a rs —_—_ ee

ox 3 w
e oc xo
0 x
34 P2
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xXxPLCOo a
Ace <i
AACOAUM FO 94.00 UUTA CUI OC DAH YMA EE CZ O00 ON OW LH UMN HE 3D >> BBXX>-NN OD >
Y rad
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a 25 -
mw fee 7
oa gue 5
a0 ous
nov a
=oOu Dams oo auMs roams osm oAumsoeoAumsnorosumsrno 5 oS
3 & OAM TW) AS Aaa AAA ATUNUURUURUTUTUM MMMM Ss or TU DIO Og
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awa
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o DAUM sor OAM or osaUMsnooroAaumsnHoarosaumsnorarosa = &
A Gams nome r eases aru UUUURUURUUMM MMM MMMM Ss or oo Tino B®
i


“BuruyyBy 0, asuodses yswao jeordA \
YI SMOYS YeENS jeIUOZIIOY e “YBN ey UO syUN OM} 942 UL “ODIX3W NI 3HIFdWVI JO 47ND JHL JO S3UVIE SVD 91 auNDId



The pixel number is based upon the fact that the pixels are obtained at
equal increments of time. The line was thus derived and found to provide
a close approximation to the city positions, except for a slow drift

away from the correct value as the nadir distance increases, (Figure

14, upper right.) From header parameter Z (Figure 2), it was found that
the spacecraft was higher than the nominal altitude at the time of this
test, by the amount which correctly accounts for the mismatch between

the lines and the dots on Figure 14,

In the Air Force film imagery, the along-track pixels are reproduced
at equal intervals, but the across-track pixels (the ordinate in Figure
14) are nonuniformly spaced across the film to maximize the similarity
between distance on the film and distance along the earth's surface.

The objective of the film-making system has been to simplify the task
of the personnel who search these records for evidence of weather, For
the digital data user, our study indicates that a more accurate across-
track pixel location can be achieved (at the expense of more processing)
through the use of the aforementioned formula of the sinusoidal scan.
All plots derived from digital data in this report make use of uniformly
spaced pixels in the across-track direction. As compared to the surface
geography, these plots contain.a geometric distortion matching that of
the line on Figure 14, Some of this effect can be seen in film-to-map,
the comparison shown in Figure 13, where the horizontal-to-vertical
proportion (ratio) changes with respect to the east-west position. It
is indeed fortunate that the scan of the DMSP mirror is so perfectly
Sinusoidal, making it possible to determine across-track direction
accurately through the use of a fairly simple formula. The correction
for satellite altitude, exemplified by Figure 14, could be obtained
either from the comparative lateral positions of several fixed lights or
it could be obtained from parameter Z in the scan header,“

DMSP image processing involves the use of the Cassini map projection

whereby distances in the cross-track or Y direction are made true to scale.

This is the same approach used by Croft in defining the Y direction.
However, this results in an affine map projection since scale becomes
larger in the along-track or X direction as one moves away from the
orbital path center line. The USGS recommends the use of conformal
Projections such as the Space Oblique Mercator (SOM) rather than the
affine Cassini for such applications. The SOM has been implemented thus
far only for Landsat imagery. (A, P. Colvocorresses, Topographic Divieios, USGS, Reston VA)

35 ‘



were included in report one, it was tentatively speculated that the
lights might be some peculiar manifestation of eccentric thunderstorms,
However, by the time of the Scientific American article” we had received
confirmation from the Japanese Fisheries Agency that the lights were due
to the squid fishing fleets equipped with incandescent lights of great
power,” The changing locations of the lights followed the known patterns
of the changing locations of the fishing fleets in that sea. The fleets
seek squid whose locations are dependent on the thermal conditions of the
water. Their positions can be determined with some useful degree of
confidence from an examination of the DMSP infrared pictures which show
water temperature. 1° The same Japanese Fisheries experts advised us

that a smaller fleet worked near New Zealand, and the Scientific

American article attracted letters from New Zealanders who had

witnessed the fleet. Recently, in tracking down a UFO incident that
occurred in New Zealand, Shaeffer!+ investigated the possibility that the
UFO lights which had been reported were actually due to the Japanese
fleet, with the light travelling under peculiar propagation conditions,
In response to technical advice from the author, Mr. Shaeffer obtained a
DMSP image (the central one shown in Figure 18) and kindly provided a
copy of it to the author.

The location of the fleet as revealed in these three images is much
like that reported by another correspondent 7 who said that the Japanese
fished off the west coast of South Island. Because the boats clustered
along the 12-mile limit, he suggested that DMSP image examiners might
mistake them for a coastal population strip; indeed, a slight error in

drawing the coastline in Figure 18 would lead to such erroneous conclusions.

Mr. Wilkes advised that the lights were very bright--that "even 12-miles
away they throw distinct shadows, and if there are enough boats offshore
it is possible to read a newspaper outside on a moonless night." Later
in the season, the fleet heads toward the east side of South Island and
apparently becomes more dispersed. During the period depicted in Figure
18, Mr. Wilkes reports observations of a fleet numbering about 200,

concentrated between 12 and 25 nautical miles offshore between the towns
‘of Westport and Hokitika, (The bright spot between islands is located |

“at the Maui gas field, and may be a flare) =
44



The parameters H and C concern the infrared sensor. Spacecraft
altitude above earth and attitude relative to the sun are coded in
parameter Z and a photomultiplier calibration is given by P; only the
altitude was needed in this study. (Image positions were adequately

determined by interpolation among known locations.)

Many of the important header parameters are not adequately
represented by the octal digits. In particular, some of the parameters
require the octals to be brokén into binary equivalents. Subsets, no
longer in the original groups of three, must then be extracted. We have,
therefore, implemented one further step in the processing of the header
which is designed to make Basic data handling practical. This step is
described by Figure 3, in which an octal header is converted to the SRI
system, a mixture of binary, octal, and decimal representations. The
treatment of line count preserves the frame digit as an initial number,

a matter of convenience. Parameters Q and M are rendered in binary,
because in that form tneir interpretation is clearest. Time, probably
the key parameter, is preserved beth in its original octal form (useful
when referring back to more primitive data listings), and it is converted
to decimal and then divided by 1024, in which form it is simply the
number of seconds past U.T. midnight. The gain (G) is converted to
decimal as the most convenient form; however, H and C and some zeros are
maintained in their octal form as we do not expect to use them.
Parameters Z and P will be needed in future computer programs and we have
therefore converted them to decimal, the form which is needed by Basic
programs. Finally (1), which identifies the spacecraft, has not been
preserved in the SRI system. The spacecraft identification is inherently
included in the overall data file titling system.

Figure 3 thus illustrates how the header is converted into nine
numbers, each of which is in a convenient format. In the next stage of
processing in the SRI system, the scans are divided into successive
groups of 64. Associated with this group of scans, the 64 headers are
preserved as a single entity. The 64 scans are called a "register" and
80 the group of 64 headers are called the "register headers," identified

by a register number and an image number. The register size of 64 was

11



IX CONCLUSIONS

The digital data which can be provided by the Air Force are of
practical use, and they have potential value. For the most part, this
benefit stems from the preservation of the photometric parameters. In
this study we have endeavored to develop a method for orderly handling
of the digital data, and to develop the techniques for recovering photo-
metric data--at least to the point where the practicality of the recovery
could be assessed. The scale of this effort has occupied all our re-
sources, with the result that we have not had the opportunity to apply
the method to a specific study at the time of this writing. It is clear,
nevertheless, that the potential value is there. Examples are the unique
lightning-stroke record on the right of Figure 16, and the gas flare
signature statistics in Figure 17. Given the present state of our de-
velopment of these data, it would be a straightforward matter to perform
a quantitative analysis of either phenomenon by direct manipulation of
the units of data. If further digital records of the same regions could
be made available at useful intervals of time, we could then measure the
temporal evolution of gas-flaring through the comparison of successive

images, For such a study, the digital records would be essential,

The moderately large size of the DMSP pixels (as compared to those
of Landsat, for example) offers the user some benefit, in that the
quantity of numbers which must be handled is not overwhelming. With
Landsat, we would be dealing with about ten million numbers to represent
an area equivalent to one unit (having 4608 pixels) in this study. One
of our conclusions has been that the analysis of DMSP data -is indeed
practicable. The main element lacking is an avenue by which the digital
data may be obtained. To our knowledge, the image tapes supplied for
this analysis are the only ones ever released for unclassified study.

