Sandau R. Digital Airborne Camera: Introduction and Technology

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7.1 The ADS40 System 305

Fig. 7.1-33 Aalen, Germany, Orthophoto 1:1000, altitude 1.2 km, GSD 12 cm

Fig. 7.1-34 Eiderstedt, Germany, False-colour orthophoto for the update of biotopes 1:5000,

altitude 2.0 km, GSD 20 cm

306 7 Examples of Large-Scale Digital Airborne Cameras

Fig. 7.1-35 Eiderstedt, Germany, False-colour orthophoto for the update of biotopes 1:2500,

altitude 2.0 km, GSD 20 cm

Fig. 7.1-36 Eiderstedt, Germany, False-colour orthophoto for the update of biotopes 1:1000

(zoom of 7.1-35), altitude 2.0 km, GSD 20 cm

7.2 Intergraph DMC Digital Mapping Camera 307

7.2 Intergraph DMC Digital Mapping Camera

The Intergraph Digital Mapping Camera (DMC) is a large format digital mapping

camera, based on frame sensor technology. The workﬂow for mission planning

and photo ﬂights is very similar to analogue ﬁlm cameras. Image post-processing

is required for the raw image data. Aerial images from the DMC have a central

perspective and the photogrammetric workﬂow is similar to that for scanned ﬁlm

images.

7.2.1 Introduction

The DMC (Fig. 7.2-1) is based on a CCD frame sensor design with central perspec-

tive and stable inner geometry. At the time the DMC was designed, it was the clear

intention to develop a very versatile digital airborne camera to cover a wide range of

applications from large scale mapping with 5 cm GSD from high-resolution imagery

up to small-scale application with 1 m GSD for remote sensing.

Owing to its frame sensor design, the DMC does not require the sophisticated

post-processing of line sensors necessary to rectify the raw image data. It is also not

dependent on GPS/INS technology, although most of today’s photo ﬂight missions

demand high-precision GPS and very often direct georeferencing capabilities as

well. But not depending on constant IMU data for post-processing, gives the user

Fig. 7.2-1 DMC camera located in a T-AS gyro stabilized mount

308 7 Examples of Large-Scale Digital Airborne Cameras

one more degree of freedom and reduces technical risks affecting the success of the

photo ﬂight mission.

The design of the DMC has a frame sensor basis and allows the user to continue

with the same workﬂow as for scanned ﬁlm images, which means that analogue ﬁlm

cameras can be operated in parallel with the DMC for the same production line.

7.2.2 Lens Cone – Basic Design of the DMC

Since it is not (as yet) economically possible to install a single, large area, digital

colour array CCD that could reach the format of a 230 × 230 mm ﬁlm camera, the

DMC solution is to incorporate several compact camera heads, each with its own

lens and CCD sensor. The modules are then aimed at the scene at slightly displaced

ﬁeld angles. Four panchromatic modules in the centre of the frame are surrounded

by four multispectral camera heads at the periphery of the frame. Figure 7.2-2 illus-

trates the ground coverage taken by the four panchromatic camera heads. The DMC

employs four 7 k × 4 k large-area CCD chips, each with an f4 120 mm focal length

Fig. 7.2-2 Ground coverage of the DMC with four panchromatic camera heads

7.2 Intergraph DMC Digital Mapping Camera 309

lens operating in the panchromatic (visual response) mode. The overall panchro-

matic resolution (over the ground) is 13,824 pixels across track and 7,680 pixels

along track. The cross-track FoV is 69.3

◦

,formedfromtwo7kpixeldimensions,

and the along-track FoV is 42

◦

,formedfromtwo4kpixeldimensions.

The multispectral array is made up of four f4 25 mm focal length lenses exposing

four channels, R, G, B and NIR. The multispectral chips are of lower area resolution,

each being 3 k × 2kpixels.

All CCD sensors are Dalsa models with 12 μm square pixels (>12-bit dynamic

range) and have a special architecture with four readout registers placed at the cor-

ners of the chip. This provides for high readout rates, such that an image repetition

frequency of one image every 2.1 s can be achieved, an important feature to acquire

aerial images for stereoscopic viewing at high forward overlap and high ground

resolution.

7.2.3 Innovative Shutter Technology

An electromechanical inter-lens shutter system is employed on each camera, with

precise synchronisation of all camera exposures to exclude geometric errors. These

shutters are of a design customized for airborne operation, with few mechanical

parts and very high reliability. There is no need for frequent shutter exchanges as is

the case for other types of aerial camera.

All eights shutters are continuously monitored by microcontrollers and adjusted

in real time for any kind of variance, to maintain a stable exposure cycle over time.

