SENSOR CHARACTERISTICS

1-4 COVERAGE CHARACTERISTICS

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.

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.

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