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PHOTOGRAMMETlUC ENGINEERING
surmountable geodetic problems. The Photogram metric Triangulation Systems employed
in the tests summarized above perform with
high reliability in achieving these solutions.
These developmen ts represen t a breakthrough in the geodetic sciences that will increase in importance rapidly. Credit for
initiating these applications belongs to
AFCRC's Geodesy and Gravity Branch.
Success in test is also a result of close cooperation with Air Photographic and Chart
Service at Orlando AFB, the Air Proving
Ground Center, Eglin AFB, Air Force Missile
Test Center, Patrick AFB, and the 1371th
Photomapping Squadron, Turner AFB,
Georgia.
Satellite Photography with
Strip and Frame Cameras*
DONN L. aCKERT,
IIead, Research & Development Div.,
Photogrammetry, Inc.,
922 Burlington Dr., Silver Spring, Md.
ABSTRACT: The problems associated with satellites as camera carrying vehicles
are quite dig'erent from those of conventional aircraft. This paper discusses the
the use of strip cameras and frame cameras in satellite operations. The desirable and undesirable characteristics of each camera type are discussed and
compared. It is shown that both cameras will have a place in future satellites
depending upon the purpose of the photography.
It RTlFICIAL earth satellites have been pro-
n
posed as camera-carrying vehicles ever
since the successful orbiting of Sputnik I.
Some of the problems associated wi th such
proposals have been mentioned before (1), but
little has been said about the type of camera
that would best suit satellite operation. Assuming that a satellite carrying an operational
camera can be placed in orbit, and assuming
further tha t the ca mera can be made to poi n t
toward the earth, some question arises as to
what type of camera should be used. The advantages and disadvantages of frame cameras
and strip cameras peculiar to satellite operations are discussed here to partly answer this
question.
Both frame cameras and strip cameras
perform the same general function; i.e., they
expose film to an image presented by a lens.
They both advance film and control exposure
time, but the method of accomplishing these
and other functions are far different. 'vVhere
the frame camera exposes an entire format or
section of film at one time, the strip camera
exposes only a line across the film at anyone
instant, and a section of film is exposed only
after su fficien t ti me has elapsed to allow a
translation of the camera across or over the
area being photographed. A frame camera
may be said to "stamp" the image on the
film, and the strip camera to "paint" it on.
A frame camera without image motion compensation works best under static conditions,
but a strip camera can work only when a
relative motion exists between the object
and the camera lens. As a result of these
characteristics, frame cameras have been employed in relatively high-flying aircraft where
the angular velocity of an object about the
lens is low, while strip cameras have been
employed in low-flying aircraft where the
angular velocity of an object is high. The
compatibility of the strip camera to highspeed flights has made it valuable for military
reconnaissance photography, but it has never
enjoyed the simplicity or precision of the
frame camera which has been exploited for
mapping photography. It should be noted,
then, that the end use of the photography as
well as the restrictions of satellite operation
* Presented at the Society's 26th Annual Meeting, Hotel Shoreham, Washington, D.
23-26, 1960.
c., March
SATELLITE PHOTOGRAPHY WITH STRIP AND FRAME CAMERAS
should influence the choice of these cameras
for satellite photography when it becomes a
reali ty.
vVhile there are restrictions imposed upon
a camera by the satellite, there are ad vantages as well. The ever-present vibration
existing in conventional aircraft should be
nearly non-existent aboard a satellite. A satellite does not experience changing wind nor
downdrafts, and the forces that change its
orientation are all internal. However, because the satellite in orbit is "floating" or
really at the balance of forces, Newton's first
law is important and a very small force like
winding film can change its attitude and the
aim of the camera. Therefore, the mechanical
operation of the camera is important. That
attitude is important is seen from the fact
that a 1 minute angular change for a satellite
hundreds of miles up means shift in the principal rayon the ground on the order of
hundreds of feet; for example, a 1 minute
change for a satellite 100 miles high means a
shift of 158 feet for the principal ray or optical axis on the ground.
