ABSTRACT:

An observatory for the highest energy cosmic rays (> 10^19 eV) is under construction and beginning to take data in the southern hemisphere. It is a hybrid detector which observes the cascade of particles produced by a primary cosmic ray both by nitrogen fluorescence in the atmosphere and by particle detectors on the ground. The sensitive area of the observatory is 3000 km^2. At the moment about 100 surface detectors and several of the fluorescence telescopes are in operation. The observatory will be completed by the end of 2005 with 1600 surface detectors and 24 fluorescence detectors A comprehensive study of these cosmic ray requires a complementary northern observatory since at the highest energies the propagation distance of the particles is limited by the Cosmic Microwave Background and the distribution of matter within the allowed propagation distance is not uniformly distributed over the sky. The properties of the observatory will be described and the nature of the data will be discussed.