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Author Topic:   Basic GPS and SONAR Tutorial
Moe posted 03-20-2004 03:11 PM ET (US)   Profile for Moe   Send Email to Moe  
Here's a short and very basic tutorial on GPS and SONAR.

A GPS unit tells you where you are. You can get a very basic non-charting unit that gives you your latitude and longitude and then you can plot that on a waterproof chart.

The basic GPS unit can give you headings to "waypoints" which are latitude/longitude pairs of positions you are interested in, and program into the unit.

The basic GPS unit can also give you your speed over ground (which can be different than speed over water due to currents), and it can give you the course you have been on. Most display this as true, but some have a setting for magnetic variation in the area and have an option to display it as magnetic course. This isn't necessarily the same as the boat's heading if the boat is having to be steered into current, waves, or wind. A GPS unit cannot give you the boat's heading when stopped, unless it has an electronic compass inside, and most don't. Electronics and electrical power can fail. No electronic device is a complete substitute for a good magnetic compass.

Due to many factors, including effects of the atmosphere, the position determined by GPS can have error in it. There are two systems set up for transmitting corrections that compensate for this error. One is the USCG-DGPS, which requires a separate receiver and antenna, and for that reason, it is seldom used by recreational boaters. The other is the FAA's WAAS system, which is transmitted on the same frequencies as the basic GPS info, and doesn't require a separate receiver and antenna. If the GPS receiver has the internal software to use this correction information, it is "WAAS-enabled." This adds very little to the cost of a GPS unit, and should be a "must-have" on a marine GPS.

The GPS unit can tell others where you are if you have it connected to a VHF radio with DSC capability. This can be from an emergency call you make, or as a response to a fishing buddy's "position request" to your radio.

Beyond the basic GPS is the GPS/chartplotter. Chartplotters have maps/charts stored in internal memory and display your position on these charts. This removes the step of you having to plot your position on an actual chart. The bigger the screen, the more expensive it is, but the easier it is to read, especially when the boat is moving around. Color screens, like color paper charts, are also more expensive, but much easier and quicker to read.

The default ("base") maps/charts stored in internal memory are crude, with poor coastline detail (missing little coves, etc) and missing many nav-aids, but they can show you a rough idea of where you are. Most better fixed-mount chartplotters can accept memory cards, either with preprogrammed charts in them, or blank, where you load the charts you want off of a CD-ROM. Most handheld chartplotters, and some fixed mount, have varying degrees of blank memory in them for loading charts from a CD-ROM. These charts have much more detail and pretty much all of the nav-aids. Electronics and electrical power can fail. No electronic device is a substitute for having a current chart, plotting tool, and dividers, and knowing how to use them.

Some chartplotters can automatically plot the position of other boats who transmit a VHF DSC signal, if the receiving VHF radio can output this position. This received position can be from another boat's distress call, or from a buddy's reply to a "position request" you transmitted to him.

Fishfinders transmit a sonic pulse downward and objects below, such as fish and the bottom, reflect a portion of the signal back up. These days, the reflected signals are generally plotted on a display giving a side view of the underwater situation, with the surface on top and the bottom on the bottom. Fish are shown at the depths they are at, however, where the fish are laterally, i.e. on which side of, or whether forward or aft of the transducer isn't exact. They are somewhere within a widening cone.

The "transducers" that transmit and receive this signal can use different frequencies and angles of radiaton. One is better for shallow water and the other for for deeper water. There are "dual-frequency" transducers than can do both, even simultaneously. The results can be displayed separately or simultaneously. For your use, you'd want dual-frequency

Some folks, with plenty of console space, prefer the more expensive solution of a separate fishfinder and GPS/chartplotter, but for those with limited space like on the 170, these are available in less expensive combination units, where the display can be switched back and forth between chart and fishfinder, or the display can be split showing chart and fishfinder simultaneously. Like so: . Interconnected or combination GPS and fishfinder units can store depth, and sometimes water temperature, in addition to latitude and longitude, in a "waypoint."

Most marine electronics talk to each other (share data) over one-way serial connections using the standard NMEA language. This consists of widely accepted "sentences" that all brands understand. However, many manufacturers additionally have proprietary sentences that can only be understood by equipment of the same brand. High-end electronics (for larger boats) can also talk to each other over ethernet, just like a computer network.

You can find lots of detailed information on the web by doing an Internet search, such as Google. But the best reference I can give you is to go to the website of the manufacturer of the unit you are interested in, and download the User Manual for that unit. Unfortunately, Garmin doesn't have the user manual for the upcoming 178C on-line yet, but downloading the manual for the 188C will give you a good idea of how it works. Lowrance and Furuno also have their user manuals online.

