Author Topic: Adding optional GPS sensor to Helix  (Read 1802 times)

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Offline mako9man2

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Adding optional GPS sensor to Helix
« on: July 24, 2016, 09:29:06 AM »
I am in the process of upgrading to the Helix 12 si.  While I am adding this unit should I add an optional GPS Sensor?  The only reason I am even thinking about this is my transducer is located on the transom about 8ft aft of the Head unit.  It seems to me if these GPS receivers are accurate to +-8ft and my transducer is 8ft away when I mark a WAYPOINT  I am potentially +-16ft away from the WP when I try to find it again.
Having this optional GPS sensor mounted on the stern above the Transducer should cut the search area back down to +-8ft.
Am I thinking about this correctly?   


Offline Bob B

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Re: Adding optional GPS sensor to Helix
« Reply #1 on: July 24, 2016, 09:47:09 AM »
Your thinking is absolutely correct.

The HS GPS would give you an even better upgrade.
**Looking for the one that makes it all worthwhile**

Offline mako9man2

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Re: Adding optional GPS sensor to Helix
« Reply #2 on: August 17, 2016, 08:27:01 PM »
Your thinking is absolutely correct.

The HS GPS would give you an even better upgrade.

I got the HS GPS, have it installed and tuned.  Looking at the GPS Diagnostics Screen it is apparently picking up at least 7 satellites and showing a position error of 2 to 4ft.  Am I interpreting this correctly.  Can this unit position me within 2ft of a waypoint?  That would be terrific as it is now mounted directly above the transducer on the stern.

Offline newkid4si

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Re: Adding optional GPS sensor to Helix
« Reply #3 on: August 17, 2016, 10:24:55 PM »

      Here are some quotes about GPS accuracy from previous post.

The Fix Type does not always directly translate into a certain accuracy level.  It should but does not all of the time.  Example: the GPS Receiver could be tracking every satellite in the sky but due to not being able to receive a direct signal from some of the satellites (multi-path error) its accuracy will suffer.  The best indicator of position accuracy is the HDOP number which we translate (as does every manufacturer using their own formulas) to an estimated position accuracy number.  All the Fix Type tells you is how many or what type of satellites the unit is tracking.

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Greg Walters at Humminbird

The following post are from Adam Broughton. He had the finest explanations about many Humminbird features I have seen. Unfortunately his site is no longer up. The pictures won't load and don't
click on the links. They don't work either. Just reading may give you an idea of what we are up against. 

GPS Positioning Details

Wednesday, 30 March 2011 22:34 | Written by Adam Broughton | PDF | Print | E-mail
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At best, most GPS units that you install on your boat can resolve your position to about +/- 1 meter, or about 6 feet in diameter. GPS precision is limited by the US Department of Defense to 1/1000th of a minute of angle of the earth’s geometry, which, if you do the math, works out to a little over 6ft. You can note that your position on your fishfinder shows your position to exactly this degree: three decimal places or 1/1000th of a minute of angle.

With GPS, you can imagine that you are wandering around on a grid with gridlines spaced approximately 6 feet apart laterally and longitudinally. Sometimes you’re directly between two lines, sometimes on top of a line, and sometimes closer to one than the other. GPS can only say that you are on “this” line or “that” line. It cannot tell you if you’re somewhere between the lines.

Having said that, to say that your GPS “can” always resolve your position to within about 6 feet would not exactly be correct. The ability of the GPS to resolve your position depends on the data it is receiving from the GPS satellites orbiting the earth. If you turn your GPS on and let it run with the boat stationary, say on the trailer and parked in your garage, then you would see the GPS plot your position around an area, not precisely at the same point over and over. The scatter of points is due to natural variability that beyond the control of the GPS antenna. What is done with this (scattered) data varies widely from brand to brand, and can have a big impact on the accuracy of the position shown to the user.  I’ll explain…
A plot of the raw data looks something like this:
You can imagine that if your GPS plotted the raw data, you’d be jumping around all over the place. The unit has to do some kind of smoothing before it can resolve your position cleanly. Some manufacturers do this with simple averaging, which results in a shown position for a moment, then a jump to a new position for a moment, then another jump, and so on. Other companies will use statistical  confidence intervals that work based on variability: if the variability of data is high, it keeps listening. If the variability is low, it says “Ok, I know where you are with X% confidence”.

