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Technical description and specifications of the Nortech ATLAS

1. Background
2. VL System Overview
3. VL System Specifications
4. SL System Overview
5. SL System Specifications
6. Projects and Deliverables

1. Background

Nortech Geomatics Inc. has been a pioneer in the development of airborne laser profiling, and terrain contour modeling for more than a decade. The first generation systems involved the integration of large, expensive and complex inertial attitude sensors with mechanically stabilized platforms. As technology progressed, Nortech developed several subsequent generations of profiling systems which include; the Airborne Laser Terrain Profiler (ALTP), Digital Video Geographic (DVG) and All Terrain Laser Acquisition System - Vertical Laser (ATLAS-VL) vertically stabilized profiling systems. These systems evolved by incorporating the latest technology available such as; GPS satellite positioning, advanced computer processing and high-resolution SVHS video imagery. The currently operational stabilized platform systems have been developed into compact, flexible systems that have been engineered around an in-depth knowledge of the requirements for operations, logistics and cost effect functionality.

The VL systems produce a single linear profile of the terrain directly below the helicopter flight path and are typically flown in parallel flight lines to permit topographical surface modeling using advanced modeling techniques. The vertical stabilization aspect of this system makes it ideal for projects located in rugged and/or heavily forested areas. However, this advantage is not cost effectively transferred to areas which are flat or are sparsely vegetated since the multiple flight line aspect contributes to higher aircraft costs. To permit entry into this market, Nortech has developed a swath profiling system, which includes high-resolution digital imagery. This combination with the addition of a precise attitude reference system permits single pass corridor profiling and minimizes the number of parallel passes required in a block area project. Time and cost savings realized are passed directly to the client. Additionally, the final product is improved since the type and accuracy of the data collected allows for a wider variety of post project deliverable options.

2. ATLAS-VL Vertical Laser System Overview

The ATLAS-VL system is a vertically stabilized, helicopter-borne, profiling platform, which performs coordinated laser profiling and high-resolution videography for linear projects and harsh or operationally difficult environment areas. It permits non-intrusive field mapping information gathering to occur in sensitive areas or areas that do not facilitate ground operations.

The system incorporates a two axis vertically stabilized platform housing a high-repetition rate ranging laser, and a high resolution SVHS video camera system. The information from these systems are coordinated with the data from the satellite based GPS, global positioning system, and a helicopter airframe attitude reference. A second SVHS video camera is also mounted in a forward oblique angle providing a perspective view supplementing the corridor information.

Primarily, it has been the location and types of projects, which has driven the necessity for the use of helicopters as opposed to other aircraft. Since most projects are in heavily forested areas, the ability to slow down the forward data collection speed is important to ensure adequate laser penetration of the forest canopy. The present ATLAS-VL system has been flight safety approved by the U.S. FAA and Canadian DOT for operations in the Bell 206B, Aerospatiale 350 and the Aerospatiale MBB 105B helicopters. These airframes have been selected for their international availability and reasonable operating costs. Recent requests from clients have prompted an evaluation of potentially developing a fixed wing mounting kit for longer corridor projects.

3. ATLAS-VL System Specifications

The laser provides a 2,000 Hz eye-safe infrared pulse stream, which is vertically aligned to the gravity vector and stabilized by a 2-axis platform. The platform uses real-time feedback from a vertical reference unit (VRU) to drive the platform motors to ensure that the laser is always pointing in the vertical position. This vertical aspect is crucial for accurate DTM modeling in areas of heavy forestation. The platform also contains the high-resolution video camera for imaging of the project area's terrain features. The video imaging is recorded onto a SVHS video recorder where each frame has been time tagged by a VITC code for coordination with the GPS positioning information.

Normal flight operating parameters for the system will be 100m AGL in elevation and a forward velocity of 80 kph. This translates to a laser range density, with a forward separation of 0.01m. The beam spot size at this altitude is 25cm and can produce multiple surface reflections when passing through foliage. To accommodate this, the multiple returns are analyzed to record the longest, second longest and shortest range returns. This produces an ability to extract the canopy profile as well as the ground contour, which can be used in forest volumetric aspects.

The specifications of the laser ranging are typically 20cm in absolute accuracy and 2cm in relative accuracy. The vertical profiling lines are flown in combination using a series of parallel flight lines to provide adequate data for the construction of surface model. The spacing of these parallel lines is dependent on the type of terrain encountered and the overall requested accuracy desired by the client. Nominally the spacing is 50m in flat areas, 25m in low slope rolling hills and 10m - 15m in steep/erratic terrain. This laser ranging data combines with the post mission differential phase GPS positioning and results in a system which provides half meter DTM modeling with relative accuracy approaching the 20cm level.