We have further concluded that potential applications for nighttime

imagery exist which cannot be met by the DMSP. In the main, this conclusion

54



‘parameter M, we know that the gain control was linear and so the bright-

ness of a parameter having sixol S was (3.74314 x 107?) (0, 238095), or

about 10°? watts/cm@-sr, In DMSP descriptions, this level is often cited
as the threshold, :

E. Comparison to Landsat

As was described in report one, Landsat has been operated at night,
with the result that most of the pixels were completely unexcited; that
is, zero in all colors. The exception was caused by gas flares which
strongly excited the sensors; many pixels were fully saturated. The de-
tails of the pixel illumination patterns as a function of latitude, longi-
tude, and spectrum, were included in report one, At that time, we lacked

information concerning the photometric parameters of the DMSP and there-
fore a comparison was not possible,

Among the sensors on the Landsat satellite, the most sensitive
should be that associated with band 5 of the multispectral sensor. When
operated in its high gain mode, this band exhibits a maximum radiance of
0.00067 watts/cm?-sr, 13 The telemetered output is a 6-bit number, but
the sensitivity to the dim light is somewhat increased through the use of

"compression," by which means the numbers of a 7-bit code (0-127) are non-
linearly mapped into a 6-bit code (0-63). Examination of the "decompres-
sion tables" reveals that this mapping is linear for the first few codes
(the dimmest) so that, from the standpoint of determining the minimum

light ievel detectable by Landsat, it is equivalent to the use of a 7-bit
code,+ -

The minimum among the minima is then achieved by band 5 when it is
in a high gain mode, at which “ime the maximum radiance (all binary ls)
corresponds to 0,00067 watts/em* “sr. The minimum detectable level is

‘then 1/128 of this amount, or about 5,2 x 1076 watts/cm?-sr. The

reason for the relative appearance of insensitivity of the Landsat records

at night is thus made clear; it is 4 orders of magnitude less sensitive
than DMSP,

52



Table 2, THE GAIN ASSOCIATED WITH EACH PIXEL VALUE IN THE LINEAR AND LOGARITHMIC MODES OF OPERATION
(When multiplied by the radiance from Table 1, this yields the pixel radiance.)

Sixol Octal Gainflin} — Gaintlog}
none. o 1 1
1 589127 7295)

2 2 E254 863588

2 3 +5238) 803085

3 4 «936508 + PHYA

4 5 <20635 +b9385b

5 b <3047b2 +BU4SUE

b ? 888889 559484

? w 873016 +S5722b

4 nu -8S7143 52794?

3 Re -841270 48143?

a 13 +825397 447500

A Ww 809524 -425956

b us +7535) + 38635

8 be 22278 +3593)

c nr +7610 334048

iq 7 20 + 746032 + 310501

d 21 + 730155 +288613

> 22 +7128 2682659

e 23 +b98413 245359

E 24 +b82540 +231781

f 25 +bbbbb? «215443

F a +b50754 +200256

3 27 -b34921, + 286140
. G EA] +bLA048 173019

h 31 +b03175 +140823

# R 587302 W548

i 33 57424 2238549

I 4 +SSSSSb «129155

j 3s 53983

J 2» + 523810

k 37? -50793?

kK 40 «452064

1 4. 4B

L 42 40318

B 43 MAGS

q 44 4RBS72

n 4S 4L2B58

N 4b + 396825

° uN? + 380552

° 50 + 365079

p SL “34920”

Pp 82 + 333333

q 33 33 74b0

@ Su 301587

r $5 285714

R Sb +PEAUL

s 8? +253%8

s i) -23809S

t bl +222222

T be 206344

u 63 +19097%6

u bu +474b03

v bS +188730

v bb -142857

. &? 126584

u x) +211111

x a 095238

x cc +079365

y B 063452

¥ TM 047619

z 7S 031745

z % 015873

none nr Q

50



V THE GENERATION OF GRAY-SCALE MAPS

In order to generate gray-scale maps of the same information shown
on the units, SRI set as its objective the application of the same
Diablo OCR element used for the sixol plots. As a first step for
achieving this objective, every OCR character was repeatedly printed in
a rectangular array, from which a small portion of the resulting record
is shown in Figure 8. This chart was then examined by means of a photo-
meter (United Detector Technology, Inc., 80X Opto-Meter, with sensor
6288), and the brightness of the reflective light from each rectangular
array was objectively determined. Each OCR character was thus associated
with a certain level of darkness that would appear if that. character
were used to represent a certain range of the available 62 photometric

values.

The human eye cannot perceive 62 shades of gray in light reflected
from a page, nor do the OCR symbols provide 62 levels. Typically, in
these plots, we attempt to provide nine or ten levels of gray. Better
gradation was achieved in trials using two-symbol overstrikes, but for
simplicity ‘the single symbols are used here.

Using the OCR symbol thus selected by direct photometric measurement,
the generation of a visually pleasing image from the units of digital
data requires only a modification of the sixol plotting algorithm, A
range of digital values must now lead to a single symbol. For convenience,
a one-unit array of such symbols is here called a "map." By such means,
and using only seven symbols (of which the blank is one), a digital
image (map) of Atlanta, Georgia has been printed with Diablo OCR
characters in Figure 9(a). In this map, one can see the outline of the

city, the major access roads, and the smaller cities around it.

22

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i



eS O—— rr

Fe. Flare Location by Computer

Another large gas flaring region is shown in Figure 17(a), in which
the large bright region is Edmonton, Alberta. To the south and west of
Edmonton lies a large gas and oil field, in which a great deal of gas
flaring has long been visible in the DMSP images. Figure 17 provides a
first look at this region in digital data, Immediately, one sees a
characteristic which is unlike that of the populated regions of earth;
there are small isolated spots so bright that they saturate the sensor.
The clue to this is the presence of blank regions (digital code zero)
with black regions immediately above and below.

To demonstrate this isolation of zeros, the sixol plotting program
was modified, In statement 160 of Figure 7, the zero sixol was changed
from a blank to a filled black rectangle. Using this program, the
western unit of Figure 17(a) has been rendered as a sixol array, shown
in Figure 17(b). Now the ‘gas flares stand out clearly as black spots
that are isolated. From this, a pattern never evident among the lights
of cities away from oil fields, it 1s clear that a computerized pattern
recognition program could readily be devised to locate the flares.

}
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:
i
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TABLES

The Radiance of a Saturated Pixel for
Differing Values of VDGA Gain . se 0 © 2 ee we we ew we

The Gain Associated With Each Pixel Value
in the Linear and Logarithmic Mode of Operation... +.

iv

49

50



By measuring the locations of these cities geographically, and by
determining their pixel number along the scan, we have undertaken to
evaluate the optimum method for pixel-by-pixel determination of along-
scan location. Figure 14 shows the resulting comparison. The drawn
line shows the calculated relationship between lateral geographic
distance D from the nadir (the abscissa) and the pixel number n, (the
ordinate):

D=R + TR | 90 - cost [peu + sin (a sin cbn)) | - asin (bn) i

where D = surface distance, along scan, from nadir

= radius of earth. D and R are in the same units

= altitude of the DMSP satellite, in the same units

= 57.85°, the full angle of mirror swing

0.20888 for 3 n.mi. pixels; twice that for 1.5 n.mi.
= pixel number counted outward along scan from nadir.

a cc @ mw

Each of the major cities, from Fort Worth and Dallas to the sea, is
then shown by a dot which was located, with ressect to the ordinate, by
determining the pixel number of the city center on the DMSP image. The
location of each dot, relative to the abscissa, was determined by
measuring the geographic distance of the city from the subsatellite track.
A few of the cities were east of the track, but most were west.

As the satellite passes overhead, it sees the earth by means of a
mirror which is suspended on springs and is oscillating from side to
side. In engineering parlance, this mirror spring system has a Q of
approximately 500, from which parameter an engineer will conclude that

the oscillating mirror position as a function of time is very close to
the ideal sinusoidal.

The calculated line on Figure 14 was determined by the above
formula, which relates the sinusoidal oscillation of the mirror to the
distance along the surface from the nadir, assuming a nominal satellite

altitude, and accounting for earth curvature as if earth were a 6370
km sphere,

33



The sixols are in effect a natural extension of the 4-bit symbols
called hexadecimal. Similarly, 3-bit symbols are called octal and 1l-bit
symbols are called binary, From the author's studies on this matter, it
appears that 7-bit symbols are not practical because of the limitations
of both symbolism and mechanisms, Therefore, sixol probably represents
the ultimate achievable numbers of bits which can be converted into a
single-character code. (Superimposed pairs of symbols could serve to
encode 7 or 8 bits, if a need arises.)

In order to make possible the use of the sixol symbols as depicted
in Figure 5, it is necessary to use a type font which distinguishes
between zero and the letter oh, and which distinguishes between the
number one and lower case letter el. From experimentation with the
machines which were already available within the author's facility, it
was determined that 3 or 4 type fonts met the criterion, but that the
optical character reading (OCR) version of the Diablo wheel was the best
for purposes of publication. This has been used to generate Figure 5 and
all the plots given herein, This approach offers an additional advantage;
any reader equipped with the necessary mechanism to read OCR symbols can
then reconvert these sixol plots back into numbers, and thereby recover
the DMSP digital data in their full detail, Since the printed page is a
compact form for the presentation of such data, it is the writer's
expectation that future computer centers will include optical character
printers and readers as a normal means for the preservation and

communication of data.

A unit of data in a sixol array is given in Figure 6. The blank
region in the lower edge is Dallas and Fort Worth, whose central lights
were so bright that the DMSP sensor saturated, yielding all zeros (which
are here represented by blanks, in order to increase the.visibility of
saturated regions). The seeming featurelessness of this ‘array, except
for the blank region noted, is misleading. A careful examination will
veveal many small regions that are brighter than others, but such an.
examination is tedious and time-consuming. We shall see that a slight
modification of the array yields a black and white map which makes the

features stand out much as they do in a film image. Then from a

18



The spaces between the plotted symbols hinder pattern perception, and so,
using a convenient feature of the DIC computer terminal, we have
eliminated the spaces in Figure 9(b). In this map, one of the earliest
examples of the SRI trial imagery from a DMSP tape, the outline of

Atlanta and its neighboring population centers becomes readily apparent.

