A new approach to mapping the earth’s surface just might have important benefits for ocean voyaging sailors. The upcoming Shuttle Radar Topography Mission (SRTM), scheduled to fly in September 1999 aboard the Space Shuttle Endeavour, will map 80% of the earth’s land surface during its 11-day mission. This radar data could be used to update the worldwide and U.S. nautical charts produced by the National Imagery and Mapping Agency (NIMA) and NOAA (more on that below).
For centuries, the biggest problem a mariner had was knowing his location. Many a ship was lost because the ship’s captain had only the roughest idea of his vessel’s actual lat/long position. Today, however, a GPS receiver no bigger than a paperback book can give mariners their lat/long positions down to 100 meters (down to 10 meters if differential GPS is available). Positioning problem solved.
Or is it?
Unfortunately, in many areas of the world even the amazing capabilities of GPS don’t correct an underlying problem: outdated charts. Many places that voyaging sailors are most interested in, like the South Pacific, the Caribbean, and Indian Ocean areas, have charts based on very old survey data. This survey information was collected when celestial navigation and lead lines were the only methods for position fixing and chartmaking. According to the British Admiralty’s chartmaking branch, some of its charts have errors of as much as seven miles. Thus, while GPS is accurate, the charts you’re using may not be.
The best solution to the problem of inaccurate charts is to resurvey those areas that have charts based on old data. Using traditional survey methodsdeploying survey ships and airplanesis expensive and time-consuming. Many small countries, like the island nation of Kiribati in the central Pacific, for example, don’t have the funds to perform the survey work required. Even the wealthy U.S. doesn’t seem willing to devote the resources necessary to completely resurvey its own waters to GPS levels of accuracy (some survey work is being done, but it is limited to the main ship channels of a few major ports).
What makes the SRTM project so exciting is that it will perform an end run around much of the survey problem (of land features anyway) by placing a highly accurate interferometric radar aboard Endeavour. This radar setup will illuminate the planet’s surface from 126 miles up and use the returning echoes to assemble digital maps of all land masses between 60° N and 56° S. These digital maps (similar to the maps acquired during the Magellan radar mapping mission to Venussee the SRTM web site found at www-radar.jpl.nasa.gov/srtm/ for examples of radar interferometry images) will show all land features 30 meters (98 feet) or larger. This data will have great potential for improving the piecemeal survey data that NOAA and NIMA still use for drawing charts. “It’s going to be a tremendous advantage to have this data,” said Dr. Drew Smith, a geodesist for the National Geodetic Survey, a branch of NOAA. “For example, we’ve got a shoreline map of Hawaii that’s about 30 years old that is off by 50 kilometers in some places.” The high quality of SRTM data is possible because of two radar mapping techniques: synthetic aperture radar (SAR) and radar interferometry.
The value of the SAR method is that it allows the shuttle radar to work like a very long antenna, but without the weight, cost, and complexity. By storing sets of data as the shuttle moves along and then correcting for the Doppler shift of the received signals by factoring out the shuttle’s orbital speed, a kilometers-long synthetic antenna can be created and the echoes can be “focused” on a single point. And, just like the marine radar you have on your boat, the wider the antenna, the greater the resolution in horizontal beamwidth and the smaller the target that can be resolved.
While SAR uses signal processing to achieve its really big antenna, the radar interferometric method requires that two antennas have some physical separation between them. Thus, once the shuttle is in orbit, it will extend a 200-foot mast off to one side of its payload bay. At the end of this mast will be a receiver antenna. This antenna will work with a receive antenna in the payload bay. When reflected radar energy arrives at the different receive antennas, there will be a phase difference in the echoes depending on the topography of the surface. While the phase difference is small, it’s enough to resolve vertical differences quite accurately. However, to process all the raw phase-difference data into a radar image requires substantial computing power.
Thus, in a single 11-day shuttle flight, all this land and coastline data will be gathered. After the mission returns to earth, with about 9,800 gigabytes of data stored on approximately 300 high-density data tapes, the number-crunching will begin. Astronauts will swap tape cartridges as the data is gathered. (Let’s hope they pay attention. “I thought you were going to change the tape.”) The amount of information is huge. “It will take us nearly a year to process all this data,” said Major Earl White of NIMA.
The processed SRTM data will be broken into two data sets. One set will have a resolution of 30 meters, while the other set’s resolution will be 90 meters. Maps and data for areas inside U.S. borders will be released to the public with the full 30-meter resolution. For areas outside the U.S., however, only 90-meter resolution will be available for non-military applications. Note how this complements GPS selective availability specifications: the GPS signal is degraded to 100-meter accuracy for civilian users.
Interestingly enough, NIMA doesn’t have any immediate plans to correct its worldwide database of nautical charts based on SRTM information. Part of the reason for this is because SRTM’s primary mission is military intelligence. The digital elevation models (DEM) derived from SRTM data will be used to provide digital maps based on the WGS 84 datum standard (WGS 84 is the “native” datum of GPS) for flying airplanes and cruise missiles at low altitudes and with pinpoint accuracy. Another reason NIMA isn’t planning to use SRTM data is that updating nautical charts for out-of-the-way islands perfect for voyaging sailors is not a high priority for NIMA, which is interested in deep-draft locations likely to be used by U.S. Navy vessels. “Updating charts hasn’t been the purpose [of SRTM],” said Chris Andreasen, chief hydrographer at NIMA.
The National Ocean Service, the chartmaking branch of NOAA, would like to make use of SRTM data. “We’re very interested in it,” said Capt. Dave MacFarland, chief of the mapping and charting branch. “We plan to exploit it as soon as it becomes available.”
After the Department of Defense has its DEM coverage, it seems reasonable that selected parts of the SRTM data set could be used to update worldwide nautical charts. Updated charts, with all the small Pacific islands properly located according to the WGS 84 datum, would be an excellent civilian marine spin-off of SRTM.
