GPS WAAS Antenna Location : Masthead or Sternrail?

[noelex 77]

Most GPS antennae are active these days so there is no concern about coax loss.

No firm data, but I can imagine situations where my masthead speed would have little relation to the boat speed. Such as sailing in light sloppy conditions downwind. It would need a much larger averaging delay to even out the masthead oscillations.

[Rustic Charm]

Originally Posted by noelex 77

Modern GPS units are very sensitive and will still work satisfactorily with this sort of obstruction. However, the GPS is such an important instrument that I prefer to get the best possible information from it. It does make a (small) difference when high levels of precision are needed such as an anchor alarm or retracing a track, especially in conditions such as heavy rain.

I just tried my Garmin HH for you and the satellite signal strength went from 10 satellites visible with an average signal of 95% (from the unmarked scale) and a reported accuracy of 2m in open sky to still all 10 satellites visible with an average signal strength of 65% and a reported accuracy of 5m. This was directly under the centre of a solar panel.

Brilliant! I really appreciate you doing that for me. It's enough for me to leave it as it is for this year and I'll look at moving it next year when I pull her up on the slip again.

A guy I was talking to when I re-installed it today, didn't think it would work so that's why I thought I'd ask.

[a64pilot]

It's been a few years ago when I installed my Garmin 740S in a center console boat. Since I was installing it in the console and it under a T top, I was concerned I might need an external antenna. I kept the boat stored inside of a full metal aircraft hanger.
When I turned it on, it received and tracked a full list of satellites, from inside of a closed aircraft hanger. Signal strength was down some, but I no longer worried about an external antenna.
Now on my sailboat, I have an old Raymarine plotter, with an external antenna and it will sometimes lose position and never tracks as many satellites or have as good a signal as the Garmin did. Maybe it's just old, or maybe the Garmin is better at receiving signals? Point being is that there is a lot of variables between machines, some work better than others.

[witzgall]

Any GPS sensor installed from new in the last few years is extremely sensitive, and can often be mounted below the deck of a boat. So the stern rail would be fine, and the mast head would, IMHO, be overkill.

Chris

[a64pilot]

I'm thinking a mast head mount would not be good at all, due to the excessive movement up there at times.
Most of my GPS receivers work just fine inside of the boat, for my Ipad I use an external bluetooth GPS and leave it inside of the cabin on charge Universal Bluetooth GPS Receiver for Today’s Most Popular Handheld Devices | Dual Electronics works very well and is a better receiver than most built in GPS chips

[continuouswave]

Originally Posted by CaptTom

...WAAS data is embedded in the satellite signal, so it's as good as the location data.

This is not quite right. The GPS signals come from 31 satellites in medium earth orbit. The WAAS signals come from three satellites in geostationary orbits, which are much higher.

[gbgreen59]

Multipath becomes more of a problem with antennas mounted very low. You may see 11 satellites, but multipath will be affecting accuracy. I wouldn't put the antenna in the cabin unless I didn't care about accuracy to about 20 meters or so. Advertised accuracy of 2 meters assumes a limited level of multipath. Multipath comes off of surrounding metal like masts, booms, shrouds, stays, etc. It is worst in marinas where there is a forest of masts and booms sailboats booms. So, there is a trade between limiting multipath (top of mast) and accuracy (reduce swaying from mast).

[continuouswave]

Originally Posted by gbgreen59

Multipath comes off of surrounding metal like masts, booms, shrouds, stays, etc. It is worst in marinas where there is a forest of masts and booms sailboats booms.

When you are moored to a dock in a marina, you probably don't have to worry too much about any inaccuracy in your position from GPS.

[Dockhead]

Originally Posted by Wotname

Does anyone have firm data on GPS signal degradation when the GPS antenna is mounted on the mast head.

Clearly this gives the best view of the sky and clearly there can be some significant coax loss. The coax loss can be calculated and compensated for by using different coax or a higher gain antenna. I'm not convinced by the mast head movement argument with some firm data. Automobile and aviation GPS units seem to work quite OK in a highly dynamic environment.

But I could be persuaded by some solid data backing up the current viewpoint

GPS receivers (rarely antennae) don't use coax. They are connected to your network by either an NMEA0183 cable or to the N2K backbone.

No problem with that at the masthead, but a RPITA (that's a Royal PITA) to put it there and run cable, and to get to it in case you need to service it. A horrible place for it in my opinion.