You may also see physics models built into the software, that assume you can only accelerate or decelerate at a certain rate. It understands that your boat can’t jump 12 feet in a split second, and smooths the data based on physics equations. These are very nice to use and do an excellent job of accurately and precisely plotting your location.
Although you never see the software code of a manufacturer’s product, you can note these effects in the performance of a GPS unit. The end result is the level of confidence that you have in the product’s ability to resolve your location.

So what does all of this have to do with you and fishing? Just note that where your GPS says you are and where you are actually located on your map (or chart) may not be exactly the same.  ...but it should be close!

GPS Antenna Installation Location

Wednesday, 30 March 2011 22:47 | Written by Adam Broughton | PDF | Print | E-mail
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In GPS and Sonar Targets we made the point that there is some error associated with the position of an actual target and the position of a waypoint that was marked from that target, resulting in potential displacement between where you think your target is and where it actually is. In GPS Positioning Details we made the point that there is some error in the GPS’s ability to resolve your position. Here we discuss how the location that you mount your GPS on your boat can impact the precision to which you mark sonar targets.
When most people are installing the GPS puck on their boat, they choose the location that is first: convenient and second: providing a descent view of the sky. The premise of choosing a location is often “the GPS is on the boat, I’m on the boat, therefore the GPS position is my position.” Unfortunately, the displacement between you, the GPS puck, and the sonar transducer you’re using to mark targets may have a significant impact on your ability to return to targets that you have marked.
One convenient place where GPS pucks are often installed is the rear deck of the boat. We illustrate it here as the worst case scenario: GPS puck mounted on the extreme rear of the boat, and the transducer mounted on the trolling motor.


Let’s say the GPS puck and the trolling motor transducer are displaced by 20 ft (in the plane of the boat). You might think this arrangement would result in a 20ft discrepancy of marked waypoints each time you hit the MARK button. If this were true, you’d be able to always get yourself within 20 feet of the target, right?
You could probably guess my answer was “no” and here’s why: with the boat arranged as in the image above, with a 60° sonar cone, in 40 feet of water, will begin showing sonar targets when they are on the fringe of the sonar cone, 20 feet away. This scenario is illustrated as follows:


If you immediately mark the target before getting on top of it, you’d have a 20 ft. error attributable to sonar, and another 20ft error attributable to the GPS puck installation. You’ve then placed your waypoint 40 feet away from the target. Note that in deeper water, this error is even larger.
Now consider it is some time later and you want to return to your waypoint. With the boat approaching from the other direction you may or may not note that you pass over the target on your way. Once you reach your target, you are now looking between 40 and 80 feet away from your target---not so insignificant now, is it?

In order to minimize the error between where you place your GPS points and where sonar targets are ACTUALLY located, you should mount your transducer and GPS antenna as close to each other as possible.  The location of the display is irrelevant, unless your sonar control head has an internal GPS. That is to say, if you are mounting a control head, a GPS antenna, and a sonar transducer, then try to place the GPS and the sonar transducer as close to each other as possible. You can mount the control head (display) wherever you like.

In the image below, left, the GPS is mounted with the sonar on the bow. In the image at right, the GPS is mounted on the stern, sonar at the bow. Marking and locating targets is obviously problematic.


If your control head has an internal GPS antenna , then mount the control head and transducer as close together as possible. Most fish-finders with internal GPS antennas will support an external antenna so that you can mount the GPS puck closer to the transducer, but not all of them do. For example, a Humminbird Fishing System (700 Series and above) will support external GPS, but any 500 Series and below does not.

Here is an example installation where the control head, GPS antenna, and sonar transducer (on the bottom of the trolling motor) are all mounted close to each other. This installation makes for a very nice, compact setup that is sure to mark and re-locate targets very accurately.


In this installation, GPS signal is networked to the console, and the console unit reads a transducer at the transom, 20+ feet away. We just made the argument that this is bad, so what are the possible solutions? –well it depends. If you’re primarily navigating from the console and using sonar to monitor depth, then no big deal, just go as-is. However, if you’re marking sonar targets from the sonar display, such as when using Side Imaging, then your GPS points are going to be 20+ feet off target! The only real solution is a 2nd dedicated GPS puck mounted near the rear transducer. Otherwise you’ll have to know that your sonar targets are “near” your waypoint and you’ll have to go back with the trolling-motor mounted transducer and find it (time-consuming!).

