def guess_platform(product_id):
    """Guess platform of a product according to its identifier."""
    if len(product_id) == 40 and product_id.startswith('L'):
        return 'Landsat'
    if product_id.startswith('ASA'):
        return 'Envisat'
    if product_id.startswith('SAR'):
        return 'ERS'
    if product_id.startswith('S1'):
        return 'Sentinel-1'
    raise ValueError('Unrecognized product ID.')