GPS antenna (for both AIS and Chartplotter)

[sailingharry]

So, you may recall I trashed my arch in an horrible accident outside the Port of Montreal. An entire new arch is to be built, and that means I have a clean slate on installation.


One issue is solar and interference with my GPS antennas. The existing system has 3 antennas:
* Chart plotter, antenna mounted on centerline on the arch.
* AIS, mounted about 3.5' off center
* Backup, also mounted about 3.5 feet off center.


The off center ones are on a cluster mounted basically in line with the edge of the transom, the upright of the arch. This cluster holds my AIS antennas (GPS and VHF) and my backup GPS system (an ancient but very low power system with nothing but a numeric display and 0183 outputs). In the new arch, it will also hold my stern light, my new anchor light, my high-gain cell antenna, and possibly a mount for a future StarLink. Quite a nice cluster!


My issue is the main chart plotter antenna. This is mounted dead center on the arch. Also on the arch are 3 solar panels. The GPS antenna makes the center panel (the one least affected by shade, of course) half the size of the other two. If I moved that antenna to the cluster, I'd gain a 20% increase in solar capacity.


Beyond some "philosophically pure" answer, is there really a benefit of keeping the GPS antenna centered? In some places (south coast of Newfoundland, for instance) I've seen charts that are 100' or more offset from reality, so 3 feet of error really doesn't matter.


I'm really inclined to move it to the cluster, and not worry about it. But I'd hate to drop $20K on the arch and wish I'd done it "right."


An alternative, that I"m pondering, is to stick a "prodder" on the front of the arch. An 18" long tube that sticks forward from the arch under the solar would allow me to mount the antenna just forward of the solar, but it would be a challenge to fish the wire.

[rslifkin]

Does your chartplotter have an internal GPS antenna? If so, have you tried it without the external antenna to see if performance is adequate?



Being that we're only talking about a couple of feet, I wouldn't be worried about mounting the antenna off-center personally.

[kayakerChuck]

It's a sailboat- close enough is plenty good for this. Mount the GPS antenna where convenient and with a decent view of the sky.

[ranger58sb]

Originally Posted by sailingharry

One issue is solar and interference with my GPS antennas. The existing system has 3 antennas:
* Chart plotter, antenna mounted on centerline on the arch.
* AIS, mounted about 3.5' off center
* Backup, also mounted about 3.5 feet off center.


The off center ones are on a cluster mounted basically in line with the edge of the transom, the upright of the arch. This cluster holds my AIS antennas (GPS and VHF) and my backup GPS system (an ancient but very low power system with nothing but a numeric display and 0183 outputs).

Can you not select your AIS GPS as source for your plotter?

FWIW, our AIS GPS antenna turns out to be something like 25-40' more accurate than the internal GPS antenna inside one of our MFDs (and maybe the other MFD too, both shaded by hardtop)... possibly significant for some places along the AICW for instance.

-Chris

[capt jgw]

GPS is only accurate to around 10 feet, sometimes better, sometimes worse. Plenty good enough for navigation, and being a couple feet off center won't matter. And a lot of chart programs and displays have an offset you can program in, tell your system where the antenna is located and it will correct your position to match, fore and aft and side to side.

[wholybee]

Can't you adjust the offset in your chartplotter and AIS? You should be able to put the antenna anywhere and adjust the offset.

I would look into using the AIS as the primary for the chartplotter. And if it works, the Chartplotter internal could be the backup. Then you only have one antenna to deal with, and still have a backup.

[Sailer_Med]

Have you given serious consideration to using a couple of simple GPS pucks mounted “someplace?” My boat has three GPS antennas, none of which are GPS “antennas”, they are the little pucks (about 2” square). Two are mounted inside the hull in different locations in proximity to the electronics that want them. The third is inside my Axiom +12. They all work fine and yield the same location. Simple, cheap, etc.

[sailingharry]

The chart plotter system, a Furuno, does not have an internal GPS.


Indeed, the AIS does provide position data. When my external antenna dies (twice so far -- once, as it came with the boat, with a short in the N2K network that killed the GPS data stream, once when the GPS rollover killed it), the AIS is my immediate backup. However, I suspect that the AIS position is a little less accurate and a little less frequent. The AIS is an older unit. But more important, the AIS antenna is already on the cluster, so shifting to that doesn't help.


I am pretty sure the AIS allows a port/starboard offset. But I don't think the chartplotter can. Not sure how it even would, because until there is motion, it doesn't know where port/starboard is (well, truthfully, it has compass heading from the autopilot, so if it's smart enough it could figure it out even if I'm stationary).


I already own the antenna, so keeping it is only the cost of the mounting point and the labor to install.


I'm really strongly leaning to putting it on the cluster. Very little downside, lots of upside.


=================



About that short on the AIS network. Just to fill in for the curious. This was a real hair-pulling event (I'm bald, but regardless...). The N2K "network" was the antenna and the chart plotter. Dumb simple. The power injector was a Garmin cord, all neatly cast in one chunk of rubber. The failure, once I found it, was a short between the power ground and one of the data wires, inside the Garmin cord. Once I found it, I simply grabbed onto that pin of the plug (the shorted signal pin) and pulled it out -- since this was a 2-wire power cord, the signal pins do nothing. Absolutely none of the troubleshooting algorithms look to check continuity between power ground and signal wires. It was basically a happenstance that I found it.

