My GPS is mounted on the flybridge dash under a bimini, (see Avatar), and it does not interfere at all. However, some earlier GPS units were less good in picking up signals that are weak, and did not pick up as many channels either, so were not good if shrouded. My first car GPS (circa 2004), was pathetic at this, and only worked right up in clear line of sight under the windscreen. From the way my iPad and iPhone now work anywhere just about, I suspect any more modern unit would be fine - which is why they almost all now appear to have in-built antennae.