Tim Furniss/LONDON
ACCORDING TO DR MICHAEL Kobrick of NASA's Jet Propulsion Laboratory, California, "-we have a better global map of Venus than we do for the Earth". He has conceived a joint Space Shuttle mission with the Defense Mapping Agency, of Fairfax, Virginia, and the US Department of Defense.
The aim of the mission is to map 80% of the Earth's surface in close-up and it could be flown earlier than its present projected 2000 launch date if a slot in the Shuttle manifest can be found. The Venus map resulted from the work of the Magellan mapping satellite which, in 1990, went into orbit around the veiled, hot, planet, its radar piercing the thick carbon-dioxide atmosphere. The Shuttle mission will also use a radar system.
THREE-DIMENSIONAL MAP
The 11-day mission, called the Shuttle Radar Topography Mission (SRTM), will use a modified version of the Spaceborne Imaging Radar-C (SIR-C) system which was flown on two Shuttle Radar Laboratory missions, the STS59 and STS68, in 1994. The SRTM is designed to gather three-dimensional (3-D) data of 80% of the Earth's surface, except near the poles, flying in a high-inclination low-Earth orbit, to produce the most accurate and complete topographic map ever to have been assembled.
The 15m-vertical/30m-horizontal resolution (or sharper) images, covering areas which accommodate about 95% of the Earth's population, will be gathered by a radar antenna on a 60m-long mast, an additional C-band imaging antenna and improved navigation and tracking devices on the modified SIR-C. The mast, derived from the International Space Station's truss structure, will extend sideways out of the Shuttle's payload bay, allowing the radar antenna to acquire stereo-like images by means of a technique called interferometry.
MAKING COMPARISONS
Reflected radar from a point on the Earth is received by two sources on the Space Shuttle and data on surface relief are gathered by comparing the difference in the distance which the radar has to travel to the receivers. This technique - not unique to the Shuttle - was tested successfully during the second SIR-C/STS68 mission. The NASA portion of the SRTM will be part of the space agency's Mission to Planet Earth programme.
The 3-D images will then be used in the computer-generation of topographic maps, which will then be used for a range of scientific, civilian and military applications. "With the exception of measurements from weather satellites, the topographic information produced from this mission will be the most universally useful data set about the Earth that NASA has ever produced," says Dr Miriam Baltuk, a space-agency programme scientist.
"Possible applications range from scientific uses such as planetary geophysics or hydrologic drainage-system modelling, to more realistic flight simulators for military aircraft; for mission planning, simulation and rehearsal, to commercial uses, like finding better locations for cellular-telephone towers, and improved maps," says Baltuk.
Kobrick says: "Since radars can see right through clouds, the STRM's 11-day flight will give us enough data to produce an image of the Earth 30 times more precise than any that currently exist, and the best part is that the image will be in 3-D."
CIVIL AND MILITARY USES
The Defense Mapping Agency, which will pay for most of the $150 million mission, plans to use the radar data to fulfil a joint defence requirement for a digital global-terrain-elevation map, with data points spaced about every 30m (100ft). This compares with an agency digital terrain map of 65% of the Earth's surface with data points every 100m. Completion of this data set has been hampered by the lack of cloud-free photos over major areas of the world.
Although there are several other radar satellites, such as the European Space Agency's remote-sensing satellites and Canada's Radarsat, there is no uniform global coverage of the Earth in the detail which is expected from the SRTM. Other sources available to the Defense Mapping Agency are multi-spectral and panchromatic images from remote-sensing satellites such as France's Spot and the US Landsat. The X-band synthetic-aperture-radar system, built by Germany with Italy, and which was flown on the Spaceborne Radar Laboratory Shuttle missions, is not yet included on the SRTM, but may be if NASA agrees to a German proposal.
The US Department of Defense and the National Reconnaissance Office have other sources of data sets. These are classified images taken from various spy satellites using high-power panchromatic and multi-spectral cameras, such as the Advanced KH-11 (a Hubble Space Telescope which at the Earth, rather than at the stars) and the Lacrosse radar-reconnaissance satellite. Images from the French-Italian-Spanish Helios and from Russian reconnaissance satellites may also be made available.
SENSITIVE IMAGES
Some of this imagery is being used in the production of the terrain maps and for other defence applications, but is not attributed as such, nor released for commercial use. Similarly, some sensitive images from the SRTM will remain classified. Some images from the SRL were withheld by NASA at the Pentagon's request.
Source: Flight International
