CHRIS/PROBA Introduction

INTRODUCTION

CHRIS PROBA
The Project for On-Board Autonomy (PROBA) small satellite mission launched on 22 October 2001 is a technology proving experiment to demonstrate the on-board autonomy of a generic platform suitable for small scientific or application missions. A number of different Earth observation instruments have been included in the payload to test the platform pointing and data management capabilities, and these present opportunities for using the data for scientific Earth observation purposes. The instrument payload includes a Compact High Resolution Imaging Spectrometer (CHRIS), a radiation measurement sensor (SREM), a debris measurement sensor (DEBIE), high resolution and wide angle Earth pointing cameras, a star tracker and gyroscopes.

The CHRIS imaging spectrometer system has been designed principally to provide remote sensing data for land applications, and is intended to demonstrate that low cost compact imaging spectrometers can be viable instruments when combined with agile small satellite platforms. Figure 1 is a photograph of the instrument. CHRIS will provide a spectral coverage from 400 to 1050nm with a minimum spectral sampling interval ranging between 1.25 and 11nm and a ground sampling interval of 17m at nadir. The PROBA satellite platform will provide along- and across-track pointing.

PROBA is equipped with an on-board memory of 1.2 Gbit, of which approximately 1 Gbit will be available for recording CHRIS data. An S-band ground station, located in Redu, Belgium, will have satellite visibility approximately six times every 24 hours. The ESA Kiruna ground station will also acquire CHRIS data.

The exploitation of CHRIS data to be acquired as part of the PROBA mission will be organised by the CHRIS Steering Group, the BNSC, ESA's Earth Sciences Division, selected PIs and key participants from industry.

Scientific Objectives
The objective for CHRIS is to provide Earth surface reflectance data in the visible/near infrared, at high spatial and spectral resolution, for atmospheric, land and coastal studies. The instrument will use the PROBA pointing capabilities to obtain reflectance measurements with a wide range of different viewing configurations, and provide Bidirectional Reflectance Distribution Function (BRDF) data for selected test sites on the Earth's surface.

Atmospheric science objectives focus on aerosols, which as well as being important for weather and climate, are also a consideration for accurate atmospheric correction of satellite data. Previous research using the 2 different viewing angles of the ATSR-2 instrument on ERS-2 has shown that accurate aerosol retrieval is possible using a pointable multi-spectral instrument. Compared to the ATRS-2 instrument, CHRIS offers additional look angles and additional bands, which will permit a better estimate of aerosols. The core activity on aerosols is funded by NERC, UK, with the science lead being provided by Dr. Jeff Settle, University of Reading, UK.

Land studies will concentrate on the estimation of land surface biochemical (chlorophyll content) and biophysical (LAI/biomass, albedo, fAPAR) properties using multi-angle reflectance data. To date most land surface studies have used information from the spectral and temporal domains, but there is increasing interest in using directional reflectance properties. Theory suggests that directional reflectance is controlled by the spatial and geometric structure of the reflecting surface, as well as by the optical properties of its component elements. CHRIS will be used to acquire sets of hyperspectral images covering the same area of ground with different sensor view angles. Surface biophysical parameters will be estimated from the CHRIS data using a number of different techniques, ranging from traditional (the so-called vegetation indices), through more recent developments (such as red-edge position), to more advanced techniques (notably BRDF model inversion). The science leader for land studies is Professor Mike Barnsley, University of Swansea, UK.

There is also strong interest in the potential of CHRIS data for coastal studies. CHRIS radiometric performance is suitable for distinguishing a number of different water constituents in Case II waters.

The Announcement of Opportunity
An Announcement of Opportunity (AO) was issued in October 1999 to increase the involvement of European researchers in the PROBA/CHRIS experiment and build on the core programme of scientific experiments being led by Dr. Settle and Professor Barnsley.

The main aim of the AO is to widen the science and applications interest, in particular encouraging the use of the instrument for coastal studies. Following an analysis of the proposals that have been received, the CHRIS data exploitation plan has been further developed. A CHRIS-PROBA planning meeting, including the successful AO respondents, was held on 4 and 5 July 2000, provided a forum to make further progress.

Sub-groups
The scientific proposals were divided up into sub-groups as a means of ensuring the efficient use of the relatively limited amounts of data which will be available from CHRIS:

Aerosol and Calibration
Agriculture and Soils
Forestry
Coastal and Inland Water

A data acquisition plan has been prepared on the basis of discussions involving the sub-groups to determine a set of sites for which data will be acquired. The agreed sites have been prioritised as:

Core
Additional
Reserve

The acquisition plan takes account of the sub-group data requirements which are represented by the various CHRIS operating modes which have been developed.