def vmax_dual(wavelength, prf1, prf2):
    """Doppler velocity [m/s] from dual PRF scheme radar (+ or -).

    From Jorgensen (1983), Eqn 2

    Parameters
    ----------
    wavelength : float
        Radar wavelength [m]
    prf1 : float
        First Pulse repetition frequency [Hz]
    prf2 : float
        Second Pulse repetition frequency [Hz]

    Notes
    -----
    In the case of a mobile platform (such as the NOAA P-3 aircraft, the
    Doppler velocity must be adjusted for movement of the scanning platform.

    The fore/aft angle is defined as the angle fore or aft from a plane
    normal to the direction of motion
    """
    return wavelength / (4 * (1.0 / prf1 - 1.0 / prf2))
