National Physical Laboratory


CIMEL calibration at NPL
CIMEL calibration at NPL

Project Dates: 2014 - 2015

Vicarious calibration and validation using ground reference sites is a key technique in Earth Observation, both for the cross comparison of sensors (including calibration of lower accuracy sensors against sensors with better radiometric calibration) and for monitoring long-term drift (effectively by comparison against themselves using a stable ground reference). It is common to use accessible desert sites - for field-campaign measurements that are made to coincide with a satellite overpass.

The aim of the ACTION project is to set up a reference calibration site on behalf of the European Space Agency (ESA) in Gobabeb Namibia. This site will be characterised through an extensive field campaign (see details below) and by permanent instruments providing regular automatic measurements of the ground surface and the atmosphere. It is the European (ESA / CNES) contribution to the RadCalNet network of calibration sites.

A four-month characterisation of all the instruments that will be used in the field campaign, as well as the instruments that will provide the permanent monitoring, has been undertaken with full traceability to SI standards at NPL.

The Gobabeb Research and Training Centre
The Gobabeb Research and Training

Field Campaign at the Gobabeb Site (Namibia), November 2015

Staff from NPL, in collaboration with CNES, successfully completed a field campaign in the Namib Desert in November 2015. The purpose of this site characterisation was to find a perfect location for a permanent measurement site that will be installed next year to continuously monitor the atmosphere and surface reflectance.

NPL and CNES team members at the Gobabeb Research and Training Centre
NPL and CNES team members at the
Gobabeb Research and Training

After a 15-hour flight and a six-hour drive on gravel tracks, the team arrived at Gobabeb Research and Training Centre in the Namib-Naukluft National Park. Nestled between the dunes and the gravel plains, the research station became the team's home for two weeks. Every day, they drove out to the chosen measurement site, 10 km to the east. Reflectance was measured using an ASD portable spectroradiometer and GRASS , the Gonio Radiometric Spectrometer System.

Figure 4: ASD measurements being taken at the Gobabeb site
ASD measurements being taken
at the Gobabeb site

The ASDs were used to assess the uniformity of the site over a wide area. Between the CNES and NPL teams, small scale and large scale areas were covered, and long transects measured.

GRASS enabled the team to measure the HDRF (Hemispherical Directional Reflectance Factor), or the angular distribution of reflectance over two selected sites. The data will be compared to a permanent instrument that will be installed in the near future.

GRASS deployed at the Gobabeb site
GRASS deployed at the
Gobabeb site

After a fruitful campaign, the data allowed the team to select a potential location for a permanent instrumented calibration site. The team is looking forward to returning to Gobabeb next year to install the mast, sun photometer and weather station that will stream data to NPL.

The NPL team included Claire Greenwell, Aga Bialek and Maxim Lamare, who is a CASE PhD student at Royal Holloway University of London (RHUL) and NPL.

For more technical information about the preparation to this site characterisation see: 'Preparation of a New Autonomous Instrumented Radiometric Calibration Site: Gobabeb, Namib Desert', Proc. SPIE 9639, Sensors, Systems, and Next-Generation Satellites XIX, 963919 (12 October 2015); doi: 10.1117/12.2194885

Funding source: ESA

Contacts: Aga Bialek and Claire Greenwell


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