As an illustration of the kind of qualitative map change which is
easily accomplished with these methods, Figure 9(c) shows the same
digital information now represented by 9 shades of gray and with a digital
partitioning which yields a lighter image. This is equivalent to an
increase of the exposure of the film or, alternatively, a change in the
gain settings of the DMSP. A marked change occurs in the appearance of
the map and, in fact, similar qualitative changes are often observed in
the film imagery. At times, such exposure changes make the recognition
of familiar cities and gas flares more difficult. The value of such
scale changes in the study of digital data lies in the fact that the
limited range of visually perceptible gray variation can be fully
utilized by adapting the digital partitioning to the particular needs
of each image. In this case, for example, a requirement to locate cities
and roads would best be met by the partitioning of Figure 9(b) while a
requirement to study the darker details or the albedo of the surrounding
countryside would be better served by Figure 9(c),

The elimination of spaces among the plotting symbols is not advisable
when sixols are to be plotted. The example in Figure 9(d) shows the
same information as in 9(b) or (c), plotted to the same size, and the
sixols become difficult to identify. The sixol symbols must be kept
separated, as in the example of Figure 6. _This is only a minor
inconvenience, as the analyst can readily relate individual sixols to
individual gray map symbols, despite the different proportions of the
sixol and map plots, By photo reduction, it is possible to make the
sixol plot and the map of equal size, but this serves only the somewhat

"cosmetic" purpose of appearance and is not necessary in a working
environment.

25

i
|
H
i
i



REMARK Sixol.decimal February 1979 Croft SRI
REMARK set unit label at 7b0
REMARK place sixols in WS for decimal data

r=

[atl at gla ld
Se Rea a as
1)
>
>:

DATA ee ee ee Le

DATA falalAlatblatBlatclatCratdls'Diafel ate!
T Telitrrye "SIGhATHT THT STi al Ti al jtatdl

DATA TT Ce Er 2

DATA (pl atPratqralQtatriatRiatstalShatth attr

DATA fulastUlalyl ati atwla What xl at Xi alyl at Ve

DATA '2?,tZt

GOSUB ?S0

REMARK,

FOR M=L TO

Asst

FOR N=L. TO 72

READ D

AS=AS+US{D}

NEX’

PRINT AS

NEXT OM

GOSUB_?50

GOTO 794

PRINT

PRINT 'UNIT.8.102.15b'

PRINT

RETURN
END

print sixols, 72 across and b4 down

Paci aie

ono

LY]

LULU Lt

BESE

FIGURE 7 COMPUTER PROGRAM CONVERTING THE DMSP DATA UNITS TO |
SIXOL ARRAYS ‘

21



100 200 300 400 S00 600 700 800
T T T T T T T
: DISTANCE ALONG SURFACE FROM NADIR — nmi
340+ <FT.WORTH, i
1X
e
a20 | DALLAS, TX i
© GRANVILLE, TX |
i
{ 300 + ‘© TYLER, TX i
'@ LONGVIEW, TX |
280 | ® MARSHALL, TX
|
‘® SHREVEPORT, LA i
260 +
‘® RUSTON, LA |
240 + LINE SHOWS VALUE * {
{ CALCULATED FOR Ne
= MI Ri
zor § nen aMeyEe VICKSBURG, MS
IN,
200 |- § JACKSON, MS .
' 180 -
g MERIDIAN, MS © SHOW CITY CENTERS LOCATED
g IN DMSP DIGITAL DATA OF
veo 5 29 MARCH 1978 (designated SRI 148)
4
wok X
= SELMA, AL
120
MONTGOMERY, AL
100 |
AUBURN, AL
80; COLUMBUS, GA
6oL
| ‘ALBANY, GA
or SAVANNAH, GA (east of nadir)
TIFTON, GA
2b
DOUGLAS, GA DISTANCE ALONG SURFACE FROM NADIR — km
1 i 4 L Su 1 L 1 L L L n 1 i _L.

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

FIGURE 14 COMPARISON OF ACROSS-TRACK DISTANCE (calculated from mathematical
description of the instrument scan) AND THE SAME DISTANCES DERIVED BY
LOCATING SPECIFIC CITIES ON A DIGITAL IMAGE AND USGS MAP. The
small, but gradually increasing separation of the curve (theory), from the dots
(measurement) is attributable to altitude of the DMSP satellite (not nominal at

i the time of this observation).

\

34



illogical interpretation of all subsequent numbers. No repetition of
this error has yet been found, but if it occurred it would upset the

reading sequence, and this second line in the unit would then be changed.

The last two numbers in the second line of the unit provide the
count of the included pixels in the across-track direction. The count
begins from the eastern edge of the picture, even though the scan began
from the western edge. This unconventional ordering is caused by the
fact that the spacecraft recorded the image on a tape recorder that did
not réwind before playback; rather, the numbers were sent to earth during
playback in the reverse direction, so that the order of all numbers (that
is, the sequence of pairs of octal digits) representing pixel brightness
wag reversed, The header information was not reversed, however. In the
SRI units of data, this reversal has been again reversed so that the
numbers read from left to right and from top to bottom (that is, from
west to east and from north to south). Registers are still numbered from

south to north.

A complication will arise when data become available from the newest
DMSP satellite (F4) because it makes its nighttime passage while traveling
from south to north! In the SRI system, concern about these matters is
minimized, because the units of data are always preserved as described,
occurring in the same orderly aystem that words appear on this page. The
complication of dealing with all the possibilities of order is then

relegated to the computer program which generates the units.

Summary of the Data Handling Procedure--We have described a program
for unitizing a long stream of numbers by means of a system that lends

itself to simplified processing without compromising quality. All of the
essential information is preserved. The map area, as represented in the
original image by a film, can be visualized as subdivided into small
rectangular areas, herein called units, Roughly, a unit width of 72
‘pixels is about 100 miles, and approximately 20 units are required to
compose an entire across-track scan. A unit length of 64 pixels is, also
roughly, 100 miles, The processing of areas larger than a unit requires
the separate treatment of several units and the subsequent blending of
results, Many examples of this will be depicted here.

15

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i
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All this preceding work has depended on film exclusively, carrying
with it the inherent limitation that pixels cannot be accurately discerned
either in latitude, longitude, or brightness. The latitude and longitude
dimensions have been satisfactorily (for many purposes) treated through

matching of the imagery to known benchmarks,

The Air Force data and filming process is designed so that sensor
gain varies in a way which compensates for the changing conditions of
lighting, with the object of producing a film whose gray shades are
proportional to the albedo of the surface. This approach is ideal for
weather sensing; the processing machines produce a final product
requiring neither calibration curve applications nor other corrections.
The remote sensing worker who wishes to determine source brightness must
undertake to determine how the gain of the amplifiers has been varied in
order to yield the albedo map; the calculations must then be reversed in
order to recover the brightness parameter originally measured by the
sensor. In effect, the Air Force system has been made practical through
the incorporation of preprocessing (some of which takes place in the
satellite); the preprocessing must be reversed to recover the primary
information, The reversal is not difficult except in the case of one
operational mode called "along scan gain control” (ASGC), in which there
is not sufficient recorded information to permit an exact reconstruction
of the original measured brightness, Fortunately, ASGC is seldom used in

the circumstances which are of interest to this study,



*(1y6js eddn) uoysog pue ‘(woN0g) “p°q ‘uo BUIYseNy
*Q21) UBingsnig “vOINaWV JO S3ILID HOrVW awos SNIMOHS LHOIN YV319 WY NO VLVG dSWO 4O SLINN N3aLXIS ZL SYHNDId-

31



|

VI IMAGES DERIVED FROM DIGITAL DATA

In order to lessen the quantity of numbers which must be handled,
many of the Air Force operational processes make use of

in which a single brightness value is derived from the average of 4 of

the original values. Because the initial pixel size is nominally 1,5
nautical miles on a side,

miles. Because of the nonuniform scan speed, these nominal values differ

from the true pixel size in some regions of the image. A more detailed
discussion of the lateral scan geometry will be given shortly.

A. The Two Resolution Scales

Two images of San Francisco and the surrounding cities are given in
Figures 11 (a) and (b). The former was obtained on September 8, 1978,

and is a 2 x 2 average, having 3 nautical miles resolution. The latter

was obtained on October 3, 1978, and it preserves the full 1.5 nautical

miles resolution. The greater detail of the full resolution image is

clearly shown by this comparison. It is concluded from a study of both

types of data that the remote sensing community will, for most purposes,
be willing to pay the price of dealing with a larger mass of numbers in

order to achieve the 1,5 nautical mile resolution. Any averaging that

may be desirable for the purposes of specific study will readily be
performed by the investigator. The utility of the 3-nautical- -mile tapes
is based solely upon fact that they are less expensive to Process per
unit area on the surface, and this criterion is comparatively unimportant
in this work, whereas it is of paramount importance to the operational

community. One of the conclusions of this study, therefore, is that the

digital data user will prefer the 1,5-nautical
3-nautical-mile averages,

“mile tape rather than the

28

"2 x 2 averaging,"

this averaging yields a resolution of 3 nautical



Both geometric and photometric calibration methods are evaluated.
An image can be considered as a 3-dimensional array, X, Y, Z, in which
X and Y are the coordinates of a picture element (pixel) and Z is the
brightness at that location. A method of approach to handling these
parameters, particularly Y and Z, is developed in a form quite different
from that which serves the operational applications.

