iX3m and GPS

[gr8jab]

Hi all,

I'm quite frustrated. As an engineer, I've tried to analyze all possible explanations before I call BS. Maybe others here can help.

Situation

Local DM and shop owner claims that his iX3m GPS works underwater. He claims it has pointed him back to the boat (which he previously marked with a way-point), accurately at depths as deep as 15 feet, and maybe even deeper.

The GPS screen on the iX3m includes GPS strength and satellite lock count. He claims he's never looked at these readouts and cannot confirm or deny signal acquisition below the surface.

Known

• The iX3m supports several satellite networks, including the USA, Russian, and Chinese.
• All these networks use 1.2 to 1.6 GHz RF frequencies.
• Assuming no reflection of the signal at the surface (which is almost impossible, even at if the satellite is directly overhead), signals of that frequency would penetrate less than 1 meter of salt water.
• iX3m has an internal compass
• iX3m has some tilt sensors, as described in the Apps Manual
• The user manual specifically states:

"It is necessary to come to surface to allow the iX3M to link to satellites; the GNSS signal doesn’t cross the water’s surface." and "To save battery the satellite connection is active only when GPS App is running."​

What am I missing? Does the iX3m have an accelerometer and attempt to do inertial navigation? Highly unlikely. Does it do some crude navigation guesses based on the compass and tilt sensors?

I have suggested to this DM that he perform an experiment where he records the GPS strength and satellite lock count and depth during ascent or descent. He scoffs, and claims "well, it seems to be working".

So I ask you, the great Scubaboard community:

1. Has anyone used the iX3m to navigate underwater?
2. If there is no good GPS data, why would the iX3m display possibly erroneous navigation guidance? I would think this is dangerous.
3. Is the DM smoking something? (In other words, is he just confused or wrong?)
4. Am I missing something?

 

[BoltSnap]

I have the iX3M Tech + GPS. GPS of any kind in the civilian market does NOT work underwater AT ALL!!!!!!!!!

It doesn't work even few centimeters underwater, not at all!!!

Just in case I wasn't clear above, GPS doesn't work underwater and neither does the GPS in Ratio Computers, not at all!

You can communicate with Ratio's support directly if you want, support@ratio-computers.com

It will take them a couple of business days, on average, to get back to you.

 

[BoltSnap]

Check the bottom right of this picture taken from Ratio's website:

 

[gr8jab]

@BurhanMuntasser: Any idea what information was displayed to this DM while diving? Obviously, the DM thought it was valid directional information for 'back to the boat'. Isn't displaying questionable or bad information a recipe for disaster?

Maybe you could recreate the situation and take some underwater photos of the GPS screen (if that kind of thing floats your boat).

 

[BoltSnap]

Any idea what information was displayed to this DM while diving?

Only compass information, nothing else. The compass can give you your reciprocal course if you "save" your heading out.

 

[gr8jab]

Only compass information, nothing else. The compass can give you your reciprocal course if you "save" your heading out.

Ah... maybe that's what this guy is confusing with real navigation. Explains a lot. Thanks!

 

[dmaziuk]

According to their glossy brochure ix3m has 3D accelerometers and tilt sensors which suggests a full (-ish?) IMU. I have not seen any information on what they do with it (even if they log the data), but theoretically it can be used for dead reckoning, and even lead a well-calibrated reference diver in perfectly still water back to the boat.

 

[gr8jab]

According to their glossy brochure ix3m has 3D accelerometers and tilt sensors which suggests a full (-ish?) IMU. I have not seen any information on what they do with it (even if they log the data), but theoretically it can be used for dead reckoning, and even lead a well-calibrated reference diver in perfectly still water back to the boat.

I would think if they had INS, they would be selling the heck out of that feature.

 

[dmaziuk]

I would think if they had INS, they would be selling the heck out of that feature.

If it worked. As far as we know it can't work on e.g. drift dives.
(Hence "in perfectly still water")

 

[stuartv]

If it worked. As far as we know it can't work on e.g. drift dives.
(Hence "in perfectly still water")

Why does the water have to be still?

I mean, obviously, if the boat is drifting, a system that gets you back to your starting point is not going to get you back to the boat. But, I don’t get the feeling that was your point. Why would inertial navigation care if you accelerated by finning versus accelerating by being pushed by water (I.e. a current)?