code: def get_null_snr(self):
  		
  		null_snr_sq = (numpy.asarray(self.get_sngl_snrs().values())**2)\
                               .sum() - self.snr**2
  		if null_snr_sq < 0:
  			return 0
  		else:
  			return null_snr_sq**(1./2.)
docs: Get the coherent Null SNR for this row.