The user of digital data will need the film images which are
generated by the Air Force from the same data as is provided on digital

tape. In the first stages of analysis, the films provide both a

convenient index and a guide to identification of large patterns in the
data. Additionally, the infrared (8 to 13 p) film provides a valuable ~
indicator of cloud cover.

Two general conclusions are drawn from this study, Firstly, the
digital DMSP data have great potential value but their cost, in terms of . . |
the interruption of the present operational routine, is quite high.
Therefore, if a program is undertaken to provide for the open availability
of an archive of digital records, great care must be exercised in selecting
only those records which have unusually high value in order that the
effort will be cost-effective. Secondly, it is concluded that several
aspects of the program, well designed for Air Force operational purposes,
are not adapted to earth-sensing needs. This is probably inevitable,
since the two applications are largely different and in some ways
incompatible, For example, the nighttime visual sensor saturates in the
center of major cities and in moderately large fires (such as gas flares).
This saturation prevents the analyst from integrating photometric
parameters, For weather observation, this inability is unimportant, and

acceptance of such saturation makes feasible a decrease in the data rate.

Such limitations in the data will probably be overcome only through
modifying the existing system or the implementation of a similar system
designed specifically to serve earth-sensing needs.

\


I SUMMARY

The U.S. Air, Force has operated its Defense Meteorological Satellite
Program (DMSP) for nearly a decade, and film images from the system have
been openly available since 1973. Films are well suited for the study of
weather, and users of such films have derived much useful data. For many
potential remote sensing applications, however, a quantitative measurement
of the brightness of the imaged light patterns is needed, and it cannot
be extracted with adequte accuracy from the films, Such information is
contained in the telemetry from the spacecraft and is retained on digital
tapes, which store the images for a few days while they await filming.
For practical reasons, it has not heretofore been feasible for the
Air Force to provide a remote-sensing user with these digital data, and
the quantitative brightness information has been lost with the erasure

of tapes for re-use.

For the purpose of evaluation of tapes as a means for remote sensing,
the Air Force recently did provide to the author six examples containing
records of nighttime DMSP imagery similar to that which has previously
been evaluated by SRI International in a film format.+ The digital data
create many new applications for these images, owing to a combination of
several factors, the most important of which are the preservation of
photometric information and of full spatial resolution. In this evaluation,
stress has been placed upon determination of the broad potential value of
the data rather than the full exploitation of any one aspect of it. The
effort was guided by an objective to develop handling methods for the
vast body of numbers~--methods which will be practical-for'use in a
research or engineering environment where budgets are limited, and
specialized capabilities and image reproduction equipment has not already
been developed. We report the degree of success obtained in this effort,
pointing out the relative strengths and the relative limitations, as
compared to the sophisticated, weather-oriented data processing which is
well suited for the Air Force requirements,



IV MAKING PHOTOMETRIC DETAIL ACCESSIBLE

In the study of a picture, the questions which arise are largely in
this form: the analyst examines an image and then, from pattern
recognition within the image, inquires concerning the brightness of
comparatively small subsets of pixels. Usually the outline of these
subsets does not follow a mathematically simple form, but rather it is in
a form which follows the outline of the surface phenomenology. In most
instances, we expect that the user of DMSP digital images will need a
capability to pluck (from the discernible data) the brightness values of
individual pixels selected from a map. If it were necessary to do this
by computer programming, the problem could become quite complex. SRI
has endeavored to develop a practical alternative in the form of a symbol
plot and a corresponding map, having a one-to-one geometric layout :
similarity. The user may then examine the map, determine the regions of
interest and refer to the symbol plot to determine the original photometric
values, pixel by pixel.

The "Sixol" Plot--It will be recalled that the original DMSP
brightnesses were included in two octal digits but that the values 00
and 77 were not included. Here, this numbering system will be called
“modified octal," as it includes only 62 values, In the units of data,
SRI found it convenient to represent these 62 values by the decimal
numbers 0 to 61. Each of these values, when plotted on a map, will yield
one symbol, A convenience therefore arises if we form a system of 62
individual symbols to represent the 62 brightness levels. This was done
as depicted in Figure 5, showing decimal, modified octal, and "sixol."
The latter term is an abbreviation of "six-bit symbol" which we introduce
here in a form having only 62 assigned values, Other applications for
sixol will require all 64 values, achieved through the use of two more
symbols which follow Z. :

16

i
H
i
Hy



In order for the Air Force to release digital magnetic tapes which are
compatible with standard computers, it was necessary for the Air Force
personnel to execute special computer programs, written only in the last
year, by means of which the full information on an original data tape can
be transferred to a more conventional format. This entails a considerable

effort by both the operational personnel and their computers, already fully

engaged in meeting practical needs,

As one of the objects of this evaluation has been the determination
of the practical use of digital tapes in place of films, the foregoing
mention of the effort required to make computer-compatible digital tapes
is relevant. It was clear that the existing facilities, and the
personnel assigned to operate them, could not support a major program of
supplying tapes. Any program for supplying digital data on a somewhat
routine basis will require the implementation of some screening procedure

so that only the most useful segments of imagery are digitized.

As a first step in this evaluation of nocturnal images, SRI under-
took a study of the films (under a previous contract) with the object of
establishing cartographic benchmarks and a systematic identification of
the fixed lights which appear on the surface in those images which show
tand.? (Croft, 1977, herein called “report one.) Many bright, fixed
lights are the major cities of earth and, somewhat surprisingly, the
brightest are those due to the major gas flares which are distressingly
plentiful. The other source of cartographic information in nighttime
pictures is, of course, moonlight. It illuminates the surface and reveals
a picture much like that in daytime. Under such circumstances, little
more is needed to aid in identifying geographic Tocations. On mooniess _
nights, however, the surface bears much resemblance to a star field in
the sky; the determination of latitude and longitude then requires a
“best fit" of imagery to the locations of known light patterns, An
atlas of such light patterns was provided in report one. This work was
adjudged to be of significant intrinsic interest, and as a result the
author was invited by Scientific American to write an article on the
subject.” This article in the July 1978 issue probably did more to
bring the DMSP to the public awareness than any other single event.

4



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FIGURE 8 REPRESENTATIVE ARRAYS OF SYMBOLS USED IN PHOTOMETRIC
MEASUREMENTS TO DETERMINE THE APPEARANCE OF DARKNESS
{the Gray Scale) IMPARTED TO PAPER THROUGH PRINTING OF
THOSE SYMBOLS. Results of photometric darkness measurements were
used for selecting grayness symbols used in subsequent DMSP image
generation (herein called “mappingTM).

23



Final Report

August 1979

THE BRIGHTNESS OF LIGHTS ON EARTH AT

NIGHT, DIGITALLY RECORDED BY DMSP

SATELLITE

By: THOMAS A. CROFT

Prepared for:
U.S, GEOLOGICAL SURVEY —

RESTON, VIRGINIA 22092

USGS PO57301

Approved by:

ROBERT S. LEONARD, Director
Radio Physics Laboratory

DAVID D. ELLIOTT, ecutive Director
Systems Research and Analysis Division

SRI International

333 Ravenswood Avenue - Menlo Park, California 94025+* U.S.A.

(415) 326-6200 + Cable: SRI INTL MNP + TWX: 910-373-1246



This operational approach represents a point of departure between

the practical needs of the U. S. Air Force and the contrasting needs of
remote sensing workers for comparatively small amounts of the same data.
The latter wish to know the brightness of the light coming up from the
surface in absolute terms, i.e., in watts per square centimeter per

steradian, and as a function of wavelength. ‘In nocturnal images, much

light originates at the surface and then the very concept of albedo is
inappropriate, As a result, the digital data user needs to take the

sixols and convert them into some measure of radiance.

A. A Decibel Trap

It is convenient to digress briefly, to review the rationale for the
decibel, because we shall see that the concept is used in a potentially
confusing manner during DMSP brightness control calculations. This usage
permeates the documentation of the system. In electrical calculations,
one often deals with a power ratio that equals the square of a
corresponding voltage ratio. For example, a power ratio of 100 results
from a voltage ratio of 10, As many technical descriptions concern
ratios of powers or voltages, there is ample opportunity for error to
arise from imprecise wording, with the result that a number intended to
Yrepresent power ratios would be applied to voltages or vice versa.

Use of the decibel precludes such errors. Ratio identifications
are clarified because the decibel is formulated differently for voltage
than for power, The decibel is 20 times the log of a voltage ratio but

only 10 times the log of a power ratio. A statement in decibels is
therefore unambiguous.

In the DMSP spacecraft, the fluctuating level of power in the light “s
seen by the spacecraft is converted linearly into.a valtage; therein lies
the key to this discussion. In subsequent amplifications of this signal,
changes in the voltage level are expressed in decibels, using the
voltage formula which incorporates a 20-multiplier., The same decibel
count applies to the power ratio in the light, for which one would use a
10-multiplier. Readers of USAF DMSP literature should be aware of this
convention, All decibels herein refer to the voltage; thus, a 20 dB
change represents a ratio of ten in light power. i

46



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With this preview of what is to come, we return to the raw data
tapes which are, basically, exceedingly long lists of numbers. In the
original form, the individual bits are organized in groups of three and,
as a result, the simplest printouts of the data represent everything in
octal digits. An initial listing of the original tapes will then yield
pages of octal digits which seemingly exhibit little order. However a
closer examination will reveal that there are 2,988 octal digits which
exhibit little order followed by 60 which are readily recognized by their
repeating patterns of digits. The first set, the 2,988 octal digits,
represent 1464 pixels which lie along a single across-track scan, The
brightness of each pixel is recorded as two octal digits, or 6 bits.
Associated with each across-track scan is a header, comprised of the
60 orderly digits previously mentioned. In this header are given the
essential physical parameters required to locate the scan within the body

of data, and to record some of the more temporally variable calibration

parameters,

After some initial processing of the data, the scans take the form
shown as Figure 1. Two and one-half scans are shown here, separated
from each other by a blank line, The first line of numbers in any one
scan is the header; all the subsequent numbers within each scan are pairs
of digits representing the brightness, now converted to decimal (rather
than octal) numbers, In the interest of keeping the list short, we have
chosen not to put spaces between the pairs of decimal numbers representing —
the pixel brightness; in practice, when one wishes to read such a list,
an overlay is applied which contains vertical lines that separate the

numbers into their correct pairings.

