Advanced LIGO and Virgo have so far detected gravitational waves from 10 binary black hole mergers ( BBH ) and 1 binary neutron star merger ( BNS ) .
In the future , we expect the detection of many more marginal sources , since compact binary coalescences detectable by advanced ground-based instruments are roughly distributed uniformly in comoving volume .
In this paper we simulate weak signals from compact binary coalescences of various morphologies and optimal network signal-to-noise ratios ( henceforth SNRs ) , and analyze if and to which extent their parameters can be measured by advanced LIGO and Virgo in their third observing run .
We show that subthreshold binary neutron stars , with SNRs below 12 ( 10 ) yield uncertainties in their sky position larger than 400 ( 700 ) \mathrm { deg } ^ { 2 } ( 90 % credible interval ) .
The luminosity distance , which could be used to measure the Hubble constant with standard sirens , has relative uncertainties larger than 40 % for BNSs and neutron star black hole mergers .
For sources with SNRs below 8 , it is not uncommon that the extrinsic parameters , sky position and distance , can not be measured .
Next , we look at the intrinsic parameters , masses and spins .
We show that the detector-frame chirp mass can sometimes be measured with uncertainties below 1 \% even for sources at SNRs of 6 , although multimodality is not uncommon and can significantly broaden the posteriors .
The effective inspiral spin is best measured for neutron star black hole mergers , for which the uncertainties can be as low as \sim 0.08 ( \sim 0.2 ) at SNR 12 ( 8 ) .
The uncertainty is higher for systems with comparable component masses or lack of spin precession .