| Interferometric Synthetic Aperture Radar Technology for Mining
By John Ellis In September 1998, AeroMap U.S. of Anchorage, Alaska, was awarded Grant No. 98-3-101 from the Alaska Science and Technology Foundation. The purpose of this project was to evaluate innovative remote sensing technologies, with the intention of demonstrating new solutions to mining problems. Project Goals
For this project AeroMap acquired data, developed products, and provided samples of the products and original data to Alaskan miners for their evaluation and use as a learning and product-familiarization tool.
A 100-square-mile area that surrounds the Fort Knox Mine was selected as the primary site. This is a large gold mining site located about 25 miles from Fairbanks, Alaska. The primary reason for selecting this site was the wealth of available survey information used to ground truth project data, and the geophysical data and geology maps of the area as provided by the Division of Geophysical and Geological Surveys. Fairbanks Gold Mining Inc. agreed to provide ground surveys and existing map data, and to help evaluate the products as applied to mining. Alaskan Weather
The weather in October at the selected site is typically overcast or partly cloudy, with little opportunity for cloud-free and snow-free aerial or satellite image acquisition. The Intermap Technologies Inc. STAR-3i active system uses microwave electromagnetic energy, instead of light, to generate image and spatial data. The active sensor provides its own radar illumination, guaranteeing collection of data in a timely manner. Unlike photographic systems, the Intermap system is not dependent upon clear weather. All flights were scheduled to acquire data during the night to avoid turbulence and adverse flying conditions. STAR-3i System
The Data Acquisition portion of the contract was accomplished using a SAR interferometer installed on Intermap's Learjet 36A. ERIM and JPL developed the system known as IFSARE (Interferometric Synthetic Aperture Radar Elevation). In transition to operational status, the system has been named STAR-3i by Intermap, identifying the system as the third generation of SAR systems operated by the company. System operation typically requires a field support crew of two to three people, depending upon the type of project. The aircraft is typically flown at altitudes ranging from 20,000 to 40,000 feet above ground level (AGL) depending upon the specifications of the project. For this project, the aircraft was flown at an altitude of 20,000 AGL.
The STAR-3i system is comprised of sensor and processor segments. The sensor segment consists of a dual-channel SAR, an inertial motion measurement unit (IMU) and a Global Positioning System (GPS) unit housed in a Learjet 36A. A second differential GPS unit, used to provide ground control reference data, is located on the ground within 200km of the collection site. The processor segment consists of the position/attitude, image formation, and digital terrain elevation processors. This combination of hardware and software makes it possible to compute very high accuracy results in the Digital Elevation Model map products. IFSARE - How it works
Radar is a ranging system. It measures distances based on the time it takes for the radar beam to reach the target and return to the antenna. The system uses the phase difference between the signal response to antennas #1 and #2 to compute an accurate elevation calculation using an interferogram. The combination of X-band radar and interferometry provides for very accurate elevation calculations, resulting in a very high resolution DEM and an orthorectified radar image (ORI).
The differential GPS and the IMU systems and processing are used for precision orientation measurements of the sensor platform (the Learjet 36A). The combination of these precise measurements and IFSARE processing provides superior accuracy for calculating elevations and their horizontal locations. Quality Control - Airborne GPS/IMU
For this project, two independent, quality control measures for verifying vertical and horizontal data accuracy were used. Ground control was provided by Fairbanks Gold Mining Inc. AeroMap conducted an independent airborne survey using their Cessna aircraft equipped with GPS/IMU sensors. A base station unit was used to complete the differential GPS survey.
Each of the ground control surveys was overlaid on the STAR-3i-derived DEM of the 100-square-mile project area. The quality control surveys extended beyond the project boundary. The two surveys verified that the Intermap data met performance specifications. The two surveys were independent of each other, with multiple common points. The common points were used to validate the surveys. Digital Elevation Model (DEM)
Next the DEM was received with nominal one-meter vertical accuracy and nominal 2.5-meter horizontal accuracy with five-meter postings. The flying height was 6100 meters above mean terrain. Based on these tolerances, the data is suitable for 1:5000 to 1:10,000 scale map products. The STAR-3i terrain elevation data was delivered in ASCII format.
The DEM product covered more than just the 100-square-mile area of interest. The area needed was clipped out and prepared for a series of products. ERDAS Imagine software was used for the initial work, but the goal was to use the data in a GIS. So the data was exported to ARC/Info, ArcView, PCI, ER Mapper, and other formats.
A series of contour products was prepared: generating five- and 10-meter contour maps, and overlaying them on a shaded relief map that was also derived from the DEM. These contours defined the terrain in surprising detail. A two-meter contour product was developed to demonstrate the great detail for defining terrain. The exceptional detail is due to the dense five-meter posting of the points.
The DEM was used to prepare an orthophoto from 1:30,000 scale aerial photography and compared it to a photogrammetric method. No appreciable difference for 1:5000-scale orthophoto products was found. Orthophotos were used to register various other sources of image data, including airborne multispectral and satellite data.
A series of topographic maps were prepared from DEM and ORI data tiles that covered the 1:25,000-scale Fairbanks (D-1) NW, Alaska, quadrangle map. The mapping exercise was accomplished to demonstrate various products that could be developed directly from the DEM and ORI data, all without much additional editing. The map products created include a DEM map; a monochromatic shaded-relief map; a 10-meter topographic map; a five-meter topographic map; an orthorectified radar image map; a thematic map with 10-meter contours; a thematic map (the USGS themes were updated using the ORI); and a color elevation image map. Updating the USGS thematic layers-using the new ORI as a reference and copying the text took the longest of all the tasks, about eight hours. Compiling the map products took approximately two hours each. Of course, further refinement of the contours could be accomplished by using a stereo softcopy system, but a project objective was to demonstrate the value of the raw data. Vector Linework and ORI
Intermap prepared samples of vector mapping that covers a portion of the Fairbanks mining district (Figure 4).
Map compilation was accomplished by using stereo mapping techniques at a softcopy photogrammetric workstation, where the ORI was viewed in three dimensions over the digital elevation model produced by the STAR-3i system. Viewing the data in 3D allows the workstation operator to position the linework and point features in their correct perspective and location, much the same way traditional maps are produced from aerial photography.
Closely comparing this linework with the scanned USGS quad sheet easily indicated where significant changes occurred in both cultural and natural features. The most notable differences were seen in the road network, located on the hillsides immediately north of the city. Summary
Based on feedback from the Alaska mining community, the STAR-3i data products were easily the most useful and versatile for solving mining problems. The high-resolution digital elevation model (DEM) was used to evaluate or enhance many of the other data products, including geology maps, geophysical data, various satellite data, and photographic orthophoto products.
The innovative interferometric SAR technology, operated as the STAR-3i system by Intermap, offers valuable map products as related to exploration, development, production and reclamation for the mining industry. Whether it is for industry or government, it was found that there are as many new applications for this new technology as there are map-related problems. The results of the Alaska Science and Technology Foundation study, conducted by AeroMap, are accessible via their web site at http://www.astf.aeromap.com. Details on the Intermap STAR-3i system are available at http://www.intermaptechnologies.com. Back |