The shutter speed is variable and can be selected in steps between 1/50 and 1/300 s.

7.2.4 CCD Sensor and Forward Motion Compensation

Forward image motion (FIM) is an important inﬂuence on image resolution in both

analogue and area-array digital images, where, with poor light levels or low ﬂying

height, the image exposure time may be too long to avoid signiﬁcant image blur

(Graham et al., 1996). FIM is a function of a number of parameters:

FIM = f

∗

t/H

where f = focal length of lens, V = ground speed of aircraft, t = exposure time (i.e.

shutter speed) and H = height of aircraft above mean ground level.

In terms of aerial ﬁlms, the usual accepted limit of FIM is that it should not

exceed 25 μm, but this is usually less in cameras with forward motion compensation

(FMC). For the DMC, a shutter speed of 1/200 s at 150 knots (77.25 m/s) airspeed

and 5 cm GSD would amount to an image blur of 8 pixels, i.e. 96 μm on the CCD

sensor.

A fully electronic forward motion compensation (FMC) of the digital image is

available by means of the time-delayed integration (TDI) mode during exposure. In

310 7 Examples of Large-Scale Digital Airborne Cameras

Fig. 7.2-3 ForwardmotioncompensationbyTDI

this way, compensation for image blur in low altitude and high resolution applica-

tions is assured (FMC has been standard for analogue aerial cameras since 1982).

FMC is implemented on the DMC in a completely electronic way. In normal read-

ing out of CCD pixels in an area-array imaging system, the charge contents of each

CCD pixel are transported line-by-line to the readout registers (Fig. 7.2-3).

Fig. 7.2-4 Resolution target

with FMC

Fig. 7.2-5 Resolution target

without TDI

7.2 Intergraph DMC Digital Mapping Camera 311

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

0.7000

0.8000

0.9000

1.0000

300 400 500 600 700 800 900 1000 1100

PAN

NIR

Red

Green

Blue

Fig. 7.2-6 DMC spectral sensitivity

7.2.5 DMC Radiometric Resolution

All of the DMC modules have a total CCD resolution of 12 bits, which enables better

exposure sensitivity, allowing details to be recorded on the CCD even in bright or

dark exposure conditions due to reﬂections, shadowing or clouds, thus increasing

the number of ﬂying days considered acceptable.

Each multispectral camera has its own colour ﬁlter. There is a slight overlap

between the individual bands to achieve a better native colour impression. The

colour ﬁlter sensitivity is as follows (Fig. 7.2-6):

Blue: 400–580 nm

Green: 500–650 nm

Red: 590–675 nm

NIR: 675–850 nm

7.2.6 DMC Airborne System Conﬁguration

A typical DMC installation is shown in Fig. 7.2-7, which is almost identical to

existing ﬁlm camera installations. The DMC camera body has similar dimensions

to the RMK TOP aerial ﬁlm camera and is designed to ﬁt into the existing gyro-

stabilised mount T-AS. The sensor management system, with real-time controller

hardware and pilot display, is used to control the camera system. An optional IMU

can be integrated into the DMC, opening up the possibility of direct georeferencing

and working without ground control or with a reduced set of ground control points

(GCPs).

Raw image data is stored on a set of 3 FDS Flight Data Storage units. These are

ruggedized storage systems with hardened pressurized enclosures, each of which

312 7 Examples of Large-Scale Digital Airborne Cameras

Fig. 7.2-7 DMC system installed in an aircraft

includes two 300 GB SCSI disk drives. A set of 3 FDS can store up to 4,400 images,

where one FDS is connected to 2 panchromatic camera heads (each 7 k × 4k)or

to 4 multispectral camera heads (each 3 k × 2 k). The data interface is a high speed

1 Gb copper-based ﬁbre-channel interface. The FDS is certiﬁed to be operated at

up to 8,000 m altitude in an unpressurized environment. Special environmental tests

following the RTCA/DO 160 aircraft standard guarantee high reliability and high

data safety for the user.

In the aircraft, each FDS is installed in a special designed carrier (“FDS shoe”)

and can be easily removed without unplugging any cables or connectors. The “shoe”

can be mounted to the aircraft seat rails for crash load safety.

For image data post-processing the FDS is either connected directly to the post-

processing software servervia ﬁbre-channel connection or hooked up to a ﬁeld copy

station for mobile data copy to removable hard disk drives or USB disk drives.

7.2.7 System Calibration and Photogrammetric Accuracy

The DMC is certiﬁed by USGS (United States Geological Service) to meet the

US national mapping standards and accuracy. The basis for this high quality level

is a robust mechanical and optical design and an intensive quality test program.

