[macguillicuddy]

My understanding is it depends on the amount of visibility, plus what type of approach they were on. One type of approach, an ILS, has big radio transmitters pointing from the runway into the air and allows the plane (either pilots or autopilot) to get close enough to the runway without visibility, and with enough precision, to land. In many circumstances ILS isn't available and an alternative is Required Navigation Performance (RNP) which uses GPS plus a ton of other inputs to give some amount of precision to the same end. If they're on an RNP approach but suffer a reduction in navigation accuracy then I imagine it's a policy 'go-around'. Even if there's enough visibility it allows the pilots to brief a 'visual' approach before attempting it.