Article
Airborne Digital Frame Cameras
The Technology is Really Improving!
As recent visits to the respective factories in Heerbrugg and Oberkochen have shown, at the present
time, the pre-eminent airborne imagers used for the acquisition of image data for mapping and GIS purposes are the metric photographic film frame cameras from Leica (in the shape of its RC30) and Z/I
Imaging (with its RMK-TOP). Both of these film cameras are still enjoying substantial sales and the
offering high-quality film processing and scanning services - which means that the air survey companies do
not have to invest in these facilities themselves.
Examples of these bureaux include HAS Images and
Precision Photo & Imaging, both located in Dayton,
Ohio, home of the Wright-Patterson Air Force Base with
its Aeronautical Systems Center and its famed reconnaissance laboratories.
newly-built examples join the many hundreds of their predecessors that are in everyday use world-wide.
They will continue to be of great service to the mapping community for a long time to come. However
digital frame cameras are now beginning to be used more widely, especially in the United States. Format
sizes and ground coverage from a given flying height and for a stated ground resolution are still quite
small in comparison with those attainable with the metric film cameras. Nevertheless, slowly but surely,
digital frame camera technology is improving and, especially with the smaller formats, there now exists
a significant market for airborne digital frame cameras supported by an enthusiastic band of users. The
purpose of this article is to provide an overview of some of the current developments in airborne digital
frame camera technology as they apply to mapping and GIS operations. Not every digital camera that
has been mounted and operated from airborne platforms will be covered in this account. As other recent
visits to the United States have revealed, new digital cameras and applications are continually appearing.
So only a representative sample of these cameras will be covered in this article. Airborne pushbroom
line scanners such as the DLR HRSC-A series, the Leica Geosystems ADS40 and the Japanese Starlabo
Three Line Scanner (TLS) will not be considered in this account - since they are line scanners, not frame
cameras, with quite different geometric and imaging characteristics.
by Prof. Gordon Petrie
Film v. Digital - Pros and Cons
The advantages of digital frame cameras over film
frame cameras are fairly obvious and quite well known
- chiefly that there is no film! Thus there is no need for
the organisation operating the camera to buy rolls of
expensive stable (metric-quality) wide-format photographic film and to have it chemically processed after
its exposure in the camera. The film processing itself is
a demanding and quite costly procedure since it
requires an expensive processing machine; the use of
lots of different chemicals having a short shelf life; a
(a)
dedicated darkroom-cum-laboratory; and highly skilled
staff. Besides which, the finally developed film needs to
be scanned in an expensive high-quality film scanner if
it is to be used in a digital image processing system or
a digital photogrammetric workstation (DPW) - though
not if it is to be used in an analytical plotter or analogue stereo-plotter, which are still in use in surprisingly large numbers in many mapping organisations.
However, in this particular matter of film processing
and scanning, it must also be noted that many air survey companies in the U.S.A. now use specialist bureaux
(b)
Fig. 1 - The large-format (23 x 23cm) metric film frame cameras that form the present standard for airborne imaging for
mapping purposes against which the performance of the new airborne digital frame cameras is being measured - (a) the
Leica Geosystems RC30; and (b) the Z/I Imaging RMK-TOP. (Sources: Leica Geosystems & Z/I Imaging)
18
October/November 2003
Besides the economies in facilities and staff that result
from using digital cameras, the radiometric quality of
the 11- or 12-bit images with their greater dynamic
range is (or should be) superior to those produced by
film-based cameras - though this loudly trumpeted
advantage is not always too apparent to the user.
Going further with this discussion, the storage, archiving and proper maintenance of numerous rolls of largeformat film is another matter of concern and expense though the reliable handling, storage and back-up of
the huge amounts of digital image data generated by
airborne imaging and mapping is probably a matter of
equal concern. On the other hand, metric film cameras
do have some decided advantages, notably their large
formats, which allow much greater areal coverage of
the ground in a single exposure from a given flying
height and for a given ground resolution. Besides
which, at this early stage of development, those few
airborne digital cameras that come anywhere near the
performance of metric film cameras cost at least twice
as much as their already expensive film-based equivalents. Hopefully this situation will improve as the digital
technology develops and matures. However, if cameras
with small or medium formats are indeed suitable for a
specific airborne image data acquisition and mapping
task, then already digital cameras offer much both to
the organisation operating the camera and to the user
of the resulting digital image data. In fact, it would be
true to say that, for small formats, digital frame cameras are now approaching a dominant market position though, of course, large numbers of existing small- and
medium-format film cameras still remain in service with
air forces and other organisations.
The production of colour and false-colour images is a
special problem with digital frame imagers. With film
frame cameras, to change from a monochrome (blackand-white) photographic film to a colour or false-colour
film is a simple matter - just insert the appropriate roll
of film into the empty film magazine or cassette and the
camera is ready for action. But a comparable procedure
is not possible with digital cameras - since the CCD
areal arrays that are used in digital cameras are inherently monochromatic. Various techniques have been
devised to overcome this particular characteristic. One
of the most popular is the use of an interpolation procedure in conjunction with a mosaic of tiny (pixel-sized)
colour filters placed over the array of detectors.
However this affects the quality of the resulting image.
Another commonly employed solution is to use multiple
digital cameras - with each camera recording a specific
spectral band to make up the final composite colour or
false-colour image which is produced by image processing. Needless to say, having to use multiple cameras
and filters is costly and may be inconvenient. Usually it
results in the format size being reduced with consequent reductions in the ground resolution or ground
coverage of the resulting aerial images. There does exist
Article
(a)
(b)
Fig. 2 - (a) Mosaic Filters. In order to capture colour or false-colour images, a mosaic of tiny (pixel-sized) coloured filters is
placed over a single layer of CCD detectors. Each detector (pixel) can only capture radiation in a single spectral band either red, green or blue. The values for the other colours for each specific pixel position are interpolated from the values of
the surrounding pixels.
(b) The Foveon X3 technology comprises three separate layers of CMOS detectors, with each layer capturing a complete
image in a different spectral band or colour. (Source: Foveon)
a new digital imaging technology using a three-layer
CMOS areal array corresponding to the three-layer
colour or false-colour photographic emulsion. This is the
X3 areal array developed by the Foveon company from
Santa Clara, California with a small format size of 2.3k x
1.5k pixels = 3.5 Megapixels. So far, this technology has
not been adopted for airborne imaging. However it is
still early days for such an application, since the product was only introduced to the market in 2002.
Airborne Digital Frame Cameras
Since the size of the areal array of detectors is, at present, the single most important factor that controls the
suitability, availability and usage of digital frame cameras in the aerial mapping field, they will be considered
here under three main headings:(I) small-format cameras, typically generating images
with formats of 1,000 x 1,000 to
2,000 x 3,000 pixels, i.e. between 1 and 6
Megapixels;
(II) medium-format cameras with image formats typically around 4,000 x 4,000 pixels =
16 Megapixels; and
(III) large-format cameras having a format of 6,000 x
6,000 pixels = 36 Megapixels or larger.
I. Small-Format Digital Cameras
In particular, the developments in this field have resulted in a rich variety of solutions that generate multiband images. Four different types of system can be distinguished:
(i) single cameras equipped with a mosaic filter and
producing their colour
or false-colour images by interpolation;
(ii) single cameras equipped with rotating filter wheels
to produce multi-band images;
(iii) single cameras fitted with three CCDs and a beam
splitter and suitable filters to produce colour or
false-colour images; and
(iv) multiple cameras coupled together and equipped
with the appropriate colour filters to produce multiband images from which colour or false-colour
images can be produced.
