Tim Furniss/LONDON

One of the few Space Shuttle flights guaranteed a launch date next year is the 11-day mission of the STS99/Endeavour.Because the flight is not related to the International Space Station (ISS) project, it is not subject to the schedule uncertainties that have plagued the ISS.

The STS99 will be dedicated to ecological mapping of almost the whole of the earth's surface, using a global digital elevation model with two radar instruments operating simultaneously. This radar interferometry is the most technologically advanced method of gathering topographic information.

The crew of the STS99 Shuttle Radar Topography Mission (SRTM), scheduled for 16 September 1999, has been named. Commander Kevin Kregal and pilot Dominic Gorie will be joined by mission specialists Janet Kavandi and Janice Voss, Japan's Mamouri Mohri and the European Space Agency's German astronaut, Gerhard Thiele. Germany's connection with this flight is not surprising because the most significant payload is being supplied by Deutsche Aerospace Space Systems in Friedrichshafen and Munich.

The German X-Band Synthetic Aperture Radar (SAR) antenna, produced with Italian assistance, is being shipped to NASA's Jet Propulsion Laboratory in California to be assembled with the US-supplied interferometer and a Spaceborne Imaging Radar SIR-C instrument. The SRTM is a co-operative project between NASA and the US Department of Defense (DoD) National Image Mapping Agency, marking the first large DoD association with the Shuttle programme since the STS53 Discovery mission in December 1992 was dedicated to military applications.

The SIR-C and SAR-X instruments flew twice on the Shuttle Endeavour missions STS59 and 68 in 1994, to test the radar technology and provided radar images to monitor changes on the earth's surface.

Both missions demonstrated that this type of radar data could be used for exact land use classification, determination of the biomass of forests, and of the extent of floods, oil spills and ice on the earth's oceans.

The SRTM will go further. The key SRTM technology is radar interferometry, which will compare two radar images taken at slightly different locations to obtain elevation or surface-change information.

The SRTM will use single-pass interferometry - so the two images will be acquired at the same time, one from the radar antennas in the Shuttle's payload bay, the other from the radar antennas at the end of a 60m (200ft) mast extending from the Shuttle. Combining them produces a three-dimensional image.

The STS99 crew will extend the 60m telescopic mast and align it precisely with the other antennas in the payload bay. They will also control 3,000 gigabytes of data, providing an almost complete survey of the earth's surface. The Endeavour will fly in a 57° inclination orbit to meet the objectives of the SRTM mission, which is designed to produce a digital elevation model of the land surface between about 60° north and south, using a 30m horizontal resolution with about 10m vertical errors, providing the most precise "picture" ever of the land surface.

The radar will bounce signals off the surface, the reflected signals being received by two on-board antenna systems and combined by computers at a ground facility to produce 3D images. Because the Shuttle will fly over most of the globe's surface, enough data will be acquired to generate the most complete topographic map of the land surface ever, says NASA.

Imaging radar was used by NASA's Magellan spacecraft from 1989-94 to produce spectacular pictures of the surface of Venus using radar. The surface is veiled by a thick carbon dioxide atmosphere that has caused an out-of-control greenhouse effect. SRTM's goal is to produce a similarly detailed map of the earth yielding enough data to produce a digital model that is more detailed than models now available. The SRTM will collect radar data over nearly 80% of the earth's land surface.

Analysts will use the SRTM data to generate 3D topographic maps called digital elevation models. These can be combined with other data about the area gleaned from other sources - including other types of satellite image - for analysis. The SRTM data will also be used to generate 3D visualisations of the earth's surface that will be used for studies of flooding, erosion, land-slide hazards, earthquakes, ecological zones, weather forecasts and climate change.

The data's military applications include mission planning and rehearsal, modelling and simulation. Other possible uses include optimising locations for cellular telephone towers and improving topographic maps for backpackers, fire fighters and geologists.

Source: Flight International