GPS and Sonar Targets

Wednesday, 30 March 2011 22:15 | Written by Adam Broughton | PDF | Print | E-mail
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Ever marked a waypoint, then couldn’t go back and find it?
A few years ago I was listening to a seminar by “Triton Mike” Bucca, and he pointed out that the sonar cone angle that you select can impact your GPS accuracy. I have to give credit to Mike for this line of discussion, as I previously had not given much thought to GPS, sonar, and boat positioning. It is an important topic, and one that most people overlook when choosing a GPS location. Here is the point that Mike makes:

Hypothetically speaking, lets assume your GPS is mounted directly over your trolling motor transducer (we’ll discuss alternative locations in Part III). Let’s also assume you’re in about 40 feet of water, giving you approximately 40 feet of bottom coverage with your 60° sonar cone. You are navigating to a brush pile that you have marked with your GPS, and once located, you begin to fish directly above it, as you would when drop shot or spoon fishing.
Although the 60 degree cone gives you excellent coverage, your precision in locating that target is very poor. In fact, there is an inverse relationship between better sonar coverage and better GPS precision. That is to say, the wider the sonar coverage, the worse your ability to locate targets that you have marked with your GPS.
A 60° beam in 40 feet of water would look something like this:



This illustration is the arrangement that we visualize. We navigate to our waypoint, and when we see the brush pile on our sonar we believe we are directly over it, as in the picture above. However, because the sonar cone is wider at the bottom, we could very well be tens of feet to one side; resulting in something like this:

In this scenario, we are as much as 20 feet off of the target, but we show the same return on the display. In situations such as drop-shot fishing in late summer when fish don’t want to move to get their meal, you might be too far away from the target to elicit a strike. Considering other error factors such as the drift of your lure off target, GPS resolution, and the precision of your (previously marked) waypoint, you could be even further off than you think.
One way to improve your accuracy is to turn your sonar to the narrow beam setting and try to locate the brush again on your sonar. Given the same scene as above, you now must maneuver the boat to find the brush again.

Note also when trying to re-locate your brush pile that, although you marked the brush on your sonar, you may not know to which side you need to maneuver. The three identical brush piles shown below would, individually, have identical sonar signatures, yet the center of each pile is 30 feet from each other.

You can use the movement of the boat to help resolve the position of the target you’re trying to find while still maintaining the benefits of a wide beam. For example, continuing on a straight path over the waypoint several times, you may note that the strength of the target is sometimes weak, and sometimes strong, corresponding to the brush pile on the fringe or in the main sonar beam strength respectively. You would have to be significantly displace to put the brush pile on the other edge of the sonar---so much so that your GPS would be able to tell you that you were off target.
You can also supplement your positioning with dead reckoning or by using a marker buoy. By using these GPS, sonar, and boat positioning theories, you can maintain a broad area of coverage while improving your interpretation of where the target is located in your sonar cone, and improve your overall accuracy.

Our article on GPS Position Details explains how the GPS itself can be off your waypoint.

Be sure you install the GPS puck with respect to the sonar transducer you'll use to mark waypoints. Click here to read why it is so important.


Offline rnvinc

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Re: Adding optional GPS sensor to Helix
« Reply #4 on: August 17, 2016, 11:49:09 PM »
Here is the "scatter plotting" diagram HB documents on a stationary GPS receiver over a 24 hour period ...which clearly show the inaccuracy in our grade of GPS receivers ...

(The above GR 50 would be similar to the present AS GRP and the AS GPS HS antenna) ...

Of course companies make more accurate GPS receivers for farming and surveying ... at much more $$$$ than fishing consumers would be willing to pay ...

« Last Edit: August 17, 2016, 11:52:51 PM by rnvinc »

Offline mako9man2

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Re: Adding optional GPS sensor to Helix
« Reply #5 on: August 18, 2016, 01:03:44 AM »
Thanks for the thorough explanation Mike and Rickie.  I appreciate it.
Joe B

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