387 
selected, the width and positioning of each band 
being selectable by software. 
2.4 Radiometric Resolution 
The pushbroom principle of linear array 
imagers offers greatly improved radiometric 
responsitivity, compared to optical-mechanical 
scanners. High radiometric sensitivity and 
dynamic range allow small changes in features to 
be observed. The dynamic range for all of the 
channels of the MEIS II sensor, for example, is 
better than 4500:1, and close to 7000:1 for four of 
the channels (Till et al., 1987). The FLI and 
CASI systems have dynamic ranges better than 
4000:1 (Borstad et al., 1989). 
This higher sensitivity to spectral intensity 
with airborne linear array sensors is the result of 
the incorporation of multiple detectors in an array, 
of which each is able to integrate over one pixel 
for the whole scan line. The integration period or 
each pixel is approximately 3000 times longer than 
optical-mechanical scanners allowing smaller 
radiances to be measured due to improved noise 
equivalent radiance (McColl et al, 1983). This 
higher spectral sensitivity provided signal-to-noise 
ratios greater than 100:1 over forested terrains 
using the MEIS II system (Till et al., 1986a). 
2.5 Geometric Performance 
The superior geometric quality of the 
output imagery from linear array systems, when 
compared to equivalent optical-mechanical imagery, 
allow such data to be more easily integrated with 
geo-referenced computer-based inventories. The 
MEIS airborne system, for example, provides inter 
channel pixel registration to within 0.15 pixel, over 
the complete range of aircraft altitudes. 
For applications relating to cartography or 
multitemporal registration, additional processing of 
the data is required to correct for other changes in 
aircraft motion such as changes in velocity, pitch 
and yaw, etc. A processing system has been 
developed to correct MEIS imagery using flight 
navigation data. Based on algorithms developed 
by Gibson (1984), aircraft inertial navigation data 
are used to compute positions for all input pixels 
and to resample the imagery to a user selectable 
geo-referenced output grid. 
Geo-coded imagery has been produced 
from FLI imagery using Moniteq’s AIR-2 software 
and flight parameters recorded simultaneously 
with the imagery. Approximately one pixel 
precision has been achieved with spatial mode 
data (Buxton, 1988). 
2.6 Stereo Capability 
In addition to the nadir-mode of operation, 
the MEIS system can be used to acquire 
continuous fore-aft stereo imagery, by using the 
precision stereo mirror module. In the stereo 
mode, two of the eight channels look fore and aft 
at angles of +/-35° while six channels look nadir. 
The data may be combined with aircraft altitude 
data to provide digital terrain information and 
standard geometrically corrected products. 
Fore-aft stereo acquisition may also be 
able to provide information appropriate for forest 
stand height determination. With geometric 
correction capability stereo imagery is also suitable 
for base mapping and topographic mapping. 
2.7 Digital Format 
The digital format of airborne linear array 
imagery allows easier integration with digital 
databases such as geographic information systems 
and lends itself to automated processing of the 
imagery. In a digital form the imagery can also 
be more easily enhanced to improve visual 
interpretation. Digital imagery can also be easily 
mosaicked to provide large area coverage or ortho 
photos. 
3.0 CURRENT LINEAR ARRAY SYSTEMS 
The discussion of available linear array 
sensors will be restricted primarily to systems 
developed in Canada which have application to 
forestry. The discussion will not include systems 
developed for monitoring the atmosphere or 
systems developed primarily for ground based use 
(e.g., non-imaging spectrometers or radiometers). 
3.1 MEIS History and Description 
The MEIS (Multispectral Electro-Optical 
Imaging Sensor) is an airborne linear array sensor 
system developed for the Canada Centre for 
Remote Sensing (CCRS). The MEIS airborne 
multispectral imager was the first solid state 
linear array imager to be developed and provide 
data to the remote sensing community (Zwick et 
al, 1978). It was part of a multi-year Canadian 
research and development initiative to evaluate 
pushbroom imager technology and to develop 
applications. 
Development was started in the late 
1970’s with the prototype, two channel, MEIS I 
sensor. The sensor had two optical channels each 
with a 512-element photodiode linear array. MEIS 
I was successfully operated in the laboratory and 
in the air, and provided a useful demonstration of 
the pushbroom imager concept. Results were 
significant enough to lead to the development of 
the current eight channel MEIS II. This version 
was developed under contract to CCRS by 
MacDonald, Dettwiler and Associates Ltd. of 
Vancouver, British Columbia. In the four year 
period from 1983 to 1986, close to two hundred 
missions were flown with MEIS II throughout 
Canada, USA and Australia (Till et al., 1986b).