# Delta-K velocities

Radar processing with "Delta-K" is a different processing from the pulse pair, and is very similar to what can be obtained from a coastal HF radar. The principle of delta-K is that the interference of radar echoes at different radar wavenumbers k_{1}=2π/L_{1} and k_{2}=2π/L_{2} corresponds to a wavenumber ΔK = |k_{1}-k_{2}|=2π / L, that can be much longer. The phase evolution of these interferences should thus probe the velocities of much longer waves, exactly like HF radars. In the case of SKIM a L=4.8 m correspond to the same horizontal velocity as a 24 m wavelength radar.

It turns out that the phase speed of waves changes with their wavelength and with the current, such that the no-current phase speed that is C_{0}=√(gL/ 2π) is shifted by a current averaged over typically 1/6 times the wavelength. Thus, in principle, "Delta-K" velocities for different choices of ΔK can give access to estimates of the variation of the current on the vertical (the vertical shear).