In the novel, it is clear that the transmission is picked up by a series of probes, which allowed the direction and destination to be picked up.
And now Deep Space Monitor 79 had noted something strange - a faint
yet unmistakable disturbance rippling across the Solar System, and
quite unlike any natural phenomenon it had ever observed in the past.
Automatically, it recorded the direction, the time, the intensity; in
a few hours it would pass the information to Earth. As, also, would
Orbiter M 15, circling Mars twice a day; and High Inclination Probe
21, climbing slowly above the plane of the ecliptic; and even
Artificial Comet 5, heading out into the cold wastes beyond Pluto,
along an orbit whose far point it would not reach for a thousand
‘After all these ages, one would naturally assume that it was inert.
But soon after lunar sunrise, it emitted an extremely powerful blast
of radio energy. We believe that this energy was merely the by-product - the backwash, as it were - of some unknown form of radiation, for at the same time, several of our space-probes detected an unusual
disturbance crossing the Solar System. We were able to track it with
great accuracy. It was aimed precisely at Saturn.