Latest News? Visit www.geoinformatics.com
I.1 Single Cameras with Mosaic Filters
veys and mapping of vegetation and with environmental monitoring. Many of these DCS-420 and -460 cameras are still in productive use for airborne imaging
today.
By contrast, the later DCS-560, -660 and -760 series of
cameras all utilized 3k x 2k pixel = 6 Megapixel CCD
areal arrays based on an indium-tin-oxide (ITO) alloy
that did not have a sensitivity in the near infra-red part
of the spectrum. Thus they could not be used to generate false-colour images, though, of course, they could
still generate true- colour images. The last in the series
- the DCS-760 - has just been discontinued and is
being replaced by Kodak's new DCS Pro 14n digital
frame camera. Again this is built around a 35mm SLR
camera body manufactured for Kodak by Nikon.
However the DCS Pro 14n features a much larger CMOS
(instead of CCD) areal array sensor providing images
with a format size of 3,024 x 4,536 pixels = 13.89
Megapixels. Each detector is 8µm square in size. It will
be most interesting to see if this camera will come into
use for airborne imaging in the same way as the previous DCS-460, -560, -660 and -760 series.
(a) European Examples
In the case of the GeoTechnologies consultancy from
Bath Spa University College in the U.K., it devised its
ADPS (Aerial Digital Photographic System) based on
the use of the older Kodak DCS-420 and -460 cameras.
To the camera was added a specially designed mount
with anti-vibration pads, an electronics control unit and
a small inexpensive GPS set that could be used for inflight navigation and control purposes. A small Psion
palmtop computer was also used to carry out flight
track planning and mission planning. These ADPS systems have been used very successfully from light aircraft for local use where rapid response is a key issue e.g. during flooding or forest fires - or in situations
where frequent flights are being undertaken over limited areas - e,g, for coastal zone monitoring. Thus the
ADPS systems have been sold mainly to research
groups in universities (Cranfield, King's College London,
Ulster, etc.) and environmental research agencies. In
1998, Professor Maas, then at the Technical University
of Delft (and now professor in Dresden), also reported
on his pilot studies in the Netherlands based on the
use of the DCS-200 and -460 cameras mounted on a
helicopter in a similar manner.
Kodak's DCS Series
Although several other amateur or professional digital
cameras have been used to acquire images from the air
for mapping or monitoring applications, the main smallformat digital frame cameras that have been used for
the purpose have been Kodak's DCS series. The DCS200 from 1992 was an early example. More popular
have been the later DCS-420 and DCS-460 models that
were first introduced in 1994 and 1996 respectively.
Both of these cameras have been out of production for
some years and have been replaced - though not with
quite the same functionality - by the later DCS-560, 660 and -760 models, the last dating from 2001. The
DCS-200 and -420 models both incorporated Kodak's
smaller M5 silicon-based CCD areal array producing
images with a format size of 1,012 x 1,524 pixels = 1.5
Megapixels, with each detector (pixel) on the array
(b) Canadian Examples
being 9µm square in size. The later DCS-460 cameras
Yet another university developed system is that built at
were fitted with the larger M6 silicon-based CCD areal
the University of Calgary in Canada. This comprises two
array producing images with a format size of 2,036 x
of Kodak's DCS-420 cameras, the one mounted in the
3,060 pixels, amounting to 6 Megapixels in total, again
nadir pointing position, the other mounted in an
with 9µm square detectors in the array. In each case,
oblique position 30° from the vertical. These have been
the CCD array was housed in the body of a Nikon
interfaced to a Honeywell strapdown IMU and an
35mm single lens reflex (SLR) camera body.
Ashtech GPS receiver, which provide accurate in-flight
Furthermore, the arrays incorporated Bayer mosaic filters with a separate filter placed over
each individual detector in the array.
With a Bayer pattern filter, the tiny
green, red or blue filters are intermingled in a systematic pattern with
50% of the resulting pixels being
green, 25% blue and 25% red. For
the DCS-460, these produce, via
interpolation, colour or false-colour
images amounting to 3 x 6
Megapixels = 18 Megapixels over the
three bands - either blue, green and
red (for true-colour) or green, red
and near infra-red (for false-colour).
Fig. 3 - The ADPS (Aerial Digital Photographic System) based on the Kodak DCSThis latter capability was and is
460 CIR digital frame camera which is produced by the GeoTechnologies consulespecially attractive to those field
tancy. Besides the camera, the system features an anti-vibration mount and an
scientists such as foresters, botanists intervalometer and power pack, together with a Trimble GPS unit. (Source: Bath
and ecologists concerned with surSpa Univ. College)
October/November 2003
19
Article
(a)
(b)
Fig. 4 - (a) The Tetracam ADC digital frame camera developed specifically for the capture of small-format false-colour frame
images from light aircraft, flown at low altitudes for agricultural applications.
(b) The ADC camera uses a CMOS areal array in conjunction with a Bayer mosaic filter and features a colour LCD
viewfinder and display on the back of the camera. (Source: Tetracam)
positional and attitude data that can be used for image
rectification purposes. A further development, also
undertaken in Calgary by the Alberta Research Council
is DORIS (Differential Ortho-Rectification System). It
comprises a Kodak DCS-520 camera, an Applanix
POS/AV IMU/DGPS unit and an on-board NovaTel DGPS
receiver. However, to this system has been added an
Optech airborne laser scanner (lidar). The IMU/DGPS
combination gives the data needed for direct geo-referencing, while the lidar generates the DEM on which the
final ortho-rectification of the frame images is based.
Still in Calgary, a commercial company, VeriMap, operates a Kodak DCS-420 (or alternatively, a Mitsubishi
thermal IR camera) in conjunction with a Litton IMU &
DGPS combination and a lidar system. This has been
flown extensively in the U.S.A., as well as Canada.
(c) American Examples
In a parallel but quite independent development at the
University of Florida, a system has also been built comprising two cameras for use in the Florida Gap Project.
The first of these is a video camera linked to a video
recorder mounted in the aircraft. The second is a Kodak
DCS-420 camera linked to an on-board computer. An
Accupoint flight navigation and camera control unit is
installed that accepts inputs from a Garmin DGPS
receiver and a Watson digital gyro system that measures the pitch, roll and heading of the aircraft. The
control unit is used to help direct the flight path of the
aircraft and to trigger the cameras at the appropriate
positions. The resulting imagery is being used to collect
land cover samples and transects to assist in the overall classification of Florida's land cover.
Turning next to commercial developments in this field
in the U.S.A., Positive Systems of Whitefish, Montana
also developed various ADAR systems that were similar
to the ADPS system of GeoTechnologies. They were
also based on the DCS-420 and -460 digital frame cameras. These have been sold to various universities and
research organisations in the U.S.A. and in Central and
South America. Furthermore a large American company,
Landcare Aviation, based in Oriskany, New York, has
operated no fewer than 15 small Cessna single-engined
and Piper Aztec twin-engined aircraft, each equipped
with a Kodak DCS-460 or -560 camera linked to a dualchannel GPS set; a precision Litton 200 fibre-optic
based IMU; and an on-board GPS-based flight management system. These systems have been operated by
Landcare mainly on behalf of the Emerge company, formerly part of the large Litton electronics group and
now part of the huge ConAgra food company. Emerge
has been concerned heavily with land use management
and with agricultural applications of the resulting colour
and false-colour imagery.
