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I recently met with Jeff Leonard, President and General Manager of EarthData International in Frederick, Maryland, and Tom Harrington, President and General Manager of EarthData Aviation in Hagerstown, Maryland, to discuss the history and the status of GeoSAR (see sidebar on page 77 for a complete explanation of GeoSAR, what it is, and how it works). GeoSAR was first used commercially on a 40,000square-kilometer NOAA Coastal Services Center project in Southern California (Southern California Coastal Water Resources Program). Completed in May of this year, the project also was the first large-scale collection of GeoSAR data. Deliverables included DEMs and image maps in USGS digital orthophoto quad format.
The GeoSAR system operates on some of the same radio frequencies used in highly populated areas like Southern California. For example, air traffic control and military communications utilize some of the same frequencies as GeoSAR. Prior to each collection, to avoid interfering with the operations of other frequency users within the project area, EarthData mission planners obtain permission from the national frequency manager to transmit on the GeoSAR frequencies. Through a unique feature of EarthData’s flight planning software, frequencies not approved for use can be "notched," or disabled, in the program that automatically operates the radar system. While notching P-band frequencies reduces foliage-penetrating capability, notching complies with the requirements to protect other users and optimize GeoSAR operations by avoiding the frequencies not approved for use. In unpopulated, and often unmapped, areas where there are no frequency restrictions, GeoSAR’s powerful continued > P-band can be used to its full potential.
The perfect example of the P-band operating at its full potential is a recent NGA project in Colombia, South America, where EarthData used GeoSAR to map the Cano-Limon oil pipeline, a project that covered nearly 94,000 square kilometers. Up until that time, even the best maps of this inaccessible jungle area contained voids and were outdated. While this was an ideal project for EarthData’s second commercial GeoSAR project, there were a number of serious challenges.
The dramatic variation in terrain elevation–5,000 meters from the coast to the Andes–and the ruggedness of the terrain presented the first challenge. Careful flight planning ensured that the rugged terrain did not "shadow" the radar coverage of the ground. Additionally, because area maps were outdated, precise locations of international borders were unknown. Concern for creating an international incident manifested as extreme caution in maintaining a buffer zone to prevent inadvertent border crossing. Flight planners used NGA maps and ancillary data sources such as SRTM data to make certain flight crews acquired complete coverage with high-quality data.
Difficult Project Conditions
The installation of ground radar reflectors presented another challenge. Although plans indicated the placement of six reflectors to provide control, limited access and heightened security concerns resulted in the ground crews installing only three reflectors. Similarly, planners would have preferred three GPS base stations but ground crews could establish only two.
One of the biggest challenges was the security of personnel, aircraft, and equipment. EarthData took extraordinary precautions, even hiring a local security company for added protection. The company coordinated all flight activity with civilian and military agencies that controlled Colombian airspace. Leonard said the project would not have been successful without the cooperation and support of these groups.
22 Terabytes Collected
After months of planning and preparation, project staff completed all the pre-deployment logistics, and the mission began. The crew of three pilots, one aircraft mechanic, three radar technicians, and a GPS operator arrived in Colombia ready for flight operations.
At the end of the project, 478 tapes were needed to store the more than 22 terabytes of data collected and returning the nearly 3,000 pounds of tapes to the United States proved to be quite a challenge. To keep data from being lost or damaged during shipment, a courier periodically handcarried tapes back to EarthData’s processing facility in Frederick, Maryland. This laborious procedure proved to be an effective way both to secure the data and to return it as soon as possible for processing. G PS data, on the other hand, was much easier to transport. After processing the GPS data in the field, the technician used a high-speed Internet connection at a nearby Colombian Navy facility to send the data back to Frederick each day.
GeoSAR is just one of the mapping tools in EarthData’s suite of airborne sensors, yet its capabilities for global mapping are revolutionary. Tropical jungle areas–some with triple canopy layers–can now be penetrated by GeoSAR technology, enabling mappers to view ground features such as trails, buildings, and other man-made structures heretofore hidden from view.
With the successes of these first commercial projects, EarthData plans to use GeoSAR on other U.S. and international projects. Foreign national mapping organizations, as well as oil companies, mineral exploration companies, archeologists, and strategic partners and affiliates throughout the world have expressed interest in acquiring GeoSAR data. Even with these successful missions, EarthData continues to be cautious and conservative in mission planning and product development. Unlike lidar, which now has many equipment vendors and service providers, GeoSAR remains a one-of-akind system; its complexities and capabilities continue to be tested and developed as new applications arise and new users come forward.
