The Egyptian pyramids, easily the most famous monuments of the ancient world, are scattered in the desert on the Nile's west bank. Built during the Old Kingdom period (about 2800-2200 B.C.) and Middle Kingdom (2040-1780 B.C.), the pyramids are heavily studied, but they still hold riddles aplenty.

It is reasonable to suppose that, despite some 200 years of archaeological explorations, some pyramids remain to be discovered in the area, since the tombs of at least three kings — Menkauhor (of the fifth dynasty), Neferkare (seventh to eighth dynasty), and Ity (ninth to tenth dynasty) — have never been found. If we can find these pyramids in their original state, the purpose and manner of construction and other lingering questions might be answered.

Tokai University Research & Information Center (TRIC) has for decades been using satellite data to survey and study archaeological remains and paleoenvironments around the world. Recently we used data from space, for the first time in Egyptology, to explore the "pyramid zone" — the Nile's west bank from Abu Rawash to Maidum.

A central question of our study is whether pyramids like those of the three giant monuments of Giza could remain undiscovered. Khufu's pyramid, known as the Great Pyramid, is about 230 meters (755 feet) on a side and 146 meters (479 feet) high. It is, of course, difficult to envision such a huge complex being buried under that desert and escaping detection for more than 4,000 years.

Such a pyramid almost certainly would have been looted, and perhaps even demolished, during the social unrest that followed the end of the Old Kingdom. Some collapsed and unfinished pyramids — which look like mountains of rubble or simply foundations without the dramatic stone superstructure — have been found; such ruined structures became the main target in this study.

	The site map of the Dahshur North excavation reveals an elaborate tomb, probably that of an important personage from the eighteenth or nineteenth Egyptian dynasty.

The View from the Sky

A wide variety of satellite data in both optical and microwave wavelengths was analyzed to search for traces of collapsed and unfinished pyramids and other artificial objects invisible from the surface. In general, each wavelength provides different information, and comparing the various satellite views of the same area provides a rich source of data.

Microwave images in different wavelengths (each providing slightly different information) were obtained for the target area from the Japanese Earth Resources Satellite (JERS-1) and the European Union's European Remote Sensing Satellite (ERS-1). These were studied to determine the microwave signature of bedrock, limestone blocks, mud bricks, and other archaeological objects, including pottery sherds and stone fragments. We also examined optical satellite data from the French SPOT satellites, American Landsat, and Russia's KVR-1000. In each case, the images were analyzed to clarify the shape and other characteristics of known archaeological remains, especially collapsed or unfinished pyramids.

The microwave images are made with "synthetic aperture radar" (SAR), a process in which the spacecraft's motion is tapped to simulate a very long antenna, which improves resolution. Short, repeated pulses of radio waves (as many as 1,700 per second) are transmitted from the satellite, and the backscatter (the echo from the ground) is collected and processed to create the image.

The density of the object on or under the ground is determined by the backscatter strength of the microwave signal; strong backscatter is associated with artificial constructions, including pyramids and other monuments. The European satellite uses C-band microwaves that display only the surface undulations of the desert. The higher-frequency L-band images of the Japanese spacecraft were much more effective at detecting buried artificial constructions; the satellite readily sees beneath the dry desert sand — where optical images revealed nothing. We therefore emphasized the JERS-1 images in our search for hidden objects.

Optical satellite images were used to clarify the shapes of known collapsed or unfinished pyramids and their formation. We first identified the spectral characteristics for the pyramids and other limestone monuments in the Landsat image. We used the SPOT images to verify the resolution needed to identify these structures. We found that monumental constructions showed strong differences from the surrounding sand, and a 10-meter (33-foot) resolution is sufficient to identify pyramid-size monuments. The two-meter (6.5-foot) resolution of the Russian KVR-1000 was used to define the surface shapes of collapsed or unfinished pyramids, which turn out to be quite distinctive.