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ge rate of 
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ay became 
became bigger year by year. The daily change rate of 
mean anomaly of NOAA satellite became faster. 
Consequently, the number of passes in one day became 
less and the recurrent period became shorter. In case of 
NOAA -11, the local time at descending node was around 
1:40 (corresponding to 13:40 in ascending mode) just 
after the launch in 1988, but the delay reached to 2.5 
hours after 5 years, whichresulted in the change of solar 
zenith angle even in the same seasons. Just after the 
launch, the recurrent period of NOAA-11 was about 10, 
which meant that the satellite passed through the almost 
same orbit after 10 days. After 5 years later, however, the 
period became about 8. 
AVHRR is an optical mechanical radiometer with 2 bands 
in visible and near-infrared regions and 3 bands in thermal 
region with the spatial resolution of 1.1km at nadir. The 
FOV is 55.4 degrees and the ground swath is about 
3,000km. 
2.2 ADEOS/OCTS 
ADEOS will be launched in August 1996 by NASDA. One 
of the main objectives of this mission is to acquire data 
on worldwideenvironmental changes in order to contribute 
to international global environmental monitoring. To 
achieve the objects, some kinds of sensors such as 
optical sensors, microwave radiometers, scatterometers 
will be mounted. Among these sensors, OCTS will be 
expected to be used for vegetation monitoring in a large 
scale. 
OCTS is an optical radiometer for the frequent global 
measurement of ocean color and sea surface 
temperature. It has 8 bands in visible and near-infrared 
regions and 4 bands in thermal region with the spatial 
resolution of 700m. The main target to be observed is the 
ocean, but the land areas will be also observed. The 
sensor gain willbe changed whetherthe main target is the 
ocean or the land. The operational schedule for 
Switching sensor gains is not determined as of the end of 
March in 1996. 
OCTS adopts an optical mechanical system with a 
rotating mirror for achieving the wde FOV with + 40 
degrees. 10 lines are scanned simultaneously in 1 swath. 
It covers about 1,400km on the ground and can observe 
the same area every 3 days. An unique characteristics of 
OCTS is to tilt a scan line along track by 20 degrees for 
avoiding a sungliter. 
2.3 ADEOS-II/GLI 
ADEOS-Il is the successor to the ADEOS mission and 
planed to launch in early 1999. GLI among five sensors 
carried on ADEOS-Il is the successor to OCTS. GLI is 
also an optical radiometer aiming at observing the 
reflected solar radiation from the earth's surface including 
land and ocean and the infrared radiation. It has 22 bands 
In visible and near-infrared regions, 5 bands in short- 
Infrared region and 7 bands in thermal region. The spatial 
277 
resolution is 1km at nadir, but some bands from visible to 
short-infrared regions has the resolution of 250m. It 
observes about 1,600km in width with the FOV of +45 
degrees. 12 lines are scanned at atime. The same areas 
are observed every 4 days. It has also the tilting function 
by 20 degrees. 
3. METHOD OF COMPOSITE FOR 
NOAA/AVHRR(SUMMARY) 
The author already proposed the method of image 
composite for vegetation monitoring using NOAA/ 
AVHRR (Hashimoto, et. al., 1992). The method is 
summarized as follows. 
1) NDVI is sensitive to the solar zenith angle, especially 
in big angles (Singh, 1988),( Matsumoto, et al.,1992). 
To weakenthe effect of solar zenith angle on NDVI, the 
satellite data wth odd number is primarily used 
because such a satellite on afternoon passes gives the 
lowest solar zenith angle in a day. 
2) The marginal areas of an image with big scan angles 
are seriously distorted both geometrically and 
radiometrically. The central part of a line is primarily 
used to prevent large image distortions. The area of an 
image wth the scan angle up to 30 degrees are 
preferable. 
3) The NOAA satellite with an odd number revolved in 
about 9 days (strictly speaking, this period varies with 
satellite number and the elapsed time after launch). 
The shift of ascending nodes on adjacent two days in 
longitude is about 3 degrees (this value also varies with 
the elapsed time). The coverage widthon the equator is 
about 26 degrees in longitude, corresponding to about 9 
times of the daily orbital shift. So the period for a 
composite is set to 9 days. 
4. COMPARISONOF THREE SENSORS 
WITH RESPECT TO SCAN GEOMETRY 
The effects of the scan angle on the radiometric and 
geometric distortions on three kinds of sensor data are 
similar. So the scan angle up to 30 degrees is adaptable 
to every sensor data for the priority of use for image 
composite. 
The period for image composite will be also supposed to 
be set as same as the case of AVHRR proposed in 
chapter 3, that willbe 9 days. Figure 1 shows the relation 
of orbits and observed areas. The symbols ‘0’ and 
triangles indicate the orbit positions and the observed 
areas with the FOV of +30 degrees. If the composite 
image covering the area in the equatorial regions 
observed on N-th day is prepared, the numbers of scenes 
acquired in nine days willbe five, three and six for AVHRR, 
OCTS and GLI, respectively. The dates are as follows; 
N-2, N-1, N, N+1 and N+2nd for AVHRR, N-4, N and 
N+4th for OCTS, N-1, N, N+1, N+4, N+5 and N+6th for 
GLI. In the areas in higher latitude, off course, more 
scenes can be utilized for a nine days composite image. 
Figure 2 shows the variations of the scan angles at the 
same point which is located on the nadir on the equator 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996