Marc Cheves is editor of the magazine.
Sidebar:
GeoSAR By The Numbers
Most East Coast aerial data acquisition flights take place around mid-day in the winter when leafless trees and high sun angle provide the best conditions for sensors to collect ground feature data. Once the leaves emerge, acquiring data that accurately portrays the ground surface and elevations beneath the tree canopy becomes virtually impossible. This challenge is especially great in areas of the world where seasonal change has little impact on foliage canopy cover and where year-round cloud cover further obscures the ground.
A new airborne mapping technology, GeoSAR, introduced at Washington’s Reagan National Airport in the summer of 2001, has virtually eliminated this problem by using radar to gather both the elevations of the tops of the trees and the ground beneath them. GeoSAR is a dual band, interferometric synthetic aperture radar (SAR) system that was developed in cooperation with the California Institute of Technology Jet Propulsion Lab (JPL), the National GeospatialIntelligence Agency (NGA), and EarthData. Work on the system began in 1995 under a contract from the Defense Advanced Research Projects Agency (DARPA). From the beginning, the project’s primary goal was to develop GeoSAR into a fully operational system to be used in commercial applications, and this is where EarthData came in.
Two of the end-products of this technology are Digital Elevation Models (DEMs) and orthorectified radar reflectance maps (also known as magnitude image maps), which provide information for applications like land use/land cover mapping and terrain analysis, among others. Topographic mapping with 3-meter contours is possible, and planimetric mapping can be don
e at 1:24,000 scale and larger. Military uses include mobilization planning, target detection and other specialized analyses. GeoSAR terrain models also can be used to orthorectify satellite imagery and supplement data collected by NASA’s Shuttle Radar Topography Mission (SRTM), which has a ±10-meter vertical accuracy.
160 Square Kilometers per Minute
Installed on EarthData’s Gulfstream II jet aircraft, the GeoSAR system consists of radar transmitters, receivers, and recording equipment located in the cabin as well as antennas mounted outside the aircraft. The X-band system, with antenna pods located under each wing, emits 3-centimeter-wavelength pulses, which are reflected off the tops of vegetation (or any other "first surface" encountered). P-band antennas, located in wingtip pods, emit 86-centimeter-wavelength pulses capable of penetrating vegetation. Both X- and P-band pulses are reflected back to the antennas and then routed to a tape system in the cabin where the data is stored. As shown in the diagram, each antenna is capable of covering a swath on both sides of the aircraft, allowing simultaneous collection of X- and P-band data in each of two swaths, each approximately 10 kilometers width. Aerial operations overlap flight lines to provide coverage of the space between the swaths directly beneath the aircraft. As a result, some points on the ground are covered eight times.
With the aircraft operating at over 30,000 feet above the ground, even minute movements of the wing-mounted antennas can greatly displace the swath on the ground. To compensate for this, the system designers developed a laser system, mounted in a pod beneath the fuselage of the aircraft, that precisely measures the position of each of the four antenna to an accuracy of ±0.3 millimeter. This data is recorded onboard the aircraft and subsequently incorporated when the radar data is processed.
Data from the four antennas is processed using interferometry, which has been used in astronomy and for industrial applications for many years. In airborne remote sensing, SAR interferometry determines the topographic height of the observed surface by combining the phase history of the radar data collected by two separated SAR antennas looking at the same surface at the same time.
Terrain "canopy" elevation data obtained by interferometric processing of GeoSAR’s X-band data yields vertical accuracies of 0.5-1.2 meters and planimetric accuracies of 1-3 meters. Ground elevation derived from the P-band yields vertical accuracies of 1-4 meters and planimetric accuracies of 3-5 meters, depending on the type of vegetation cover (height, moisture, density, etc.).
Capable of acquiring data regardless of cloud cover, day or night, the system gathers data an at enormous rate. The two 10-kilometer-wide swaths are collected at a rate of 160 square kilometers per minute. The high-speed tape system in the aircraft stores data at the rate of 1gigabyte per second, which means a flight’s worth of data can amount to one terabyte.
A 1.516Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE