def snow_permittivity(rho, fs, m=0.0, fw=9070000000.0):
    """
    Calculate the dielectric permittivity of snow
    Kendra et al. (1998), IEEE

    Parameters
    ---------
    rho:        snow density       (g/cm3)
    fs:         radar frequency     (Hz)
    m:          snow wetness        (%)
    fw:         relaxation frequency of water at 0C

    Output
    ---------
    eps_ds:     snow permittivity
    """
    eps_s = 1.0 + 1.7 * rho + 0.7 * rho ** 2.0
    eps_s += 0.02 * m ** 1.015 + 0.073 * m ** 1.31 / (1 + fs / fw)
    return eps_s
