The radio transmissions it hears are very faint, and the thermal noise of its drive plume is very loud. P.E.R.C. records every scrap of radio it hears, and runs it through signal processors several times, carefully checking whether any individual noise is the plausible corruption of a Network protocol that it recognizes. There are a few positive matches, but P.E.R.C. is pretty sure they're false ones.

Too much unknown traffic, and it all matches well to simple analog transmissions in unknown language(s).

P.E.R.C. carefully reads through its first-contact packet to make sure the whole transmission still makes sense, and hasn't been corrupted by the years its spent between the stars.

By the time P.E.R.C. is a little under a six light months away, it thinks it has a good enough triangulation on the sources to hit them with the narrow-beam communications antenna, so it pulls back the protective cover and sends three messages:

On standard Network frequencies, a digital burst reading "P.E.R.C. vessel 170E9A, reporting unknown location, unknown hardware errors. Intending to enter polar orbit on <trajectory data>. Please respond with navigation instructions."

On an unused frequency near the other frequencies it's heard transmissions on, its prepared first-contact package, which starts with defining peano arithmetic, works its way up to the lambda calculus, and then uses that to describe how to compute various physical quantities and some basic game theory.

One octave up from that frequency, copies of the most recent radio broadcasts that it has successfully decoded (continuously updated), so that people can work out its distance and velocity by watching the time delay.