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Project 3: Shuttle Radar Topography Mission (SRTM)

Jim Kompanek

Introduction:

In February 2000, the Space Shuttle Endeavor obtained near-global elevation data during the eleven day Shuttle Radar Topography Mission (SRTM) (U.S. Geological Service 2004). The SRTM was an international venture lead by the National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA) (Jet Propulsion Laboratory 2006). International sponsors included German Aerospace Center (DLR, Deutsches Zentrum für Luft- und Raumfart) and the Italian Space Agency (ASI, Agenzia Spaziale Italiana) (U.S. Geological Service 2004).

This project mapped 80 percent of the earth's surface (Figure 1) in three dimensions, at a level of detail unprecedented at this near-global scale (U.S. Geological Service 2003), resulting with the "most complete near-global high-resolution database of the Earth's topography" (Jet Propulsion Laboratory 2006). Data points were located approximately every 30 m (1 arc second) with vertical accuracy of elevation of 16 m (90% confidence level) (U.S. Geological Service 2004). The tremendous scale of the project allowed for a standardization of data on a global scale. This is in contrast to previously, when neighboring countries may have used entirely different methods to generate topographic data, which greatly limited the possibility of regional or global topographic studies (Jet Propulsion Laboratory 2006).

Figure 1. Map indicating SRTM coverage area, primarily between 60 degrees north latitude and 54 degrees south. Color denotes the number of time the landmass was covered. Water was scanned for the purpose of calibration and determination of mean sea level. Image courtesy of NASA/JPL-Caltech (Jet Propulsion Laboratory 2006).

The radar system onboard Space Shuttle Endeavor contained both C-band and X-band antenna panels (Figure 2).  Digital Elevation Models (DEMs) at thirty meter (1 arc second) intervals were processed at the Jet Propulsion Laboratory and are available through the USGS Seamless Data Distribution System. According to the U.S. Geological Service (2004), areas 30 deg by 30 deg in size (1.6 gigabyte files) are available for download at no charge in 100 Mb increments from the Seamless Data Distribution website. Data can also be ordered on CD, in ArcGrid, BIL (integer data), and TIFF (32 floating point grid) formats for $82 per CD ($32 for CD, $45 for processing, and $5 shipping) (U.S. Geological Service 2004). Data obtained through the X-band antenna panels, which are slightly higher resolution but with smaller coverage than the C-band, were processed and distributed by the German Aerospace Center. The minimal order size for the German Aerospace Center is 150 Euro (German Aerospace Center 2006), or as of 15 March 2006, equivalent to approximately $183 USD (XE.com 2006).

Figure 2. Comparison of X-band versus C-band scanning of the earth's surface.  Image courtesy of NASA's Shuttle Press Kit (Shuttle Press Kit 2000).

Shuttle Radar Topography Mission DEMs are of higher resolution and higher accuracy than other existing digital elevation models. ETOPO2 is a global DEM which includes all of the earth's landmass and seafloor, with grid spacing at 2 minute intervals (National Geophysical Data Center 2006) or resolution of approximately 3.7 km at the equator (DiBiase 2006). Raw data is available for free download or on CD ROM for $75 from the NOAA National Data Center (U.S. Department of Commerce 2006). GTOPO30 is a global DEM that includes all of the earth's landmass, which unlike ETOPO2, does not include the seafloor.  GTOPO30 is of higher resolution than ETOPO2, with grid spacing at 30 second intervals (U.S. Geological Service 2006a), or a resolution of approximately 1 km at the equator (DiBiase 2006). GTOPO30 was derived from a variety of vector and raster source data (U.S. Geological Service 2006b). As a result, vertical accuracy varies dramatically, dependent upon the source data, but is no better than 30 m at any point. Although SRTM data only covers 80% of the earth's landmass (see introduction), its resolution and accuracy are significantly higher than other global digital elevation models (Figures 3 and 4).

Figure 3. Comparison of SRTM C-band data (left) and GTOPO30 (right) showing the differences in resolution. Image courtesy of NASA (National Aeronautics and Space Administration 2006).

