375
achieved by the socialist countries.
4) Use of scanner and array systems
Space missions with passive scanner systems (line scanner and pushbroom
scanner) are characterized as one dimensional line images. The total
image becomes integrated by movement of the platform with the scanner.
Advantages:
The scanner can operate as a digital system which need not carry film.
The data may be transmitted. Therefore scanner systems are more
adaptable to long-life satellites and to multitemporal imaging of cloud-
free regions. Multitemporal images provide the possibility of change
detection.
The information can easier be separated into different spectral bands.
Spectral information may also be significantly extended beyond the vi-
sible ranqe. The digital form of the data is well suited for automa-
tion.
Linear pushbroom scanners as compared to line scanners with rotating
mirrors have the advantage of a perspective imaging capability for the
imaged line.
The disadvantages of scanner systems are
1) an expensive and complicated technology of data transmission, data
receiving, data storage and data processing;
2) limited geometric resolution
The resolution of a scanner system is limited by the data transmis
sion rate of the satellite system. Thus either an extended coverage
or an extended resolution may be aimed for. Ground resolution is
dependent on the orbital height. Landsat has sofar been operated at
a 15 Mbit/sec data rate, Spot will operate at 64 Mbit/sec. The tech
nical limit for data transmission at still affordable cost is at
about 200 Mbit/sec.
The present technical limitation at an orbit of about 300 km can
reach a minimal ground pixel size of approximately 8 m (panchroma
tic) for an image strip width of 80 km. This also corresponds to a
minimum integration time limit considering the satellite velocity.
This actually corresponds to a photographic ground resolution of
about 20 m. Therefore the cartographic application of such a scanner
system may be suitable for scale ranges between 1:100 000 and
1:250 000.
The ground pixel size of Landsat 1 and 2 (MSS) imagery is 79 m, which
is comparable with a photographic resolution of about 220 m. The
positional accuracy of Landsat rectification indicates a result of
i 50-80 m (1 Pixel).
Landsat 3 is additionally equipped with a Return Beam Vidicon TV-
camera (RBV) (ground pixel size 40 m, photographic ground resolu
tion of about 110 m); while Vidicons may be suitable for higher
geometric resolution, their radiometric resolution is poor.
3) high reguirements_for_the_stability of the spatial attitude of the
attitude of the senior and thi continuity of forward movement of the
platform.
The geometric accuracy of satellite scanner images may be restored to
± 1-2 pixel accuracy.
5) Use of active scanner systems (RADAR)
The advantages of RADAR, the possibility of cloud penetrating cannot
be compensated by the disadvantages of a restricted general topographic
detail detectability, e.q. for buildings. At present radar images from
satellite are not even suited for mapping of scales 1:250 000. The re
solution of radar signals is determined by the pulse length of the ra
dar system perpendicular to the platform movement and by the (synthe-