In the original data, the pairs of digits which represent brightness
were allowed to vary only between the values of 1 and-76,‘octal. The
values 0 and 77, although present in the original data and used in
Air Force filming, have been eliminated from the tape-recorded data by
changing all 00s to Ols and all 77s to 76s. As a result, the brightness
data should never include the successive digits "00" or "77", This is a
useful property, since it helps both the computers and the humans to spot

the header (which does contain such pairs), In converting these octal



VII PATTERN RECOGNITION IN THE IMAGERY

Typically, the analysis of these data can proceed only after the

sources of the imaged light have been identified. Such image identification

is straightforward on moonlit nights, requiring only comparison to a
map, while at moonless times it may become truly puzzling. In any case,
the identification phases of the project probably require the use of
the Air Force visual or IR films, unless the analyst has resources
sufficient to construct an equivalent image. A complete reconstruction
by units is impractical; the units have been devised for small-area

studies of, say, 1000 miles or less in extent,

On moonless nights, the lights are first identified with respect to
the known locations of cities and fires. It is a great aid to have
several images of the same region taken at different times, because the
changing locations of some lights reveal them to be transient. (The
rate of change in brightness or location is often a major clue to
identification.) One such example is illustrated in Figure 18, using
three images taken on the first three midnights of 1978. Off the west
coast of New Zealand, there are bright lights which change in position
on successive nights. Lacking other data, one would be hard pressed to
explain these lights. As it happens, the author knew in advance that
they would be there, and obtained these images as a result of forewarning.
These examples provide evidence that, at times, the images alone do not
provide enough clues to make identification possible. One must then

resort to ad hoc measures.

The author first saw larger but similar lights in the Sea of Japan.
By coincidence, one of the author's colleagues, Dr. E. T. Pierce (now
deceased) was an internationally recognized thunderstorm expert who
lived for a time in Japan and wrote about the "eccentric" thunderstorms
in this same region of the Sea of gapan.® When the Japanese sea images

42



B. Pixel Saturation and its Consequences

The population center of America is depicted in Figure 12 at 1.5
nautical miles resolution, derived from the digital image obtained on
October 3, 1978. The more southerly rectangular section of this image
is composed of five units across and two units down, while the upper
rectangle is composed of three units across and two units down. Thus,
the image contains 16 units in all. On the far left is Pittsburgh,
Pennsylvania; at the bottom is Washington, D.C., and from there toward
the upper right one sees Baltimore, Philadelphia, New York, and Boston.
This map illustrates the severity of the pixel saturation problem; all
the white areas on this image are composed of pixels which are fully
saturated and thus uninformative, The extent of our knowledge is that
the white regions were brighter than the maximum recorded value, but
this is not a very useful parameter. The presence of so many saturated
pixels prevents the analyst from integrating the light output from
cities. Since there are many potential practical applications of such
a procedure, the saturation is unfortunate. At the present time, we are
requesting a special run of a DMSP satellite in a mode which will not

Saturate at city centers, but it is a mode which does not serve the primary

function of the DMSP satellite.

C. The Y Dimension: Details of the Scan

With each digital tape, the Air Force kindly supplies a film image,
In Figure 13, we demonstrate the similarity between the images derived
by OCR character mapping (a) and those derived from film (b), Here, a
dozen units of data have been converted into a strip of image which
extends from the Dallas-Fort Worth population center all the way to the
sea, beyond Savannah, Georgia. This is an especially valuable region for
study, because a line of cities extends in a roughly ‘east-west direction
which happens to be nearly colinear with the DMSP scan.

30



Be Level Changes

The visual sensor for the Fl spacecraft, which provided all the
digital data discussed in this report, was able to operate in four modes,
each of which provided a different range of brightness. in sunlight,

a solid state detector is used (but it will not be discussed, because our
attention is restricted to imagery near midnight.) The remaining three
modes of operation make use of a photomultiplier tube (PMT). Roughly
speaking, the sensor area within this PMT can be adjusted to full size

or one-ninth size, Three modes of operation are called PMI high, PMr
low, and PMI 1/9; the latter uses one-ninth of the PMI. :

The process of controlling the gains of the amplifiers to achieve
the desired result requires the separate setting of amplifiers, choices
of modes, and the synthesis of many considerations. In contrast, the
derivation of a brightness level from a pixel value (that is, froma
sixol) is fairly simple. It begins with a reference level of radiance
and requires simply the multiplication by three numbers which are
gathered from various sources.

1. The reference level for spacecraft Fl is 0.042 watts /em?-sr.
Each spacecraft has a unique reference level, but one which is
generally within a few percent of this value; furthermore, the
radiance is determined based upon the spectrum of sunlight.
For moonlight, a slightly different value of radiance would
apply because of the different spectrum of moonlight. The
photomultiplier tube of each spacecraft in the DMSP series is
measured to determine its spectral response, and the relative
Tesponse to moonlight and to sunlight is then determined,
Generally, the difference is only a few percent. For satellite
Fl, the moonlight radiance values are 4% lower than those for
sunlight. We shall neglect this small difference,

2. Associated with each mode of operation, there is a compensating
amplifier having a gain which, once more, is unique to each
spacecraft. All of the data herein was obtained in the most
sensitive mode, PMT high, for which the compensating gain of
Fl is 86 dB; the, 55st radiance multiplier is then -86 dB, or
a factor of 10.

47

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II BACKGROUND

The Defense Meteorological Satellite Program (DMSP) is based upon
a series of earth-orbiting satellites whose primary function is the
observation of weather, both day and night, by means of sensors which
make images in the visual and infrared bands, The previous work at
SRI International, and the current work reported herein, concern only
the visual sensor and only those data obtained on that half of each
orbit when the imaged surface of the earth is at night. In this condition,
and only in this condition, the electromagnetic energy sensed by the
system is in a large measure a product of the activities of the men on
the surface of the earth. In addition, there are other natural sources
of light such as lightning, aurora, and moonlight. In contrast, the
infrared sensor primarily detects one kind of radiation--the heat
radiation indicative of the temperature of the clouds or of the surfaces,
if no clouds mask the view. In a similar simplification, the daytime
records from the visual sensor are almost exclusively owing to one source--
sunlight reflected from the clouds or the surface. It is the many-
faceted character of the sources of "nighttime visual" lights, being
attributable to a variety of both natural and man-made phenomena, which
leads to their great interest and to the probability that, when properly
recorded and processed, nighttime visual images can play an important

economic role in the remote sensing of the earth from space.

Work in this field has heretofore been limited by the fact that the
only records made available from the U.S. Air Force system to civilian
analysts have consisted of film images. While each such image is
originally recorded on magnetic tape, the records are generated at a
density of approximately 30,000 bits per inch (bpi) in a format which is
not transferable to outside computer centers at the present time. The
current state-of-the-art in computer tape recording includes 9-track,

1600 bpi tapes which cannot keep up with the data flow during the

transmission of DMSP imagery, or their playback from the 30,000 bpi recorders.

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CORRECTION OF THE IMAGE ARTIFACT: (shown as-a streak) .
OCCURRING JUST NORTH OF DALLAS (corresponding unit is |
in Figure 6). A correction algorithm, described in the text, was E
used to remove this defect and generate the map in Figure 13.

37



a

of appearance. A subsequent numerical analysis of the pixels would have
to be conducted with caution, lest the insertion of the artificial line
of averages should alter the derived pixel statistics.

E. Waste Gas Flares and Lightning

In the Gulf of Campeche (near Tabasco), Mexico has discovered
extensive oil fields, and it is flaring a great deal of waste gas there.
Figure 16 is a 1,5-nautical-mile representation of this region. As
compared to city lights, the gas flares are regions of saturation
immediately adjacent to dark regions. In the case of cities, there is
normally a gradual change from brightness to darkness as one moves away
from the city center. Individual gas flares, which have a spectral
output closely matching the response of the DMSP nighttime scanner, can
readily saturate the sensor even though, on the next adjacent scan,
there is no response. This is what leads to the appearance of isolated
saturated pixels on the imagery.

At the time of Figure 16, the corresponding infrared image showed
that a cloud covered the lower right corner of this region and swept
up to the right, where a horizontal streak is seen. This horizontal
streak is a classic signature of lightning on the DMSP images, but this
one has the distinction of being the first to be revealed to us in its

.full digital representation. Notice that the lightning affected several

successive scans, and it therefore must have lasted approximately one
second. (The scans occur once each 0.42 second.) This is probably a
fairly lengthy cloud stroke of the type which is often seen in the DMSP
data, although it is seen less by surface observers, owing to the
intervening clouds below the lightning. As opportunity permits, SRI
plans to study the physical significance of these lightning stroke
records using the sixol plots. In this example, the 2 units on the right
of Figure 16 will yield the details of the light along the streak which
is caused by this lightning event.

