along with cloud patterns. Throughout the 1960s, when the Mercury, Gemini, and Apollo space programs were launched by the U.S. National Aeronautics and Space Administration (NASA), thousands of photographs of the earth’s surface were taken from space. The level of technological sophistication of the cameras used increased through time, and in 1969, during the Apollo 9 space mission, the first multispectral photographs of the earth’s surface were taken. The clear advantages that the use of space imagery offered to monitoring the earth’s natural resources encouraged NASA and the U.S. Department of the Interior to create a spatial observations program for that purpose, and in 1967, the Earth Resources Technology Satellites (ERTS) international program began. This space program would successfully launch into the earth’s orbit the first unmanned satellites, which we now know as the Landsat series.

These developments, however, also took place in the midst of the cold war, and when the need to monitor the Soviet bloc’s movements became apparent after the Cuban missile crisis in the early 1960s, high-resolution spy satellites were sent into the earth’s orbit to keep track of any military buildup. Nevertheless, civilian-run programs represent the more important suppliers of satellite imagery. Along with the Landsat series, other important satellite series are the NOAA and GEO series along with the French SPOT satellites.

Information about earth’s resources is also being obtained at an increasing rate from sensors that operate at the microwave level, that is, from radar sensors. Radar imagery can be produced from airborne or spaceborne platforms, and several series of the latter have been in operation since the late 1970s. These are the shuttle-imaging radar (SIR) A, B, and C series launched by the United States, and the Almaz-1, ERS-1, JERS-1, and Radarsat systems launched by the former Soviet Union, the European Space Agency, Japan, and Canada, respectively.

NASA has grown aware of the value that remotely sensed data may have to social scientists and has made great efforts to establish a reach-out program. More and more satellite and radar imagery has been declassified and made available to civilian researchers, and archaeologists have used Landsat images to trace ancient levee systems in Mesopotamia, map prehistoric roads in the chaco canyon in New Mexico, and detect 2,500-year-old footpaths in the Arenal region of costa rica. So far, the most successful application of satellite and radar imagery in archaeological research has taken place in mesoamerica, where the analysis of Landsat and radar images has resulted in the discovery of an extensive network of prehistoric farm fields and settlements on the Yucatán Peninsula and the discovery of “lost” cities and a complex network of causeways and platforms in the Maya lowlands. Radar imagery has also assisted archaeologists in the detection of previously unseen structures in Angkor, cambodia.

Satellite imagery, however, has some disadvantages that may constrain its applicability in archaeology. First, the spaceborne optic sensors carried in the Landsat, SPOT, and NOAA satellites depend on solar radiation to image, for instance, daylight. Second, because of the wavelength of the optical part of the electromagnetic spectrum, atmospheric phenomena like cloud formations, smoke, dust, and fog and vegetation cover represent actual barriers that prevent the sensors from distinguishing cultural features that may be underneath. Third, the level of ground resolution varies significantly depending on the sensor used by the satellite systems. The Landsat series, for example, can go from 240, 120, 80, 30, to 15 meters; SPOT satellites have a ground resolution of 10 to 20 meters; and the NOAA series has a ground resolution of 1.1 kilometer. These characteristics may pose a problem depending on the type of features the archaeologist is interested in detecting and the availability of the images.

From this perspective, radar imagery has several advantages over satellite imagery. First, because they are active sensors, they provide their own illumination (the radar pulses), which means that, unlike optical sensors, they can image any time of day or night regardless of sun illumination. Second, because microwaves—the wavelength in which radar operates—are much longer, they can penetrate cloud cover, fog,