Re: Cutting plug of NMEA and re-joining?
Please explain [why a rocking boat's GPS won't work well]. The speed at which a GPS receiver is moving on a boat is negligible. After all, GPS receivers are used on many things moving way faster like jet airplanes and missiles.
Yes. Excellent point. A GPS receiver works fine in my car at speeds of 70-MPH.
If a boat takes on a list of ten-degrees and the GPS antenna is 6-feet above the deck, and assuming the deck level was the center of the roll rotation axis, the GPS antenna moves through the ten-degree arc or a distance of
2 x pi x 6-feet x 10/360 = 1.04-feet
A car going 70-MPH is traveling at
70-miles/1-hour x 5280-feet/1-mile x 1-hour/3600-seconds = 102.6-feet/second
In order for the motion of the GPS receiver in a 10-degree roll to be at the same speed as a 70-MPH car, the boat would have to undergo the 10-degree roll in
1.04-feet x 1-second/102.6-feet = 0.01-seconds
If your boat is rolling sideways 10-degrees in 0.01-seconds, then the GPS receiver would be traveling at 70-MPH in that direction. We know that there is really no problem for a GPS receiver to operate at 70-MPH, so I don't think a ten degree roll would be much of a problem, even one that occurs in 0.01-seconds. You are more likely to break you neck if your boat has such sudden roll motions, as I doubt your body could remain standing. But the GPS is going to keep working fine.
As for the stability of the "reading" of the GPS, I think you mean the accuracy of its position solution. As I demonstrated, a ten-degree roll only moves the GPS receiver a foot. The accuracy of the GPS position solution is about 30-times less, or about 30-feet.
In any case, we have to assume the rolling motion is some sort of oscillation. If the rolling motion were fixed, it would have no effect at all. Since the rolling motion is oscillatory, its effect is just to put a random uncertainty into a position which already has a random 30-foot uncertainty. There is no basis to assume that the error of the uncertainty from the roll would always add to the error of the position solution. There is a reasonable chance it could just as likely correct that error. In addition, the error would only be a maximum of 1-foot. If we assume the boat rocked back and forth in the ten-degree roll, the change in position of the GPS receiver over time becomes zero. It's average position is the position with no roll displacement. So averaged overtime, the roll error cancels itself out. So I don't see how it can be explained that a rocking motion will make the GPS receiver not work well. Even if the roll period were extraordinarily short, a one-hundredth of a second, the speed the GPS will be moving is not a factor. The oscillatory motion of the roll tends to make any error it introduces sum to zero. Even if there were an error, it is 30-times smaller than the typical error in the GPS position solution.
I don't see much of case to be made for mounting the GPS receiver "as low as possible" to make it work better by avoiding roll motion. In fact, there is a much better case to mount it higher, in order to give the GPS receiver antenna the best view of the sky. On this basis, I have to disagree with the advice to mount the GPS antenna "lower" so that it will "work better."
Also, any arguments about position solutions from GPS being affected by speed should take into consideration that the GPS systems computes its position by measuring signals from MOVING satellites. The satellites are all in orbit and traveling quite a bit faster than any small recreational boat is likely to be able to move across the water. If there were some limitation to the accuracy of the position solution based on the relative speed between the receiver and transmitter, it would certainly come from the speed of the transmitter movement. The satellites are moving at a speed of 3,900-meters/sec, which is 8,724-MPH.
