NASA's DC-8 airborne science laboratory has been providing data and images of South America to scientists from diverse disciplines
Soaring the length of South America and beyond over the past three weeks, NASA's McDonnell Douglas DC-8 airborne science laboratory has been hunting for everything from lost civilisations to signs of climate change in what is almost certainly the most comprehensive aircraft-based survey of the region ever undertaken over such a short period.
The key tool for the mission, and the "eye" that sees in night and day through clouds, dense jungle canopies, sand and debris-covered ice, is the Jet Propulsion Laboratory-developed AirSAR (airborne synthetic-aperture radar) system. Using its high-resolution sensors to produce topographic models, scientists hope to use the data to help map and preserve resources, unearth new archaeological sites and uncover clues to climatic change. During approximately 100 flight hours, the team expected to gather polarimetric as well as interferometric data along a 20,000km (10,800nm) track, or roughly 200,000 km2 (77,000 miles2) of data on around 40 sites.
The sensor platform is a converted DC-8-72, originally delivered in 1969 as a -62 to Italian flag-carrier Alitalia. After a short career with Braniff, the DC-8 was used as a testbed for Quiet Nacelle's hushkit system before being re-engined with CFM56-2 turbofans in March 1986. The following month it officially joined the NASA test fleet and was extensively modified for its new role. Modifications include aerosol sampling pylon attachments; a gyro-stabilised pointing and tracking mirror system; a dropsonde delivery system; atmospheric chemistry sampling probes; and several reinforced ports and apertures for other experimental systems including the AirSAR. The aircraft was also retrofitted with an enhanced weather radar, integrated inertial navigation system, a satellite-based time counter, a satellite weather receiving station and a global positioning system.
Survey targets
One of the first targets was a series of overflights over southern Mexico and the Yucatan Peninsula, El Salvador, Guatemala and Honduras, during which the team searched for undiscovered traces of the Mayan civilisation using the foliage penetrating radar. The survey, which extended south to include parts of Nicaragua, Costa Rica and Panama, was principally aimed at mapping the extent and distribution of the Mayans (and other related pre-Colombus societies) which traditionally were not thought to have occupied the most southernmost parts of these modern day Central American countries. Archaeologists hope to repeat the successes of late 1997 when new evidence of a prehistoric civilisation and traces of ancient temples around Angkor in Cambodia were unearthed using imagery gathered by the NASA/JPL AirSAR team.
The main aim of the AirSAR data interpreters was to find evidence of fortifications, causeways, walls and other tell-tale traces of an advanced civilisation. The dense vegetation had proved largely impenetrable to previous remote-sensing surveys, and the terrain had limited the extent of conventional archaeological surveys. They believe that careful interpretation of the imagery, some of it revealing the underlying features for the first time in over 1,000 years, will give new clues as to the lives and fate of ancient peoples.
Human impacts
During the same overflights, the team's next aim was to investigate new techniques for the measurement of the actual "structure" of dense tropical forests. According to NASA associate administrator for Earth science Ghassem Asrar, the "AirSAR campaign provides unique data not available from other space-borne or commercial observational platforms that will help scientists characterise past and present human impacts on the landscape". The focus of the first phase of the mission is Central America, where Asrar says the region's "unique environment and irreplaceable archaeology are being altered and destroyed at an alarming rate". Ground teams surveyed flora and fauna and placed large radar reflector targets at co-ordinated locations to help calibrate the AirSAR.
Moving farther south, the team's next major stop was Santiago in Chile before it flew to Punta Arenas at the southern end of the American continent. From this base by the shores of the Strait of Magellan, survey flights headed north to the Patagonian ice fields and south to the Antarctic Peninsula, covering the ice-covered landmass down to around 70°S. Imagery and high-precision topographic data was collected to help determine how melting glaciers in the southern hemisphere are contributing to sea level rise. Some of the concern was sparked by the results of a recent climate change study in Patagonia, which showed that, compared with the previous 25 years, the overall contribution to this sea rise more than doubled from 1995 to 2000.
Over the Antarctic Peninsula the AirSAR was used to collect data on the thickness of the ice shelves, and their related contribution to sea level rise. This data will also provide a precise topographic reference for comparison with satellite laser-altimetery data from NASA's Icesat satellite and other airborne data. This latter role is a key role for the DC-8 and AirSAR combination, which was also tasked with filling in the largest "void" in the South American topography derived from the Space Shuttle radar topography mission. This was undertaken by a modified space-based radar system that flew on the Endeavour during an 11-day mission in February 2000, during which elevation data was collected for most of the Earth. This created the most complete high-resolution digital topographic database of the planet, but several pockets of territory were too hard to map. They included some of South America's extreme southern latitudes, where deep Andean valleys prevented precise data from being collected, and required a second look from AirSAR.
The rest of 2004 is already booked up for the hard-working DC-8 and its crew. Later this year it will participate in the International Chemical Transport Experiment - North America mission, which involves "sniffing" its way through the skies of the US East Coast looking for pollutants heading towards Europe. It will also take the AirSAR to survey parts of Alaska in another science mission before the end of 2004, before preparing for a new series of missions in 2005-6. By then the bulk of the data from this year's Latin American survey will be in the hands of scientists, climatologists and archaeologists - undoubtedly to the benefit of all.
GUY NORRIS / LOS ANGELES
Source: Flight International