38



To put this number further in perspective, it may be recalled that
the city centers are saturated in the DMSP records. In such records, the
"PMT high" amplifier was used (86 dB) and the VDGA was set at 55 dB. In
this circumstance, full saturation with pixel gain unity represents about

- 2
4x10 9 watts/cm'-sr, a light level some thousand times weaker than the
weakest detectable Landsat signal,

The Landsat was never intended for use at night, and these numbers
are offered here only because. they tend to provide a quantitative explana-
tion for the emptiness of the pixels among the three nighttime Landsat
pictures which have been attempted. One, from the New York area at night,
is said to have revealed all zeros, Another, taken over Algeria, was
described in report one,® where the response to gas flares was reported.
(Some pixels were saturated; i.e., fully bright.) A third, taken in the
Los Angeles-San Diego area, again revealed all zeros. While it is diffi-
cult to prove a negative point, these. sensitivity figures nevertheless
tend to support the claim by some who have said that the Landsat is in-
capable of detecting cities at night.

53

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Owing to slight imperfections in the symbol printing locations, we
have found it advisable to compress the map symbols even farther than
the examples of Figure 9(b) and (c). The remainder of the figures in
this report have horizontal rows of symbols which overlap slightly,

A computer program which produces gray scale plots, or maps, is
presented in Figure 10, together with the first part of a unit of data,
Notice that the label, printed by statement 362, includes a space
between every contiguous letter. In this manner, the label will assume
a normal appearance when laid above the map with the symbols set close
together. The program of Figure 10 is seen to be similar to the earlier
program which makes sixol plots, except for the fact that the array is
filled with only nine symbols which are assigned to blocks of numbers.
Once this array is filled, the process of laying down the map is virtually
identical to the process of laying down the sixol array.

26



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ll.
12,

13,

4,

REFERENCES

T. A. Croft, "Nocturnal Images of the Earth from Space," Final
Report, SRI International, Number 68197 (March 1977).

. 1. A. Croft, "Nighttime Images of the Earth from Space," Scientific
‘American, Vol. 239, No. 1 (July 1978).

’T. A. Croft, "Plotting Shades of Gray with a Standard Computer

Terminal," Technical Report 3208-1, Stanford Electronics Labora-
tories, Stanford University, Stanford, California (March 1974).

Maj. Lee G. Dickenson, Defense Meteorological Satellite Program

(DMSP) User's Guide, AWS-TR-74-250 (Air Weather Service, Scott Air
Force Base, Illinois, December 1974),

C. P, Pike, Auroral User's Guide for DMSP, Air Force Systems Command,
USAF (AFCRL, Hanscom AFB, Massachusetts, April 1975).

2, A. Croft, "Burning Waste Gas in Oil Fields," Nature, Vol. 245,
No. 5425, pp. 375-376 (October 1973).

M. Brook and T, Ogawa, "The Cloud Discharge," in Physics of Lightning,
R. H. Golde, ed., Vol. 1 (Academic Press, San Francisco, California,
1977).

E, T. Pierce, "Winter Thunderstorms in Japan--A Hazard to Aviation,"
Naval Research Reviews, Vol. 29, No. 6, pp. 12-16 (June 1976),

A, Suda and S, Kasahara (private communication).

R. W. Fett, "Fishing Fleet Activities Revealed by Night-Time Data
from the Defense Meteorological Satellite Program (DMSP)," Marine
Fisheries Review, Vol. 37, No. 4, MFR Paper 1131 (April 1975).

R, Shaeffer (private communication).

QO, Wilkes (private communication).

Landsat Data User's Handbook, NASA Earth Resources Program, Document

76SDS4258 (Goddard Space Flight Center, Greenbelt, Maryland, Sep-
tember 1976).

P. F, Holkenbrink, Manual on Characteristics of Landsat Computer
Compatible Tapes Produced by the EROS Data Center Digital Image
Processing System (USGS, Sioux Falls, South Dakota, December 1978).

57



10
11
12
13
14
15

16
17
18

ILLUSTRATIONS

Digital data from the DMSP magnetic tapes after
moderate processing . . 6 se ee ee eee eevee

Conventional identification of the parts of the
header associated with each scan 2... ee eee

Conversion of the header to a partially decoded
form in the SRI system 2... ee ee ewe eevee

Partial header and first few scans of DMSP data
as preserved in one unit of the SRI system .....

A 62-element digital code, called "Sixol,"
devised to preserve the full detail of DMSP data ..

One unit of DMSP data converted to Sixol symbols,
arrayed aS a map «ee ese eee ee ee eee ee

Computer program converting the DMSP data units
to Sixol arrays... ee ee eee ewe ee enn

Representative arrays of symbols used in photometric
measurementS . 6 « ee se © © ew we ee we wo ewe

Maps of the region around Atlanta demonstrating
several aspects of the plotting... ...s.sssee

Computer program which made the maps shown in this
TFeport «6 ee wwe we ee wee we ee een nne

Comparison of the two types of DMSP data; those with
3-nautical-mile pixels and 1,.5-nautical-mile pixels

Sixteen units of DMSP data on a clear night showing
some major cities of America «ss eee eee ees

Comparison of film version and digital map of the
same DMSP image across America 4.2 eee eevee

Comparison of across-track distance by locating
cities on a digital image and map... . 2 ee eee

Northwest unit from the map of Figure 13 before -.-
correction of the image artifact .....e eevee

Gas flares of the Gulf of Campeche in Mexico ....
Gas flaring in Alberta, Canada 4... se ee eee
Squid fishing fleets off shores of New Zealand ...

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10

12

14

17

19

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23

24

27

29

31

32

34

37
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41
43



VIII CONVERTING A SIXOL TO A BRIGHTNESS LEVEL

In the operation of a typical scientific instrument, equipment

settings are varied when necessary to keep the responses in the range

where the best performance is achieved. The numbers which come back from

the instrument are seldom viewed as being completed physical measurements;
rather, they are treated as a code from which one will derive useful
numbers through the application of calibration curves, scale corrections,
and the like, This "data processing" is cumbersome perhaps and

expensive, but it is often the best way to derive maximum accuracy with
limited data,

For a practical, around-the-clock operational system, the post-
measurement data processing workload must be held to a minimum. With
this objective, the DMSP ground and space systems are constructed so
that most of the necessary scale factors are applied, and adjustments
are made, as the observations ate conducted aboard the spacecraft; as a
result, the numbers sent to earth are directly useful. Precomputed
tables and computerized algorithms are used to vary the gain of the
Spacecraft amplifiers in order to achieve this goal. A crew of operators
is constantly engaged in evaluating the data, evaluating the requirements
for new data, and determining the gain-setting commands which should be
sent to the spacecraft to control these amplifiers in the coming days or
weeks. As compared to the typical research measurements, comparatively
little processing occurs after the measurements are telemetered to the
ground. Through the control of amplifier operation and’ the selection of
scale factors, the six-bit numbers sent from Spacecraft to ground and

subsequently recorded on magnetic tape or on film represent the albedo

of the surface. No need exists for any further digital computer processing

of the light intensity data,

45



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stems from the limited range of values of brightness which are retained,
coupled with the weather-oriented requirements which alone dictate the
brightness range to be recorded. As a result, the brighter regions are
typically saturated and one is prevented from making potentially productive
observations, Large cities are not imaged at all, and they are one of the
more important regions from the standpoint of the economic value of such

data,

There is little likelihood that the design of the Air Force satellite
system will be compromised with the object of meeting this need, since it
is irrelevant to the mission of the DMSP. The most obvious cure, an in-
crease in the recorded range of radiance, would necessarily increase the
telemetry data rate, which is already a limiting operational factor, It
seems likely, therefore, that the optimization of a nighttime sensor for
remote sensing of surface lights will not occur until such time as a spe-
cial sensor is built and flown for this purpose, To this end, SRI has
been appraising many applications of the images which have come to our
attention, largely as a result of the Scientific American” article. Many
letters were received from technically sophisticated readers; for example,
an Italian scientist inquired concerning the use of DMSP midnight images
as a means for selecting an astronomical telescope site in Italy. For
that purpose, the imagery would be well suited. Most applications are
more obvious, tending to relate to monitoring of natural resources,

society, or geophysical phenomena.

The DMSP system is truly an outstanding performer in the realm of
remote sensing, In this report, attention is restricted to use of the
visual sensor at night, forming one small aspect of the DMSP performance
spectrum and one which is, as a matter of fact, considered by the Air
Force to be a minor part. Another wealth of information resides in the
infrared imagery, both in the night and the day, and the daylight visual
images contain still another story. In addition, the DMSP carries non-
imaging sensors of variety and versatile capability. We wish to close
with this note of admiration for the excellence of the system, and with
an expression of gratefulness for the help provided to us by the personnel
who work the system.

