  Gamma Ray Bursts (GRBs) are a powerful probe of the high redshift Universe.
We present a tool to estimate the detection rate of high-z GRBs by a generic
detector with defined energy band and sensitivity. We base this on a population
model that reproduces the observed properties of GRBs detected by Swift, Fermi
and CGRO in the hard X-ray and gamma-ray bands. We provide the expected
cumulative distributions of the flux and fluence of simulated GRBs in different
energy bands. We show that scintillator detectors, operating at relatively high
energies (e.g. tens of keV to the MeV), can detect only the most luminous GRBs
at high redshifts due to the link between the peak spectral energy and the
luminosity (Ep-Liso) of GRBs. We show that the best strategy for catching the
largest number of high-z bursts is to go softer (e.g. in the soft X-ray band)
but with a very high sensitivity. For instance, an imaging soft X-ray detector
operating in the 0.2-5 keV energy band reaching a sensitivity, corresponding to
a fluence of ~10^-8 erg cm^-2, is expected to detect ~40 GRBs yr^-1 sr^-1 at
z>5 (~3 GRBs yr^-1 sr^-1 at z>10). Once high-z GRBs are detected the principal
issue is to secure their redshift. To this aim we estimate their NIR afterglow
flux at relatively early times and evaluate the effectiveness of following them
up and construct usable samples of events with any forthcoming GRB mission
dedicated to explore the high-z Universe.