[ranger58sb]

Originally Posted by sailingharry

However, I suspect that the AIS position is a little less accurate and a little less frequent. The AIS is an older unit.

FWIW, our AIS GPS antenna is our most accurate. Although probably partly because it's mounted on top of the hardtop, no obstructions.

I think it's Furuno GP-33 or some such...

Continuous reception, not linked to periodic AIS VHF transmissions...

-Chris

[rslifkin]

Most AIS units I've seen only have a 1hz GPS. Better units (either built into better chartplotters or standalone) will give a 10hz GPS signal, which gives significantly better / smoother data for speed and course over ground.

[sailingharry]

Originally Posted by ranger58sb

FWIW, our AIS GPS antenna is our most accurate. Although probably partly because it's mounted on top of the hardtop, no obstructions.

I think it's Furuno GP-33 or some such...

Continuous reception, not linked to periodic AIS VHF transmissions...

-Chris

Close -- I bet it is the GP-330. That's the one I had driving our chartplotter until the rollover killed it (ones built after about 2015 or so, I forget the date, are immune to the rollover -- ours built in 2012 or so died an expensive death in barely a decade). I replaced it with one by Quark that has a built-in compass for a lot less.


Our AIS uses an actual antenna, not a receiver. It sends a signal down to the AIS, that decodes the position. Data sent to the chart plotter is 1hz at best, probably closer to .1 hz. The position update on the display is decidedly intermittent. For sending contact information (the design purpose!), it's quite suitable. Like I said up-thread, it's a bit old......


I do like that our antenna is not integrated, since as you suggest, having it with a clear view of the sky is certainly not a bad thing.


In our particular suite, that one antenna provides data to both chart plotters -- not sure how well a nav station chart plotter works with an integrated antenna, although with a networked system it could theoretically tap into the helm unit.

[BMD21]

That's a lot of antennas for those functions. So, lets look at the various functions and the receive/transmit frequencies. I think you only need 3 antennas,max.



GPs: rec only, frequency L1, at 1575.42 MHz, and L2, at 1227.6 (UHF)

VHF rec/trans, frequency 155-174 MHz

AIS rec/trans, frequency 162 MHz.


So, the vhf requires it's own antenna. The deck mounted antennas have a 6' 6 dB (measure of performance). Mast mounded have 3' 3 dB, giving 1/2 the performance. VHF is LOS as far as you can see and 3 dB will do good.


The tr/rec AIS has an internal GPS (see below) for position, having it's own GPS antenna. The AIS transmit frequency is close enough to VHF that it can share an antenna with the VHF using a splitter. I use a separate AIS antenna, they're cheap and small and don't require a splitter.


GPS uses a much higher frequency thus has a much smaller patch antenna. The pucks have both an antenna and a processor, outputtiing GPS data. A separate antenna only sends the unprocessed signal down.


So, the GPS input to the chartplotter need GPS data, since it doesn't have a GPS, though most chartplotters do. So, you need a GPS antenna and data processor. They may be separate, but I've never seen that. The VHF needs it's own antenna, mounted as high as possible. The AIS should have its own antenna. That makes a total of three (3), covering 3 separate functions. Mount as you choose, considering that the GPS needs a view of the sky up, the other two need views horizontal.

Boris

[sailingharry]

BMD21,

I'm not entirely sure I follow you. My VHF antenna is at the masthead. My AIS antenna is on the arch. I have three GPS antennas, one which is a requirement for my older ais, another dumb antenna for my secondary GPS system, and the integrated antenna for my chartplotter. Admittedly, the secondary GPS is a bit redundant and came with the boat. But it would shine on an offshore passage where the chart plotter is a big power drain with no benefit. It is simple enough to reinstall with not much benefit for removing it. So three GPS antennas, none of which can be removed, and an AIS radio antenna.

There is certainly a bit more clutter on that mounting pad, with a stern light, an anchor light, an iridium GO antenna, a cell phone booster antenna, and a mounting pad for a skylink. But none of those can really be dispensed with.

As an aside, AIS receivers by regulation cannot share their GPS input with any other device. They can output it to other devices, but the antenna must be dedicated.

[donradcliffe]

When i installed my AIS transponder, I noticed that the required GPS for the AIS was exactly the same as used for my Garmin chartplotter. Ir was a remotely powered antenna with 5 volts between the coax shield and center wire. I put in a T and tied one antenna to both devices, and it worked perfectly. I kept the extra antenna as a spare.

[svmobert]

Don't over compllicate it..

1.) Get a really good 10hz NMEA GPS sensor and put it on the network. Mount it ANYWHERE it can see the sky, except keep it away from sources of major RF interference.

2.) The AIS's antenna location offset affects the data it sends out to other vessels. The AIS's GPS to NMEA output is not compensated as far as I am aware.

3.) Assuming your boat is less than 50ft, the overall position of the GPS antenna makes little real world difference. Fast updating, support for multiple constellations, fast acquisition, and overall accuracy is the key. You can see every end of the boat from practically anywhere ON the boat, so you don't need to worry so much about the antenna position.

If you had a 100ft boat, then I'd be more worried, and possibly even run GPS antennas in multiple locations feeding independent chartplotters, but that's probably overkill as well.