Satyam is Truth and Rtam (pronounced as Ritam) is the law that governs the working of that Truth. So, although both words basically mean the same, there is still a subtle difference between the two.
One Mantra can be cited here from the Rig Veda, that has both Rtam and Satyam in it:
Rtam cha satyam chAbhidhdAt (1), tapaso adhyajAyata (2)
Truth was born and the Law of Truth (1), From the kindled fire of
Energy of Consciousness.
Rig Veda 10.190.1
Another one from Atharva Veda:
satyam brihat rtam ugram, dikshA tapo brahma yajnah, prithivi
The Truth (satyam), the Vast (brihat) and the Truth-in-movement
(rtam), strength (ugram), initiation, askesis, mantra (brahma) and
Yajna- These uphold the Earth.
Atharva Veda 12.1.1
On the Rig Vedic Mantra, the commentary by Dr R.L.Kashyap (who is with the Sri Aurobindo Kapaly Shastry Institute of Vedic Studies) is as follows:
The Supreme Reality is not a mere existence, immutable and
featureless. It is supremely aware; it is a Consciousness. And this
Consciousness is again not a mere awareness. It is dynamic, it is a
Power. When this Consciousness as Power moves into action, creation
ensues. First the Truth (Satyam) behind the creation, formulates
itself leading to the Law of the working of that Truth (Rtam). This
self-determination of Truth is the seed of creation and it's Law lays
down the lines of manifestation and governs its development.
So, i think this commentary sufficiently explains the concept of Rtam and also how it is subtly different from the concept of Satyam?