def lon(source):
    """Convert source bytes to longitude (deg, mim) pair.
    Longitude: 08014.5267 = DDDMM.MMMM
    
    >>> lon(b'08014.5267')
    (80, 14.5267)
    """
    if len(source) == 0: return None, None
    dd= int(source[:3])
    mm= float(source[3:])
    return int(dd), float(mm)