I friend of mine has a never used Raymarine tridata with transducers and a garmin 19hvs gps antenna that he has offered to me for $475. He will also install it for me for free. He purchased it about a year ago for a boat that he wound up selling. I have the old Signet marine units right now (depth and speed) and neither of them is working. He explained to me that with the GPS antenna I could tell my speed over ground instead of my apparent speed with the current passing over the paddlewheel as well as the boats motion. I know that the price is a good one especially since he will be installing it with me. My real question is are these good units and also is the antenna necessary since I don't have an actual gps.