National Mapping Programs are Affordable Again
The Need for Improvement
The need for improved remote sensing technologies that can quickly determine accurate terrain elevations over large areas evolved from military requirements. Operational requirements dictate the need for an all (reasonable) weather, day/night, high-speed collection and processing capability. To address these needs, the United States Defense Advanced Research Projects Agency (DARPA) initiated a program in 1993 to develop operational airborne IFSAR capability. Intermap acquired this technology in 1997 In response to DARPA’s mandate to ensure commercial utilization of the technology, Intermap invested several millions of dollars in the IFSAR technology. Intermap, which has been involved in airborne radar mapping since the mid-1970s, recognized the commercial benefits of interferometric radar to generate Digital Elevation Models (DEMs) and thus topographic map products, as well as fully Orthorectified Radar Images (ORIs). Since acquiring the technology in 1997, Intermap has been responsible for producing 1 million square kilometres of IFSAR generated DEMs and ORRIs. IFSAR Technology The interferometric technology is based on utilizing two radar antennae displaced by a known distance. This antenna separation is referred to as the interferometric baseline. One antenna acts as both a transmitter and receiver; the second as a receiver only. The baseline provides a slightly different path length in the reflection of the radar pulses from terrain points back to the antennae. This path length difference, or phase difference, coupled with precise aircraft positional data, provides the information required to measure the terrain elevation points IFSAR for topographic mapping uses two apertures separated by a "baseline" to image the surface. The phase difference between the apertures for each image point, along with the range and knowledge of the baseline, is used to infer the precise topographic height of the terrain being image. Intermap’s IFSAR system, is a 3cm wavelength, X-band interferometer operating on Learjet commercial aircraft. Typical data acquisitions are for areas of 10 km across-track (range direction) and 50-200 km along track (azimuth direction), collected at a coverage rate of up to 100km2 every minute. The output of high precision IFSAR datasets is accomplished by on-board laser-based inertia measurement data navigational and differential global positioning system (DGPS) processing to determine the precise position of the Learjet. This IFSAR system was recently modified to increase DEM relative performance, achieving up to 50 cm vertical RMSE, and to increase the orthorectified radar image pixel resolution to 1.25 meter from 2.5 meter. The STAR-3i system is capable of collecting +/- 1-metre vertical data from a flight altitude of 30,000 feet and +/-50-cm from a flight altitude of 20,000 feet. The image resolution remains constant, regardless of the flying altitude and has a positional accuracy of better then 3m.  Figure 2: Learjet and Aero Commander equipped with IFSAR mapping technology |