Nav platforms are named in accordance with letters of the Greek alphabet. Starting with Alpha (α) which operates just 10 days in front of the Ark (t+10 days), through Beta (t+50 days), Gamma (t+100 days), etc... all the way up to extremely remote ones operating a couple of light-years / many centuries in front of the voyage.
These robotic devices serve as critical "trail blazers", lighting the way ahead through the vast cosmic darkness yawning between our Solar System and the distant shores of Alpha Centauri. The fact that both our own Solar System and Alpha Centauri might each contain vast shells of overlapping Oort Clouds, containing myriads of uncharted material and debris, was postulated by Abdul Ahad in the original blueprint. He then designed the voyage with nav platforms, so that they would pre-alert the Navigation Team inside the Ark in anticipation of the risks, should the Ark be on a potential collision course with such debris. Micro-maneouvers could be effected, both to avert disasterous collisions, as well as to intercept such icy material as a source of water / volatiles essential to top up the biosphere's life supporting ingredients. This is discussed by Commander Zakarov and the crew of the Navigation and Control Complex on pages 84 and 85 of First Ark to Alpha Centauri - the novel.