def yaw(ax, ay, az):
    """Angle of z-axis relative to ground (psi)

    Args
    ----
    ax: ndarray
        x-axis acceleration values
    ay: ndarray
        y-axis acceleration values
    az: ndarray
        z-axis acceleration values

    Returns
    -------
    yaw: ndarray
        Yaw angle in radians
    """
    import numpy
    return numpy.arctan2(ax, ay)
