I would just add that many GPS units support only a few of the more common nema standard sentences,but you will usually find that the more comprehensive information is obtained through a proprietry binary serial interface as is the case with the zodiac range of GPS units.Which interface standard you choose to use is up to you but in many ways the proprietry one is easier.The main drawback is that you need a larger buffer for the proprietry standard as some of those queries return 110 bytes in reply.