55

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24 423430.38540546 51371404439 438593543434536,28530,9443b
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33922529534 433 539434 4392599044 4344294399542 .44597599099
3? 930436143438 134 4397045429542 444 4305974905291 35 5984S
33533523530144398.35.42.32536.32.32.394,24,07,31,21.208
744951052) .301295485494 545524 520535535591.393130.35.39b
3944443140928 427427429135 5454.44 9749749759 9543495490
34129538 542.435335490.33527.398532.995394550535.3b.97.39b
37328525534540.398.27 536 137.393592.424405915494450,99.20
2924358546 52)528128 427527 438439013901251227391598539

QU .33_aN.us 233. 2u IL 3g 9b 2a au Lun a a. Aaa uu an.aa

B28 |

PARTIAL HEADER AND FIRST FEW SCANS OF DMSP DATA AS PRESERVED IN ONE UNIT OF

THE SRI SYSTEM. Unit includes all information for a region 72 pixels in the across-track direction
and 64 pixels in the along-track direction. Four data lines comprise one scan.

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CONTENTS

LIST OF ILLUSTRATIONS . 2 6 2 ew ee we ew eee

LIST OF TABLES

a

I SUMMARY 2 es we we ee ee ee we ewes

Il BACKGROUND «6 6 2 ee we ee we we we wwe

Ill RAW DATA: THE PRIMARY RECORDS ON TAPE ..

IV MAKING PHOTOMETRIC DETAIL ACCESSIBLE oe

Vv THE GENERATION OF GRAY-SCALE MAPS... .

VI IMAGES DERIVED FROM DIGITAL DATA .....

A.
B.
Cc.
D.

E.
Fe

The Two Resolution Scales ee wee
Pixel Saturation and its Consequences
The Y Dimension: Details of the Scan

An Example of the Versatility of
Digital Unit Processing .. +. see

Waste Gas Flares and Lightning ....

Flare Location by Computer... es

VIL PATTERN RECOGNITION IN THE IMAGERY ....

VIII CONVERTING A SIXOL TO A BRIGHTNESS LEVEL .

A.
B.

A Decibel Trap. ee eee eevee

Level Changes 2. 4 ee ee eee eee

The Maximum Detectable Level... .

The Minimum Detectable Level «2. . ee

Comparison to Landsat . 4. eee eae

ix CONCLUSIONS «4 we ew ew eee ee eee

ACKNOWLEDGEMENTS 2. 6 we eee eee eee eevee

REFERENCES . oes eee eee eee eee eens

ii

iii

iv

16
22

28

28
30
30

36
38
40

42

45
46
47
48
51
52

56

57



— a

ed

D. An Example of the Versatility of Digital Unit Processing

As an illustration of the ease with which the units of digital
data can be processed, we wish to demonstrate the removal of an
artifact from the DMSP data. In the film version in Figure 13, just
north of the Dallas-Fort Worth complex, a horizontal streak runs
across the image. In Figure 15, we show the unit of data from the upper

left corner of Figure 13 as it was originally mapped. The unit was in
Figure 6, where the artifact details may be seen,
line of picture elements which are defective.

It contains a single

This was a 3-nautical-mile digital tape consisting of 2 x 2
averages, and so a test was first conducted to see if only a single one
of the original two 1,5 n. mile scans had been all-zero or had incurred
some form of systematic error. From a study of the numbers which make
up the streak in Figure 15, as compared to the numbers which make up
the lines above and below, it was determined that there was no sensible

pattern within the artifact. The values for each picture element in the

streaked area, therefore, were obtained by averaging together the values
of the picture elements above and below.

This process is much like that which takes place in television

image generation, using the delay device called a “dropout compensator."
In the television systems of today, such compensation merely substitutes
the values in the preceding line if the current line appears to be
defective. (The primary cause of such a defect is a tape "drop-out"

and thereby the name is coined.) Our process, benefiting from the Luxury
of digital computer implementation, is one step better; the missing data
are refilled by interpolation between the values before and after, rather
than by simple repetition of the values before. The digital images of

Figure 13 lack this defective streak because it has been removed. by this

“form of compensation. This removal required the writing of approximately

20 lines of code into the computer program presented in Figure 10,

Despite this demonstration of the ease of correcting the appearance of
an artifact, it must be kept in mind that the information thus inserted
in the streak is artificial; this correction is useful only as a matter

‘ 36

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*sjoquiAs j0jd se Aqisuep awes syr yim pajulid ‘sjoquiAs joxIS (p) ‘sepoo jex5IP dSWO
‘sjoqwas sseudein (q) ‘peyesedas Ajseajo sjoquiAs sseuAesd (e)

yi 0} peuBisse sjanaj Aes yuasayy1G (9)

fAjsnonByu0? paced

*(9X0} 03 412324) ONILLO Id 3HL JO SLO3dSV TIVY3SARS ONILVHLSNOWSC WINVILY GNNOYV NOIDSY JHL JO SdVW

Sarat

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DOO r eee ANe omnes ee

ASHMONSEREN+ oSSEnes
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6 auNDdId

24




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3. Operating under command from the ground is a video digital
gain amplifier (VDGA) whose gain can vary between zero and
63-7/8 db, in 1/8 dB steps. The imagery presented herein was
obtained under a computer control mode known as "along-track
gain control (ATGC)", under which the VDGA gain remains
at a fixed value throughout each whole scan. The value of
gain is recorded in the header and identified as G. (This
will be found in Figures 2 and 3.) In the SRI system, the
number has been converted to a whole decimal number which must
subsequently be divided by 8, in order to derive the second
multiplier, the decibel setting of the VDGA. For ready reference,
the reference level and the first two gain values have been
combined in Table 1, giving a value of radiance associated with
each whole dB of VDGA gain, and also a value for the maximum,
63-7/8 dB. This table provides half of the necessary
information for a manual conversion of pixel code into brightness.
A different table would apply to each spacecraft, but the values
therein would differ by only a few percent from those given
here.

4, lastly, the pixel value encodes the third multiplier, telemetered |
from the spacecraft, and preserved herein as a sixol. If the
first binary bit of header parameter M is zero, then the code
is linear and the pixel value represents a gain between unity
and zero in 64 equal steps. Alternatively, if the first bit
of parameter M is unity, then an additional logarithmic
amplifier was used. As a result, the pixel value represents a .
gain between unity and 1/100, in 64 logarithmically equal steps.
As expressed in decibels, the use of the logarithmic amplifier
subdivides 40_d5,jnto ‘64 equal steps; therefore, each step
represents 10, *“ or about 7.5% in terms of radiance. This
third type of gain, herein called "pixel gain", is fully
evaluated in Table 2, which applies to all spacecraft. When
used together with Table 1, this mechanizes the conversion
from pixel value (either in octal or sixol) to radiance. It
is only necessary to multiply the radiance from Table 1 by the
gain from Table 2, (Alternatively, the radiance conversion
could easily be performed by computer or calculator.)

C. The Maximum Detectable Signal .

The maximum detectable signal (radiance) is not quite’ 0.042 watts/cm?-sr;
that value is the zero intercept of a straight line which represents the
response of the system, and as a result it is a convenient reference

number. To avoid citing this unattainable reference, many system
descriptions make use of a reference level of 0,0212, at which response

point the pixel value is roughly 6 to 9, depending on the spacecraft.



“Ule1eY Pequosep wayshs Buysn ‘syuN ASW UeZop & WO pesodwoo s| dew ‘wily
e104 sy etn si eBeun) 406187 “yY3S 3HL OL 1SV3 SvTIVad WOus VoINaWY ssOouoV
FOVWI dSWO AWYS 3HL JO d¥W WWLIDIG GNY NOISUSA Wild 4O NOSINWdWOD El 3HNDId

32


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OOKHF Lr igOmF o@MHd 511k jOUBZEMEGKFE:

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BURG JGAMPRO TEI sBequasTuTs tovHeD Spot

Eason dhs teeeey Pee ie aoe
3 PATE TRAE Satoh ae ae Oka SOT ae peat aie
Pts tae a gine, yee ipa raat PaO eM

FIGURE 17 GAS FLARING IN THE VICINITY OF EDMONTON AND CALGARY IN ALBERTA,
CANADA (a) AS SEEN IN A MAP. Modified sixo! plot (b) shows these flares

(b)

fa)

readily identified because they saturate the sensor while affecting only small regions.

Other light sources saturate only when they affect larger regions.

41

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10° «REN GRAY.DECIMAL JULY 79 CROFT

20° REN SET LABEL IN 370 }
30 DIM Berne i
yO Beioyerf :

SO FOR K=1 TO 3
bD Be{k}=".?

