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Author Topic:   GPS and Radio Interconnection
jimh posted 03-18-2004 12:06 AM ET (US)   Profile for jimh   Send Email to jimh  
Interconnection of a GPS and a Radio is generally along these lines:

GPS Receiver to VHF Marine Band Radio Transmitter: Modern VHF Marine Band radios may provide for the transmission of encoded data containing latitude and longitude information. It is intended that such data transmission be made as part of MAYDAY transmission in order to aid in vessel location. That is, the vessel's radio transmits its position as determined by the vessel's navigation equipment. It is not necessary that the position be determined by GPS. The actual interface to the radio is on a standard marine data bus (NMEA), where the radio listens for navigational information and retransmits it as an encoded signal in the radio's modulation when the radio operator engages a special function. The position information can be supplied by any device that transmits on the data bus, but typically this function is fulfilled by a GPS receiver.

VHF Marine Band Radio Receiver to GPS Receiver: No connection.

Differential GPS Radio Receiver to GPS Receiver: Data output from specialized low-frequency (300-KHz band) radio receivers tuned to local transmitters provide an error correction signal that improves the accuracy of the GPS receiver position finding.

VHF Marine Band Radio Receiver to Graphical Chart Plotter: Modern VHF Marine Band radios may provide for the reception of specially encoded position data received from other stations. The position data received by the radio is then typically displayed on the control panel of the radio (as latitude and longitude numbers) and also transmitted on the vessel's NMEA bus to which the radio is connected. Other devices on the bus can receive this data. One such device is a chart plotting device, often part of a GPS receiver and navigation instrument. The chart plotter can then display a position indication of the vessel transmitting this data. This provides great assistance in locating a vessel transmitting a distress or MAYDAY broadcast. It may be possible to make use of this function in non-emergency situations, so that, for example, two vessels could exchange their positions via such radio transmissions and each could plot the position of the other on their chart plotter. One possible situation for this might be to exchange information about location of good fishing conditions without having to make a voice broadcast. Implementation of such a system is not believed to be widespread at the moment in normal recreational vessel radio and navigational equipment.

jimh posted 03-18-2004 12:20 AM ET (US)     Profile for jimh  Send Email to jimh     
Also note:

Not all VHF Marine Radios have the ability to transmit position data. I believe that this ability is intertwined with the Digital Selective Calling (DSC) capabilities. DSC allows for special selective calling and receiver squelch control.

Even fewer VHF Marine Radios have the ability to receive position data. The last time I surveyed the market, about two years ago, only the top-of-the-line radio from STANDARD had this ability. This function may now be available in more radios.

Moe posted 03-18-2004 12:39 AM ET (US)     Profile for Moe  Send Email to Moe     
The "a" version of the Icom 502, i.e. the Icom 502a (as indicated on the outside of the box) has the capability of outputting NMEA data to a GPS.That includes DSC data from emergency broadcasts and replies to position requests.

[Consolidated Moe's comments in another thread and added them here.]
If you have a Furuno radar with chartplotter (Navionics or C-Map), you can plot the boat's position on that chartplotter, via NMEA data, from any NMEA-capable GPS receiver, including some of the Furuno GPS/USCG-DGPS units such as the GP37.

If you want to overlay the radar image on the chartplotter, you will need to provide the unit with the boat's heading as determined by a digital compass or heading sensor, such as the KVH 1000 digital compass or Furuno PG1000 heading sensor. Some autopilots also provide the boats heading to the chartplotter.

There are several advantages to connecting the GPS to a DSC capable radio, such as the Icom 402S. First and foremost is sending your position in an emergency message. The second is automatically plotting the position of a received distress call on your chartplotter if your radio also has NMEA output, such as the Icom 502a (the Icom 402S doesn't have this capability).

If you know the MMSI of a friend, you can use a DSC-compliant radio to send him a position request, which his radio can reply to if receiving position info from his GPS. If your radio outputs NMEA data to the chartplotter his position will automaticaly be displayed. And vice versa.


