change. Once in orbit, post-launch calibration is vital to validate the instruments' performance. This is often achieved by using field instruments that measure the light reflected from 'calibration sites' - such as deserts or grassland - which can be viewed by satellite sensors.

NPL works closely with groups such as the Natural Environmental Research Council Field Spectroscopy Facility (NERC FSF), to ensure that all their equipment is calibrated and traceable to NPL standards. We also provided calibration support and measurement guidance for a national field campaign organised for the Network for Calibration and Validation of Earth Observation data (NCAVEO), through the use of the NPL Transfer Standard Absolute Radiance Source (TSARS) .

The Optical Technologies group is also working in conjunction with the (NERC FSF) to develop a new field instrument to support unequivocal calibration of various satellite sensors operating in the optical region and the validation of their data. NPL's new field instrument, Gonio Radiometric Spectrometer System (GRASS) , is intended to provide quasi-simultaneous, multi-angle, multispectral measurements of reflected sunlight from the Earth's surface.

A NPL Helicopter is also being developed, in connection with a local school. The helicopter will take spectral measurements of the Earth's surface that will support satellite Earth Observation missions.

Research Areas

Transfer Standard Absolute Radiance Source (TSARS)
The radiance calibration of a spectrometer requires a large area, uniform source of known radiance. NPL has developed a range of new radiance transfer standards specifically targeted to meet these requirements, known as the Transfer Standard Absolute Radiance Source (TSARS).

Gonio Radiometric Spectrometer System (GRASS)
Field instruments, such as GRASS, provide the in-situ data to validate the satellite measurements.

NPL Helicopter
Earth Observation satellites are the main source of global data about the Earth, it is essential that satellite performance can be validated and in some cases calibrated, using ground targets calibrated directly by in-situ measurements and local surveys. A novel approach to this problem is being developed at NPL using a remote controlled helicopter.

For further information, please contact: Nigel Fox