In 2007 and 2010 two radar-imaging satellites were launched by the German space agency DLR, TerraSAR-X and Tandem-X respectively. After 2010 both orbited in close, side-by-side formation, sometimes as little as 200 m apart. With one acting as a both a transmitter and receiver of microwave pulses, the other as a receiver, this set up allowed the two signals returning from the Earth’s surface to be matched. The slightly different positions of the platforms results in a time difference at which a pulse reflected from a point on the Earth’s surface reaches the two receiving antennas. This difference varies according to the topographic elevation of the point – in effect analogous to the parallax shift captured in conventional stereoscopic images but measured by the interference between the two signals. Although involving far more complex computation, such radar interferometry produces estimates of each point’s elevation and ultimately a 3-dimensional image of the Earth’s surface. After a period of commercial operation, DLR has decided to make part of the data available free of charge. Both systems use microwaves with a wavelength of around 3 cm (9.65 GHz frequency), which allows topographic elevation to be measured to a precision of ±1 m. Using orbits that cross the poles, each at an angle to the Equator, allows swaths from the dual system eventually to cover the whole planet, in the manner of winding a ball of string. Eventually, the data will permit the detection of vertical movements of one kind or another when multiple coverage of the Earth becomes available. However, the expected lifetime of the platforms is limited, so DLR plans to launch two 23.6 cm interferometric radar satellites to assess dynamic processes occurring on the Earth’s surface.
The resolution of radar interferometry in the two dimensions of a map depends on many factors, some of which stem from the complex processing of the raw data. DLR global data is presented at three resolutions (pixel size): 12 m, the finest; 30 m and 90 m. For local acquisition even finer resolution is possible. Only the 90 m version is being released for free use. The first interferometric radar elevation data to be made freely available was from the NASA Shuttle Radar Topography Mission (SRTM) that was accomplished from the US Space Shuttle Endeavour in 2000, using a single instrument that incorporated two antennas separated by a 60 m long mast deployed from the Shuttle. SRTM acquired data only between latitudes 60° N and 60° S, using 23.6 cm L-band radar. As well as omitting high latitudes, the SRTM design limited actual elevation precision to about 4 m compared with the ±1 m from TerraSAR-X/TanDEM-X. SRTM data with a two-dimensional resolution of 30 m are freely available from the US Geological Survey.
Full global elevation data with a 30 m 2-D resolution and elevation precision of ±9 m have also been produced by the optical stereoscopic potential of the US-Japan ASTER imaging system and are freely available to all via the US Geological Survey. Unlike data produced by radar missions, the optical stereoscopic data from ASTER depend on cloud-free, daytime conditions, and accurate derivation of parallax can be prevented by areas of rugged terrain in deep shadow at the 10 am local-time when images are acquired.
Despite the limitation of TerraSAR-X/TanDEM-X elevation data to a 90 m 2-D resolution, and the consequent loss of textural detail in landscapes, they appear to have the edge in terms of completeness and vertical precision. To get elevation data from DLR requires personal registration after reading a lengthy screed of documentation about data acquisition.