Another result of the high-altitudes is that
the camera must operate through almost the
entire depth of the atmosphere so that scattering, haze, and reduced contrast will be
prevalent and taking its toll on the resolution
obtainable just when high resolution is desirable for the small-scale imagery. With the
satellite in mind a look at some of the pros
and cons of each camera from the standpoint
of operating it in an artificial earth satellite
will show how they compare.
Looking at the frame camera first, it is interesting to note that the advantages tend to
be concerned with metrics and the disadvantages to be mechanical in nature. Of
course the metrical qualitites of the frame
camera have been exploited and developed
for years in the mapping field along with
stereoscopic viewing. Advantages of the frame
camera are:
1. Whole frames are exposed at the same
time. Thus, attitude of the camera is the
same for all images in the frame, and a
smear in one portion results in a smear
over the whole format. Measurements
from the center of one smeared, but
identifiable image, to the center of
another are as valid as measuremen ts
between distinct images.
2. Simple mathematical relations exist between image space and object space. As
a direct result of the central projection
inherent in frame photography, the
3.
4.
5.
6.
593
laws of perspective geometry provide a
simple means for data reduction requiring little machine time. This is no
small advantage considering the volume
of photography that could be obtained
from an orbiting camera.
Frame photographs may be employed in
radial triangulation. The individual
perspective frames lend themselves well
to radial triangulation over large areas
of poor control. e.g., parts of Africa.
Note that the direction of the radial
triangulation is not restricted. The overlap area or the sidelap of photos taken
on differen t passes over an area may be
used.
Conventional aerial three-dimensional
triangulation may be accomplished with
frame photographs. Again, the triangulation may go in whatever direction that
the overlap of several passes may be the
best. Triangulation is not readily adaptable to strip photography.
Frame photography is compatible with
existing equipment. Viewing devices,
measuring devices, plotting devices, and
library data reduction programs are
available for any of several formats.
This is not true on a large scale for strip
photography.
A mi ni mum demand is placed on a bsolute time. As long as an image is received, it is immaterial just when a
particular photograph is taken. One
exposed yesterday will match very well
any overlappi ng portions of a photo
taken three weeks ago or one taken a
year ago. Therefore, photographs used
in a triangulation scheme may be collected over several weeks or months
from the repetitive passes over the area
of interest. In this way photographs
from one pass may fill in for those obscured by clouds in a previous pass.
Disadvantages of the frame camera in satellite
operations are:
1. Cyclic film advance. The intermittent
film advance creates forces which must
be balanced or an attitude change occurs. In addition, the film advance introduces a low frequency vibration into
the vehicle which is hard on resolution
if not damped. Damping equipment
costs weight in the satellite.
2. Operation of the shutter creates vibration in addition to that of the film advance. Although the shutter leaves are
small, they too create vibrations which
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PHOTOGRAMMETRIC ENGINEERING
must be damped or isolated from the
film-lens system. Isolation and damping are difficult in a small vehicle.
3. A frame camera must maintain a flat
field. This is only a disadvantage when
comparing a strip camera which exposes
only a line at one time, because the
existing systems for maintaining a flat
field are compatible with satellite operations.
While the disadvantages of the frame camera tend to be mechanical in nature, the
mechanical operation of a continuous strip
camera is well sui ted for satelli te operations,
but the metrical aspects of the strip camera is
the major concern of the disad van tages. The
advantages of the strip camera for satellite
operations include:
1. Continuous film advance. The film in
the strip camera is advanced at more or
less a constant rate with smooth transitions from one speed to another when
they are required. The intermittent
operation of the frame camera with its
attendant shocks and vibrations is
avoided; however, some vibration and
banding can sometimes occur through
the film drive mechanism.
2. Shutterless operation. An opening and
closing shutter of the conventional type
is not used in the slit camera. Exposure
time is controlled by the width of the
sli t. Al though the wid th of the sli t is
continually changing in many strip
cameras, the shock of opening or closing
a shutter is avoided, and complex damping equipment is not required.
