def _as_stac_instruments(value: str):
    """
    >>> _as_stac_instruments('TM')
    ['tm']
    >>> _as_stac_instruments('OLI')
    ['oli']
    >>> _as_stac_instruments('ETM+')
    ['etm']
    >>> _as_stac_instruments('OLI_TIRS')
    ['oli', 'tirs']
    """
    return [i.strip("+-").lower() for i in value.split("_")]