The beam spectrometer of the NA62 experiment consists of 3 Gigatracker (GTK) stations. Each station comprises a pixel detector of 16 cm 2 active area made of an assembly of 10 readout ASICs bump bonded to a 200 ฮผm thick pixel silicon sensor, comprising 18000 pixels of 300 ฮผm x 300 ฮผm. The main challenge of the NA62 pixel GTK station is the combination of an extremely high kaon/pion beam rate, where the intensity in the center of the beam reaches up to 1.5 Mhit s -1 mm -2 together with an extreme time resolution of 100 ps. To date, it is the first silicon tracking system with this time resolution. To face this challenge, the pixel analogue front end has been designed with a peaking time of 4 ns, with a planar silicon sensor operating up to 300 V over depletion. Moreover, the radiation level is severe, 2x10 14 1 MeV neq. cm -2 per year of operation. Easy replacement of the GTK stations is foreseen as a design requirement. The amount of material of a single station should also be less than 0.5% X 0 to minimize the background, which imposes strong constraints on the mechanics and the cooling system. We report upon the design and architecture of the 2 prototype demonstrator chips both designed in 130 nm CMOS technology, one with a constant fraction discriminator and the time stamp digitisation in each pixel (In-Pixel), and the other with a time-over-threshold discriminator and the processing of the time stamp located in the End of Column (EoC) region at the chip periphery. Some preliminary results are presented.