GPS antenna location

[AntarcticPilot]

I wouldn't be too concerned about the effect of rolling and pitching unless there is a gyro included in the GPS reciever, the GPS signals travel at light speed - in cars it works fine up to 320kmh (I haven't managed to test them any faster )

Good explanation, but this last point isn't quite right. Yes, the signals travel at light-speed, but they are transmitted on a schedule, and different satellites need not transmit at the same time. AFAIR, the cycle of transmissions is 2 seconds long - I may be wrong about the exact time, but it's about that. So, if your antenna moves unpredictably during that cycle, it will throw the accuracy of the fix off. Receivers can compensate for slowly changing velocities, but the effects from (say) the top of a mast might be a bit too much!

I doubt if this effect really matters very much, but why put the antenna in a bad place when a better one is so much easier

 

[noelex]

My experience with an aluminium boat ( and steel will be similar) is that the old gps units will not work down below at all. The new units will get a position but the error is much higher, making anchor drag alarms and speed display useless.

Even on a fiberglass boat I think you want the best GPS signal possible. A lot of boats seem to put up with a poor gps signal with larger errors no EGNOS etc. it's an important tool and should be able to deliver its best.

Mount it outside, with a clear view of the sky. Try to mount it close to the roll center, not too far from the ends, or too high. For a good anchor alarm nearer the front is better, but this is often difficult to achieve with the other objectives in mind.

If you install a back up (and on a crusing boat you should) mount it on the oppisite side so in the event of a collision, or dismasting, one is likely to survive.

 

[noelex]

I wouldn't be too concerned about the effect of rolling and pitching unless there is a gyro included in the GPS reciever, the GPS signals travel at light speed - in cars it works fine up to 320kmh (I haven't managed to test them any faster )

The problem with mounting the gps too far from the roll and pitch center of the boat is that roll and pitch physically move the the antennae. This movement is reported as changes in position and in speed and direction. The speed of the roll and pitch movements can be similar to the overall speed of the boat, but in a very different direction.

Filtering and averaging the gps signal can overcome the variations,but at the expense of decreased sensitivity and responsiveness, particularly in COG and SOG.

 

[Baggywrinkle]

So, if your antenna moves unpredictably during that cycle, it will throw the accuracy of the fix off. Receivers can compensate for slowly changing velocities, but the effects from (say) the top of a mast might be a bit too much!

I wasn't advocating top of mast installations - anywhere on or near the deck should be fine and I'd go for uninterupted view of the sky over boats COG every time. (uninterupted view and COG would obviously be ideal).

We've found from measurements in vehicles that even when stationary they don't actually appear so from the GPS receiver point of view and the individual fixes need to be damped (averaged) and compared to gyro information as a plausibility check - reflected signals, trees, buildings, vehicle speed, atmospheric effects etc. all have an effect on the fix. Reflections should be less of an issue on a boat, but it would be poor system design if an algorithm wasn't employed to filter out the roll/pitch effect on SOG/COG. We do it for cars which is essential for deployment in places like Tokyo where the road systems are 3D and an accurate 3D fix helps the other positioning algorithms determine exactly which road and which lane the car is in.

A boat on the sea is a much less complicated environment so a bit of rolling and pitching should be no problem for the design engineers

In short, the rolling and pitching on a small yacht looks pretty much like normal GPS innacuracy to the positioning algorithms - without gyro information the difference between antenna position movements and GPS fix innacuracies can't be easily determined AFAIK.

 

[DaveKnell]

I wouldn't be too concerned about the effect of rolling and pitching unless there is a gyro included in the GPS reciever, the GPS signals travel at light speed - in cars it works fine up to 320kmh (I haven't managed to test them any faster )

The GPS on my phone has worked perfectly well with a very restricted view of the sky at 37,000' and around 600 mph, plonked on the chart table (GRP boat), by the helm and by my bed in the aft cabin on anchor watch. At least, I assume it worked OK for the last of these.

--Dave

 

[noelex]

The GPS on my phone has worked perfectly well with a very restricted view of the sky at 37,000' and around 600 mph, plonked on the chart table (GRP boat), by the helm and by my bed in the aft cabin on anchor watch. At least, I assume it worked OK for the last of these.

--Dave

Ofen when the GPS signal is poor their is still a position fix, but the accuracy suffers.

 

[mcframe]

3mm of fibreglass can have no effect, or lots, the devil is in the detail as we say.
3 mm of grp with a good film of salt water running over it can be in a different league.

GPS is a digital system. It works up to a certain level of signal degradation then fails catastrophically.

Personally, I think the pushpit is a good compromise.

Ours is deck-mounted, just inboard of the quarter, away from cleats, but *under* the pushpit - I fitted it in winter, and didn't quite notice how close it is to the O/B bracket, but I've seen no ill effects.

Maybe the lower rail of the pushpit is also a good compromise between getting signal and getting bashed/tangled?

(Swinging mooring, so we board over the stern.)