def get_default_signal_estimate(sicd):
    """
    Gets default signal for use in the RNIIRS calculation. This will be
    1.0 for copolar (or unknown) collections, and 0.25 for cross-pole
    collections.

    Parameters
    ----------
    sicd : sarpy.io.complex.sicd_elements.SICD.SICDType

    Returns
    -------
    float
    """

    if sicd.ImageFormation is None or sicd.ImageFormation.TxRcvPolarizationProc is None:
        return 1.0

    # use 1.0 for co-polar collection and 0.25 from cross-polar collection
    pol = sicd.ImageFormation.TxRcvPolarizationProc
    if pol is None or ':' not in pol:
        return 1.0

    pols = pol.split(':')

    return 1.0 if pols[0] == pols[1] else 0.25