Chuck Tribolet posted 03-18-2004 09:04 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Differential GPS receivers are getting very rare, having been
replaced by WAAS (Wide Area Augmentation System) which does
much the same thing, but uses geosynchronous satellites to
transmit the information on the same frequencies used by
GPS, which allows it to be implemented for essentially no
manufacturing cost. Almost all modern GPSs support WAAS.

There are two circumstance where a VHF radio will transmit

The first is in response to pressing a Distress
button. Most modern console mount VHF radios implement this
-- it's required for a new radio to get FCC type
certification. Very few handhelds implement this.

The second
is in response to a DSC position request call. Not all VHF
radios will generate or respond to a position requrest call.
The Standard Spectrum, Spectrum+, Intrepid+, Matrix, and
Quantum will do both, the Standard Eclipse+, Intrepid LE,
and Intrepid will not do either, The Standard Quest
will reply to but not generate position requests. The Icom
M402, M502, M502a, and M602 do both, the M127 does neither.
At least some Icom radios require explicit user interaction
(and it's more than just a OK button push) to respond. On
at least some Standards, the radio can be setup to reply
automatically or require a button push approval.

It should be noted that the accuracy of a DSC distress
tranmission is only one minute of lat/long, about a mile.
I think this applies to position requests also.


Moe posted 03-18-2004 10:17 AM ET (US)     Profile for Moe  Send Email to Moe     
To respond to a position request on the Icom 402S, you have to push the DSC button, certainly not as intuitive as having an "OK" button.


Chuck Tribolet posted 03-18-2004 10:48 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
On my buddy's ICOM radio, it's more than just a single DSC
button to respond to a position request. It's also multiple
button pushes to answer a voice DSC call (on the Standards
you just pick up the mike and talk. It's all set to the
working channel.

ICOM needs a lesson in human factors.


phatwhaler posted 03-18-2004 11:52 AM ET (US)     Profile for phatwhaler  Send Email to phatwhaler     
I used to have a ROSS DSC 500 DSC radio on my old boat and it was "Top of the Line." It was a class A DSC radio that could:
1. Send/Rcv Positions
2. Talk in scramble mode
3. You could input all three key pieces of a distress call.
a. Position via GPS input
b. Manually input people on board
c. Manually input type of distress.
Then when you hit the distress button all three key pieces are broadcast over channel 70.
4. 30 watt loudhailer
5. Request and Send your position to another DSC radio. I could actually scroll down to my friend’s number and request his position. My radio would call his on Chnl 70 and get the info.
6. Hook it up to a computer and you could poll other DSC radios and show their position on electronic charts.
7. Group calling
8. Call waiting.
9. Listened to two channels at the same time, not just scanning back and forth.
The list goes on and on....

Ross used to be a small company in Largo Florida , but it was bought out by L3 Comms.

If anyone is looking for the "Cadillac", here it is:

I basically just used it as a normal radio because hardly anyone I knew had DSC capabilities.

phatwhaler out.

peetmin posted 03-18-2004 12:11 PM ET (US)     Profile for peetmin  Send Email to peetmin     
I just traveled this path reciently by connecting My Standard Horizon Intrepid to my Garmin 182. Works great, my first thought was what do you say to the coast guard when one of the kids accidently pushes the wrong button.
It brought to mind a story told to me by a friend. Evidently they had anchored and happy hour had run long into the evening when someone decided it would be a good time to discharge the flare gun into the air. The details get a little sketchy but I guess the Coast Guard responded and was not impressed when the crew member who had fired the flare told the coast guard that they had run out of ice.
BEACHPANDA posted 03-18-2004 07:53 PM ET (US)     Profile for BEACHPANDA  Send Email to BEACHPANDA     
I've got a Standard Horizon and a Lowrance GM 7000C. I haven't been able to pair the two together. How is the connection made?
Chuck Tribolet posted 03-18-2004 08:18 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
What model SH?


peetmin posted 03-18-2004 08:18 PM ET (US)     Profile for peetmin  Send Email to peetmin     
The GPS has a NMEA output (2 wires + and -) And the radio has an imput pair of wires that need to be connected in order for the radio to recieve information from the GPS.
Chuck Tribolet posted 03-18-2004 09:17 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
What model Standard Horizon? It's important because Standard
Horizon changed their color code at some point, at least once.
I've got the PDF for your GPS, and PDFs of all the SH VHFs
(I'm a manual packrat), and once I know what radio you have,
I'll tell you what wires to connect.

