  Radio detection of air-showers is a mature technique that has gained momentum
over the past decades. With increasingly large-scale experiments, massive
air-shower simulations are needed to evaluate the radio signal at each antenna
position. Radio Morphing was developed for this purpose. It is a
semi-analytical tool that enables a fast computation of the radio signal
emitted by any air-shower at any location, from the simulation data of one
single reference shower at given positions. It relies on simple electromagnetic
scaling laws of the radio emission (i.e., electric field) at the antenna level
and then an interpolation of the radio pulse at the desired positions. We
present here major improvements on the Radio Morphing method that have been
implemented recently. The upgraded version is based on revised and refined
scaling laws, derived from physical principles. It also includes
shower-to-shower fluctuations and a new spatial interpolation technique, thanks
to which an excellent signal timing accuracy of a fraction of nanosecond can be
reached. This new implementation, provides simulated signals with relative
differences on the peak-to-peak amplitude of ZHAireS simulations below 10\%
(respectively 25\%) for 91\% (99\%) of antennas while the computation time was
reduced by more than 2 orders of magnitude compared to standard simulations.
This makes Radio Morphing an efficient tool that allows for a fast and accurate
computation of air-shower radio signals. Further implementation of Askaryan
emission or enabling to use an input value of the geomagnetic field should
reduce relative differences with ZHAireS by few percents and make the method
more universal.
