PROBA-1 carries onboard several Earth observation and space environment scientific instruments. In view of the project’s aim to demonstrate technology, several platform elements and systems have also been included as technology experiments.
Extensive use is made of automated functions onboard the spacecraft. Full onboard flight dynamics computation, in conjunction with automated ground segment functions such as pass automation and high-level user interfaces, satisfy the spacecraft autonomy requirements.
Automated functions onboard handle nominal spacecraft operations, plan and schedule activities, and manage payload resources. Onboard flight dynamics include orbital navigation as well as computation and control of instruments, camera pointing and scanning in order to meet user-defined targets (latitude, longitude and altitude).
CHRIS is the prime instrument of the PROBA-1 mission and its applications cover atmosheric, land, agriculture and oceanic and coastal observations. The technology objective is to explore the capabilities of imaging spectrometers on agile small satellite platforms.
The methodology used in the CHRIS-PROBA project is to sample top-of-atmosphere radiation in appropriate parts of the spectrum, and from different directions nearly simultaneously. Aerosol reflectance depends on the extinction coefficient. This in turn depends on the aerosol particle size distribution and on the wavelength of the light.
The aerosol properties are retrieved by inversion of radiative transfer models. The enhanced spatial, directional and spectral sampling of the CHRIS-PROBA system promises further improvements.
CHRIS provides information on land cover type and areal extent and on the directional properties of surface reflectance, and hence infers properties about surface structure, especially of vegetation canopies.
CHRIS measures carbon fluxes in relation to global climate change, forestry productivity and forest condition.
CHRIS data is highly relevant to forestry studies. Of particular interest is the dual capability of CHRIS to acquire both hyperspectral and multi-angle data. The hyperspectral data supports species identification and mapping, as well as providing information about the impact of pollution and other environmental factors on forest production.
CHRIS provides data for studies into the relative contribution of leaf area index (LAI) and chlorophyll to the wavelength response provided by individual forest canopies. This has particular relevance to monitoring stress in forests.
CHRIS provides high spatial resolution and multi-temporal ‘ocean colour’ imagery.
All the priority water sites have in-situ sampling (optical and bio-geochemical) during the CHRIS acquisition periods. There are also:
- Sun photometer measurements
- other forms of remote sensing (additional airborne/satellite systems)
The priority test sites are well characterised and represent a range of Case 2 waters*.
(*The classification of ocean waters into “Case 1” and “Case 2” began with Morel and Prieur (1977). They wrote that two extreme cases can be identified and separated. Case 1 is that of a concentration of phytoplankton high compared to other particle. In contrast, the inorganic particles are dominant in case 2. In both cases dissolved yellow substance is present in variable amounts. An ideal case 1 would be a pure culture of phytoplankton and an ideal case 2 a suspension of non-living material with a zero concentration of pigments. Morel and Prieur emphasized that these ideal cases are not encountered in nature, and they suggested the use of high or low values of the ratio of pigment concentration to scattering coefficient as a basis for discriminating between Case 1 and Case 2 waters. Although no specific values of this ratio were proposed to serve as criteria for classification, their example data suggested that the ratio of chlorophyll a concentration (in mg m-3) to the scattering coefficient at 550 nm (in m-1) in Case 1 waters is greater than 1 and in Case 2 waters is less than 1. Importantly, however, Morel and Prieur also showed data classified as “intermediate waters” with the ratio between about 1 and 2.2.)