# Calculate Landspeed Underwater

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Hi People, I am hoping someone might be
able to nudge me in the right direction (apologies for the long post but wanted
to get all the information I have gained so far down.

Basically I am after a solution to record
the path my vessel took under water for later analysis…like a bread crumb
trail.

Requirements:

Ideally have a range of at least 30meters
however if there were no other options I would accept down to 10meters.

Working fresh and salt water.

The vessel is (25cm x 8cm) so size and
power consumption are a factors.

It would be traveling roughly parallel to
the sea bed at variable distances from the sea bed (range of 0-30 meters)Does not need to be super accurate, anything
less than 5 meters would be fine.

Measurement speed range of 0 – 4 mph.

Measure direction the object was moving in
(i.e. forwards, sideways, backwards)…I am planning to use a compass to
ascertain N, S, E, W heading.

Options I have discounted:

Accelerometers:

https://www.robotshop.com/en/10-dof-16g-triple-axis-accelerometer-gyro-barometric-sensor.html

This was my initial thinking but in doing
some reading it seems they are not suited to my needs (unless you spend loads
of money, and then the solution would end up being too heavy anyway).

Optical Flow:

https://pixhawk.org/modules/px4flow

Looks too new (from a consumer perspective)
/ complicated. I don’t know what its range would be like. Also requires

Current favorites:

Sonar:

http://www.alibaba.com/product-detail/1mhz-waterproof-transducer-underwater-ultrasonic-sensor_1911722468.html

Simplest use is distance from object,
however can use doppler effect to analyse speed of a moving object.

40m range, nice!

Presumptions:

If I fired this at an angle to the seabed I
could deduce the speed the floor was ‘moving’ below which would give me the
speed of my vessel?

I am also presuming that I could interpret
direction of movement from the data?

I presume that the sensor would need to be
aimed at an angle of around 45 degrees down to the seabed?

Laser Rangefinder:

http://www.parallax.com/product/28043

Although it works differently to the Sonar
the premise of use looks the same, and thus I have the same queries with this
as I do with the Sonar above.

Presume if I mounted the sensor behind high
quality glass (to waterproof it) then performance would not be impacted that
much.

This is a lot more costly so if it does not
give me any advantage over sonar I guess there is no point.

Water Flow Meter:

Super low cost and simple compared with the
other options, I would potentially use a funnel to increase water pressure if
it needs more sensitivity at low speed.

Would then just need to calibrate the
pulses from the sensor to a speed reading.

Significant limitations of this is it would
be registering a speed of 0 if the vessel was simply drifting with the current….its
speed over the seabed that I am interested in.

Current favorite option is sonar (with the
option of using water flow meter as second data source)…however are my sonar
presumptions correct, have I missed anything?

Any better ideas?

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A pilot in an airplane or a

A pilot in an airplane or a captain on the ocean has the same problems.  It is easy to know your speed relative to the wind, but the wind is always blowing somewhere so those numbers don’t mean much.

A pilot uses either a GPS (or something like a GPS to calculate exact position via triangulation) or uses certain types of fixed stations to calculate position via triangulation and then speed relative to the ground (ground speed).  Since a GPS doesn’t work under water, I think you are left with using a couple of fixed stations on dry land to triangulate and a GPS when on the surface.  A pilot also uses accurate maps of the ground and can use ground features to calculate ground speed pretty accurately.  I know on several flights the numbers would be with a few knots of what the gps said was the average speed.

The nuke submarines of the cold war used to have antennas that they drag behind them which stay close to the surface so they can always know where they are.

An interesting problem to try to solve.  Let us know what you finally do!

Regards,

Bill

I can tell you that the only

I can tell you that the only reasonable way to do this without having an external acoustic positioning system (underwater GPS) is to use what’s called a DVL. They are designed specifically to solve this problem. They are also insanely expensive, and I’ve never seen one that would fit inside the space you are talking about.