Image Processing & Rectification
Since the ground coverage of each DCS frame image is
tiny, very large numbers of images need to be acquired
20
October/November 2003
to cover any substantial area of terrain. In response to
this particular characteristic of these very small-format
images, image processing packages such as DIME
(Digital Images Made Easy!!) from Positive Systems and
COBRA from Inpho's Finnish associate company have
been developed that can handle the huge numbers of
small-format images in a highly automated manner to
generate a single large and homogeneous orthoimage
mosaic in digital form for the end-users. Of course, this
operation also requires the availability of a DEM, which
is normally supplied from some other existing source if
a full ortho-rectification needs to be implemented.
Tetracam
Tetracam is an independent manufacturer of small
multi-spectral cameras that is based in Chatsworth,
California. Previously it produced its MCA camera system which employed four separate imaging channels
with separate lenses and filters for each channel to
produce multi-band images from which colour or falsecolour images could be generated. This system is now
being re-designed. However, currently the company's
main product is its ADC multi-spectral camera designed
specifically for agricultural use. The ADC is a single-lens
camera featuring a Motorola CMOS areal array that generates a 1,280 x 1,020 pixel = 1.3 Megapixel image. The
array is equipped with a Bayer mosaic filter. The
resulting colour image can be seen directly in the aircraft on a tiny screen fitted to the back of the camera
and is stored on a flash card. The camera is also
equipped with a USB port to allow it to be connected
to a laptop computer and a serial port that gives connection to a GPS. The body of the tiny battery-operated
camera measures 5.5 x 3 x 2 inches (14 x 7.5 x 5cm)
without the lens and has proven to be very popular for
agricultural applications. Thus it has sold widely to
individual farmers and crop consultants as well as to
large organisations such as Ag Canada and the U.S.
Dept. of Agriculture and to agriculture and agronomy
departments in several American universities.
These are based on Kodak's MegaPlus monochrome
(black-and-white) digital camera range, which includes
numerous different models, each with a different size of
CCD areal array. They include the MegaPlus 1.0 (providing 1 Megapixel images); 1.4i (with 1.4 Megapixels); 1.6i
(1.6 Megapixels); 4.2i (4.2 Megapixels); 6.3i (6.3
Megapixels); and 16.8i (16.8 Megapixels). In the case of
SenSyTech's AA456 Airborne Digital Camera (ADC), the
Megaplus 4.2i with 2,020 x 2,041 pixels = 4.2
Megapixels forms the basis of the system to which
SenSyTech has added a camera mount; a system control unit (including a built-in GPS set); and an operator
control and display unit for its use in airborne imaging.
The SenSyTech ADC system generates monochrome
images only.
This was then followed by the AA497 Airborne Multispectral Digital Camera (AMDC) which was developed
by SenSyTech in the late 1990s in cooperation with
ERIM International. This featured the same basic units
as the AA456, but to it were added coloured optical filters mounted on a fast rotating filter wheel that was
placed in front of the camera lens, together with an
IMU to provide attitude data. With the ability of the
MegaPlus 4.2i camera to acquire digital images at the
rate of two per second, this arrangement allows the
AA497 system to acquire between two to five individual images of the ground in a rapid burst with between
80 to 98% overlap between them. The actual overlap is
dependent on the speed of the airborne platform and
the number of individual spectral images being captured in the burst. Nominally the images are supposed
to cover the same piece of ground, but obviously each
individual spectral band image is taken from a slightly
different position in the air as the airborne platform
flies forward. The consequence of this sequential
method of image capture is the requirement for special
software to carry out the rectification and registration
of the individual spectral images needed to produce
the final multi-spectral image. This software was developed by ERIM. The resulting images are then available
to the user either as separate band images or as
colour or false-colour composite images.
I.3 Single Cameras with a
Beam Splitter & 3 CCD Arrays
Redlake Multi-Spectral (MS) Cameras
A quite different approach to the acquisition of digital
colour and false-colour images from airborne platforms
using a single camera has been taken by the Redlake
company located in San Diego, California. This is based
on the technology developed by the DuncanTech company that was taken over by Redlake in November
2002, Redlake having previously acquired Kodak's
I.2 Single Cameras
with Rotating Filter
Wheels
SenSyTech ADC & AMDC
Cameras
The SenSyTech company is
based in Ann Arbor,
Michigan and was formerly
known as Daedalus
Enterprises. It was well
known for its early multispectral and infra-red linescan imagers that date from
the early 1980s onwards.
However its current product
line also includes two digital frame camera systems.
Fig. 5 - The SenSyTech AMDC (Airborne Multispectral Digital Camera) comprising the
digital frame camera with its filter wheel and IMU; the control unit with an integral GPS
receiver; and a display and keyboard housed in a portable case. (Source: SenSyTech)
Article
offered by Airborne Data Systems of
Wabasso, Minnesota. This is based on the
MS3100 or MS4100 camera combined with
an electronics control unit, a GPS set and
flight control and navigation software.
Similarly the Australian company, Integrated
Spectronics from Baulkham Hills, New South
Fig. 6 - (a) The Redlake (formerly DuncanTech) MS4100 digital frame cam- Wales - which is well known for its HyMap
era fitted with a single lens and three separate CCD areal arrays to
hyperspectral scanner - has produced its
record images of the same piece of ground in three separate spectral
ISAACS (Integrated Spectronics Airborne
bands simultaneously to generate either a true-colour or false-colour
Acquisition Camera System) which is also
image. (Source: Redlake)
based on Redlake's MS3100 and MS4100
(b) A diagram of the beam splitting and colour separating prism used in
models. To these, the company has added a
the MS4100 camera allowing the simultaneous capture of the three spectral bands required for the production of true-colour or false-colour frame ruggedized mount, a control unit and conimages at full resolution and without interpolation. (Drawn by Mike
nections to an Applanix POS/AV unit and
Shand)
the Fugro Omnistar DGPS system, as well as
providing its own image data acquisition
and processing software.
EAMD division with its range of MegaPlus cameras a
year or so before. Unlike the various digital cameras
that have been discussed up till now - all of which use
a single CCD array to collect their images - the
I.4 Multiple Camera Systems
DuncanTech/Redlake cameras use three quite separate
CCD areal arrays to collect simultaneously the three
A quite thriving area, especially in the United States,
individual band images that make up a false-colour
has been the development of multiple small-format
(CIR) image. This is achieved through the use of a sinframe cameras that are coupled together with their
gle lens in conjunction with a beam splitting and
optical axes set parallel to cover the same piece of
colour separating prism that is located immediately
ground simultaneously to form a multi-spectral imaging
behind the lens. The three CCD areal array sensors capsystem. The basis for all of these systems has been to
ture the entire frame image simultaneously in different
use relatively low-cost small-format monochrome CCD
spectral bands - which is a necessity when the images
cameras and fit them with appropriate filters to generare being captured from a moving airborne platform.
ate the multi-band images from which the final trueThese cameras overcome the limitations of the two precolour or false-colour frame images can be generated.
vious techniques where the true-colour or false-colour
Currently a considerable number of these systems are
images were being generated either by using a Bayer
operational.
or other mosaic filter screen and subsequent interpolation or through the use of sequential images via a
(a) STI Services
rotating filter wheel. Furthermore, the use of this
A centre for the development of these multiple camera
arrangement means that three full planes of colour sepsystems has been Terra Systems which is based in
arated image data are being collected and integrated
Hawaii and was taken over by STI Services in 2001.
to form a single true-colour or false-colour image
The company's original system was called TerraSim-1
instead of using the interpolation process where the
(TS-1) and comprised four small-format (1,024 x 1,024
final true-colour or false-colour images are made up
pixel) CCD cameras equipped with synchronized eleconly partially of actually measured values, while the
tronic shutters. Each camera was also fitted with a difremaining values have been interpolated.