The DTM model is then used in the feature extraction form the video imagery. The SVHS recorded imagery is reviewed and captured in a frame by frame technique in the areas that contain features of interest. These frames are then coordinated with laser range data to permit horizontal spatial placement of all visible features. The SVHS videography can also be annotated with information in a post mission process to add client requested information to the videotape for subsequent review and analysis. Both the forward and vertical video can be annotated since the GPS time is encrypted in a binary fashion on the top line of each video frame. In this fashion, the initial tape recording can be re-mastered into different series of tapes containing different information such as; time, position, chainage, IP proximity, waypoint, etc. These tapes have been utilized for project planning, environmental impact assessment and vegetation analysis.

4. ATLAS-SL System Overview

The ATLAS-SL system has been designed around a multi-platform installation, which includes aircraft, both fixed and rotary wing, as well as terrestrial vehicles such as truck and train rail car options. The system incorporates a high-repetition rate scanning laser, a high-accuracy, digital 3-dimensional inertial attitude reference and GPS positioning system, a Class 1 digital camera and an advanced navigation/data logging computer interface. These component systems culminate in an integrated system that combines swath laser profiling with digital imagery in a "mathematically" leveled methodology.

Since no mechanical stabilization is attempted, the integration and subsequent component packaging can be reconfigured to meet any installation requirement. The initial installation has been designed around a fixed wing airframe to facilitate operation in the established market for airborne mapping and profiling. Subsequent installation kits such as a truck mounted are under scrutiny to evaluate data collection for highway inventory systems. This design flexibility of the ATLAS-SL system permits an ease of adaptation to different scenarios driven by the client's requirements.

5. ATLAS-SL System Specifications

The laser provides a 12,000 Hz eye-safe infrared pulse at a scan rate of 20 Hz over a 60° field of sweep. Normal flight operating parameters for the system will be 260m AGL in elevation and a forward velocity of 180 kph. This translates to a laser range density of 200 points for each scan line. Each scan line will contain successful laser ranges, which have a forward separation of 0.004m and a lateral separation of 1.5m. The combination of the subsequent scan lines will produce a pseudo-grid of approximately 2.5m x 1.5m assuming stable flight. The specifications of the laser ranging are typically 5cm and at worst case 20cm in absolute accuracy and 2cm in relative accuracy. This combines with the attitude and positioning accuracy, better than 20cm typically achieved by present GPS differential phase techniques, to result in a system which will confidently provide sub half meter DTM modeling with relative accuracy approaching the 10cm level.

Digital imagery will be combined with the DTM produced by the scanning laser to permit ortho-rectification of the digital imagery and subsequent planimetric feature mapping. The digital imagery is captured by an 8 bit, 2000 x 2000 pixel, CCD array camera using a 42° field of view lens. This combination results in a typical image size of 200m x 200m at the previously specified flying parameters. Since the imagery is not set to provide stereoscopic height information, there is no need to capture the imagery at high overlap ratios. The data capture rate for providing 10% image overlap from the preceding image would be an interval of 4.4 seconds at a 1/1000 shutter speed to minimize pixel blurring due to forward motion. The image resolution at these parameters produces a pixel size of 10cm x 10cm. Higher resolution is possible by reducing the data acquisition altitude of the aircraft.

6. Projects

As in the previous laser mapping systems, ATLAS - SL has been designed primarily for linear route location, mapping and design. However, the modifications and improvements in technology incorporated into ATLAS - SL do allow for greater scope as well as improvements in the product deliverables.

There are a number of areas which ATLAS - SL technology is applicable to. These broadly defined areas are as follows:

• Electric Utility (Transmission Lines)
• Industrial (Pipelines)
• Mining and Minerals
• Environmental

6.1 Final Deliverables:

Forward-Looking Digital Video - annotated with the following items:

• Time/Date
• Position
• Along Track Chainage
• Cross Track Offset
• From/To Stations
• Voice Over Commentary

Generation of Digital Terrain Models:

• Ground Contours
• Extracted Profiles

Planimeteric Data from Digital Imagery:

• Vector Files of Digitized Features - (e.g. roads, buildings)
• Measurements of Features - (e.g. tree crown diameter)
• Mosaicked Imagery
• Images on CD (for GIS import)

Mapping

• Plan/Profile Sheets
• AutoCad DWG. or DXF Format
• MicroStation .DGN files
• Digital and Hard Copy Plots
• Mosaicked Alignment Sheets