Single GPS antenna, multiple devices

[solder monkey]

Hi all,
I have several devices on-board (eg. radar, navtex etc.) that offer increased functionality with a GPS input in NMEA 0183 format. I’d like to add a GPS input to each of the devices, however to avoid a ridiculously large cluster of GPS antennae what I’d like to do is share a single GPS source signal with all the devices, ie. use one GPS antenna to feed all the devices.
Can anyone advise on the best way to do this? What I’m thinking is that it should be possible to use a GPS splitter box (powered if required to boost the signal strength) to convert a single GPS input into approximately 4 outputs.
Is this possible, and if so what’s the best way to do it?
Cheers in advance for any advice,
Mike.

 

[prv]

You’re asking about sharing an antenna with several receivers (which is certainly possible, though not that common in leisure use), but you say the devices are expecting NMEA data from one receiver. That’s what you‘re actually trying to do - you don’t need splitters and boosters, just a normal GPS “mushroom” receiver with built-in antenna, and some twisted-pair wire.

Pete

 

[solder monkey]

Hi Pete, yes, what I have in mind is a single antenna/receiver (something like this:
Ship GPS receiver Antenna Module NMEA 0183 baud rate 4800 DIY Marine | eBay )
that can be used to feed GPS data to each of the individual devices.
Is it simply a matter of splicing the relevant signal wire from the mushroom into several separate feed wires, each one of which carries the GPS data to a particular device (eg. navtex), or am I being too naive?

 

[Spirit (of Glenans)]

NMEA0183 supports exactly this arrangement, in which there is one "talker" and multiple "listeners". AFAIK, you could "daisy chain" the feed from the antenna from one instrument to the next. The ones that will accept data will receive it, while not sending their own data down the line.

 

[AngusMcDoon]

NMEA0183 supports exactly this arrangement, in which there is one "talker" and multiple "listeners". AFAIK, you could "daisy chain" the feed from the antenna from one instrument to the next. The ones that will accept data will receive it, while not sending their own data down the line.

Up to 4 listeners, so for example one NMEA-0183 GPS antenna can be daisy chained to a VHF radio, a chart plotter, a GPS repeater and, erm, struggling to think of a 4th device! That's the spec anyway, but NMEA-0183 is a muchly abused standard, and the GPS antenna may not have enough oomph to communicate with 4 devices, so it's suck it and see.

All the connected devices must be at the same baud rate, usually 4800, and no other device in the chain must be putting any data on the line other than the GPS.

 

[prv]

At a glance that looks like a suitable choice of receiver. And yes, you can just connect a few “listening“ devices in parallel like that - it really is that simple ?

Pete

 

[rogerthebodger]

have done exactly what you plan by feeding from a RS232 NMEA GPS mouse to both my SH VHF radio and to Angus MOB tag receiver.

 

[Graham_Wright]

As expected but what about the reverse case where there are two sources of gps data producing an NMEA string that may contain two (slightly) different positions?

My AIS transducer needs (I think) its gps information from a dedicated receiver but then the derived target information is transmitted on NMEA.
The plotter, NASA AIS and VHF take gps data from a Raymarine receiver. Is there some redundancy here?

I claim a higher tuned lack of knowledge on this so welcome any elucidation.

 

[AngusMcDoon]

As expected but what about the reverse case where there are two sources of gps data producing an NMEA string that may contain two (slightly) different positions?

You need a multiplexer to combine two signals from two sources, otherwise you'll end up with garbage. But why would you want to combine two GPS sources anyway?

My AIS transducer needs (I think) its gps information from a dedicated receiver but then the derived target information is transmitted on NMEA.
The plotter, NASA AIS and VHF take gps data from a Raymarine receiver. Is there some redundancy here?

Yes, but it doesn't matter. Use 1 GPS source to feed into the AIS and the other to feed into the rest. You could get rid of one GPS and use it to feed into everything as the number of devices you are sending the GPS feed to is 4.

 

[john_morris_uk]

You need a multiplexer to combine two signals from two sources, otherwise you'll end up with garbage. But why would you want to combine two GPS sources anyway?



Yes, but it doesn't matter. Use 1 GPS source to feed into the AIS and the other to feed into the rest. You could get rid of one GPS and use it to feed into everything as the number of devices you are sending the GPS feed to is 4.

Although electronically possible, the regulations for AIS do not allow you to combine to combine GPS signals. More specifically, the AIS transponder MUST have a separate independent GPS receiver. There are good technical reasons why which I read about once upon a time, and now can't remember the details.

As far as the other matters are concerned, the answers already been given. Our GPS receiver feeds the MFD, the DSC radio and the radar. Simples.

 

[laika]

My AIS transducer needs (I think) its gps information from a dedicated receiver but then the derived target information is transmitted on NMEA.
The plotter, NASA AIS and VHF take gps data from a Raymarine receiver. Is there some redundancy here?

Not quite sure I understand whether you already have a dedicated GPS for an AIS transceiver which is different from the "NASA AIS" you describe above but for a general case where you have GPS from a "normal" source but want to use the GPS info from your AIS as a backup to supply the VHF/plotter if the "main" GPS breaks...

