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Mcmurtry, G. J.

Petersen, G. W.

Rambert, C. E.

English

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VAde *atae dind r NIASA sponserskip E 7. 4 - 1 0. O. a 3in the interest of early and w; e d /s-
semination of Earth Resources Survey
Interim Report Program information and without liability
for any use made thereof."
ORSER-SSEL Technical Report 24-73
ANALOG TO DIGITAL CONVERSION AND PROCESSING OF MSS DATA
USING A HYBRID COMPUTER
C. E. Rambert and G. J. McMurtry
ERTS Investigation 082
Contract Number NAS 5-23133
INTERDISCIPLINARY APPLICATION AND INTERPRETATION OF ERTS DATA
WITHIN THE SUSQUEHANNA RIVER BASIN
Resource Inventory, Land Use, and Pollution
Office for Remote Sensing of Earth Resources (ORSER)
Space Science and Engineering Laboratory (SSEL)
Room 219 Electrical Engineering West
The Pennsylvania State University
University Park, Pennsylvania 16802
Principal Investigators:
Dr. George J. McMurtry
Dr. Gary W. Petersen
Date: May 1973
(E74-10033) ANALOG TO DIGITAL CONVERSION N74-11162
AND PROCESSING OF NSS DATA USING A
HYBRID COMPUTER Interim Report
(Pennsylvania State Univ.) 5 p HC $3.00 Unclas
CSCL 09B G3/13 00033
https://ntrs.nasa.gov/search.jsp?R=19740003049 2020-03-23T14:04:38+00:00Z
ANALOG TO DIGITAL CONVERSION AND PROCESSING OF
MSS DATA USING A HYBRID COMPUTER
C. E. Rambert and G. J. McMurtry
The objective of this investigation was to design and implement
a process for the conversion of multispectral scanner data from FM
magnetic tape recordings to digital magnetic tape recordings of a pre-
viously defined format. Specifically, the problem was to design a
system which will directly accept analog data from FM magnetic tape
recordings produced by the Bendix eight-channel multispectral scanner,
sample the analog data, convert those samples to digital numbers,
arrange those digital values into standard ORSER format, and output
the data onto digital magnetic tape in standard IBM System 360 nine
track tape mode of 32 bit words written in 800 bits-per-inch density.
The design was further restricted to implementation on existing systems
available to ORSER. It also was required that the multispectral scanner
and the conversion system maintain a high degree of accuracy in the
measurement and sampling of the spectral response of the subject. Thus,
simultaneous sampling is considered advantageous over simple multiplex-
ing in which each channel is sampled as it is input to the analog-to-
digital converter, no two channels ever being sampled at the same instant.
The system used for implementation was the hybrid computer operated
by the Hybrid Computer Laboratory of The Pennsylvania State University.
The hybrid computer consists of a Digital Equipment Corporation PDP-10
digital computer and an EAI 680 analog computer mated by a special
interface unit to translate data and control signals from one computer
to the other. The sampling and conversion of the multispectral scanner
data is controlled by a digital program. The maximum sampling speed
is limited by hardware speed, including not only the maximum speed
(31,700 samples per second) of the converter, but also the instruction
execution speed of the controlling digital computer. Prior to this
project, the maximum sampling speed available was limited by existing
subroutines to approximately 11,000 samples per second. For the Bendix
data, a theoretical minimum rate of 10,000 samples per second was required.
1
Normally, up to five times the minimum sampling rate is used for convenient
interpretation. Thus, up to 50,000 samples per second could be used. The
11,000 samples per second is therefore an undesirably limited maximum
sampling rate when the hardware is theoretically capable of a maximum of
31,700 samples per second.
A new subroutine was developed so that data rates approaching the
hardware maximum might be achieved. The sampling periods and rates
obtained with the special subroutine depend upon the number of channels
sampled and that part of core in which the program is located. However,
the sampling rates for eight channels of data were increased to 19,544
samples per second (2443 samples per second per channel) in fast core
and 17,496 (2187) in slow core. This is an obvious improvement over the
previous maximum of approximately 11,000 samples per second.
Besides the higher speed, there are other features in the new sub-
routine designed.for the specific application of multispectral scanner
data conversion. One such feature affects simultaneous sampling of all
channels. Also, the time between samples may be fixed (constant) for
all samples, or an adaptive sampling rate, such as that required to
geometrically correct multispectral scanner distortions, may be utilized.
Once the multispectral data is converted into raw, digital numbers, it
must undergo another conversion into the specified ORSER format before
