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SHUTTLE RADAR MISSION SUCCEEDS

Working in shifts in orbit: Divided into two teams of three astronauts each, the crew of the Shuttle Endeavour accomplished the Shuttle Radar Topography Mission in February - one of the most spectacular Shuttle missions ever flown. In 24-hour operations, team Blue and team Red, which included the German ESA astronaut and mission specialist Dr. Gerhard Thiele, ensured the continues support to the high-tech payload of STS-99.

The Shuttle's payload bay was filled by the 12 meter long radar installation which Endeavour used to collect terrain data for a precise threedimensional and digital world map. Radar sensors have distinct advantages in such tasks as compared to other, for example optical measurements. Radar systems work day and night and also can see through clouds.

The goal of the Shuttle radar mission was a global measurement of terrain elevation points every 30 meters with a precision of up to 6 meters vertically. Up until the SRTM mission, the best available complete data set of Earth terrain elevations only supplied elevation points every kilometer with a vertical precision of 100 meters. More exact terrain data were only available from selected regions of the world, especially from areas in the industrial countries.

But even SRTM, from the mission design, could not cover the entire globe. With its inclination of 57 degrees, Endeavour could cover the entire surface of the Earth in an area between 60 degrees of Northern latitude and 56 degrees of Southern latitude. This is the entire area from Cape Horn in the South to southern Scandinavia in the North. This equals to round about 80 percent of the land mass on which 95 percent of the world's population live.

The SRTM mission concept goes back to two successful Shuttle flights which took place in the spring and in the fall of 1994 during which part of the SRTM radar technology flew into orbit the first time. For one this includes NASA's SIR-C radar (Shuttle Imaging Radar C) which operates in the C-band (5,3 Gigahertz, wave length: 6 centimeters). Secondly the X-SAR system (X-Band Synthetic Aperture Radar, 9,6 GHz, wave length: 3 centimeters) was on board the shuttle. The system was designed by the German Research Center (DLR) and the Italian Space Agency ASI.

When applying a radar with synthetic aperture, the engineers take advantage of the forward movement of the Shuttle to increase the reception quality. Because of the movement there is a virtual antennae length of about 600 meters as compared to the actual equipment length of 12 meters. This means that the synthetic antenna is 50 times as big as the real one.

In order to generate threedimensional terrain data, it is also necessary to measure the same spot on Earth twice. During the two flights in 1994 this was accomplished by flying twice over the same point and then overlaying the two measurements. However, this procedure has a distinct disadvantage: Since there is always a time gap between the two overflights the radar return characteristics of the specific surface area might change because the ground is either more wet or drier. This would change the radar echoe making the generation of a terrain elevation model impossible. Also, with this "multi-pass" flight profile the orbits had to be very accurate and could only differ by just a few hundred meters.

Because of these constraints, the European-American radar installation flew in a new configuration for the SRTM mission, allowing a stereo reception, i.e. the data measurement from two different looking angles in one overflight (single pass). For this, small 6 meter long receive-only antennae were installed on a 60 meter long extandable mast to complement the SIR-C and X-SAR transmit-and-receive installation in the orbiter's payload bay.

This application of the so-called radar interferometry technology is the technologically most modern and advanced process for generating terrain elevation data. Germany's DaimlerChrysler Aerospace subsidiary Dornier Satellitensysteme was the main industry contractor for building and expanding the X-SAR system. The extandable mast was built by a company in the USA.

The radar mast, which was stowed in a container inside Endeavours cargo bay for launch and landing was one of the decicive factors that contributed to the mission's success. Its structure is made of carbon-reinforced composite with stays of titanium rope. The mast was the longest structure built up in space yet. While there was some concern before the mission that the mast would not be stable enough, the construction proved more stable in flight than expected. This was important since because of the antennae base line of 60 meters the structure was supposed to move as little as possible. Even the smallest deflections had to be recorded and measured and will have to be taken into consideration when the elevation models are computed.

Extending the mast after Endeavour reached orbit on 11 February was accomplished without any problems. During the mission a problem with a cold gas thruster developed which was located at the tip of the mast and which was needed for the attitude control and positioning of the mast. Because of this problem, the crew had to use the orbiter's own attitude control system more than planned, such increasing the fuel usage of that system. In the first days of the flight, it looked as though the mapping part of the mission had to be cut short because of this.

However, several measures to reduce the fuel usage where implemented into Endeavour's operation, allowing to complete the mission as planned. For one the intertia movements of the orbiter generated by the daily process of dumping surplus water overboard was used to aid to the attitude control maneuvers. Secondly the daily so called flycast maneuvers, which were needed to keep the Shuttle at its very low orbit of 233 kilometers of altitude, were optimized, allowing to omit one of the maneuvers entirely, again saving on the fuel. After the mast was retracted back into the container, Endeavour returned to Kennedy Space Center on 22 February.

After a contious mapping operation of 222 hours and 23 minutes, Endeavour's crew returned with 332 high density tapes, enough data to fill 20000 CDs. Alltogether, the SRTM mission covered 113 million square kilometers of the Earth's surface. Many areas were overflown two or even three times. The evaluation of the data will take approximately two years. There is a clear agreement between NASA, DLR, ASI and NIMA; the US National Image and Mapping Agency which was also a partner in this mission, in how the data will be made available. The scale of possible users is big. One area of application which is relevant to aviation safety are improved terrain warning systems.

From page 52 of FLUG REVUE 5/2000