def lat_lon_determine(cube):
    """
    Test whether a diagnostic cube is on a latitude/longitude grid or uses an
    alternative projection.

    Args:
        cube (iris.cube.Cube):
            A diagnostic cube to examine for coordinate system.

    Returns:
        trg_crs (cartopy.crs.CRS or None):
            Coordinate system of the diagnostic cube in a cartopy format unless
            it is already a latitude/longitude grid, in which case None is
            returned.

    """
    trg_crs = None
    if (not cube.coord(axis='x').name() == 'longitude' or
            not cube.coord(axis='y').name() == 'latitude'):
        trg_crs = cube.coord_system().as_cartopy_crs()
    return trg_crs