iX3m and GPS

[dmaziuk]

Why does the water have to be still?

Because of that damn Einstein, that's why. As IIRC @Akimbo would say, pox on his house. If it weren't for him, your IMU would indeed know if it's stationary relative to an anchored boat, or relative to a drifting one.

PS. and dead reckoning would "just work" and and all those refrigerator-sized gyros @KWS described would never have been invented.

 

[stuartv]

Because of that damn Einstein, that's why. As IIRC @Akimbo would say, pox on his house. If it weren't for him, your IMU would indeed know if it's stationary relative to an anchored boat, or relative to a drifting one.

PS. and dead reckoning would "just work" and and all those refrigerator-sized gyros @KWS described would never have been invented.

So... you can't explain why. Got it.

You start on the surface. Your nav unit knows your position and velocity (which is speed and direction) via GPS.

You drop over the side of the boat into still water. You use your fins and accelerate in a certain direction and then you carry on finning at your new velocity for a while. Your IMU registers the short acceleration at the start, and the time. No further acceleration means a steady velocity over that amount of time. It calculates the direction and distance back to the starting point.

You drop over the side of the boat into a current. The current accelerates you in a certain direction and then you carry on at your new velocity for a while. Your IMU registers the short acceleration at the start, and the time. No further acceleration means a steady velocity over that amount of time. It calculates the direction and distance back to the starting point.

Why does the IMU work in the first case but not the second?

 

[gr8jab]

So... you can't explain why. Got it.

You start on the surface. Your nav unit knows your position and velocity (which is speed and direction) via GPS.

You drop over the side of the boat into still water. You use your fins and accelerate in a certain direction and then you carry on finning at your new velocity for a while. Your IMU registers the short acceleration at the start, and the time. No further acceleration means a steady velocity over that amount of time. It calculates the direction and distance back to the starting point.

You drop over the side of the boat into a current. The current accelerates you in a certain direction and then you carry on at your new velocity for a while. Your IMU registers the short acceleration at the start, and the time. No further acceleration means a steady velocity over that amount of time. It calculates the direction and distance back to the starting point.

Why does the IMU work in the first case but not the second?

Yea, I'm not following @dmaziuk either. At best, he seems to have confused his physicists, maybe thinking of Newton, not Einstein. But also, he seems to be talking about how to return to a boat drifting in a current, which, of course, is impossible since you can't know the position of the boat. But, he might also be talking about how you set your origin and static reference (without GPS augmentation). Mathematically, an INS needs to know a non-motion reference point (sort of like TARE or ZERO on a scale). Even a moored boat will swing and drift. How do you ZERO an INS? Do it on shore before boarding the boat?

Theoretically, current shouldn't matter for exactly the reason you described, the INS should continuously add all the acceleration vectors, integrate over time, and know your exact position. My old flight instructor (retired from the Air Force) said his aircraft INS was so good he could taxi to his spot on the tarmac after a long flight. He was prone to hyperbole, but the guy was experienced and an educated engineer and pilot.

But, there seems to be some practical limit to the technology and dealing with currents. The guys at ARIADNA.TECH Diver Navigation seem to be struggling with current compensation too. In their FAQ, they comment about it. I wonder what the underlying issue might be. Maybe the resolution of the IMU?

"Sea currents can be compensated by activating the sea current compensation function while swimming against the current and maintaining a steady position for a few seconds. The speed and heading of the current are measured and taken into account in subsequent navigation calculations."​

 

[stuartv]

Yea, I'm not following @dmaziuk either. At best, he seems to have confused his physicists, maybe thinking of Newton, not Einstein. But also, he seems to be talking about how to return to a boat drifting in a current, which, of course, is impossible since you can't know the position of the boat.

Theoretically, current shouldn't matter for exactly the reason you described, the INS should continuously add all the acceleration vectors, integrate over time, and know your exact position. My old flight instructor (retired from the Air Force) said his aircraft INS was so good he could taxi to his spot on the tarmac after a long flight. He was prone to hyperbole, but the guy was experienced and an educated engineer and pilot.

But, there seems to be some practical limit to the technology and dealing with currents. The guys at ARIADNA.TECH Diver Navigation seem to be struggling with current compensation too. In their FAQ, they comment about it. I wonder what the underlying issue might be. Maybe the resolution of the IMU?

"Sea currents can be compensated by activating the sea current compensation function while swimming against the current and maintaining a steady position for a few seconds. The speed and heading of the current are measured and taken into account in subsequent navigation calculations."​

The only way I can see current being an issue is if you aren't able to let the unit establish a fixed point of reference before you start tracking movement. I.e. if you first turned it on and were already in a current, it would have no way to know that. But, if you start the unit when it is on the surface, and it has GPS to establish the initial position and velocity, then current should be a non-factor. No difference between finning in a certain direction or being carried in that direction by a current. Or, if you don't want to do GPS, start the unit and let it "zero" itself while still on shore and standing still. But, GPS is so small, cheap, and low-powered, I can't see NOT using it. Or, as implied by the Ariadna snippet you quoted - grab a rock (the anchor, etc.) on the bottom and hold on to be stationary while the unit establishes "zero".

 

[dmaziuk]

The only way I can see current being an issue is if you aren't able to let the unit establish a fixed point of reference before you start tracking movement. I.e. if you first turned it on and were already in a current, it would have no way to know that.

Good so far

But, if you start the unit when it is on the surface, and it has GPS to establish the initial position and velocity, then current should be a non-factor.

Yes, if you do that, it should work reasonably well.

So now the question is, did OP's DM go through that "boot-up" procedure before their ix3m took them "right back to the boat". There's nothing on the ratio site to suggest ix3m can do that. It might be in the manual which I did not read.

 

[gr8jab]

So now the question is, did OP's DM go through that "boot-up" procedure before their ix3m took them "right back to the boat". There's nothing on the ratio site to suggest ix3m can do that. It might be in the manual which I did not read.

Based on communications with Ratio and comments here, I'm pretty sure the DM mistook 'reciprocal course' feature for underwater GPS. Nothing indicates that the iX3m has INS or GPS underwater.

 

[dmaziuk]

Based on communications with Ratio and comments here, I'm pretty sure the DM mistook 'reciprocal course' feature for underwater GPS. Nothing indicates that the iX3m has INS or GPS underwater.

Now I read the manual: they have accelerometers "mainly for the compass: to compensate for tilt", but they can be accessed separately via "pitch and roll app". So it looks like they may have enough sensors for INS, but certainly no software. Possibly no processing power either.

They do say "GPS doesn't work under water" for people like your DM.