The ULTRA experiment has been designed in the framework of the EUSO project to provide quantitative measurements of the reflected/diffused signal produced by the EAS impacting on the Earth surface. The detector is a hybrid system consisting of an UV optical detection unit, the UVscope, and an array of scintillators, the ETscope (EAS Telescope). The UVscope is used to detect Cherenkov diffused UV light from EAS which are detected in coincidence by the ETscope array. The atmospheric transmission

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... roperties will also be studied using the UV light detector and a laser emitter. Moreover, measurements of light from meteors are feasible with the same instrumentation.In this paper the various physical items involved in this research are introduced and discussed, and a detailed project of the experimental setup together with preliminary estimations of the expected performances are presented.