  Over the last decades, radio detection of air showers has been established as
a promising detection technique for ultrahigh-energy cosmic rays and neutrinos.
Very large or dense antenna arrays are necessary to be proficient at collecting
information about these particles and understanding their properties
accurately. The exploitation of such arrays requires to run massive air-shower
simulations to evaluate the radio signal at each antenna position, taking into
account features such as the ground topology. In order to reduce computational
costs, we have developed a fast computation of the emitted radio signal on the
basis of generic shower simulations, called Radio Morphing. The method consists
in the calculation of the radio signal of any air-shower by i) a scaling of the
electric-field amplitude of a reference air shower to the target shower, ii) an
isometry on the simulated positions and iii) an interpolation of the radio
pulse at the desired position. This technique enables one to gain many orders
of magnitude in CPU time compared to a standard computation. In this
contribution, we present this novel tool and explain its methodology. In
particular, Radio Morphing will be a key element for the simulation chain of
the Giant Radio Array for Neutrino Detection (GRAND) project, that aims at
detecting ultra-high-energy neutrinos with an array of 200 000 radio antennas
in mountainous regions.os with an array of 200 000 radio antennas in
mountainous regions.
