Re: External vs. Internal GPS antenna
Searun1<br />The internal antenna works pretty well thru glass and a soft canvas or vinyl top. It will not work thru metal windsheild frames, other electronic gear, Your body.<br /><br />I find my GPSMAP 76 works very well in my 21 foot Runabout. I have it mounted on the dash on the starboard side not too far from the aluminum window frame. I just have a vinyl top. Other electronics in my case do block the signal as they are higher and just to the left. Still I never loose position out on a lake or out to sea.<br /><br />I suggest if your only going to use it in your boat then get the external antenna. If you are going to put in your truck also while on the road then the internal antenna may be better. <br /><br />Why use an external? There are 24 GPS satellites up in low earth orbit. To get a position you need 3 birds. To also get elevation (3-D) your need 4 birds. These satellites are in low earth orbit and with 24 up you will not have trouble getting 4 for a 3-D position when out on the open water. There are also 2 Geostationary satelites that you can receive. (Garmin #35 AOR-W and Garmin #47 POR) Since these birds are Geostationary they are 22300 miles up and over the equator. From your location this means they will always be low on the horizon and a long ways away. These are the WAAS satellites used for error correction. If You can not receives these birds then your GPS accuracy is about 9 meters at best or 30 feet. With the WAAS error correction your accuracy is about 3 meters or 10 feet. In my opinion at your location to receive the WAAS birds consistently you will need to use a external antenna up high enough to not be blocked by other iteams on the boat. Somewhere near the top of the windsheild unless you have a radar. If mounted on the dash very often someone or something will block the WAAS birds and with WAAS birds accuracy builds over time. So for best accuracy go with an external mounted where it has a clear view of the entire horzion.<br /><br />Two WAAS birds your can see from your location are Garmin #47 POR (Pacific Ocean Region) at location 0 degrees Latitude and 178 degrees East longitude.<br />Also Garmin #35 AOR-W (Atlantic Ocean Region West) at location 0 degrees latitude and 54 degrees West Longitude.<br /><br />Garmin 172C will work on you dash with the internal antenna but to get the best accuracy I suggest the external antenna.