70 NEXT K

80 FOR K=4 TO b
30 BS{k}etsr

200 NEXT K

320 FOR K=? TO 9
220 BS{k}='+!

130 NEXT K

WHO FOR K=10 TO 12
250 Betk}='|"

360 NEXT K

270 FOR K=13 To 15
280 Be{k}='T!

150 NEXT K

200 FOR K=lb TO 18

230 FOR K=19 TO 21

eS0 NEXT K

2b0 FOR K=22 TO bL

270 BS{k}='gr

e280 NEXT K

300 Ren The B® list is filled with plot symbols
2

3b0 PRINT i
362 PRINT ‘Albertas SRI 356’ H
3b4 PRINT

370 PRINT 'MAP.4.301.15b"

it THEN BOO

2

PRINT

FOR M=L TO bu

Ager

410 FOR Nel TO 72
READ A

430) AS=AS+B6LA}
NEXT _N

520) REN DASP data follow: te

£

we

o

a]

2

ux
z Z4
4 a

>

s

2000 REN = 400002000722400077370174 4O0002000b13400077370174 949 378

BOLD DATA 494335254394532.23520122422528 58514 852552658 120528 |
2OCO DATA 34122484 .22.27 437 507 OS OSs LE LALA os b 208530 a 050149
2030 DATA 24413520529 ,29.3? .2b 046127137130 .2) 116,50 s2ablalb
2OWO DATA 4249513.8520528.22422520131523.24 129530022.6,15525

2OSD DATA 48448432432 5394375985295 58s 2b 8442922023534 420925025

2OB0 DATA 20518523426417+2841758.16125.43,9),00,50,38.3b5391.200

2O7D DATA 16 438529438513525 526.2) 53004125,34 babbaldsdu baal

2080 DATA 18523430138.24435 1265236126137 334250225 bb 8 s20012

BORD DATA 27429424 432429432139. 4 5 42035495 0255094094 54 Use Pak

RLOQ DATA 32524.52533551522 5951459361753 7 927 930.48 .80504 499590

3320 DATA 20519518523,28,29545525.34 527.2909) .8 2443204415027

BRED DATA 30.20.0327 524 529538419527 027 9027.24.27 ebb 2) ,22,21 5356 i
BL3D DATA 27424 536427423433544.385385295275354

4b1b5127427438.3b

FIGURE 10 COMPUTER PROGRAM WHICH MADE THE MAPS SHOWN IN THIS REPORT
AND A FEW SCANS OF DMSP DATA FROM ONE UNIT (four printed lines
per scan)

i

27



pixel brightness values to decimal, a value of 1 has been subtracted,
So that the decimal values range from 0 to 61. Header recognition is
then achieved in the SRI system by format control, where it is practical

in the scientific application, because a more wasteful tape format can be
afforded.

As depicted in Figure 1, each header consists of two parts separated
by a space. In the original records, the first part of this header
preceded the scan of data and the second part followed it. The header
is explained in greater detail in Figure 2, where each octal digit has
been broken down into its three component binary bits, shown directly
underneath, The meanings of the bits or the octal digits are explained
under the heavy brackets. The following comments about these parameters
are offered as a rough guide, although we have not attempted to provide a
complete explanation here. (Such explanations will presumably be

provided with digital tapes to those users who receive them. )“*9

° The first binary digit will always be 1 since only visual data are

contemplated herein.

e The frame count, although it occupies many binary bits, seldom
gets larger than a few. The author speculates that the long
string of bits, being mostly zeros, provide a primary recognition
feature for finding the header in the original digital data
strings,

e The line count simply counts the across-track scans and thus

it will increase by unity in each successive header,

@ Word two has never varied from the value shown in this figure,
among the examples thus far witnessed by the author. The
parameter Q gives a small-scale scan correction, already

incorporated in the data and not needed in this work.
e Universal time of day is specified in word 125,

e The gain of the amplifiers is G and the mode of operation is
M, These will be discussed later.



Table 1. THE RADIANCE OF A SATURATED PIXEL FOR DIFFERING VALUES OF VDGA GAIN

(This applies only to satellite Fl, since it assumes a reference
level of 0,042 vatts/em2-sr and PHT high gain of 86 dB.)

YDGA GAIN RADIANCE
as dB as inverse watts/em2-sr
(+) factor (pixe1=000000)
° 1 2,10495E-06
1 1.12202 1,87604%E~06
2 1, 25893 1.67202E-06
3 1.41258 1,09019E~06
4 1.58490 1, 32813E-06
5 1.77828 1, 183708-06
6 1.99527 1,05497E-06
1 2.23873 9. NO2U4E-07
8 2.51189 8.37993E-07
9 2.81839 7,46B62E-07
10 3.16228 6.65642E~07
"1 3.5uB 14 5.9325NE-07
12 3.98108 5.28738E-07
3 446685 4.71238E-07
14 5.01189 4.19991E-07
16 5.62343 3.703 17E-07
16 6.30960 3.33611E-07
7 7.07949 2.97331E-07
18 7.94331 2.64996E~07
19 8.91255 2.36178E-07
20 10,0000 2. 10N9UE~07
21 11.2202 1,87603E-07
22 12.5893 1.67201E~07
23 1, 1255 1,4901BE~07
24 15.8490 1, 32813E-07
25 17.7829 1, 18369E-07
26 19.9527 1,05497E~07
27 22,3874 9. 8O2K0E~08
28 25.1190 8.37990E-08
29 28.1840 7.46859E-08
30 31.6230 6.65639E-08
31 35.4816 5.93251E-08
32 39.8110 5.28735E-08
33 4.6687 4.71236E-08
34 50.1191 4.19989E-08
35 56.2346 3.743 16E-08
36 63.0962 3. 33609E-08
37 70.7951 2.97330E-08
38 79.4335 2, 64995E-08
39 89.1258 2.36177E-08
ho 100.001 2. 10493E-08
4 112,203 1,87602E-08
42 125,894 1.67201E-08
43 144,255 1, 49018E-08
uy 158.491 4,328 12E-08
45 177.830 1, 18369E-08
46 199.528 1,.05496E-08
47 223.875 9.W0236E-09
48 251.191 8, 37986E~09
49 281,842 7. W6855E-09
50 316.231 6 .65636E-09
51 354,818 5.932U8E-09 -
52 398.112 5.28733E-09
53 an6 .689 4.71233E-09
54 501.193 4. 19987E-09
55 562. 348 3.783 14E=09
56 630.965 3.33608E-09
57 707.955 2.97328E-09
58 794. 338 2. 6U994E-09
59 891.262 2.36176E-09
60 1000.01 2. 10492E-09
61 1122.03 1,87601E-09
62 1258.94 1,67200E~09
63 3412.56 1.4901 7E~09
63.875, 1562.27 1. 34737E-09

49



The 0.0212 radiance is about the brightest that one could expect in a
daytime scene, corresponding to a cloud having an albedo of 0.8, lam-
bertian reflectance, and no absorption of light in the intervening atmo-
sphere. This alternative reference point is not of practical significance
at night, but it is often mentioned in the DMSP literature, and hopefully
this brief mention will tie together the two explanations. For the
present purpose, it is simpler to specify the intercept, 0.042 for Fl,
with a brief note in passing that the gain of a system cannot be zero dB
in the modes of operation commonly employed.

D. ‘The Minimum Detectable Level

At midnight the "PMT high" gain of Fl would be 86 dB, and the VDGA
gain might reach 63-7/8 dB. A pixel value of binary 000000 (signifying
a pixel gain of unity) would thus represent a brightness level of

~86 | 63: 7/8
0042-10 2 39 20 = 1.35 x 10°? watts/cm@-sr.

The DMSP can subdivide this radiance to 1/63 in the linear mode, or 1/100
in the logarithmic mode, To determine the true minimum detectable scene
brightness available on tape, it is necessary to account for the fact
that the lowest level, binary 111111, has not been preserved. The lowest
recorded value is 111110, octal 76, In turn, this is hot a measure of
brightness, as the value indicates only that the recorded scene was that
dark, or darker. The measurable minimum, therefore, is represented by
pixel value of binary 111101 or octal 75. In the linear mode of opera-
tion, this represents 4.3 x 10721 watts/cm*-sr, In the logarithmic

mode, this represents 1.6 x 107} watts/cm?-sr, These are the minimum
detectable levels for the instrument, but such low leveis have not been

detected in the tapes available to this study; noise and natural light
sources exceed this level,

A more typical value found on the tapes is a value of G equal to
440 which, when divided by 8, yields the VDGA gain of 55 dB. In Figure 6,
a sixol symbol S$ is fairly typical. From the first bit in header

31

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chosen because our digital tapes are inherently organized by successive
groups of 64 scans, Two other coincidences supported this choice of 64;
firstly, the map display was judged to be most convenient if the units of
data were not much longer than 64. Secondly, SRI had earlier been
provided with Air Force tapes in which groups of 4 pixels had been
averaged together, (''2 x 2 averaged’) and in such tapes the records

(that is, the groups of numbers separated by gaps on the magnetic tape)
contained 128 scans having precisely the same number of pixels as does
the aforementioned register. By defining a register as equivalent to a
record in the 2 x 2 averaged tapes, several convenient numbers describing
the data became whole numbers, The significance of this decision as a
guide to future work probably does not lie in the fact that SRI chose

64 scans, but rather in the experience that an optimum choice of unit
size can simplify much of the work that follows, The choice of 64, which
worked weil in this exercise, might not be optimum if the tape formats
change. Such a change does not seem unlikely, and partly for this

reason we have made no attempt herein to provide a detailed mapping from

the original tapes into the units of data.

The first few scans from a unit of data are depicted in Figure 4
where they take the form of a Basic data set. This was formed
automatically, by the same computer program which read the original
Air Force tape. In this set, the first line is a remark which provides
a numerical identification for the unit, encoding its location both with
respect to the along-track and across-track directions on the original
film and the identity of the original film. This numbering system has
worked well, The set of data is called a "unit" meaning that it lists
the brightness of its pixels in two-digit decimal numbers ranging from
0 to 61. The designation "Unit.9.61.154" means that it isthe ninth
unit in the acrossstrack direction, that it is from register 61, and that
it is derived from the image identified as SRI number 154. The associated
film supplied by the Air Force is also identified as SRI number 154.

The second line in the unit provides the repetitive part from the
first and last headers, and is included in order to give confidence that
no data artifacts occurred in the register. On one occasion, in one tape,
three anomalous pixel values were found in one register, causing an

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13