GPS antenna placement

I ran into Mike Meyers who sets up Rangers for Frankie’s Marine. There seems to be a bit of confusing about placing GPS antennas’s that needs clearification.
If you were mapping, the best theoretical placement of the antenna for accuracy would be to place one like the Lowrance Puck directly over the transducer. As with all things in life, that’s theoretical. Unfortunately in real life there are far more important factors. One of these is whats called shadow zones created by objects between the satilite and the antenna. This can be created by almost anything at that high of freq, even down to a person. But the big problem with installing the antenna back by the transom is the motor, an almost perfect black hole. Unless the puck was mounted on a pole so the antenna cleared the top of the engine, which is impractical, there is almost a 90 degree wedge on the horizon raising almost to straight up that will prevent any signals from coming thru, and that’s just the motor.
Second, motors are notorious for creating whats called RFI, or Radio Freguency interference. With the new fuel injection systems you have more than just the good ole spark plug problems. The nice part is that getting even half a boat lenght way can help if you have that problem, but there is now way to tell if that might be causing problems other than poor acquisition of satilites.
So you can see, having the GPS antenna and the transducer close to each other would be nice, its not a real good idea in most cases.
And one more fact. Lowrance has done a great job with its abilities to collect data so you can make your own maps, but the system is not that accurate that you need to worry about having the antenna that close to the transducer.

WarrenMN

Even WAAS GPS systems are only acurate to around 30ft at best. So if you think putting the antenna over the transducere is going to bring you back to that excact spot, you are wrong. Some days even with good satelitte reception the best accuracy you can get is within 100ft. These are invalueable tools without a doubt, but alot of people buy these thinking they are acurate within inches. Then when they are using them and they only get close they think the units are broke.

Good points guys. GPS is a great tool but you can’t assume it’s dead on accurate. I keep my antenna on my console without any problems.

If you look at the spec’s, GPS are suppose to bring you back with in 7 to 10 feet with WAAS, but here’s where puck placement does affect accuracy. When you come back to the same spot, where ever the puck is, thats where the position is. So if you have it on the bow and see a spot under the transducer, on these big boats, that position is recorded 20 feet ahead of where the transducer is. So if you were going east and did that and came back going west, the transducer would be 40 feet from where it was when that puck got back exactly where it was before. Thats why I say theoretically, but why worry about accuracies so close it only tells you which side of the boat to fish. If the winds blowing, 5 seconds and your off the mark any way. It comes down to a saying, Penny wise and pound foolish.
With the troubles the motor could cause, the gps’s view of the sky could be so hindered you’ve lost any thing you gained. Add in that there must be a bit of a delay between when Lowrance records the lat/lon and the depth, thats why you need to go 2 mph or lower for ok maps. Its still the best tool out there for most. I don’t think people are interested in stepping up to the tens of thousands of dollars equipement.
WarrenMN

I have a Lowrance LMS332C with the Navionics Premium Gold I believe it is?? I noticed that the map and my GPS location as shown on it when zoomed way in is not as accurate as I would hope. It is close however and helps a great deal, I like to fish small reefs shown on the map but I have to run split screen just to double check my depth. There is a bouy shown on this map perhaps for navigation purposed on the lake I mostly fish. If I position my boat with GPS puck as closely as I can to this bouy Can I calibrate my GPS to this bouy??? Has anyone done this or is it do-able?
Thanks, Satch

That likely is the last place I’d check against as bouys move alot. If they aren’t picked up in the fall and placed in the spring, then the ice movement over winter and in the spring when it goes out would likely move it.
I’d have to say the best place to check your gps is the easiest too, at the boat ramp, as long as it doesn’t have the problems Oahe has.
WarreMN

Doc always gets after me for going over every ones head, even some times his, but this might be a good time to bring up some thing on the accuracy of your GPS. There should be a number you can look up on your GPS when your out with it that can give you an idea of just how good the answer it’s giving you is. There is HDOP (2D) and PDOP(3D) which are kind of a circle of area that covers how much the different satelites disagree.

You can look at the signal from a satilite as a string with a specific length from its position. You take that string and scribe an arc through the reciever and that should fall exactly where you are. Now do that with three satilites and they should all intersect in one spot and that would give you a 2D answer. Now do it with 4 satilites and it should even toss in your altitude or a 3D point. Problem is there are some things make that distance from the satilite take paths that aren’t exactly straight for one reason or an other. You can see one of the effects some times on weather radars. When coniditions are right there are all sorts of ground reflections showing up.

There are other factors too as in if the how accurate the sat’s clock is, how good your GPS’s clock is and how old its values (Almanac?) for figuring out where the satilite is are. One of the two sites the Navy has to set the sat’s clocks is in Rosemont I think.

All these things add up to error in figuring what the string length is. Now when you scribe the lines they don’t all cross at one perfect point but cover an area. My gps gives me the HDOP, Horizontal Dilution of Precision, for 2D and PDOP, Position Dilution of Precision, for 3D.

What it amounts to is a value to show how big an area was needed to include all the intersections. So the bigger the value the more error is in that location the gps is giving you.

Here’s the a good place to look up some of the terms you may see and wonder what they are for.

http://www.schriever.af.mil/GpsSupportCenter/glossary.htm

WarrenMN