Figure 4. Comparison of SRTM data and GTOPO30 showing differences in resolution. Image courtesy of NASA's Shuttle Press Kit (Shuttle Press Kit 2000).

Interferometry was the basis of the Shuttle Radar Topography Mission.  According to the U.S. Geological Service (2006), interferometry is "the study of interference patterns created by combining two sets of radar signals." Specifically, the SRTM was a fixed-baseline interferometry configuration, using active sensing. Active sensing allows for data to be obtained at any time of the day and through cloud obstruction, versus passive sensing (such as aireal photography) which requires daylight and a clear line-of-sight, free of cloud obstructions (Jet Propulsion Laboratory 2006). As mentioned above, the SRTM used both C-band and X-band radar, with X-band being of slightly higher resolution (see Figure 2).

With this configuration, radar pulses were emitted from a single antenna but collected from two separate antennas, a set distance apart (Jet Propulsion Laboratory 2006) (Figures 5 and 6).  n the case of the SRTM, pulses were transmitted from an antenna located in Space Shuttle Endeavor's cargo bay. This antenna also contained panels to receive the radar signals after they bounced off the surface of the earth. The second antenna was located at the end of a long mast (also referred to as an arm). According to the Jet Propulsion Laboratory, this mast was approximately 60 m in length and was also the longest "rigid structure every flown in space" (Jet Propulsion Laboratory 2006). This outboard antenna was equipped with panels to allow it to receive the same signals (from a different vantage point) as the one located in the cargo bay. The result of the two vantage points is a 3 dimensional view of the earth's topography.

Figures 5 (left) and 6 (right). Depictions of shuttle arm configuration on board Space Shuttle Endeavor (U.S. Geological Service 2006).

High resolution topographic data has near limitless applications (Table 1).  There are numerous scientific applications, including geologist, soil scientists, volcanology, seismology, hydrology (National Geospatial-Intelligence Agency 2006, U.S. Geological Service 2004). There are a wide variety of military applications, including battlefield simulators, logistical planning, and weapon and guidance systems. I can personally attest to the benefit of accurate military simulators, as months before serving as peacekeeping in Kosovo, my Bradley Fighting Vehicle crew was able to patrol a simulated American sector in a M2A3 Close Combat Tactical Trainer. This allowed combat vehicle crews to experience the extreme terrain of the Balkans, in a safe, simulated environment. 

There are also numerous civilian applications, such as accurate topographic data for aircraft ground warning and navigation systems, line-of-site determination for the cellular phone industry, as well as maps for backpackers and hikers.

There are numerous examples of how high resolution topographic data is useful in the field of Cultural Resource Management. As mentioned previously, line of sight-of-sight determination is important for the cellular industry. As a clear line of sight is critical for cellular reception, a clear line-of-sight of a cellular tower is not desirable for historic properties, districts, or monuments (Historic Preservation Office 2006, Joseph and Roberts 2001). As such, prior to construction of a cell tower, highway, or any other undertaking requiring a federal permit or funding, an analysis of all historic properties must be performed to determine if the potential undertaking will be within its viewshed. The more accurate the digital elevation model is, the more useful the viewshed analysis will be.

The field of archaeology is also greatly benefiting from the SRTM. The accurate DEMs allow for precise study of regional settlement patterns, as well as predictive models for site location. As a result of the SRTM, archaeologists are now able to study regional landscape and terrain in areas where accurate topographic maps are either unavailable (or not available) prior to the SRTM (Shuttle Press Kit 2000).  Shuttle Radar topographic data acquired during the 1980's and 1990's has been used by archaeologists throughout the world, and has aided in the identification of medieval road networks in Scotland, sites in the dense jungle of Cambodia (Bridges 1999), and (now non-existent) river drainages during the Han Dynasty in China (Holcomb 1992). The SRTM is also proving to be a valuable asset in the field of archaeology of the Near East (Figures 7 and 8).