Hope this helps,

jimh posted 03-22-2004 08:21 PM ET (US)     Profile for jimh  Send Email to jimh     
A Tip of the Hat to Moe for this concise explanation of GPS and SONAR.
Barney posted 03-23-2004 07:43 PM ET (US)     Profile for Barney  Send Email to Barney     
Moe, Thanks for taking the time to put this together. Next project is to figure out how to make my Garmin handheld GPS talk to my Furuno sounder thru the NMEA cable. Pushing electrons the correct way. Jim
Milepost43 posted 03-23-2004 08:36 PM ET (US)     Profile for Milepost43  Send Email to Milepost43     
Nice write-up. you can go to the Lowrance site and download a demo that you can play with. Worth a visit.
erik selis posted 04-05-2004 07:37 AM ET (US)     Profile for erik selis  Send Email to erik selis     
I have a Lowrance X97 fish finder in my 170 Montauk and am planning to install a GPS. I would also like to use this GPS in my car.

I have been looking at a Garmin GPSMAP 276C.

Does anyone have any experience with this model and are there others that I should consider?

Btw, great article Moe.



Moe posted 04-05-2004 02:33 PM ET (US)     Profile for Moe  Send Email to Moe     
Erik, AFAIK, the 276 isn't shipping yet. I've heard the date is end of April for the Americas version with the Atlantic version you'd want following maybe a month later.

It looks a bit like the older 176 model with the advantages of an auto-routing basemap, the new LCD, and the new swivel mount. Much nicer.


erik selis posted 04-05-2004 02:46 PM ET (US)     Profile for erik selis  Send Email to erik selis     
Hopefully they will be available over here buy the summer.
It really looks like a nice piece of electronics and worth waiting.



jcswpa posted 04-05-2004 04:47 PM ET (US)     Profile for jcswpa  Send Email to jcswpa     
Great suggestion, I was just about to buy the 188C GPS/FF and found about the 178C here, it is due in April 14 at West Marine, and its $400 less. I can wait for a week.


Moe posted 04-05-2004 06:18 PM ET (US)     Profile for Moe  Send Email to Moe     
You can probably get it a LOT cheaper on-line than at West Marine, unless they'll price match printouts of on-line prices.


Steve M posted 04-06-2004 12:06 AM ET (US)     Profile for Steve M  Send Email to Steve M     
Moe is very astute. I wish I had queried Moe before I chose my Sonar/GPS....

While NMEA 183 has been around for a while, (I think the early 90's) I was somewhat surprised by the lack of interoperability between my dealer installed Humminbird Matrix 65 (fishfinder with GPS), and the Standard Horizon Spectrum + VHF radio that I installed myself. The intent was to have DSC position functionality, where my Matrix 65 would tell my radio where the boat was using the GLL and RMC messages. I captured the NMEA data that the Humminbird was spitting out on my PC (via serial port) and sent it to a very helpful person at Standard Horizon. Standard Horizon sent me back the following:

"It appears as if the hummingbird matrix is not calculating the check sum correctly. I used a nmea simulator and output his sentence and it works however the program changed the last digits (check sum). The customer needs to report this to Humminbird.



top is original sentence, bottom is the sentence that works with the correct calculated check sum."

So I sent an email to Humminbird, and never heard anything back....

bigjohn1 posted 04-06-2004 07:31 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     

Got your other e-mail on the VHF - thanks. Per the many recommendations submitted in response to my thread on "170 electronics on a budget", I have looked into various models of GPS, Sounders, and Combo units. I know its not an absolute neccesity but I would like to get a unit and the
"chip" to show me detailed mapping of Guam and its surrounding waters. A check of Garmin units and the available "chips" shows various segements of the South Pacific but Guam is not covered on any of them. It seems they have them for nearly all of Indo-Pacific region except for the section where I am. Guam's Lat/long is 13N and 143E. I even looked at the Garmin chip that covers the area termed "Oceana" and my area is not covered. Am I looking at something wrong or could it be that I live in a region that is not covered by any manufacturer's chip?

Big John

Moe posted 04-06-2004 10:26 AM ET (US)     Profile for Moe  Send Email to Moe     
It appears there's a gaping hole in Garmin's coverage right there:

Same with Navionics:

I can find a 1:50,000 chart of Guam on a couple of C-Map products:



Look like you need to find a GPS/Chartplotter than can do C-Map.


