I spoke to Dejan Damjanovic, GeoEye's program manager for airfield and harbor mapping, to figure out what NGA wanted and how their company could deliver. One thing really makes this 3D mapping possible, he explained: the fact that IKONOS (and OrbView-3 for that matter) can take stereo images. Not all satellite sensors can do that. To create stereo imagery other systems have to take one image, then wait until they return to the same place to take a second one. That, as you might expect, produces a lesser product since things - the landscape, cloud cover, etc. - might have changed in between the two visits. IKONOS, in contrast, snaps both images at nearly the same time.
The data that come back to earth from the satellite are 3D in the sense that each pixel has an assigned "Z" value, but they still need to be read and, in the case of NGA, delivered as 3D shape files. Not only that, the data must meet NGA and several other international standards organizations' (ANSI, EUROCAE and RTCA) mapping requirements. Damjanovic is one of the experts who participates in the creation of these standards. That, he suggests, gives the company an edge in this sort of work. GeoEye, along with NGA and the Federal Aviation Administration (FAA), all use the same tools for data extraction: BAE SYSTEMS' Socet Set.
What are all the data for? "Safety of flight," Damjanovic says. In short, it's all about getting aircraft into and out of airports safely. The databases help NGA scout the safest paths and allow companies like FlightSafety to build simulators so that pilots can be familiar with hazards before they fly into airports for the first time. In fact, the whole program, called the Stereo Airfield Collection program, came out of U.S. Secretary of Commerce Ron Brown's death in 1996. Brown and the CEOs of a number of firms lost their lives when their Air Force plane crashed into the side of a mountain on the way into the Dubrovnik (Croatia) airport.
Before the initiative, such data were created from classified imagery. Today, the commercial sector collects the data and creates the products. But there's another way to collect high resolution 3D data: LiDAR. Why did NGA not want to use LiDAR? Damjanovic reminded me that the airports NGA studies are located all over the world, not just in the U.S. In the U.S., he explained, there are plenty of LiDAR resources. But overseas, there are far fewer. Also, each system is a bit different, so that even if they all met the specifications, they would not all be "the same." Using satellite images from the same sensor ensures that all the 3D databases are created from the same instrument and thus are more comparable.
This latest contract, which is the company's third opportunity to provide 3D mapping of airports, requires GeoEye to map 365 airports. How do they do that? First recall that IKONOS revisits each place on the earth every three days. So, ideally, each airport could be "shot" 120 times a year. But there are clouds, so not every pass is likely to produce acceptable imagery. GeoEye uses a global cloud model to predict when it's worth trying to get a picture of each one of the airports. If there's cloud cover, that pass might be used to capture some other area. All things being equal, any airport is likely to be captured in a one to two month window. A 15-20 person team at GeoEye is responsible for the 365 models.
Where is this sort of work headed? Damjanovic notes that the largest increase in air traffic worldwide is now in the third world, so it's likely that those airports will need mapping. He also notes that airports with more traffic will probably need to be mapped more often. The other growth area is in another transportation arena prone to traffic hazards: ports and harbors. Opportunities to map and create simulators for that sector are appearing, too. But, transportation safety is only one use for satellite capture stereo images. GeoEye clients already use the imagery to create 3D models of buildings and other structures.