def lat(source):
    """Convert source bytes to a latitude (deg, min) pair.
    Latitude: 2543.7024 = DDMM.MMMM
    
    >>> lat(b'2543.7024')
    (25, 43.7024)
    """
    if len(source) == 0: return None, None
    dd= int(source[:2])
    mm= float(source[2:])
    return int(dd), float(mm)