'Set polar motion source flag for entries in IERS table'

def pm_source(self, i):

    pmflag = self[u'PolPMFlag'][i]
    source = (np.ones_like(i) * FROM_IERS_B)
    source[(pmflag == u'I')] = FROM_IERS_A
    source[(pmflag == u'P')] = FROM_IERS_A_PREDICTION
    return source
