sensors have their advantages and the most likely 
scenario for the future will be an increased emphasis 
on data fusion as users select the sensors most likely 
to provide their information in each case and rely on 
their workstation software to use all the data 
together. The two types of data will be 
complementary rather than competitive. 
2. AIRBORNE DIGITAL SENSORS: 
REQUIREMENTS 
To have any chance of an impact in a market place 
spoilt for decades by high performance film 
cameras, an airborne digital sensor must provide: 
• large field of view and swath width 
• high resolution and accuracy, both geometric 
and radiometric 
• linear sensor characteristics 
• multispectral imagery 
• stereo. 
The first requirement, however, seems to rule out 
area CCD arrays, because readily available models 
in mid 1999 are 4Kx4K pixels or less, whereas a 
linear array of 12,000 pixels is readily available, 
requiring only one third as many flight lines. 
Considerable research work has been done in 
Germany since the 1970s, which has demonstrated 
the suitability of three panchromatic lines on the 
focal plane, with additional multispectral lines near 
the nadir. This obviates the need for multiple area 
arrays to provide a wide field of view and a 
multispectral capability (Figure 1). The left-hand 
diagram suggest how the focal plane could be 
populated using the three line principle: three 
panchromatic lines give the geometry and stereo, 
whilst additional lines, their sensitivity controlled by 
filters, give the multispectral information. In the 
right hand diagram, multiple area array CCDs and 
lenses are required to provide both the same ground 
pixel size and multispectral range as the three-line 
approach. 
multiple RGB + infrared arrays to 
increase FOV 
WÊÊMÊMÊÊÊÊÊËËtÊËÊËÈÊÊÊÎ&&iËÊtmËÊÊÊmmÊ0ÊMmm 
Fig. /. The alternatives: linear and area CCD arrays. 
3. THREE-LINE SCANNER APPROACH 
The three-line concept results in views forward from 
the aircraft, vertically down and looking backward 
(Figure 2). The imagery from each scan line is 
assembled into strips (Figure 3). The characteristics 
of relief displacement in the line perspective 
geometry of the strip approach vis a vis the 
conventional central perspective geometry are 
indicated in Figure 4, showing the line perspective 
geometry of the three-line imagery on the left and 
the familiar central perspective geometry of the film 
photograph on the right. The angles between the 
incoming information to the three lines are, of 
course, fixed. With three lines there are three 
possible pairings for stereoscopic analysis - strips 1 
and 2, 2 and 3, and 1 and 3. With film cameras, the 
parallactic angle is a function of principal distance 
and airbase. Moreover, every object appears on all 
three strips, whereas on film imagery only 60% of 
the area of any one photograph is in a triple overlap.