Extensive environmental testing is part of the manufacturing process. Each cam-

era system undergoes a laboratory calibration, where focal length, lens distortion

and other characteristics are precisely measured. After this laboratory calibration,

each camera system is subject to a burn-in ﬂight to perform the platform calibra-

tion. Finally all calibration data are stored on a calibration CD and delivered with

the system to the customer. The user can repeat at any time a veriﬁcation ﬂight to

check platform calibration. The DMC is fully ﬁeld serviceable and each component

can be exchanged on-site.

The mapping accuracy of the DMC is:

for X,Y 5 μm

∗

photo scale, e.g.2.5cm@6 cm GSD

for Z0.05% of ﬂying height above terrain, e.g. 3 cm @ 6 cm GSD

7.3 UltraCam, Digital Large Format Aerial Frame Camera System 313

This high accuracy leads to the possibility of ﬂying at higher altitudes compared

to ﬁlm cameras while meeting the same mapping requirements.

7.3 UltraCam, Digital Large Format Aerial Frame

Camera System

7.3.1 Introduction

UltraCam is a large format digital aerial frame camera. The design is unique and

based on a multi cone concept. The camera is produced by Vexcel Imaging GmbH,

a wholly-owned subsidiary of Microsoft Corporation.

The ﬁrst UltraCam was pre-

sented in May 2003 at the 2003 ASPRS Annual Conference in Anchorage, Alaska,

in its initial implementation as the 85-megapixel UltraCam

system (Leberl et al.,

2003). Three years later, at the 2006 ASPRS Annual Conference in Reno, Nevada,

the 136-megapixel format UltraCamX was introduced.

Fig. 7.3-1 Large scale aerial image at 3 cm GSD. This 700 pixel by 500 pixel sub-area of an

UltraCam image covers only 21 m by 15 m on the ground

The new UltraCamX was developed with the experience and success of the

UltraCam

during the three previous years, undergirded by a rapid evolution of the

digital sensor market and a transition to the fully digital photogrammetric workﬂow.

A review of the period since the ISPRS Congress in 2000 in Amsterdam, when the

ﬁrst digital large format aerial cameras were presented, shows a rather slow initial

acceptance of the new technology. It was not until 2004 that a signiﬁcant number of

digital large format aerial cameras was sold by the then three competing vendors, so

Microsoft Corporation operates though Vexcel Imaging GmbH. The Austrian team is denoted as

“Microsoft Photogrammetry”. In this text, we will use both “Vexcel” and “Microsoft”.

314 7 Examples of Large-Scale Digital Airborne Cameras

that worldwide count of systems in operation reached a signiﬁcant number. Since

2004 digital cameras began to have a major impact on the global perspective of the

photogrammetric all-digital workﬂow and value systems.

7.3.2 UltraCamX Produces the Largest Format Digital

Frame Images

7.3.2.1 Overview of the UltraCamX

The UltraCamX camera system is the most recent implementation of the basic

UltraCam imaging principle. It takes advantage of the most valuable developments

of the industry in the technologies of sensors, data storage and data transfer as well

as Vexcel’s in-house experience and know-how. As will be reviewed later, some

essential elements of the camera technology were special custom developments for

the UltraCam.

The signiﬁcant advantages of the UltraCamX include:

• large image format of 14,430 pixels cross-track and 9,420 pixels along-track

• image repeat rate based on a data rate of 3 gigabits per second

• excellent optical system with 100 mm focal length for the panchromatic cam-

era heads and 33 mm for the multispectral camera heads, for an exact 1:3

pan-sharpening ratio

• image storage capacity of 4,700 frames for one single data storage unit

• almost unlimited image harvest due to exchangeable data storage units

• instant data download from the airplane by means of removable data storage units

• fast data transfer to the post-processing system using the new docking station.

The camera consists of the Sensor Unit, the onboard storage and data capture

system, the operator’s interface panel and two removable data storage units. This

airborne conﬁguration is shown in Fig. 7.3-2. It is complemented by an elaborate

software system for mission planning and in-ﬂight camera operation, the Camera

Operating Software COS.

Of course, there is also an integral systems component in the ofﬁce, be it a

ﬁeld ofﬁce or the main home ofﬁce. This addresses the processing of the acquired

imagery into deliverable imagery, in the form of the Ofﬁce Processing Center OPC.

7.3.2.2 The UltraCamX Sensor Unit

The UltraCamX Sensor Unit consists of eight independent camera cones, as shown

in Fig. 7.3-3. Four of these contribute to the large format panchromatic image;

the other four, to the multispectral image. The sensor head of the UltraCamX

is equipped with 13 FTF5033 high performance CCD sensor units, each produc-

ing 16 megapixels of image information at a radiometric bandwidth of more than