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Detailed Description of SRTM Observation Principles (2)

Attitude of shuttle

The attitude of the shuttle used in this mission is very distinctive.
The main antenna in the shuttle cargo bay is installed obliquely on the support structure in the cargo bay so that the antenna face is tilted at an angle of 14 degrees with respect to the horizontal surface of the cargo bay.The outboard antenna is oriented to the north,and the shuttle flies tilted at an angle of 59 degrees with respect to the line perpendicular to the surface of the Earth.This is to maintain the orientation of the antenna and the angle of the mast at 45 degrees with respect to the line perpendicular to the surface of the Earth.This attitude increases the coverage of the Northern Hemisphere,which contains large areas of land.
The shuttle flies with the tail section facing the direction of travel,and the cargo bay facing the direction of the Earth to reduce the risk of space debris colliding with the windows of the shuttle.

Scan SAR observation image acquired using the shuttle
Scan SAR is a technology in which the antenna beam is scanned in order to increase the coverage.There are two methods of scanning the beam,one in which each of the phases transmitted from multiple phase detectors mounted on the antenna is changed so as to scan the beam electronically,and one in which the antenna itself is moved so as to scan the beam mechanically.
In SRTM,SIR-C is using Scan SAR and the beam is
scanned electronically.

*: On SRTM, the shuttle flies with the tail section facing the direction of travel.

Technical Difficulties Attendant to SRTM
  • Deployment and retraction of the mast
    The mast is 60 m long.This is the longest articulated structure to be used in space up to now.During launch and return to Earth,the mast is folded to about 1/20th of the fully extended length,and stored in a canister in the cargo bay.The technology for deploying and retracting the mast while twisting it in the reverse direction one bay at a time by means of the motor in the canister was a technical challenge for the mission.The mast starts to be extended about 5 hours and 30 minutes after lift-off,and takes 17
    minutes to fully extend.
    If the mast cannot be extended or retracted due to a breakdown of the equipment,this work will be done manually from outside the shuttle (EVA).
  • Minimizing changes in attitude due to external disturbances or vibration
    On this mission,the shuttle will travel at a speed of about 7.7 km per second with the mast extended.The main antenna and outboard antenna will be subjected to external disturbances due to the effect of gravity *1 ,and to vibration due to shaking and flexing of the mast as a result of the daily orbit control (thruster jet)of the shuttle.The alignment between the main antenna and the outboard antenna must be within +-0.03 degrees,so highly accurate control isrequired.
    Attitude control and maneuver,which greatly change the attitude of the shuttle,are performed above the sea when observations are not performed.Control is made for the influence in attitude due to the effect of gravity,by slowly and continuously releasing jets of nitrogen gas from thrusters installed at the end of the outboard antenna while observation is taking place.Misalignment of the outboard antenna during observation is corrected by the crew at appropriate intervals.

*1: A spacecraft that has a long axis and travels in a low orbit (orbit altitude of no more than 1,000 km)tends to assumed an attitude and gravity gradient such that one end faces the Earth and the other end faces the opposite direction.

History of the Synthetic Aperture Radar for Use in Space

SAR was conceived immediately after the appearance of radar technology at the beginning of the Second World War. Subsequently,the US actively researched SAR from both the theoretical and empirical aspects.As a result,by the latter half of the 1950s,six kinds of experimental aircraft SAR systems had already appeared.
Observations from space using SAR started from the US oceanographic observation satellite SEASAT in 1978.SAR observations were subsequently carried out using Japan Earth Resources Satellite-1 (JERS-1),the European Space Agency's (ESA)ERS-1 and ERS-2,and Canada's RADARSAT.
SAR missions using the shuttle started from STS-2 in 1981.The first SAR employed was the Spaceborne Imaging Radar (SIR)-A.Subsequently,an improved version of SIR-A was used in STS-41G (SIR-B),and a further improved version called SIR-C was used in STS-59 and STS-68.From STS-59, a X-SAR synthetic aperture radar that operates in the X-band was installed together with SIR-C,hence the name of the Space Radar Laboratory (SRL)was used for the name of the shuttle mission.
Year and month
of launch
Mission name Country where mission
was developed
December 1972 Apollo 17 US The moon's surface was measured using the first space SAR in history.
This SAR was called Apollo Lunar Sounder Experiment (ALSE).
June 1978 SEASAT US The first artificial satellite to carry SAR.Was designed to conduct oceanographic observation.
November 1981 STS|2/SIR|A US Shuttle-mounted SAR.
November 1983 STS|9/MRSE Germany One of the experiments in the Spacelab mission.
October 1984 STS|41G/SIR|B US Shuttle-mounted SAR.Improved version of SIR-A.
May 1989 Magellan US Venus probe.First SAR designed for planetary exploration.Three-dimensional map of the surface of Venus was produced.
March 1991 ALMAZ|1 USSR SAR installed.
July 1991 ERS|1 *1 ESA C-band SAR installed.
February 1992 JERS|1 *2 Japan L-band SAR installed.
April 1994 STS|59/SRL|1 US Shuttle-mounted SAR.Improved version of SIR-B and X-SAR installed.
October 1994 STS|68/SRL|2 US Shuttle-mounted SAR.
April 1995 ERS|2 *1 ESA C-band SAR installed.
November 1995 RADARSAT Canada C-band SAR installed.
October 1997 Cassini US Saturn probe.Ku-band SAR installed.To be used to observe the surface of Titan,a satellite of Saturn.

*1FEuropean Remote Sensing Satellite-1,2 @*2FJapan Earth Resources Satellite-1

Utilization of Three-Dimensional Maps

The high-resolution,three-dimensional topographic map obtained as a result of this mission can conceivably be used in the following three ways.
  1. A topographic map that was previously expressed by drawing contour lines on a paper map can be treated as a digital elevation model.Consequently,it is possible to combine satellite image data with various geographical information and data,such as maps and statistical information,and create a geographic information system on a computer.Conceivable applications of such a system include resource management in agriculture and forestry,understanding the environment,mineral resource
    development,conservation of the regional living environment on a local government scale,town planning
    and development,and surveying a disaster situation over a wide region.
  2. Earth observation data that was previously obtained in two-dimensional form can now be obtained as data that includes differences in topography.This data can be used for regional weather forecasting that takes account of topography,improving climate models,obtaining an accurate understanding the distribution of forests in mountains,and so on.
  3. A three-dimensional map can be used in applications that use a bird's-eye view to express the geographical relationship between points more clearly than a plane map. Examples include town planning,creation of road traffic networks,architecture that takes account of the overall landscape,car navigation systems,and determining line-of-sight areas in wireless communications.

A more practical example for improving aircraft safety is described here.

Since the 1970's, aircraft have been equipped with a ground proximity warning system (GPWS) to prevent sudden crashes into mountains or the ground. When a radio altimeter detects that an aircraft is approaching the ground, the GPWS initiates aural or visual warnings. However GPWS is not effective for steep slopes where sometimes crashes can't be avoided. Most aircraft accidents with casualties are caused by aircraft crashing into the ground before the crew becomes aware of the danger.

During the past several years, airlines have been installing an Enhanced GPWS (EGPWS). This system has global geographical data, and data of obstacles close to airports all over the world. This system is able to warn the crew of potential crashes into slopes or mountains, which considerably increases safety. Unfortunately the Earth map data utilized for this purpose has an accuracy of only 500m to 1000m. Data obtained by the SRTM mission could be applied to EGPWS to produce a system with higher precision.
Enhanced GPWS (EGPWS) EGPWS Display
Courtesy of ANA

Last Updated : September 1, 1999

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