it is ready for storage on magnetic tape. There are three operations
in this conversion. The first operation involves both a scale change
and an axis translation. The second operation, of rearranging, not only
fits the data to the required format but also reverses the order of the
samples, so that maps made from the output have the standard point of
view (that of looking down from above as if in an aircraft) instead of
the mirror image (as if looking up at the earth's surface from below),
as would happen if no change were made. The third operation reduces
the data storage requirement by a factor of four, by packing four
consecutive 8-bit words into one 32-bit word. Another subroutine
improved the speed of writing onto magnetic tape by at least a factor
of four over the existing magnetic tape subroutine. In addition, both
of the new subroutines offer the programmer more flexibility than their
predecessors as a result of their simplicity.
2
The advantages of the conversion system developed in this investi-
gation over a simple bulk digitizing system are five-fold. First, more
precise control of the sampling of the analog signals by the hybrid
system fits the data into the coordinate system more accurately than
does simple digitizing. Second, the final product of the hybrid system
can be written in the format required by any user system instead of a
single format particular to the digitizing system. Third, the conver-
sion system requires no specialized equipment for implementation; it is
strictly a software system which could be easily altered to new specifi-
cations without equipment changes. Fourth, the hybrid implementation
allows geometric corrections to be performed on the MSS data as it is
being sampled. Fifth, the analog computer provides preprocessing of
the data before conversion.
Upon completion of the conversion system implementation, the system
was used to convert the analog data from 13 flightlines flown on 5 May
1971 by Bendix Corporation. The data was originally recorded on one-
inch-wide magnetic tape at 60 ips with an Ampex FR 1600, 14-track wide-
band FM tape recorder that has a cutoff frequency of 200 kHz at that
speed. Due to the unavailability of a comparable tape playback unit
for use in the Hybrid Computer Laboratory, the data was rerecorded onto
half-inch-wide magnetic tape with an Ampex FR 1300, seven-track FM tape
recorder at the Bendix ground station in Ann Arbor, Michigan. At the
same speed, 60 ips, the FR 1300 has an upper cutoff frequency of 20 kHz.
The Bendix scanner senses eight spectral bands. The eight analog
signals plus a synchronization signal fit easily on the 14-track recorder,
but there are two signals too many for the available seven-track unit.
To resolve this problem, two seven-track tapes were made from the original
14-track recording, each containing only six data channels and the syn-
chronization channel. The four data channels estimated to contain the
most information appear on both tapes. These analog tapes were converted
to digital data and then merged on the IBM 360 at The Pennsylvania State
University Computation Center. For verification purposes, a comparison
for each flightline was made between the imagery produced on a film
recorder from the original flight tape and the computer-generated
reflectance map of the same flightline and channel, produced from the
digitized rerecorded data.
'3
The limit of computational efficiency was observed during the
digitizing runs. It was determined that the new digitizing system
results in an average of 2.75 percent of the computer time spent waiting
for the next scan line, i.e., the system resulted in 97.25 percent effi-
cient use of the computer while the program is in the digitizing phase.
There are a number of extensions of this investigation that could
prove useful. The preprocessing of analog data prior to sampling and
conversion could be treated as a feature extraction problem. The concept
of non-linear sampling to compensate for scanner geometry could be carried
further by using actual terrain contour information instead of assuming
the terrain to be flat.
As discussed above, the immediate objective of this work was to
digitize the Bendix data. This data was collected over central Pennsyl-
vania and will serve as a source of ground truth for high altitude air-
craft and satellite studies, as well as a direct source of information
in itself. The conversion system which was developed is suitable for
digitizing other aircraft data. Discussions have been held with Johnson
Spacecraft Center personnel regarding the possibility of digitizing some
data collected on the supporting C130 aircraft program.
ORSER-SSEL Technical Report 24-73
The Pennsylvania State University
May 1973


Analog to digital conversion and processing of MSS data using a hybrid computer

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