I also use the GlobalSAT unit Tom references. It was recommended by the navigation software I use. Have used one for several years and it has worked flawlessly—fast acquisition of satellites and quite accurate. It has had a hard time sometimes under our covered slip roof (metal), but once we are out, it is great. I place it on the forward window sill when at the lower helm. And being in the brokerage business, I like it as I can take if from boat to boat with my laptop and navigate completely independently from whatever system is on the boat I am moving.