THE IMPACT OF OPTICAL STORAGE TECHNOLOGY ON
REMOTE SENSING AND GEOGRAPHIC INFORMATION SYSTEMS
W. Murray Strome
PCI, 4800 Dufferin Street, Suite 202
Downsview, Ontario, Canada M3H 5S8
THE DATA STORAGE PROBLEM
The launch of LANDSAT-1 in 1972 resulted in a sudden,
tremendous growth in the amount of digital remotely sensed
data. Ground stations were faced with the problem of recording
50-100 digital LANDSAT scenes daily each containing about 25
megabytes of data at a recording rate of 2 megabytes per
second.
At that time, standard computer peripherals included magnetic
tape recorders with data densities of 200, 556 and 800 bytes
per inch (50.8, 1412, and 2032 bytes/cm) with data transfer
rates of up to 40 kilobytes per second and disk drives with
data capacities of about 100 megabytes. Clearly, such
equipment was incapable of handling the data volume provided by
the satellite. The solution adopted by all groundstations was
to use high performance analogue instrumentation recorders with
special electronics to encode and decode the digital data.
Typical recorders had 28 tracks recorded at a density of 10,000
bits per inch (25,400 bits/cm) on each track. The tape speed
used was 60 inches per second (152 cm/sec). Computer
compatible tapes (CCTs) were produced for user distribution
after the satellite pass by playing the data back through a
computer at a reduced speed. In some installations, data were
transferred to several CCT drives operating in parallel. A
single scene could not be contained on a standard 1200 ft (366
mu CCT, Thus, users would be supplied with up to 8 reels of
tape (depending on the recording density) for a single scene.
The LANDSAT-4 and 5 Thematic Mapper has provided an order of
magnitude increase in data. Fortunately, magnetic recording
technology has advanced considerably. A typical TM scene
contains about 265 megabytes of data. Data are still recorded
on specially modified multi-track analogue instrumentation
recorders. However, the recording density is now typically
30,000 bits/inch on each track (11,800 bits/cm/track) and tape
speeds of up to 100 inches/second (250 cm/sec) are employed.
Disk drives with capacities of 600 megabytes are in common
usage and the CCT industry standard is now 6250 bytes/inch
(2460 bytes/cm). However, even with this advance in
technology, a full TM scene contains more data than can be
recorded on a 6250 bpi CCT. Thus, a typical user product
consists of one-quarter TM scene on a single 6250 bpi CCT or a
full scene on four tapes.
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