The most critical part of a GPS installation is the antenna. It should be installed where it can get a clear view of the sky and not be in the shadow of obstructions. The antenna should be in an area where it will be minimally affected by multi-pathing; that is receiving unwanted reflected signals from the structure of your boat. However, you should not mount a GPS antenna on top of a sailboat’s mast. The rocking of the boat is accentuated at the top of the mast which makes it an undesirable position for the antenna because the GPS will move as the vessel heels. There is a strong possibility that the antenna of your GPS or your VHF receiver can receive interference if they are in the path of your radar’s transmission beam. In some installations, the powerful pulses from the radar can cause severe problems and interfere with your equipment.
Your GPS may have a built in antenna, or it may be designed with a separate antenna connected by a coaxial cable. The manufacturer should advise against your cutting this cable, because the cable length must remain unchanged for maximum efficiency. Follow your manufacturer’s instructions.
Most recreational vessels are relatively small and even if the antenna is a distance away from the GPS, that distance may still be relatively short. On a large ship it is possible that the antenna may be a fair distance away from the GPS display and the readings shown on its GPS relate to the position of the antenna, not the location of the GPS unit.
To learn more about GPS and Chart plotters register for the CPS-ECP Electronic Navigation in-class course at: http://www.boatingcourses.ca/course-descriptions/electronic-navigation
Or the online course at: http://parttime.stlawrencecollege.ca/stlaw/course/course.aspx?C=80078&pc=248&mc=83&sc=88