ferent colour filter to allow their images to be collected
in different spectral bands - usually blue, green, red
Several different models of this DuncanTech camera
and near infra-red. The TS-1 camera system is still
design featuring three CCD areal arrays and a single
being offered by STI Services. Its second generation
lens used in conjunction with a beam splitter are availsystem, called TerraSim-3 (TS-3), again comprises four
able, each model being equipped with a different size
cameras, each equipped with a larger 2,048 x 3,072
of CCD areal array. These include the MS2100 (with 494
pixel CCD areal array and an appropriate colour filter.
x 656 pixels); MS2150 (582 x 780 pixels); MS3100
These individual camera units are mounted together in
(1,040 x 1,392 pixels); MS4000 (1,200 x 1,600 pixels);
a single box, together with the associated electronics
and MS4100 (1,080 x 1,920 pixels). Although the camand an external GPS antenna that receives differential
eras have been sold directly to some airborne users, a
GPS signals supplied via a satellite. Optional are an
special integrated system called Agri-View is also
inertial measurement unit (IMU) for attitude measure(a)
(a)
(b)
(b)
ments and the Zeiss TA-5 gyro-controlled mount to stabilize the multiple camera unit during flight - though
the addition of these systems adds considerably to the
cost of a system. Flight planning software
(TerraMission); in-flight control and monitoring software
(TerraFlight); and post-flight processing software for
band-to-band registration and radiometric corrections
to the recorded images are also provided as part of the
overall system. STI Services also offer the less expensive DMSV multi-spectral camera system. This has a
very similar configuration, but utilizes four very lownoise video cameras with each producing a much
smaller format (578 x 740 pixels) than the CCD digital
cameras used in the TS-1 and TS-3. It appears from the
Web site that the DMSV system has been developed in
collaboration with SpecTerra Systems Pty. Ltd., based in
Perth, Western Australia. Among the users is the
AeroMap mapping company based in Alaska.
(b) Space Imaging
Terra Systems has also been the supplier of the DAIS-1
(Digital Airborne Imaging System-1) operated by Space
Imaging for certain types of mapping and land cover
classification projects. Although this is a custom-built
system, it is, in many ways, very similar to the TS-1
system. It comprises four Dalsa CA-D7-1024T digital
frame cameras, each equipped with a 1,024 x 1,024
array of CCD detectors, with each detector 12 x 12µm in
size. As with the other TS systems, the spectral filters
allow the acquisition of individual band images in the
blue, green, red and near infra-red parts of the spectrum. The DAIS-1 is mounted on a Zeiss TA-S gyro-stabilized mount and has an on-board differential GPS
receiver and an Applanix POS/AV-510 unit to provide
measured values of the external orientation parameters
(position and attitude) for each frame. As with many of
these multiple small-format camera systems, the
images are then ortho-rectified using the position and
attitude values from the DGPS and IMU instruments in
conjunction with a standard USGS-NED DEM obtained
from an internal data bank. After which, tonal balancing
is carried out to remove the variations in tone between
the different frame images within a block due to illumination geometry and viewing geometry. This is followed
by the mosaicing of the individual rectified and tonallybalanced image frames.
(c) Airborne Data Systems
This company (already mentioned above in the section
on Redlake's cameras) is based in Wabasso, Minnesota
and has been designing and building its own
SpectraView digital multi-spectral systems since 1992.
Again these systems are based on the use of multiple
small-format cameras coupled together and operated
as a single unit. A wide range of options is offered
(c)
Fig. 7 - (a) The principle of multi-band images being captured simultaneously over the same piece of ground by multiple small-format digital frame cameras. (Drawn by Mike Shand)
(b) The TerraSim-1 (TS-1) multi-band imaging system from STI Services comprising four small-format digital frame cameras, each equipped with different colour filters to record images of
the same piece of ground in different spectral bands simultaneously.
(c) The DMSV unit comprising four low-noise video frame cameras also equipped with different colour filters to record multi-band images simultaneously. (Source: STI Services)
21
Latest News? Visit www.geoinformatics.com
October/November 2003
Article
The company's AirCam
multi-spectral system
comprises four individual cameras coupled
together with a navigation and control unit
and an electronic display - along much the
same lines as the
other multiple camera
systems being discussed in this section.
The four cameras used
in the AirCam system
have either 1k x 1k or
2k x 2k CCD areal
arrays. These are then
Fig. 8 - Space Imaging's DAIS-1 multiple digital frame camera system showing the individual
fitted with the usual
components of the overall system. (Source: Space Imaging)
blue, green, red and
near infra-red filters. However some examples have
only three of the cameras equipped with blue, green
and red filters, while the fourth camera is a colour
camera equipped with a Bayer mosaic filter. While a
GPS is supplied as standard, a ring-laser based IMU
has only been supplied to a few customers - since
it is of course an expensive addition to the overall
system.
(e) ImageTecK
This company, based in Chatsworth, California, has supplied five multiple camera systems to the Boeing
backed Resource21 company that specializes in the
supply of remotely sensed imagery to the agricultural
industry. Each system consists of four cameras coupled
together with each individual camera being equipped
with a different narrow-band filter - like the other camera systems in this group. The main application has
been to supply farmers with colour images for the analysis of growth patterns and disease.
Fig. 9 - This particular example of the SpectraView multispectral frame camera system from Airborne Data
Systems is equipped with four cameras - though up to
seven cameras can be supplied with the system. (Source:
Airborne Data Systems)
using cameras equipped with CCD areal arrays having
either 1,024 x 1,024 pixels (Model SV4P); 2,048 x 2,048
pixels (SV4S2); or 4,096 x 4,096 pixels (SV4S4).
Airborne Data Systems also informs potential customers that a total of between four and seven cameras
can be fitted into their standard box, though the fourcamera version is that most commonly supplied, providing separate blue, green, red and near infra-red
band images. All of the SpectraView systems incorporate a DGPS set and an IMU. The IMU can either be a
more precise unit (type -L) combining a ring laser gyro
with the DGPS or a less precise unit (type -M) offering
a more limited 20m positional accuracy. As with the
other multiple camera systems, after the image data
has been acquired, first band-to-band registration of
the individual images is carried out. This is followed by
the use of the DGPS and IMU data to geo-reference the
composite images. Further processing steps allow the
production of mosaics or ortho-mosaics.
(f) American University Developed Systems
Besides the commercially-built and operated systems
described above, other similar multiple camera systems have been developed and built by research
groups in different American universities. One of
these is based in the University of North Dakota
(UND) and forms part of the Upper Midwest
Aerospace Consortium. This group has developed, in
partnership with a private company, Digit Inc., its
Airborne Environmental Research Observation Camera
(AEROCam). Again this comprises four Dalsa CA-D4
digital cameras, each of which is based on a 1,024 x
1,024 pixel CCD areal array of 12 x 12µm detectors.
This AEROCam system is also equipped with a DGPS
and an IMU. Still another university that has built a
small-format airborne camera system is that developed at the Environment Remote Sensing Center
(ERSC) at the University of Wisconsin located at its
main campus in Madison. However this appears to be
based on small digital cameras from Dycam in
Chatsworth, California for use in a tiny electricallypowered UAV, called DigiDot, rather than a full multi-
(d) Kestrel Corporation
The Kestrel Corporation is based in Albuquerque,
New Mexico and is very active in the biomedical
imaging field as well as developing multi-spectral
and hyperspectral imagers for remote sensing use.
(a)
(b)
(c)
ple camera system like UND's AEROCam which is
operated from single-engined or twin-engined aircraft
owned by its School of Aerospace Sciences.
