If your radar mast is gimbaled then there is no issue. However, just so you know, one of my handheld GPSs is located in the cabin and I have some degradation but not enough to kill the connection.
Off topic here but, I enjoy it when I see satellite TV antennas high on the house. As if the height is going to improve the reception. 100% of the time, if there is no interference, the dishes are placed high so the logo can be seen. Satellite dishes work as well at ground level as long as there is not blockage of the signal.
My point being, GPS is not Satellite TV nor is it INMARSAT. GPS has a fleet of satellites always moving so anywhere you put the antenna should receive at a minimum of 3 birds, which is all that is needed for a fix. So put it on the combing, the rail, the coachroof... anywhere should be fine as long as it is not in the way of safe boat operations. And, I would recommend a short cable run to whatever the antenna is connect to.