Figure 7. SRTM data showing region between Tell Beydar, Tell Chagar Bazar, and Tell Brak in Southern Mesopotamia, showing numerous evenly-spaced Tell sites (appear as small dots on the image). Copyright Andrew Sherratt 2004. All rights reserved. Used here for educational purposes only. (Sherratt 2004)

Figure 8. SRTM data showing a 10 km radius around Tell Brak. Copyright Andrew Sherratt 2004. All rights reserved. Used here for educational purposes only. (Sherratt 2004)

In the Near East, many sites were documented by classical archaeologists in the 19th and early 20th but no accurate coordinates exist for their location. Data obtained by the SRTM allowed for a "re-discovery" of the precise location of these sites, as well as aided in the identification of previously unidentified archaeological sites (Sherratt 2004). The role of the SRTM and other remote sensing applications are rapidly changing the field of archaeology and are, and will be to an even greater extent, a great asset in the prediction and analysis of such resources.

DiBiase, David (2006) The Nature of Geographic Data, Lesson 7.  The Pennsylvania State University World Campus Certificate Program in GIS. Accessed 15 March 2006.

German Aerospace Center (2006) X-SAR/SRTM Products/Digital Elevation Models. http://www.dlr.de/srtm/produkte/preise_en.htm Accessed 17 March 2006.

Historic Preservation Office (2006) About Communication Towers. State of New Jersey Department of Environmental Protection Natural & Historic Resources.  http://www.state.nj.us/dep/hpo/4sustain/cellttwr_article.pdf Accessed 17 March 2006.

Holcomb, Derrold (1992) Shuttle Imaging Radar and Archaeological Survey in China's Taklamaken Desert. Journal of Field Archaeology, 19(1), 129-138.

Jet Propulsion Laboratory (2006) Shuttle Radar Topography Mission: The Mission to Map the World. http://www2.jpl.nasa.gov/srtm/ Accessed 12 March 2006.

Joseph, J.W. and Daniel G. Roberts (2001) Visual Effects and Cell Towers: A Different Point of View. American Cultural Resources Association, 7(3), 1,6-7.

National Aeronautics and Space Administration (2006) Land Topography Picture http://learn.arc.nasa.gov/education/DATA-Earth.doc Accessed 17 March 2006.

National Geophysical Data Center (2006) ET0P02 2 minute Worldwide Bathymetry/Topography Grids http://www.ngdc.noaa.gov/mgg/fliers/01mgg04.html Accessed 16 March 2006.

National Geospatial-Intelligence Agency (2006) Shuttle Radar Topography Mission. http://www.nga.mil/portal/site/nga01 Accessed 9 March 2006.

Sherratt, Andrew (2004) Spotting Tells From Space. Antiquity 78(301).  http://antiquity.ac.uk/ProjGall/sherratt/ Accessed 17 March 2006.

Shuttle Press Kit (2000) Endeavour OV105 http://www.shuttlepresskit.com/STS-99/index.htm Accessed 17 March 2006.

U.S. Department of Commerce (2006) NOAA National Data Center http://ols.nndc.noaa.gov/plolstore/plsql/olstore.prodspecific?prodnum=G02092-CDR-A0001 Accessed 15 March 2006.

U.S. Geological Service (2003) Shuttle Radar Topography Mission (SRTM) Factsheet 071-03. http://erg.usgs.gov/isb/pubs/factsheets/fs07103.html Accessed 10 March 2006.

U.S. Geological Service (2004) Shuttle Radar Topography Mission: Mapping the World in 3 Dimensions. http://srtm.usgs.gov/ Accessed 12 March 2006.

U.S. Geological Service (2006a) Earth Resources Observation and Science (EROS) http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html Accessed 15 March 2006

U.S. Geological Service (2006b) Earth Resources Observation and Science (EROS) http://edc.usgs.gov/products/elevation/gtopo30/README.html#h17 Accessed 15 March 2006

XE.com (2006) Universal Currency Converter http://www.xe.com/ucc/ Accessed 15 March 2006

This document is published in fulfillment of an assignment by a student enrolled in an educational offering of The Pennsylvania State University. The student, named above, retains all rights to the document and responsibility for its accuracy and originality.