(g) IGN, France
During the 1990s, the French IGN national mapping
organisation carried out a research and development
programme investigating the possibilities of using airborne digital frame imagery for some of its mapping
work. Different types of digital camera were used to
acquire the necessary images. The first of these has
again involved the use of multiple cameras - in IGN's
system, three monochrome digital cameras were used.
These were aligned with parallel optical axes and fitted
on to a shock absorber mount. Each camera used a
Kodak 3k x 2k CCD areal array and each was fitted
with a different filter - blue, green, red - to ensure that,
when the three individual bands were combined, a
true-colour image would result. A second system has
involved the use of two cameras, each fitted with a
Kodak 3k x 2k areal array using the Bayer mosaic filter
and subsequent interpolation to generate colour
images. The two cameras were mounted in the aircraft
in a "split-vertical" (i.e. oblique) configuration with the
optical axis of the first camera being tilted to the right
of the flight line and that of the second camera to the
left of the line. While this increased the lateral coverage
of the twin camera system, a preliminary rectification of
the two tilted images was necessary prior to their use
for mapping. IGN's reports also mention the use of a
third type of digital camera having a 4,096 x 4,096
pixel array that generates black-and-white imagery, but,
strictly speaking, this falls into the class of medium-format digital cameras that will be dealt with in the next
section.
Summary
In summary, it can be seen that numerous small-format
airborne digital cameras are currently in operational
use. Indeed, it can be said that the technology is now
quite well established. However it can also be seen
that the main emphasis has been on the production of
true-colour and false-colour images for environmental
monitoring and agricultural applications over relatively
small areas. Even then, very large numbers of images
have to be handled and processed.
II. Medium-Format Digital Cameras
Whatever the undoubted success of small-format digital
cameras, the biggest drawback has always been the
very limited size of the format itself - with the resulting
severe limitations in the ground coverage of a single
frame image. So, during the late 1990s, with the commercial availability of larger CCD areal arrays of up to
4,000 x 4,000 pixels = 16 Megapixels at a not too
exorbitant price, there was an immediate response on
the part of the airborne imaging community. This has
mainly gone in two different directions.
(i) The first has been the development of digital backs
that could be fitted to existing high-quality film cam(d)
Fig. 10 - The AirCam system from the Kestrel Corporation showing (a) the four frame camera unit; (b) the display unit, together with a laptop computer; (c) the electronics unit; and (d) a
false-colour image of Fort Bliss, Texas acquired by an AirCam system. (Source: Kestrel Corporation)
22
October/November 2003
Article
(a)
(b)
(c)
Fig. 11 - (a) The GeoTechnologies MF-DMC medium-format frame camera system. The actual camera is in the middle of the photo. It is based on the body of a Hasselblad 555 ELD film camera and is equipped with an f = 40mm lens and a Dicomed digital back using Bayer colour mosaic filters. On the left is an Apple Mac laptop computer; on the right, a Garmin GPS receiver
sits on top of a Fugro OmniSTAR DGPS unit.
(b) A Bell 412 helicopter modified with a special nose fairing in place protecting a MF-DMC frame camera system.
(c) The protective nose fairing has been removed from the helicopter to show the MF-DMC frame camera system with its roll and pitch adjustable mount. (Source: Bath Spa Univ. College)
eras generating 6cm wide images such as those
manufactured by Hasselblad, Rollei, Mamiya and
Contax.
(ii) The second has been the development of specifically designed digital cameras using the medium-format arrays, the most obvious example being the
Kodak MegaPlus 16.8i camera already mentioned
above.
II.1 Modified Film Cameras
AIMS
A pioneering and very influential development in this
particular area has been the Airborne Integrated
Mapping System (AIMS) that was first proposed by the
Center for Mapping at Ohio State University in 1995.
The original proposal was for the development of a
fully digital acquisition system for mapping purposes.
This would comprise a digital camera together with a
tightly integrated DGPS/IMU system capable of producing high-accuracy positional and orientation data for
the digital camera and the platform on which it was
mounted. As finally realized in prototype form in 1997,
the digital camera part of the overall system consisted
of a Hasselblad 553ELX camera body equipped with a
Zeiss lens and fitted with a "BigShot" digital camera
back. This utilized a 4k x 4k pixel CCD areal array with
a 15µm square detector size, manufactured by
Lockheed Martin Fairchild Semiconductors. The integrated positioning and attitude elements of the overall
AIMS system comprised two dual-frequency Trimble
GPS receivers and a strapdown Litton LN-100 IMU.
As demonstrated by many tests and published reports
from the period 1999 to 2001, the AIMS system was a
distinct success. The digital camera produced high-quality monochrome (panchromatic) images having a much
better ground coverage than those produced by the
small-format cameras - though still quite limited by the
standards of metric film cameras. The use of the mea(a)
sured positional and attitude data from the DGPS/IMU
combination showed a good accuracy, though, as we
now know, not quite of a standard to eliminate the
need for ground control points (GCPs) and aerial triangulation, but still providing very valuable auxiliary data
for the geo-referencing process. Further tests carried
out in collaboration with the EarthData mapping company involved the use of the AIMS system together
with an airborne laser scanner (lidar) over a range in
Maryland. This proved to be an excellent combination
with the digital imagery and the lidar data complementing one another quite strongly.
Digital Backs
The undoubted success of the AIMS project has led to
many similar systems being created. On the digital
camera side, a number of digital backs have been
developed for fitting to film cameras with 6 x 6cm or 6
x 4.5cm formats from which the film magazines have
been removed. These digital backs were developed by
various companies such as MegaVision, Sinar,
PhaseOne, Linhof and Jenaoptik. Most of them have
been based on Kodak's ITO-based 4,080 x 4,080 pixel
CCD areal array with its 9µm square detector size, as,
of course, is Kodak's own DCS ProBack product. Other
suppliers have used the Dicomed silicon-based CCD
areal array which also has a format of 4k x 4k pixels,
but features the rather larger detector size of 15 x
15µm and is considerably more expensive than the
Kodak product. On the other hand, the Kodak array is
limited to true-colour only, whereas the Dicomed product is also sensitive to radiation in the near infra-red
and can be used to produce false-colour images. Both
the Kodak and Dicomed arrays use Bayer mosaic filters
with interpolation to produce the required colour
images. Another supplier of digital backs for fitting to
medium-format Hasselblad and Mamiya cameras is the
Japanese Fuji company. However its Luma II digital back
utilizes a somewhat smaller-sized rectangular CCD areal
array - with 4,008 x 2,672 pixels = 11 Megapixels than the Kodak and Dicomed products. It also uses a
(b)
Fig. 12 - (a) The Emerge DSS medium-format digital frame camera - which utilizes the body of the Contax 645 film camera
and a MegaVision digital back - placed on its mount.
(b) The complete DSS system with its Applanix supplied electronics package, including its POS-AV DGPS/IMU unit. The actual IMU is fitted to the top of the metal frame that fits over the camera. (Source: Applanix)
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Fuji mosaic filter with interlocking octagonal-shaped
pixels (instead of the square-shaped pixels used by
Kodak) to generate colour images by interpolation.