One option would be a multiplexer as AngusMcDoon suggested but you'd need one with some smarts to only allow GPS from your backup source through if the primary went down, otherwise you may end up with your plotter thinking you're jittering backwards and forwards a few metres every second or so. Some of the higher end multiplexers can do this (but are probably prohibitively expensive) or you can go open source with a raspberry pi.

The simpler option though might be manual failover using a double pole double throw switch (electrical gurus: let me know if I'm getting terminology wrong here) with each GPS source a separate "throw". Then you can do manual changeover if you need to

 

[AngusMcDoon]

The simpler option though might be manual failover using a double pole double throw switch (electrical gurus: let me know if I'm getting terminology wrong here) with each GPS source a separate "throw". Then you can do manual changeover if you need to

Sounds a bonza idea to me. Keep newfangled electronics out of the system wherever possible.

 

[TernVI]

Not sure what commercial boxes are out there, but you can use a few buffer or driver or even opto isolator chips to provide drive to more receivers.
Years ago I was on a boat which had sporadic instrument issues which were cured by buffering some of the NMEA.
It was a big annoyance when everything stopped working at once!
Nearly had to get the lead line out.

 

[Graham_Wright]

I assume the AIS transceiver outputs the target information on NMEA. Might it also output the raw GPS info? That would make redundant the other GPS source. (Of course if it doesn't output raw GPS, my original fear goes away).

 

[john_morris_uk]

I assume the AIS transceiver outputs the target information on NMEA. Might it also output the raw GPS info? That would make redundant the other GPS source. (Of course if it doesn't output raw GPS, my original fear goes away).

The regulations say there must be a separate GPS for the AIS transponder. You can't use the AIS transponder GPS for ship's data and you can't use the ships GPS for the AIS. I know because I am that sad person who actually read through the International regulations once upon a time.

 

[PaulRainbow]

The regulations say there must be a separate GPS for the AIS transponder. You can't use the AIS transponder GPS for ship's data and you can't use the ships GPS for the AIS. I know because I am that sad person who actually read through the International regulations once upon a time.

Sorry John, that's not entirely correct. The AIS does indeed have to have its own GPS source, but the GSP data can be sent to other instruments from the AIS.

 

[laika]

I assume the AIS transceiver outputs the target information on NMEA. Might it also output the raw GPS info? That would make redundant the other GPS source. (Of course if it doesn't output raw GPS, my original fear goes away).

Apologies...I had assumed this *was* your second source of GPS data.
An AIS transceiver should have a dedicated GPS receiver. On mine at least this is not a standalone-but-dedicated receiver connected via an NMEA-0183 connection, it's an external antenna which plugs into the AIS box (at least I *think* the GPS chipset is in the AIS box rather than the antenna enclosure...). Some transceivers, yes, the GPS data is output by default along with the AIS data. With my AIS500 that is *not* the case by default but thanks to this post from Kees Verruijts' blog detailing the required incantations for the(old) srt board mine uses, I enabled it with a laptop and a usb to serial connector.

Qualification to previous "switch" idea: the baud rates for the two GPS sources would need to be the same and acceptable by the thing they're feeding. Unless your VHF is configurable (mine certainly isn't) this would need to be 4800. Some AIS transceivers have a 4800 output.

Regarding john_morris_uk's post above, as I think we've discussed in previous threads, the instruction for the having the AIS's GPS used only for that purpose does exist but makes no sense in the context of a small pleasure boat. No-one from ofcom or the itu is going to rap your knuckles for using it as a backup. And Re: PaulRainbow's response....I can't find it now but I do remember reading exactly what john_morris_uk did (and thinking and if I'm not wrong discussing here that it made no sense...). I think that's why Raymarine disabled the GPS output from their (earlier at least) transceivers.

 

[john_morris_uk]

Sorry John, that's not entirely correct. The AIS does indeed have to have its own GPS source, but the GSP data can be sent to other instruments from the AIS.

My apologies if my memories failed me. However I am sure I recall that you can't use the GPS data from the AIS to navigate the ship with. I remember because I was posed with the same problem when rewiring Serendipity and the AIS supplier argued the same thing so I went to check the regulations.

 

[TernVI]

The regulations say there must be a separate GPS for the AIS transponder. You can't use the AIS transponder GPS for ship's data and you can't use the ships GPS for the AIS. I know because I am that sad person who actually read through the International regulations once upon a time.

I don't think that's correct. I think Class B AIS transponders have to have their own GPS 'built in' but they are often 'built in' to an external antenna.
The AIS will rpboably output its GPS data along with all the AIS info, but it may be wanting to do that at a much higher baud rate?

Class A AIS may take position info from nav sources other than GPS.

 

[kitling89]

Sorry John, that's not entirely correct. The AIS does indeed have to have its own GPS source, but the GSP data can be sent to other instruments from the AIS.

This is what we do on Madogwy for most of our NEMA connected devices requiring GPS. AIS Transciever outputs NEMA 0183 which the VHF, plotter, etc ingest.

The only exception would be the NAVTEX because it's antenna has a built in GPS antenna as well. Nice to have a back up anyways (Navtex itself is basically a backup source of informationn)