| SENSOR CHARACTERISTICS |
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The field of view has a maximum off-nadir observation angle of about 50.5°
(2200 km swath width).
The entire swath of the VEGETATION instrument is represented hereafter for different orbits : orbit 0 of day 1 is the reference orbit. For equatorial areas, it is shown that there is a gap between orbits 0 and 1 of the same day, while for higher latitudes, there is a large overlap. The equatorial gap is filled the next four days, so that over the entire 26 day cycle, only 5 days do not give any observation. The number of "missing days" decreases for higher latitudes and at about 32° (N or S) every day provides at least one observation.
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| Fig. 14-1 : Day 1, Orbit 0. |
Fig. 14-2 : Day 1, Orbit 0&1. |
Fig. 14-3 : Day 1, Orbit 0 & Day 2, Orbit 0. |
| About 90% of the equatorial areas are imaged each day, the remaining 10% being imaged the next day. For latitudes higher than 35° (North and South), all regions are acquired at least once a day. |
Operation specifications
Equator
crossing time: descending node : 10:30 local solar time
Image
transmission: All spectral bands at full spatial resolution acquired on terrestrial
areas will be stored onboard in a solid state memory, allowing the use of only
one receiving station to which data will be transmitted in X band. All the spectral
bands will also be transmitted in L band, for possible local receiving stations.
The time and frequency of data acquisition must be related to the evolution
rate of the processes to be characterised, taking into account limitations due
to observations from space in the solar energy domain, mainly atmospheric disturbances
and cloud coverage. These two factors force an over sampling in time so that
accumulation of acquisitions and screening of cloudy measurements leads to a
“useful” acquisition frequency adapted for vegetation studies. The effect of
these factors on acquisition reduction can only be known from statistics on
cloud coverage and atmospheric optical thickness, which is varying during the
day, with the season and with the geographical location.
To
get a minimal cloud cover, the best acquisition time is midmorning as many of
the sun synchronous satellite remote sensing systems devoted to land applications
(Landsat, SPOT).
Existing
operational systems are delivering information on vegetation or meteorological
conditions with a period ranging from 5 to 10 days. A mean interval between
useful acquisitions to measure changes in vegetation growth is considered to
be about one week : high level products are then generated to provide data with
the usual frequency, the VEGETATION system providing sufficient data to derive
the final useful information. To achieve this goal, experience from existing
systems shows that actual acquisition should be as much as possible with a frequency
of one day, to ensure coverage of the entire land areas each day. Even with
this strong constraint, cloud screening will, in some regions and for some periods
in the year, significantly decrease the useful acquisition frequency (especially
in the tropical regions during the rainy seasons). This is probably the greatest
drawback of solar energy measurements and any possibility to keep to the one
day interval should be reserved.
Frequency
acquisition is strongly related to spatial resolution, number of pixels by line
of image and field of view of the instrument. Consistent modifications of these
parameters should be discussed with the users to provide the best compromise,
current values for the specifications being the preferable combination that
was accepted today.
