Exploitation of time series
Venµs and Formosat satellite data have very similar characteristics, summed up in the table below:
|Repetitivity||2 days||1 day (but the data being expensive,|
only one image every 3 or 4 days have been ordered)
|Resolution||10 m for the best quality||8 m|
|Field of view||27 km||24 km|
|Observations||Constant viewing angles||Constant viewing angles|
|Superposition precision||3 m||8 m|
|Spectral bands||12 bands, from 412 to 910 nm||4 bands, from 485 to 820 nm|
Both satellites acquire images under the same viewing angle, and thanks to the constant viewing angles, the directional effects are minimized. Thus no correction of the viewing angle of the camera is needed. It will ensure a temporal stablility of acquisition which guaranties a very high quality of the data series. The images of a series are directly comparables one with the others and the variations of observed reflectances are due to natural and artificial variations of the surface, to the variation of the solar angle through the year and to the aerosol or cloud content in the atmosphere. Moreover, thanks to this minimisation of the directional effects and to the frequency of acquisition, a new and accurate atmospheric correction method has been implemented.
As a result, Venµs and Formosat can provide high quality time series of images, as shown in the following figure:
Surface reflectances versus time, obtained from a Formosat-2 image series,
after atmospheric correction for a fallow plot on the site of the Tensift valley in Morocco.
In blue, the blue band, in green, the green band, in red, the red band, in dark blue, the... near infra-red band.
By comparison, the following figure shows the same type of time series taken by SPOT, thus with variable angle conditions. The noise level for this time series is higher.
Surface reflectances through time, obtained from a SPOT image series,
after atmospheric correction for a wheat plot parcelle over a site in Romania
(in green, the green band, in red, the red band, in brown the... near infra-red band)