(a) European Examples
European examples of these converted film cameras for
airborne imaging include the MF-DMC (Medium-Format Digital Mapping Cameras) from the GeoTechnologies
consultancy at Bath Spa University College in the U.K.
whose earlier ADPS system has been already discussed
above. These MF-DMC cameras utilize the body of the
Hasselblad 555ELD SLR camera equipped with an electrically driven shutter and available with a range of
Zeiss lenses. The MF-DMC(1) model uses the Dicomed
16MPx array, while the MF-DMC(2) model makes use of
the Kodak ProBack. A number of MF-DMC cameras have
been supplied to mining companies in Indonesia and
South America; yet another has been supplied to an
organisation in Uganda which uses it to monitor excessive vegetation growth on Lake Victoria. In the U.K.,
further examples have been supplied to NPA Ltd. and
to the government's Environmental Agency. This agency
operates its MF-DMC camera in conjunction with an
Optech airborne laser scanner for coastal monitoring
purposes. The lidar produced DEM is used as the basis
for the ortho-rectification of the digital frame images
being acquired by the camera.
Another quite similar conversion is that carried out by
the Latvian mapping company, SIS Parnas. This comprises a Mamiya RZ67 Pro II camera which has also
been equipped with the Kodak ProBack. The camera
was then calibrated at the photogrammetry laboratory
of the Moscow State University of Geodesy &
Cartography to allow it to be used for photogrammetric
mapping. As with many of the systems discussed
above, a DGPS receiver is fitted to provide accurate 3D
positions of the camera stations. The final photogrammetric processing of the imagery is being carried out
using the PHOTOMOD DPW supplied by the Russian
Racurs company.
(b) American Examples
In the United States, many more medium-format digital
frame cameras have been constructed and come into
productive use. Prominent among these is the camera
system that originally has been produced for the
Emerge company, which, as previously discussed, has
been an extensive user of the Kodak DCS-460 smallformat digital cameras. This new Digital Sensor System
(DSS) has been constructed on the basis of a Contax
645 camera, modified to take the MegaVision digital
back based on the Kodak 4k x 4k CCD areal array with
its Bayer mosaic filter. The camera is available with
Zeiss lenses having a range of focal lengths with f =
35, 45 or 80mm. A metal frame attached to the camera
mount has been constructed to fit over the camera like
October/November 2003
23
Article
(a)
(b)
Azimuth Corporation, now part of Leica Geosystems.
However the digital camera has been designed and
built by the company's R&D section located in
Albuquerque, New Mexico. This uses a Rolta lens in
combination with a Kodak 4k x 4k CCD areal array.
Originally only panchromatic imagery was generated,
but the company has now produced a new version of
the camera fitted with a similar sized (4,080 x 4,080
pixel) CCD areal array with a Bayer mosaic filter to produce colour frame images having a 48 Megabyte (3 x
16 Megabytes) file size after interpolation. Under a
newly announced re-organisation, the 3Di and
EnerQuest companies have been amalgamated and will
operate the three DATIS systems and the RAMS system
under the banner of the newly merged company, which
is called Summit Mapping LLC.
Fig. 13 - (a) The DATIS II system developed by 3Di - with its medium-format digital frame camera and its laser scanner
together with its IMU and DGPS units - all integrated together within a single mount (seen in the foreground). The accompanying electronics package is located in the background.
(b) The RAMS system developed by EnerQuest, comprises a medium-format digital frame camera (on the right); the laser
scanner which is integrated with an IMU and a DGPS receiver (in the middle); and an on-board electronics package that
records the data acquired by the digital camera and laser scanner units (on the left).
a cap and carries an embedded IMU that is set directly
over the camera. The partnership that developed the
system under the leadership of Emerge included Pixel
Physics of Rochester, New York, which has carried out
the modification and radiometric calibration of the camera. The other major partner in the project has been
Applanix, which has developed the electronics package,
including the integration of its POS-AV DGPS/IMU system and a flight management control and display system. Recently Applanix has taken over the whole project (including the patent rights) and is now marketing
the DSS system world-wide. Pixel Physics is also offering the camera only under the name TerraPix DACS
(Digital Aerial Camera System). The latest news (from
May 2003) is that the DSS system will also be offered
by Leica Geosystems. Several of the DSS systems have
already been sold, including two to users in Japan.
II.2 Medium-Format Digital Cameras
So far, the main examples of purpose-built airborne
medium-format digital cameras have been those based
on the top-end of the MegaPlus range, first manufactured by Kodak and now produced by Redlake. A typical example is the FSE Aerial Imaging System offered
by ImageTecK of Chatsworth, California. This is based
on the MegaPlus 16.8i monochrome camera ruggedized
for flight and provided with a control unit and a computer with removable disk drives and special acquisition software, Previously ImageTecK had supplied similar systems using lower-resolution MegaPlus models to
various other users, including Nortrek Geomatics in
Canada and AeroDigital Chile.
(a) Digital Camera & Lidar - DATIS
Several other medium-format frame camera systems
that have been designed specifically to complement airborne laser scanners have been produced for American
photogrammetric mapping companies. An early example
was the DATIS (Digital Airborne Topographical Imaging
System) produced by the 3Di company at its Boulder,
Colorado facility. Two complete DATIS systems are now
operational; a third has just been built. DATIS uses
lidar systems that have been designed and built by 3Di
in-house. The overall DATIS system employs a SystronDonner IMU together with a DGPS constructed on the
basis of OEM cards rather than a complete unit bought
in from another supplier. The digital camera components of the system have also been designed in-house,
although their actual construction has been sub-contracted to an outside supplier. The cameras are fitted
with Rollei lenses and feature a 4k x 4k Kodak CCD
areal array producing panchromatic (monochromatic)
frame images. These images are referenced directly to
the DEM produced by the laser scanner, allowing rapid
production of digital orthophotos.
(b) Digital Camera & Lidar - RAMS
The RAMS (Remote Airborne Mapping System) operated
by EnerQuest of Denver, Colorado is another integrated
system comprising an airborne laser scanner and a digital frame camera. In this case, the lidar with its integrated DGPS and IMU has been supplied by the
(c) Digital Camera & Lidar - EarthData
Finally, within this group, it is worth noting the
extensive and high-profile use of another airborne
laser scanner and digital camera combination operated by the EarthData mapping company of
Gaithersburg, Maryland. In the wake of the terrorist
attack of September 11th, 2001 on the World Trade
Center in New York, EarthData flew its system on 19
occasions between 15th September and 22nd
October 2001 to collect lidar data and digital frame
imagery to produce DEMs and digital orthophotos
that were used extensively during the rescue and
recovery operations that followed the attack. The
imager that was used for this work was the Kodak
Megaplus 16.8i digital frame camera producing
panchromatic (black-and-white) imagery with 256 (8bit) grey values. Quite a number are being used in
combination with airborne laser systems that produce DEMs, on the basis of which, the digital frame
images can be ortho-rectified.
Summary
It can be seen that there is a strong move
towards the airborne use of medium-format digital
cameras which offer a substantially better ground
coverage than the small-format cameras. Quite a
number are now being used in combination with
airborne laser scanners that produce DEMs, on the
basis of which, the digital frame images can be
ortho-rectified.
III. Large-Format Frame Cameras
As mentioned in the introduction to this article, the
quality standard for the large-format frame cameras
used for photogrammetric mapping and interpretation
has been set by the current models of the metric film
frame cameras produced by Leica
(RC30) and Z/I Imaging (RMK-TOP).
