What is SRTM?
The Shuttle Radar Topography Mission (SRTM) has been a joint project between the National Imagery and Mapping Agency (NIMA) and the National Aeronautics and Space Administration (NASA). The planned objective of this project was to produce digital topographic data for 80% of the Earth's land surface (all land areas between 60º north and 56° south latitude), with data points located every 1-arc-second (approximately 30 meters) on a latitude/longitude grid. The absolute vertical accuracy of the elevation data will be 16 meters (at 90% confidence). This radar system gathered data that will result in the most accurate and complete topographic map of the Earth's surface that has ever been assembled.
How it was done?
SRTM made use of a technique called radar interferometry. In
radar interferometry, two radar images are taken from slightly different
locations. Differences between these images allow for the calculation of surface
elevation, or change. Interferometry is the study of interference patterns
created by combining two sets of radar signals. If you've ever
seen a puddle of water with a film of oil on it, you've probably
noticed bands of color on the surface. These bands of color are
caused by light rays bouncing off the smooth surfaces of the oil
and underlying water, making interference patterns. To get
two radar images taken from different locations the SRTM hardware consisted of
one radar antenna in the shuttle payload bay and a second radar antenna attached
to the end of a mast extended 60 meters (195 feet) out from the shuttle.
SRTM was a fixed-baseline interferometry mission. This means two radar data sets were collected at the same time and the antennas that collected the data were separated by a fixed distance. In the SRTM mission, the main antenna onboard the space shuttle collected one data set and the other data set was collected by the outboard antenna located at the end of the 60 meter (200 foot) mast.
When
you turn on a light bulb to illuminate a room, light rays are emitted from the
bulb and strike all the objects in the room. The waves of light that are
reflected from the objects enter your eye, allowing you to see.
For SRTM, the main antenna located in the payload bay of the space shuttle illuminated a portion of the surface of the Earth. But instead of using light rays, a beam of radar waves was transmitted. When the radar waves hit the surface of the Earth, rays were scattered in various directions. These scattered waves were collected by the two SRTM antennas. The baseline distance between the main antenna and the outboard antenna were known very accurately and remained constant. What did change is the distance to the Earth's surface in relation to the two antennas. Within the reflected radar beam, the point which represents where the reflection took place was slightly different between the main and outboard antennas.
Using the information about the distance between the two antennas and the differences in the reflected radar wave signals, accurate elevation of the Earth's surface can be calculated.
Although the intended goal was a bit larger surface area of the world, the SRTM has successfully collected data over most of the land surfaces that lay between 60 degrees north latitude and 54 degrees south latitude. That's about 80% of all the land on the Earth.
In the maps below, the colors of the swaths indicate the number of times the area was imaged by SRTM. For land, one-time coverage is green, twice is yellow-green and so on, as shown in the key at lower left. Over water, the color code is in shades of blue as shown in the key at lower right. Areas in red could not be mapped. SRTM is a topography mission, so data was mostly acquired over land. Small amounts of data were collected over the water for calibration purposes.
SoftWright Fully Supports SRTM Data
SoftWright's TAP software has been substantially restructured to fully support the SRTM data format. We are producing off the shelf products of this new high resolution data to be used within our RF system design software. For world-wide availability for STRM data click here. For close up details click the region to see available 3-arc second resolution data in North, Central and South America as well as Europe, Asia, Australia, and Africa. As more data is released we will acquire and redistribute it. If you have access to SRTM data anywhere in the world through other sources, the TAP software will read this data directly. This newly developed database is a quantum leap forward in both topography data accuracy and resolution from the previously available 30-second resolution. This evolution within our software has taken approximately one and one half years in development and is now fully included in all new TAP systems and all TAP systems that are on a current software maintenance agreement. If you are on a current software maintenance subscription, this new capability is provided at no additional cost as a regular new technology enhancement to the TAP software. If you would like to have us contact you about the details, please click "contact me" below and fill out the web form.
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