I have mine enclosed within the instrument case above my scuttle, with nothing but a thin sheet of fiberglass and a windshield between it and the sky. Easy to get to, simple cabling since the network is already right there, and perfect reception. No need to go up to the masthead.

[gbgreen59]

It is instructive to understand a couple of parameters that the boater can control to some extent. It is good to understand how the installation affects advertised accuracy. The Dilution of Precision (DOP) and multipath are things that can be controlled to some extent. The GPS antenna mounted low, like inside the cabin, is most subject to both multipath and a "poor" DOP. If advertised accuracy isn't a concern, like for AIS, then it is a fine place for the antenna. Definitely the easiest installation. An antenna on a stern pedestal or arch would have the best DOP (visibility of all satellites in view) but multipath from the mast is still going to be problematic for meeting full advertised accuracy. An antenna installation on the top of the mast would be give the best accuracy...but it is giving the position of the top of the mast with position being an average through the swaying of the mast over the interval of integration or filter time constants. So, really it is a trade between a few parameters. Full advertised accuracy probably isn't a concern to most boater, so pick and choose. For me, I would choose at stern pedestal or arch for the GPS antenna location. I don't like people standing in front of my GPS (in the car is an exception).

Here is an example of DOP issue. My brother has his GPS antenna (built into the plotter) on the console of his bay fishing boat. With he and I standing in front of it (it is at about chest level), we were cruising down a canal between rows of houses on each side with houses on stilts (Galveston area). He showed me that the GPS had us passing along the houses and not in the narrow canal. Plus, he pretty much blamed the GPS being bad. But what was happening is that he and I were blocking a good portion of the sky and the houses were blocking most of the low elevation satellites. Hence, the DOP was poor and the position had a bias in it. I have other interesting examples of DOP issues also.

I hope this brief description helps in understanding the trade-offs between different GPS antenna installations. Again, most boaters don't need 2 meters of accuracy for GPS.

Here are a couple of experiments that can be done in your spare time.
- With poor DOP, you will see a bias in the position solution that is greater than the advertised accuracy.

- With multipath, you will see the position solution wander around by an amount greater than the advertised accuracy.

Of course, ionosphere and troposphere errors can be excessive in some cases too. But we can't control those and are at the mercy of those errors.

[gbgreen59]

BTW: If anyone cares or does not know....In addition to adding a ranging signal, WAAS is most useful for providing corrections for error caused by the ionosphere. It can help position accuracy when ionosphere error is the dominate error source. If the dominate error source is not the ionosphere (dominate error caused by multipath, DOP, or troposphere), then WAAS does not provide much help for position accuracy. Again, I hope this is helpful.

[Rustic Charm]

[QUOTE=gbgreen59;1665041] An antenna installation on the top of the mast would be give the best accuracy...but it is giving the position of the top of the mast with position being an average through the swaying of the mast over the interval of integration or filter time constants. [QUOTE]

can you that in English 101 so uninitiated can understand what you just said please. (that's me).

[gbgreen59]

I feel I'm over simplifying things, but:

A GPS antenna placed at the top of the mast gives a height that is the height of the mast (as opposed to water level; which is a "don't care" for most). Plus, the horizontal position (which we are most interested) moves based on mast sway. The GPS is telling you where the top of your mast is at a given time. Most boaters would not want to see the position moving around, so a lower antenna removes some of this motion that might be seen on a plotter.

Hope this is a little clearer.

[Rustic Charm]

Originally Posted by gbgreen59

I feel I'm over simplifying things, but:

A GPS antenna placed at the top of the mast gives a height that is the height of the mast (as opposed to water level; which is a "don't care" for most). Plus, the horizontal position (which we are most interested) moves based on mast sway. The GPS is telling you where the top of your mast is at a given time. Most boaters would not want to see the position moving around, so a lower antenna removes some of this motion that might be seen on a plotter.

Hope this is a little clearer.

well, I did ask for 101 so that's fine.

Why though, would most boaters care if the location is moving around a bit, and it can only be a few meters? What's the usual inaccuracy 5M ? I can understand if your on a mooring, but on a mooring or in the river your not going to be swaying around like your out in rough open water, so why does it matter?

[transmitterdan]

It will degrade the accuracy of any SOG computations for one thing. There isn't a good justification for putting the sensor up there. The multi-path argument isn't persuasive to me.