You are setting up a system that receives a fairly weak signal from 20.000 km away.
An RF noise emitter a few cm away from the receivers, the antenna cable, or the antenna itself can easily affect the data-quality.
The M2 is made to be lightweight, so you won’t find a a stock metal enclosure, like every other GNSS receiver has. However, there are ways do this yourself, and it is very effective.
With regards to triggering, you could consider using shielded cables as well.