3. Small focal-plane. The focal-plane of the
strip camera need only be a single line
across the film. Therefore, the problem
of maintaining the film in the focal-plane
is reduced to passing the film over a
cylindrical roller whose axis is parallel
to the slit. A film flattening mechanism
as well as power to operate it is not
needed with resultant savings in weight.
4. Narrow lens field. Because the lens of a
strip camera presents an image only
across the width of the film and not
the diagonal of a rectangle or square,
the angular field required of a lens is less
for the same film width, and it is easier
to design a well corrected lens over a
smaller field.
Disadvantages of the strip camera are
primarily concerned with the metrical quality
and inherent accuracy of the photograph.
Some stem from the method of projection of
the image to the film The disadvantages of the
strip camera for satellite operations are:
1. The exposure of any fini te area is not
made at one time. The camera must
scan throughout some time interval in
order to record succeeding objects on the
ground. As a result different images on
the film may have different exterior
orientations associated with them however close they may be. Thus, sensing
and recording equipment must be available in the satellite to Cbnstantly and
continuously record the vehicular attitude for later use when measuring the
film.
2. Image-motion compensation error. The
film of the strip camera must be advanced in the camera at the same rate
the image presented by the lens advances to effectively stop the relative
motion between the two over the width
of the slit. Automatic control of the film
advance requires more auxiliary equipment and more unwanted weight in the
vehicle. In addition, image-motion compensation can effectively change the
scale in the flight direction. A small percentage error in the synchronization between the film and the image may not
result in apparent smear, but such an
error will be large for measurements
made on the film in the direction of flight.
3. Double perspective. Strip photography
exhibits the perspective resulting from
a central projection perpendicular to
the flight direction and an orthogonal
view parallel to the flight direction.
Figure 1 compares the perspective of a
frame and strip photography. Relief displacemen t is radial from the nadir of the
frame photograph, but it is normal to
the flight line of the strip section. Figure
2 shows that relief is still radial about
the nadir of the tilted frame photograph,
but the relief displacement of the tilted
strip photograph radiates about a point
only when the object is on a line perpendicular to the flight line. This dual
projection serves to complicate the data
reduction required of film measurements
and increases machine time for what will
become a tremendous amount of photography when satellites start to carry
cameras.
4. Numerous potential error sources. The
general complexity of the strip camera
and its geometry gives rise to many
SATELLITE PHOTOGRAPHY WITH STRIP AND FRAME CAMERAS
~
fiJ
~
~
+.,
A. VERTICAL FRAME PHOTOGRAPH
i
•~
8. SECTION OF A VERTICAL STRIP PHOTOGRAPH
FIG.
1. Comparison of vertical perspective between frame and strip cameras.
FIG.
2. Comparison of oblique perspective between frame and strip cameras.
A.. OBLIQUE FRAME PHOTOGRAPH
possible sources of error when measurements on the film are reduced to the
ground. An error in attitude, altitude,
or focal-length would affect the results
of any camera, but in addition to these
the strip camera results are affected by
errors in timing, image-motion compensation, and veloci ty measurements.
5. Image quality closely related to camera
attitude. It is obvious that a 90° swing
or crab in a strip camera is deleterious,
but also ruinous for metrical purposes
are relatively minor swings of a few
degrees.
This listing shows that the advantages of
one camera are not directly opposed to those
of the other.
With a small angular field and imagemotion compensation, the strip camera has
given good photography in high-speed aircraft
applications, and the camera creates little
vibration. The strip camera would appear
well-suited for satellite use, since a satellite
is no slow-moving vehicle. On the other hand,
the strip camera requires four object space
dimensions for metrical purposes, three space
coordinates and a fourth to measure the film
length. In addition, the error sources and the
double projection tend to restrict the strip
camera to general photography for nonmapping purposes.
595
!:t. SECTION OF A OBLIQUE STRIP PHOTOGRAPH
Then too, the frame camera is not a smooth
running device, and this would be poor in a
vehicle which almost demands smooth operation. It would require image-motion compensation as well. However, there are many
things that can be done to materially reduce
the shock and vibration of the frame camera;
some have already been used in aircraft.
