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1. Introduction
Digital camera technology started to attract photogrammetric researchers and system developers in the mid 1990s. Several efforts have been exerted to deploy the CCD-based digital camera technology into the airborne mapping environment. Some of these efforts focused on using the digital camera in a stand-alone mode (c.f., King et al, 1994; Mills et al, 1996) and confirmed the use of such cameras in the airborne environment.

More efforts focused on using the digital camera as a component of an integrated system (c.f., Mostafa et al, 1997; Cramer et al, 1997; Toth and Grejner-Brzezinska, 1998) where the System integration concept was initially proposed by Schwarz et al (1993) and successfully implemented for the land-based mobile mapping by El-Sheimy (1996). Other research efforts to develop airborne integrated systems were immediately directed towards the mapping industry and no sufficient publications are available albeit the validity of such research and its associated resulting mapping products (c.f., Congalton et al, 1998). Independently, the research outcome highlighted a number of conclusions:

1. The integrated airborne system approach (typically: digital camera/GPS/INS) far outperforms the stand-alone approach because of many advantages (c.f., Mostafa et al, 1997; Cramer et al, 1997; Toth and Grejner-Brzezinska, 1998)
2. A number of parameters have to be taken into account for the integrated system approach to work successfully: such as the calibration of system components, and the calibration of the integrated system as a whole. New software tools were recommended to be developed for such a purpose.
3. The existing photogrammetric algorithms and techniques are good enough to process the data delivered by these systems. However, seamless data flow between different commercial softwares was much needed back in the late 1990s and has been made possible in the last few years (c.f., Madani and Mostafa, 2001).

In this paper, The Digital Sensor System (DSS) is introduced as a dedicated product that was designed for the airborne mapping industry based on the aforementioned scientific research findings and years of experience of using such systems.

Manufactured by Applanix Corporation, the DSS is a fully operational, fully integrated all digital multi-sensor system developed for digital mapping data acquisition and processing. The DSS consists of a 4K x 4K digital camera, POS AV direct georeferencing system and a flight management system (FMS).

Unlike consumer type small format digital cameras, the DSS system has come through careful mechanical design, ruggedization, calibration, and testing. The DSS captures high resolution digital imagery together with their associated direct georeferencing data for various GIS applications such as stereo visualization, feature extraction and classification, orthophoto generation and digital elevation model (DEM) extraction.

Table 1: Specification of The DSS

Array size	4092 X 4077 pixels, 9 micron pixel size
Lenses	Standard: 55mm – Color & CIR Optional: 35mm – Color only
Shutter Speed	1/125 – 1/4000 sec
Max Exposure Rate	2.5 or 4 sec
GSD	0.05 to 1 meter (platform dependent)
Smear	< 10% typical
Housing	Ruggedized exoskeleton with lens stabilization
Positioning accuracy	0.05 – 0.3 meter, post mission
Navigation error	0.008 – 0.015 deg, post mission
FMS	TrackAir EZtrack or external third-party
Northing (cm)	80 GB removable hard drive and pressurized data brick

The DSS has several features that make it an ideal tool for many mapping and GIS applications:

• It is light-weight and easily deployable in small aircraft and pilot only operations
• It has the flexibility to inexpensively collect colour and CIR from one platform
• It takes full advantage of direct georeferencing through the built-in POS AV system and processing tools
• It is fully warranted and calibrated as a complete mapping system: camera, direct georeferencing system and FMS
• It has a low-cost infrastructure: no special computer hardware is required, all post-processing can be done on a lap-top
• It works seamlessly with many of the existing digital mapping packages such as Image Station by Z/I, ERDAS Imagine by Leica, and Socet Set by BAE

In the following sections, an overview of The DSS design, calibration and performance is presented and the system performance in DEM extraction and orthophoto generation is discussed in some details using a real mapping mission flown over southern Ontario, Canada in September, 2003.

2. The DSS Sysyem Design
Although the DSS has been designed to accommodate off-the-shelf components, its design parameters included the aerial survey requirements. Therefore, different components of the DSS have been re-designed and custom machined, such as:

• The digital back is rigidly attached to the camera
• Camera lenses are optimized for focus at infinity using a special locking mechanism
• The camera digital back is modified for both color and CIR imaging
• The camera is mounted in a proprietary exoskeleton designed to keep the lens, the camera body and the IMU rigidly attached to each other
• The CCD chip is calibrated for minimum flaws
• The system has been ruggedized to survive the shock and vibrations exposed to in the airborne mapping environment
• The camera is radiometrically and geometrically calibrated, the IMU is calibrated, and the entire system is well calibrated prior delivery to the user.

The DSS comprises off-the-shelf components. The main advantage of this approach is that there is a clear upgrade path when new CCD and camera technology is introduced into the commercial market place. The DSS specifications are shown in Table 1.


Figure 1: The DSS
3. The DSS System Calibration
Before the DSS can be used for aerial mapping, it must be calibrated geometrically to relate the coordinate frames of the imager, the IMU, and the GPS antenna. Since no agency has been established for the digital camera calibration, this is done in house right after the DSS manufacturing at Applanix through a terrestrial calibration process. Using a calibration cage that is imaged from several angles, the interior orientation parameters (focal length, lens distortions, principle point positions), and the IMU/camera boresight angles are precisely computed.

Following the terrestrial calibration, an airborne calibration is performed for the first time the system is installed onboard an aircraft. A boresight test area is flown and the POSCAL TM software is used to refine the calibration parameters. This completes the DSS calibration process and the results of the airborne calibration are used for successive mapping missions. For details, see Mostafa, 2003.

4. The DSS Applications
One of the growing demands of the GIS and remote sensing industry is for high-resolution, geometrically accurate orthorectified colour and CIR imagery, that is available upon request. While higher resolution satellite imagery is now available (0.6 m), it often takes weeks to collect the area of interest, and even then the final geometric accuracy is only a few meters, at best. (Ref?) Hence aerial imagery is really the only way of meeting the high resolution, accuracy and quick delivery times being demanded.

In order to generate the orthorectified colour and CIR imagery, consisting of either single orthophotos or orthophoto mosaics, the exterior orientation (EO) of each image is required, as well as a Digital Elevation Model (DEM) These needs are fulfilled by the DSS with its high resolution imagery and direct georeferencing system, without the need for GCP and aerial triangulation Using the EO from the POS AV, and stereopair imagery from the DSS, a DEM can be automatically or semi-automatically extracted using high density elevation point determination from photogrammetry softcopies. Since the extracted DEM contains all features in the project area, it is up-to- date and there is no need to worry about terrain changes from existing DEM due to redevelopments in the area of interest. The stereo images can also be used for visualization and feature extraction.

The all-digital workflow and built-in direct georeferencing system of the DSS also provides several new possibilities for “rapid response” applications. For example, with an existing DEM it could be possible to generate lower accuracy orthophotos on the fly during the airborne data acquisition process in near-real time (i.e., before landing). Examples of rapid response applications are forest fire mapping, oil spills, hurricanes, terrorist attacks etc. Even more interesting however, is that full-accuracy orthophoto mosaics can be generated within hours or days (depending upon project size) after landing using differential GPS and the DEM extraction.

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