The real problem here is that the marine electronics industry
didn't standardize on a color code for NMEA, or, better,
standardize on a connector. There are genderless connectors
that would have made it plug and play for two devices. The
old IBM Wiring System used these, though they were pretty
big and clunky, and not marine grade. But the concept
would have worked.

And all of this is made more complex because true NMEA devices
have + and - for both in and out, but a lot do what I call
kinda sorta NMEA (KSN) - they have an in and an out wire,
which are really +in and +out, and a ground, which may be
unique to NMEA, or may be just good old electrical ground.
Fortunately, because of the way the interface circuits work,
it still works, at least almost always.

Sorry about the brief response earlier. I needed to get to
a store before they closed at six, and Adm. Linda called to
ask whether we were diving tomorrow. I'd rather be short
with you than with her. ;-) ;-)


cmarques posted 03-18-2004 09:23 PM ET (US)     Profile for cmarques  Send Email to cmarques     
I just hooked up my Eagle gps to a Raymarine 53 radio- Had to get new power cord from Lowrance (same as Eagle) with NMEA outputs to connect. Also had to go in the setup screen of the GPS to enable the rs232 port to send data. I think with my radio it would be a little hard to accidentally send a distress call- you have to lift acover on the back of the mic and hold for 4 seconds as the display counts down then activates the call. It displays no data or other info except"nmea" to show a signal from gps.
If this is related- When you get an mmsi # like I did from Boat US, what does that info provide? Is there a master database of all id's, who retains that info and how is it obtained when needed if the coast gaurd doesn't yet monitor dsc radios?


BEACHPANDA posted 03-18-2004 09:55 PM ET (US)     Profile for BEACHPANDA  Send Email to BEACHPANDA     
My Standard Horizon radio is the GX 1255 S

I looked in both of the boxes, and couldn't find the wire, or any extra wire for that matter, that looked like a connector of any sort. All I had left over from the install/s were some screws for the gimbal mount/s.

Is there a specific part # for the connector? Who do I call with the part #, LEI or SH?


Chuck Tribolet posted 03-18-2004 09:56 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Cmarques: yes there are databases of MMSIs. There are two:
One is US only and has the free ones issued by Boat US.
There's also an international one that includes the MMSIs
and call letters issued (at a price) by the FCC.

You can access the international database at
but I know of vessels with FCC issued call letters that are
NOT in it, either by name or by call sign.

More important, does your local CG have a DSC radio? Some
don't, some do because for one reason or another they bought
one, and a few do because they are a part of an CG comms
upgrade that will take another couple of years. You can
read about it at .
This upgrade seems to be pretty high priority to the CG and
Congress as it includes a lot of other stuff that supports
their national security mission. The system it replaces is
pretty ancient.

The good news is that I've heard CG Group San Francisco say:
"The CG has received a DSC Distress signal ..." and they
aren't due for the upgrade till 2005.

And very important: there are getting to be a lot of DSC
radios out there on the water. I took a walk through the
Monterey Breakwater parking log on a recent afternoon while
everybody was pulling put and cleaning up, and about half
the radios or better were DSC.

