We present the status of advanced construction of the 2.3 t, 920 Hz and 100 mK ultracryogenic resonant bar AURIGA at the INFN Legnaro Lab. We have demonstrated in room temperature tests a pulse energy sensitivity of 12 K and the ability to resolve pulse arrival times (phase part) as short as 2 mu s. The route to approach the Standard Quantum Limit for the detection of g.w. bursts is discussed in the light of a numerically solved electromechanical model. The results show the relevance of the role of the optimal condition for the transducer mass and of the ratio Q/T of the quality factor of all resonators involved, mechanical and electrical, to their thermodynamic temperature. A few general considerations are given on the SQL of ultracryogenic antennae, as for spheres or bars of various aspect ratios, made of various materials. We give a brief presentation, with a preliminary comparison, of proposed concepts of networks of the various kinds of antennae, to get omnidirectional observatories, which would be able to confidently detect g.w. pulses with their incoming direction, and test their Riemann tensorial properties with their propagation at light velocity.
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