May 2008 saw probably the most significant announcement for geologists of this century (The Landsat Science Team 2008. Free access to Landsat imagery. Science, v. 320, p. 1011; and see landsat.usgs.gov/images/squares/USGS_Landsat_Imagery_Release.pdf). Given a broadband internet connection, it will soon be possible to download Landsat data (MSS, TM and ETM+) covering any area on Earth free of charge from the US Geological Survey, provided it occurs among the >2 million scenes archived by their EROS Data Center. This act of open-handed generosity by the USGS marks a key step in revolutionising the activities of geologists of the Third World, especially those in Africa; the least well-mapped continent. Landsat data and those from the Japanese-US ASTER instrument aboard the Terra satellite offer huge potential for mapping rocks and soils, especially in dry lands, at scales of up to 1:50 000. Africans need to know about their physical resources, especially water, instead of well-heeled mining, petroleum and consulting companies from rich countries, who have more or less monopolised (and sometimes eked out) knowledge of the continent’s riches. Now they can begin to find out for themselves.
Satnavs useful to hydrogeologists as well as white-van drivers
Microwave radiation emitted by radar remote sensing systems does not merely produce useful images of the Earth when all else fails because of cloud cover. They interact with the surface in such a way that their characteristics change, specifically when the moisture content of surface materials such as soil varies. This phenomenon has spurred development of satellite-borne estimation of soil moisture. But since the launch of constellations of satellites aimed at precise navigation, such as the well-known US Global Positioning System (GPS) and Europe’s Galileo system, everywhere on the Earth is continually bathed in weak microwaves. Researchers at the University of Colorado, Boulder have done a test of the concept using a single GPS receiver recording continuously at one site in Tashkent, Uzbekistan (Larson, K.M. et al. 2008. Using GPS multipath to measure soil moisture fluctuations: initial results. GPS Solutions, v. 12, p. 173-177).
Multipath signals are received when an electromagnetic signal arrives at an antenna, not along a direct path from its source, but indirectly due to reflection of the signal by an object or surface near the antenna. Multipath contaminates all GPS measurements, leading to small positional errors, because the receiver locks onto a signal that mixes the direct and reflected signal. It is difficult to isolate the effects of multipath in GPS carrier phase signals. However, the signal-to-noise ratio (SNR) data computed by a GPS receiver are also affected by multipath and provide an easier route to quantifying multipath effects. In fact the authors found that the amplitude of the SNR varies over time and correlates well with variations in local soil moisture following rainy and dry episodes. Although a first test of concept, the results are sufficiently encouraging that specialist GPS receivers may be developed that allow both precise positioning and accurate measurements of soil moisture – what may become a must for hydrogeologists, especially in arid and semi-arid terrains.