CAUTION: There are many inaccurate claims on eBay regarding marine GPS antennas. Like all products there is a difference in the build quality and the performance of the chipset or "engine" used. The chipset is important as it determines the number of channels, the speed of processing, time to fix and reliability in poor reception conditions such as heavy rain. In addition quality units have up to date solid state memory backup similar to computer memory devices rather than cheaper volatile battery backed memory. Some claim to have 12 or 16 channels but not all can process these simultaneously resulting in "Not Used" status for some satellites even though they are in view. All GPS units are EMC (interference protected) they have to be by law however, contrary to some claims, all are affected by radiation from Radars or VHF units if mounted incorrectly. All modern GPS units have Magnetic Declination Compensation, it is part of the GPS system and is only useful if you are using your GPS to give you a "calculated" magnetic heading, the earth's magnetic field does not affect a GPS's normal operation. Finally, not only do our units come with complete instructions for your specific application but our expert technical support team will be happy to help you install your new purchase.