def unpack_uncertainty_parameter(u):
    """Unpack uncertainty parameter (integer or single character). 

    See:
        https://www.minorplanetcenter.net/iau/info/MPOrbitFormat.html

    Args:
        u (str): Packed uncertainty parameter.

    Returns:
        int: Uncertainty parameter, or -1 if invalid.
        str: Empty string or letter if uncertainty parameter was invalid.
    """
    try:
        u = int(u)
        u_flag = ''
    except ValueError:
        u_flag = u
        u = -1
    return u, u_flag