(a)
(b)
(c)
Equipped with suitable gyro-controlled mounts and featuring forward image motion compensation in
conjunction with fine-grained highresolution film, these cameras can
produce images with resolutions of
50 to 60 lp/mm (amounting to 20 to
17µm) on the film plane. If one
takes the relationship between the
film resolution in lp/mm and the
equivalent pixel size as being 1:2
(as given by the Kell-Factor), then
the smallest possible pixel size on
Fig. 14 - (a) This diagram shows the arrangement of the 8 cameras making up the Z/I Imaging DMCS frame camera system. The 4 pan cameras
the film will lie between 7 to 10µm.
located within the central part of the diagram are all tilted from the vertical outwards in a star-shaped configuration. The other 4 cameras locatThis relationship is also reflected in
ed at the corners of the diagram all point in the vertical direction with parallel optical axes to produce overlapping images of the same piece of
the smallest pixel size that is proground in the red, green, blue (RGB) and near-IR parts of the spectrum.
vided for in high-accuracy pho(b) The ground coverage of the four tilted pan cameras.
togrammetric film scanners - the
(c) The production of a single "virtual" vertical pan image from the four tilted sub-images. (Drawn by Mike Shand)
24
October/November 2003
Article
(a)
(b)
company. Each of the four cameras has a Zeiss f = 120mm
lens. In combination, this four
camera configuration produces a
rectangular 13.5k x 8k = 108
Megapixel panchromatic image
giving an angular coverage of
74° (cross-track) x 44° (alongtrack) over the ground.
For the acquisition and generation of true-colour or falsecolour frame imagery, Z/I
Imaging's solution incorporated
in the DMCS again utilizes multiple cameras - either three or
four as required. However, in
this case, these cameras are
arranged with parallel optical
axes to cover the same piece of
Fig. 15 - (a) The Z/I Imaging DMC (Digital Modular Camera) on its Zeiss T-AS gyro-controlled mount.
ground. Each individual camera
(b) The various elements of the DMCS (Digital Mapping Camera System) installed in an aircraft carrying out demonstration flights in the U.K. The TAS gyro-controlled mount and the cradle on which the camera unit will sit are in the lower right part of the photo. The electronics unit is located at
has a filter defining a different
the left edge of the photo. (Source: Z/I Imaging)
spectral band placed in front of
it. The size of the individual CCD
areal arrays used in these multi-band cameras is much
Leica DSW500 with 5µm; Z/I Imaging PhotoScan with
III.1 Large-Format Digital Frame Imagery
smaller - 2k x 3k = 6 Megapixels - than those used in
7µm; Vexcel Imaging VX4000 with 7.5µm; Wehrli RMUsing Multiple Cameras
the DMCS pan cameras. So the cameras have been fit3 with 8µm; and Vexcel Austria UltraScan 5000 with
ted with short focal length (f = 25mm) wide-angle lens5µm. Scanning a high-resolution monochrome film
The two commercial developments that are under way
es to provide a reasonable ground coverage. The
with a 10µm pixel size on any one of these devices
in this particular area are the multiple camera systems
penalty for this is, of course, a very considerable reducresults in an image having 23,000 x 23,000 pixels =
of Z/I Imaging and Vexcel Austria.
tion in the ground resolution (cf. f = 25mm v. f =
529 Megapixels for the standard film frame format
120mm) compared with the DMCS pan camera when
size of 23 x 23cm. In commercial practice, in order to
(a) Z/I Imaging
operating from the same flying height. The final truekeep down the file sizes, the film is often scanned at
The Digital Modular Camera (DMC), now re-named the
colour or false-colour image is formed later through the
a 15µm pixel size - which still results in an image
Digital Mapping Camera System (DMCS), has been
co-registration and fusion of the individual images
with 15,333 x 15,333 = 235 megapixels. As we have
under development by Z/I Imaging for some years The
recorded by each of the three or four component camseen, these pixel sizes lying in the range 7 to 15µm
initial announcement of its development took place in
eras.
that can be obtained from scanned film images lie in
1999, while the prototype was first shown at the ISPRS
the same range as those commonly found on CCD
Congress held in Amsterdam in July 2000. Now the first
(b) Vexcel Austria
areal arrays. Thus there is little difference in the geodeliveries of the DMCS are under way to customers in
The Vexcel Austria UltraCam D camera was announced
metric resolution values that are encountered in the
the United States (3001), Japan (KKC) and Germany
in April 2003. It also features multiple cameras operatfocal plane with film frame cameras and with digital
(IFMS). The DMCS comprises two separate multiple
ed in two groups of four. The first group of four camframe cameras. The main difference lies in the much
camera systems which are integrated together inside a
eras generates panchromatic (black-and-white) images,
smaller format sizes that are possible with digital
single box so that they can be operated as an integratwhile the second group of four generates true-colour
frame cameras due to the small sizes of CCD areal
ed unit sitting within a Zeiss T-AS gyro-controlled
and false-colour images. However the generation of the
arrays that are available at the present time.
mount. The first of these two multiple camera systems
panchromatic images is carried using a quite different
Undoubtedly the situation will improve with time.
forming the DMCS is designed to produce monochrome
configuration to that used in the Z/I Imaging DMCS.
(panchromatic) imagery; the second system produces
Instead of the four tilted cameras of the DMCS, the
Current developments in large-format digital frame camcolour or false-colour imagery of a somewhat lower
UltraCam D has its four cameras all pointing in the vereras are taking place in two main directions:ground resolution. To generate the panchromatic (blacktical direction with parallel optical axes. The four cam(i) The first involves the use of multiple medium-format
and-white) images, the DMCS integrates four individual
eras are mounted in a line pointing in the flight direccameras producing multiple images that are synthecameras arranged (i.e. tilted outwards) in a star-type
tion. The shutters of each camera are triggered
sized into a single large-format image. This is the
configuration. The resulting four individual tilted images
sequentially with a very tiny time delay of 1 or 2 milapproach is that being followed by the commercial
overlap slightly and are exposed simultaneously. They
liseconds between them. This means that each subvendors of large-format digital cameras.
are then processed (i.e. rectified) to form a single perimage making up the complete frame image is essen(ii) The second approach involves the construction of
spective image. Each of the four component cameras is
tially taken from a single exposure station in the air.
single cameras fitted with the very largest CCD areal
fitted with a 7k x 4k = 28 Megapixel CCD areal array
Thus the components of the final composite image,
arrays that are available at the present time. So far,
originally manufactured by Philips in Eindhoven in the
which is made up of the different sub-images, all share
these cameras have all been constructed for military
Netherlands - though this part of the Philips organisathe same projection centre. One of the cameras is descustomers and applications.
tion has now been taken over by the Canadian Dalsa
(a)
(b)
(c)
(d)
Fig. 16 - (a) The basic concept of the Vexcel Austria UltraCam D system with the four pan cameras all pointing vertically towards the ground with parallel optical axes. These four cameras
are triggered in very rapid succession to record a set of sub-images taken from a single projection centre in the air. The resulting sub-images are stitched together to form a single large-format pan image.
(b) The three main units of the UltraCam D system with the digital frame camera at left; the computing and storage unit in the middle; and the display unit at right.
(c) The UltraCam D multiple frame camera unit with its 8 cones - 4 of these are located in-line to produce the final large-format pan frame image; the remaining 4 cones acquire the multiband images covering the red, green, blue (RGB) and near-IR parts of the spectrum to form true-colour and false-colour images.
(d) A colour image of Salzburg Cathedral taken with the UltraCam D at 1:5,500 scale and a ground pixel size of approximately 5cm. (Source: Vexcel Austria)
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October/November 2003
25
Article
(a)
(b)
company's well-known KS-87
reconnaissance film frame camera with its 5 inch (12.5cm) wide
film producing images with a 4.5
x 4.5 inch (11.5 x 11.5cm) format.
The CA-260/4 conversion dating
from 1993 used a 2k x 2k = 4
Fig. 17 - (a) The Recon/Optical CA-270 "dual-band" digital frame camera providing
Megapixel CCD array with built-in
day and night optical imaging capabilities.
