lg(E$_{FD}$/eV) 18.5  19  19.5  /VEM) $_{38 }$ lg(S  1 1.5  2  2.5  3  2 Fig. 2. The exposures of several cosmic ray experiments as a function of measured energy [11]. Fig. 1. Correlation between an energy estimator of the surface detector (S 38 ) and the energy reconstructed from fluorescence detector data (E FD ) for extensive air showers measured by the Pierre Auger Observatory [4]. The full line is the best fit to the data. at the trigger level reaches 100% for cosmic ray events $^{with the energy above 3}$× 10 18 eV. By including events with larger zenith angles (up to 80 ◦ ) in the analysis, the aperture is increased by 30%. The events recorded by the surface detector are recon- structed using the arrival time and the signal size from the shower particles reaching the tanks. The magnitude of the signal at 1 km from the intersection of the shower axis with the ground is S (1000), measured in units of vertical equivalent muon (VEM) – average signal size given by a muon crossing the tank vertically. The total signal is estimated from the lateral distribution function fit as a size parameter of the shower. Two cosmic rays of the same energy, but incident at different zenith angles, will yield different values of S (1000) due to the attenu- ation of the shower in the atmosphere. The attenuation curve can be obtained from measured data. With the subset of events detected in hybrid mode (simultaneous measurement with both surface and flu- orescence detectors) the link between S (1000) for given zenith angle and the primary energy can be established independently on simulations using data from the flu- orescence detector which provides calorimetric energy measurement (see Fig. 1.). There are currently three working observatories of UHECRs in the world. Two of them are located in the northern hemisphere – Telescope Array in Utah, USA [8] and Yakutsk in Russia [9]. The Pierre Pierre Auger Observatory is situated in western Argentina [10], but its second part is planned in the northern hemisphere in Colorado, USA [12]. The exposures of current and also of some former observatories of UHECRs are shown in Fig. 2.. The Pierre Auger Observatory and also Telescope Ar- ray will be described here. Both of them are hybrid observatories, but they differ in the type of the surface detector. The Pierre Auger Observatory has already published its results, which are discussed in following sections. Up to now, there was no result presented by the Telescope Array collaboration. The Pierre Auger Observatory is located in the south- ern hemisphere in the western Argentina province of Mendoza close to the city of Malarg¨ ue (69 ◦ W, 35 ◦ S, 1 400 m a.s.l.) [10]. The construction of this largest cosmic ray observatory in the world was finished in June 2008. The arrival direction of the primary particle is inferred from the relative arrival times of the shower front at dif- ferent surface detector tanks. The angular resolution im- proves with measured energy and zenith angle because of greater number of triggered stations. The surface array has the angular precision better than 1 ◦ in case of mea- surement of extensive air shower by at least four tanks. The fluorescence detector comprises four observation sites located atop small elevations on the perimeter of the surface detector array [14]. Six independent telescopes, each with the field of view of 30 ◦ times 30 ◦ in azimuth and elevation, are located in each fluorescence detector site. The telescopes face towards the interior of the array so that the combination of the six telescopes provides 180 ◦ coverage in azimuth. The telescopes are housed in climate-controlled buildings. Nitrogen fluorescence light enters through a large UV filter window and Schmidt optics corrector ring. The light is focused by a spherical mirror onto a camera of 440 pixels with photomultiplier light sensors (see Fig. 4.). Light pulses in the pixels are digitized every 100 nanoseconds, and a hierarchy of trigger levels culminates in the detection and recording The surface detector measures the front of the shower as it reaches the ground. The tanks activated by a cos- mic ray shower record the particle density and the time of arrival. The flux of cosmic ray muons provides a con- tinuous monitoring of the surface detector. The aper- ture achieved with the surface array for zenith angles less than 60 ◦ is 7 350 km 2 sr. The detection efficiency The surface detector is an array of more than 1 600 water Cherenkov detectors spaced at a distance of 1.5 km and covering a total area of 3 000 km $^{2}$. Each detector is a plastic tank of cylindrical shape with an area of 10 m 2 and height 1.2 m filled with purified water [13]. Each tank is operating completely autonomously and its components are shown in Fig. 3.. 3.1. Pierre Auger Observatory 3. Current Experiments