Moe posted 04-06-2004 12:10 PM ET (US)     Profile for Moe  Send Email to Moe     
Oops... I meant to say 1:150,000 scale on Guam.


Peter posted 04-06-2004 01:21 PM ET (US)     Profile for Peter  Send Email to Peter     
Erik, I have a 176C for my 15 SuperSport which I also use in the car with the Garmin friction mount. It's a nice unit with a decent user interface.

The 276C appears to be a 176C with higher resolution color screen, same swivel mount, and a software upgrade to provide auto-routing. I would have liked to have that auto-routing feature on my 176C so I would wait until the 276C becomes available and go with that over the 176C.

erik selis posted 04-06-2004 02:15 PM ET (US)     Profile for erik selis  Send Email to erik selis     
Yes, the 276C seems to be a very nice unit and the auto-routing with the audio output is just what I'm looking for.

Thanks for your comments and advise Peter.


OzWhaler posted 04-14-2004 09:13 AM ET (US)     Profile for OzWhaler  Send Email to OzWhaler     
BigJohn.... why don't you check out the Navman charting and fish finding range(ex-NZ)- I think you'll find they cover your area using C-map (
Moe posted 04-14-2004 09:23 AM ET (US)     Profile for Moe  Send Email to Moe     
Good idea, OzWhaler. I suggested the TrackFish 6600 to John in private email. But it's relatively expensive. It's an option (besides the Northstar 6000) for those looking for a combo unit who want to keep their Brunswick boat all Brunswick! It also works with Navman's fuel metering setup. Nice lookin' unit.


bigjohn1 posted 04-14-2004 10:17 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Thanks for the tip on the C-MAP capability. I have consulted some of my local contacts about equipment and it seems most simply run a standard, non-charting GPS and one of the more economically priced fishfinders. Aside from the big oceangoing tankers and other serious comercial traffic around Guam, it seems nobody runs a chart plotter there. Though I can't confirm this, I am told the C-MAP cards for Guam don't very good detail...I am going to follow up on this though and make sure.

I was a bit taken aback when I heard few high-end fishfinders are run there. Reasons given were the extreme water depths we have offshore around there (5,000ft+) and the fact that few fishfinders are worth a rusty nickel at those types of depths. Guam is very close the Marianas Trench - home of the deepest ocean depths on the planet.
I guess they simply run cheaper units and only use them when they are near shore in shallower depths.

I still haven't thrown in the towel yet though and I think there is a unit out there that will fit my purpose. Even if it won't read 5,000ft, many 3,000ft units on the market and after all, what is the purpose of needing detailed bottom and fish-finding capability at depths exceeding 3,000ft while in a 17' boat? :-)
Big John

Steve M posted 04-16-2004 09:20 PM ET (US)     Profile for Steve M  Send Email to Steve M     
Not that anybody is interested....but for the sake of completeness with regard to my earlier post in this thread.

Humminbird not long after my last post did respond and indicated that there was a software fix in Version 1.5 or later of the Matrix 65 software. (I had version 1.4). They sent me the $24.95 download cable for free which I used to upload my Matrix 65 Sonar/GPS unit with the latest softare (a version 2.x) available on the web. As a result my Sonar/GPS and VHF are now talking in NEMA 183, and my VHF is now fully DSC capable. Plus the software upgrade provided significant functionality, viewing modes and other enhancements for the Matrix 65, which in retrospect would have been worth the $24.95 by itself.

OzWhaler posted 04-17-2004 10:18 PM ET (US)     Profile for OzWhaler  Send Email to OzWhaler     

I am very keen on the new Navman 6600 combo and Navman VHF/DSC unit - although a little expensive - is brilliant with super clear sonar and chart displays in sunlight and masses of functions including engine performance.

In Western Australia we have a huge coastline with many very remote and rarely visited areas - good fishing. The 6600 unit takes two C-chart cards and when I asked about the mapping range he said "What do you want...?". They have the ability to actually programme the C-chart card for ANY area I am interested in and would be visiting ... I don't have to buy standard ones covering particular ranges of coast...

They have master CD's of the world's charts and can specifically programme the C-cards for any area. Thought this may interest you - it may be possible to get detailed charting of Guam even if it is not on a standard C-chart now.

I am surprised - I would have thought with Guam's wartime history there would be A1 charts available. I was doing development work out of Indonesia at the island of Moratai where MacArthur was based on his way to the Phillipines and, although remote, the charting available was excellent when we were locating Allied and Japanese ship and plane wrecks.

Sorry to diverge again from the thread 'Steve M' - but hope this helps you BIGJohn.. !

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