Poli, Daniela 
  
  
Figure 5. Contour lines with 200 m interval (right) in a region covered by cirrus clouds (left). 
3 CLOUD-TOP HEIGHT ESTIMATION FROM ATSR2 
The ATSR2 data were collected from the ESA ATSR NRT service (Buongiorno, 1999) during the Special Observation 
Period (SOP) of the programme MAP (Mesoscale Alpine Programme). MAP is an international research initiative 
devoted to the study of atmospheric and hydrological processes over mountainous terrain (MAP Science Plan, 1998). 
The SOP period lasted from September, 7 to November, 15, 1999 and was concentrated to three target areas in the inner 
Alpine region of Italy, Switzerland and Austria. 
The ATSR2 instrument is part of the ERS-2 satellite system which was launched in April 1995. The successor sensor, 
AATSR, will be part of Envisat which is currently scheduled for June 2001. ERS-2 is in a near-circular, sun-synchronous 
orbit at a mean height of 780km and a sub-satellite velocity of 6.7 kms !. The spacecraft is positioned to operate with a 
descending equator crossing of around 10:30 local solar time and of a ascending equator crossing of 22:30 local solar 
time. The repeat cycle is about 3 days. First, the ATSR2 views the surface along the direction of the orbit track at an 
incidence angle of 55° as it flies toward the scene. Then, some 120s later, ATSR2 records a second observation of the 
scene at an angle close to the nadir (Mutlow, 1999). ATSR2's field of view comprises two 500 km-wide curved swaths, 
with 555 pixels across the nadir swath and 371 pixels across the forward swath. The pixel size is 1x1 km at the center of 
the nadir scan and 1.5 x 2 km at the center of the forward scan. The sensor records in 7 spectral channels: 0.55pum, 
0.67um, 0.87um, 1.6um, 3.7um, 10.81um, 12.0um. The geolocation for the GBT products proceeds by mapping the 
acquired pixels onto a 512x512 grid with 1km pixel size whose axes are the ERS-2 satellite ground-track and great 
circles orthogonal to the ground-track. The resampling is done using a nearest neighbor method (Mutlow, 1999). 
Several significant data quality problems can be found in the delivered ATSR2 images which are also reported in 
(Danson et al., 1999): 
- Differences of odd/ even pixels: Fig. 5 shows a part of an enhanced ATSR2 nadir image from the 0.87|im channel. The 
difference between odd and even pixels is obvious. The reason is that odd and even pixels from the sensor are 
calibrated separately as they are obtained from different integrators (Mutlow, 1999 and personal communication). 
- Scan jitter: Correct positional registration of the 2000 pixels around a scan relies on a steady scan rotation rate. A scan 
jitter arises when the rotation speed of the scan mirror deviates from this, as can happen if the rotation is obstructed by 
debris. Irregular rotation results in a misalignment of data from successive scans (Mutlow, 1999). 
- Nearest neighbor resampling: due to this resampling method which is used to generate the GBT data, a sort of "stairs" 
is introduced in the images (Fig. 6). 
  
1166 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.