IMC capability. It could be fitted
(b) Pan (visible) and medium-IR frame images taken simultaneously with the CA-270
with f = 75, 150 or 300mm lenscamera. (Source: Recon/Optical)
es. The later CA-260/25 model
from 1996 used a much larger 5k x 5k = 25 Megapixel
ignated as the "master cone" and features four CCD
CCD areal array which could be used to take stereoareal arrays located in the corners of its overall field of
cover of larger areas. A still later model in the series is
view. The other three cameras (or "cones") acquire the
the CA-260/50 which features a rectangular 10k x 5k =
sub-images that fill the gaps between the four images
50 Megapixel CCD areal array developed by Dalsa. This
that are produced by the "master cone". The final
array provides images with a resolution of 49 lp/mm
panchromatic image, after being stitched together from
with a 2.5 frames per second read-out rate. Using an f
the master images and the sub-images, is 11.5k x 7.5k
= 75mm lens, this array provides an angular coverage
pixels in size = 86 Megapixels, with each pixel being
of 68.4° x 37.5° over the ground. Another digital cam9µm square in the focal plane. The four cameras all
era that is a conversion of a film frame camera is the
use an f = 100mm lens from Schneider Kreuznach as
KS-127 LOROP (Long-Range Oblique Photographic) camstandard, the angular coverage of the final image over
era equipped with an f = 66 inch (1.67m) lens. The
the ground being 55° x 37°.
conversion also allowed it to record its images using a
digital back.
In the case of the second group of multiple cameras
that generate the true-colour and false-colour images,
Besides these conversions, Recon/Optical has also built
once again, these are arranged with parallel optical
a series of purpose-built digital frame cameras. These
axes, with each of the lenses of the four cameras havinclude the CA-265 IR digital frame camera which uses
ing the appropriate spectral filter placed in front of it.
a 2k x 2k = 4 Megapixel array that is sensitive to radiEach of these four cameras has a CCD areal array of 4k
ation in the medium IR (Ï = 3 to 5µm) part of the specx 2.7k pixels = 10.8 Megapixels and is fitted with a
trum that allows image collection both during the day
lens having f = 28mm. These provide an angular coverand during the night. Other Recon/Optical models
age of 65° (cross-track) x 46° (along-track). The protoinclude the CA-261 Step Frame Camera which provides
type UltraCam D camera has already been flown and
extended cross-track coverage by fast-stepping movesample images are available from Vexcel Austria - which
ments of the camera in that direction. Some of the lathas now been merged with the Vexcel Corporation of
est digital frame cameras from Recon/Optical are the
Boulder, Colorado in May 2003.
so-called "dual band" or "dual-spectrum" models such
as the CA-270 (low altitude), CA-279 (medium altitude)
and CA-295 (high altitude). These allow simultaneous
III.2 Large Format Digital Cameras
images to be acquired in both the visible and infra-red
parts of the spectrum allowing day and night acquisiThe current state-of-the-art in digital frame cameras is
tion of image data. It should be noted that all of these
represented by the reconnaissance aerial cameras that
Recon/Optical cameras are generating monochromatic
have been constructed in the U.S.A. by Recon/Optical
(black-and-white) images.
and BAE Systems for American military air forces.
Recon/Optical
This company, originally known as Chicago Aerial
Industries (CAI), has been a major supplier of reconnaissance camera systems for over 70 years. During the
early 1970s, the company developed a pushbroom
scanner equipped with a linear CCD array for the USAF.
However this type of imager was not favoured by the
air force, since it meant that the reconnaissance aircraft
had to be flown in a relatively stable and non-manoeuvring path while acquiring the linescan imagery, making
them vulnerable to defensive fire. So the company has
since concentrated its efforts into cameras equipped
with large CCD framing arrays which allow the coverage
of an area to be acquired while the reconnaissance aircraft is manoeuvring. Recon/Optical now makes numerous cameras that are based on this technology. The CA260/4 was an upgrade (or conversion) of the
BAE Systems
Those framing cameras produced by BAE Systems that
incorporate 9,216 x 9,216 pixel = 85 Megapixel CCD
areal arrays give the highest resolution digital frame
images from a single array that are known about publicly. The part of BAE Systems that produces these cameras is called the Reconnaissance & Surveillance
Systems Division and is based in Long Island, New
York. Previously this particular facility formed part of
Lockheed Martin's Aerospace Electronics Systems (AES)
operation which was purchased by BAE Systems' North
American business in 2000. In fact, prior to its ownership by Lockheed Martin, the facility was a part of
Fairchild with a long tradition in the construction of film
cameras. The CCD areal array, which is manufactured inhouse by BAE Systems, measures 8 x 8cm in size and
is fabricated on a single 5 inch (12.7cm) diameter sili-
con wafer. The pixel size is 8.75 x 8.75µm and the
array provides monochrome images with a resolution of
57 lp/mm (similar to that of metric film cameras). The
array also allows a 2 frames per second read-out rate.
More than half-a-dozen F-979F and F-985C cameras
have been built that utilize this particular array. When
fitted with an f = 75mm lens, the F-979F camera provides an angular coverage of 55.8°. As with all of these
cameras fitted to military reconnaissance aircraft that
need to operate at high speeds and low altitudes, IMC
is standard. The work to develop an "Ultra High
Resolution Reconnaissance Camera" based on this
technology has been sponsored by the U.S. Naval
Research Labs (NRL). However, in presentations and
papers given at the ASPRS conferences held in
Washington and Denver last year (2002), it appears
that BAE Systems are considering the development of a
metric version of this camera designed specifically for
mapping applications. It is already apparent that, if four
of these 9k x 9k pixel areal arrays can be packaged
together, a format of 18k x 18k pixels = 338.6
Megapixels would result. This would give a ground coverage and a geometric resolution that is comparable
with that of metric frame cameras.
Summary
The present situation with regard to large-format airborne digital cameras is extremely interesting with several cameras having either just entered service or will
do so in the near future. It will be fascinating to see
the results that can be obtained when using these new
cameras for mapping operations. Nevertheless it should
be borne in mind that this new generation of airborne
digital cameras will produce monochrome images that
are equivalent to those from film cameras utilizing a 5
inch (12.5cm) wide film and producing images that are
4.5 x 4.5 inches (11.5 x 11.5cm) is size. When films are
scanned using a pixel size of 10µm (close to the pixel
size of the CCD areal arrays), this produces images that
are 11.5k x 11.5k = 130 Megapixels in size. If, instead, a
pixel size of 15µm is used, this results in an image with
7.67k x 7.67k pixels = 59 Megapixels. Thus the current
generation of large-format airborne digital frame cameras lie in the same region in terms of geometric resolution and format size as those aerial film cameras
using 5 inch (12.5cm) wide film.
Conclusion
As this article has attempted to show, real progress in
airborne digital frame camera technology has been
made over the last decade. Small-format digital frame
cameras have already become well established for airborne imaging purposes. Medium-format digital frame
cameras have now entered service in some numbers.
Almost certainly, they will come into still more
widespread use. Large-format digital frame cameras are
now being developed and they represent the current
state-of-the art - though, as yet, they still do not provide the format sizes and coverages that are standard
with metric film cameras. However the advent of comparable digital frame cameras does appear to be on
the horizon.
Professor G. Petrie, Department of Geography &
Topographic Science, University of Glasgow, Glasgow, G12
8QQ, Scotland, U.K.
Internet: http://www.geog.gla.ac.uk/~gpetrie
E-mail: [email protected]
Fig. 18 - Unit Assemblies for the BAE Systems 9.2k x 9.2k Ultra High Resolution Reconnaissance System (Source: BAE Systems)
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October/November 2003
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