jimh posted 03-18-2004 10:16 PM ET (US)     Profile for jimh  Send Email to jimh     
[Gently edited this thread to remove some spurious concepts; added some comments from Moe which were buried in another thread.]
cmarques posted 03-18-2004 10:16 PM ET (US)     Profile for cmarques  Send Email to cmarques     
The followinfg is from the online owner's manual for your gps unit. Like mine tha data cable is part of the power cable. If you only have a 2 wire power cable then the one you need runs about $39 through West marine
"The most popular DGPS system relies on a grid of ground-based transmitters
that send correction signals to DGPS receivers. These in turn,
connect to the GPS receiver (such as the GlobalMap 7000C). Lowrance
offers an optional DGPS receiver for your unit.
See the following diagrams for general wiring connections. Read your
other product’s owner’s manual for more wiring information.
To exchange NMEA or DGPS data, the unit has two NMEA 0183 version
2.0 communication ports. Com port one (Com-1) can be used to receive
NMEA format GPS data or DGPS data. Com-1 can also transmit NMEA
format GPS data to another device. Com-2 is for NMEA output only.
The three wires for the com ports are combined with the two power
wires to form the power/data cable. Com-1 uses the yellow wire to
transmit, the orange wire to receive and the shield wire for signal
ground. Com-2 uses the blue wire for transmit."

hope this helps

cmarques posted 03-18-2004 10:30 PM ET (US)     Profile for cmarques  Send Email to cmarques     
Thanks for the info Chuck.
I guess what I was wondering is: I'm offshore and have an emergency, I enable the distress function on my radio and you, 10 miles away recieve that call on your radio, do what now? I know some radios will display that lat/long that I'm sending (mine doesn't). Besides switching to ch 70? then to 16 automatically, what else am I sending? Does my mmsi # get transmitted or is it identified where someone can lookup and say thats Chris' boat in trouble? Maybe this should be in another discussion but I just want to know how joe boater reacts if sending or recieving a distress call.


Chuck Tribolet posted 03-18-2004 10:57 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The DSC distress tranmission includes MMSI and lat/long (to
one minute). Most radios will display these (The Ray 53 is
the only one I've run into that doesn't). In most cases, if
the MMSI is in the radio's "little black book", it will
display the name you entered instead of the MMSI, so you'll
know if it's one of your buddies.

In some cases, the radio can also transmit the nature of
your your distress and number of SOBs (souls on board).

Joe Boater's gonna react: "What the heck is that damn
beeping?" I think the CG should run DSC distress drills at
1200 hours every Saturday. Preannounce them on CH 16,
including the MMSI and location.

Me: I'm going to grab pencil and paper out of the dry box
and write down the position and MMSI, get fired up and
headed that way, and then, if someone else hasn't, get the
CG on 16 and report the contact to them.


jimh posted 03-18-2004 11:53 PM ET (US)     Profile for jimh  Send Email to jimh     
What is the reasoning behind only transmitting the position information to a resolution of one minute? It seems rather arbitrary. Anyone have insight into this?
Chuck Tribolet posted 03-19-2004 01:32 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I suspect that the one minute resolution has it's roots in the origins of DSC: Blue water operations on HF. One minute resolution is more reasonable there where the boats (ships) are bigger, and response times (and drift distances) longer. It may also have something to do with the accuracy of electronic navigation equipment at the time DSC was conceived.

I read somewhere that there was work underway to increase the resolution, but of course it's a committee doing it, and then there needs to be time for radios to get upgraded. If they do it right, it's just a firmware update in the radio, but it's unclear that VHF radios have field upgradable firmware like most GPSs. I've never seen a VHF firmware update on a manufacturer's website. I've seen lots of GPS firmware updates.


jimh posted 03-19-2004 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Good observation, Chuck. I suppose we should first be glad to know the Coast Guard has DSC radios in their radio watching standing facilities, then we can worry about the resolution in the position data being sent.
CHRISWEIGHT posted 03-19-2004 02:37 PM ET (US)     Profile for CHRISWEIGHT  Send Email to CHRISWEIGHT     
here in the UK all vessels which carry a vhf are required to have a qualified operator and a ships licence (yes even a 16 foot Boston Whaler) all data about the vessel and the radio mmsi are recorded by the licensing authority and this information is available to the coastguard.

From Feb 2005 the dedicated listening watch on channel 16 will be discontinued. the current advice is to operate DSC and then call mayday on channel 16.

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