The frame camera has several advantages
in its favor as a satellite camera. Actually
these advantages as listed are no different
today then they were years ago when the
simplicity and precision of a single-point
projection was recognized and then exploited.
The most important advantage of the frame
camera in satellite operations may well be its
relative independence on object-space measuremen ts of the vehicle, especially ti me. The
overlap of photographs selected from a large
number of passes over an area can be used in
radial and aerial triangulation although the
photos may have been taken months apart
from different satellites whose orbits may not
be well known at the time of the photography.
In this way a collection of occasional good
photos exposed through the haze and clouds
can be built up, and may well pay for the
multitude of poor ones. Frame photography
used in this way is far less dependent upon
orbital information than is the strip photography, and any triangulation performed need
596
PHOTOGRAMMETRIC ENGINEERING
not follow a flight line but it is free to follow
the overlap.
It is the author's opinion that the frame
camera should be employed to obtain satellite
photography when satellites become cameracarrying \rehicles, although the strip camera
may be justified for special applications.
Photography from the frame camera is useful
for many purposes in addition to general
vIews.
BIBLIOGRAPHY
1. Rosenberg, Paul, "Earth Satellite Photogrammetry," PHOTOGRAMMETRIC ENGINEERING, Vol.
XXIV, No.3. 1958.
2. Katz, Amrom, "Height Measurements with
3.
4.
5.
6.
the Stereoscopic Continuous Strip Camera,"
PIWTOGRAMMETRIC ENGINEERING, Vol. XVI I I,
No.1. 1952.
Macdonald, Duncan E., "image Motion in Air
Photography," PHOTOGRAMMETRIC ENGINEERING, Vol. XVlll, No.5, 1952.
MANUAL OF PHOTOGRAMMETRY. American Society ot Photogrammetry, Washington, D.C.,
1952.
McNeil, Gomer T., "Photogrammetric Analysis
of image Motion Compensation," PHOTOGRAMMETRlC ENGINEEIUNG, Vol. XVII, No.4,
1951.
Mussetter, William, An Analysis of the Continuous Strip Camera as Applied to the Measurement
Height and Depth. The Ohio State University
Mapping and Charting Research Laboratory,
Technical paper No. 135. Columbus. 1951.
A New Approach to
Analytical Triangulation*
J.
ALFRED STRINGHAM,
I nte!ligence Laboratory,
Rome A ir Development Center, Griifiss A ir Force Base, N. Y.
ABSTRACT: Presented in this paper is a new concept under development by the
A ir Force for utilizing auxiliary data such as stellar photography and a least
squares network analog, for analytical triangulation. The approach allows
the simultaneous adjustment of blocks containing several hundred photographs.
Finally comments are included on the A ir Force's present approach to future
automation of this concept.
T THE
outset it should be emphasized
that this paper presents only an apA
proach to photogrammetric triangulation and
not a solution. In pursuing approaches one
often discovers, through the wisdom of hindsight, other paths that would have proved
more fruitful.
A justification for embarking on proving
or disproving a concept was well stated by
Alfred North Whitehead. "A scientist does
not discover in order to know, he knows in
order to discover." Learning whether an approach constitutes a discovery fully justifies
scientific endeavor. However, our objectives
also have more concrete foundations. Briefly,
world trends establish requirements for
drastic cuts in reaction time. The concept
outlined herein seeks to fully exploit blocks
of up to a thousand photographs in hours,
not days. No existing solution possesses this
capability.
The first requirement is a penetrating look
at the problem. Basically the physical environment, described by a strip or block of
overlapping aerial photographs, is represented by a mathematical model. Subsequently this model is solved, or approximately solved, to provide numerical quantities for the desired unknowns. It is important to realize that these computations
pertain to the mathematical model, and not
the physical situation. It is therefore obvious
that accuracy is inherently limited by the
degree of exactness with which the mathematical model simulates the physical situation.
Conventionally triangulation models make
certain initial assumptions, such as that light
* Presented at the Society's 26th Annual Meeting, Hotel Shoreham, Washington, D. c., March
23-26, 1960.