# Real time tidal currents

Hi,

I have expertise in tidal current analysis and prediction using low-powered computers.

It seems to me that knowledge of future tidal currents would be helpful in mission planning, and real-time prediction may be useful as a data input to a Kalman filter or equivalent.

Is there any interest in this?

Jeremy

Да, это очень важно и интерес есть.

Hi Jeremy, welcome to the forum

This at least sounds interesting in theory, especially for open-water use where predictions aren’t dependent on surrounding structures. Would you be able to explain a bit more about what you mean by tidal currents, and how you envisage that prediction playing a meaningful role in motion estimation?

This at least sounds interesting in theory, especially for open-water use where predictions aren’t dependent on surrounding structures. Would you be able to explain a bit more about what you mean by tidal currents, and how you envisage that prediction playing a meaningful role in motion estimation?

For an underwater vehicle, there are two inertial frames of reference; the moving body of water, and the fixed underwater terrain. Propulsion is in relation to the water frame of reference. Navigation is usually in relation to the underwater terrain.

You can estimate motion by knowing what the local water body speed and direction is at any time and your measurement of motion in relation to the water body.

Tidal currents are the cyclic flow of water under the influence of gravitational fields from the sun and moon and significant harmonics of the basic solar and lunar frequencies.

Tidal currents are significant in shallow water and not so significant in deep water.

Knowing the tidal data parameters at a location enables you to predict decades in advance (or retrograde) what the tidal component of a current will be. These will be a vector of water speed and direction. This vector varies over distance and underwater terrain, but can be estimated by interpolation, and can be established precisely by measurement at fixed points for a period of time.

Tidal currents and tide height are related in that the tide height is a function of the accumulated ebb and flow of water via a tidal current.

In addition to the tidal current component, there is usually ‘noise’ caused by wind and air pressure generated currents. Over a long period, these have a fixed value, often zero amplitude.

For mission planning, you can predict the tidal current at a location over time and then plan the propulsion of the vehicle in relation to the water so that you can then navigate in relation to the fixed terrain.

For navigation, you can generate the tidal component of the water current in real-time and use that as an input to your navigation filter (often a Kalman filter). This can be subtracted from the actual water current to feed only the error values to the filter – that is non-tidal components of the current. You are effectively giving the filter the water inertial frame data values and letting it work on the differences between that frame and what is actually measured.

To some extent, this is similar to using a GPS fixed location receiver as a reference to the values measured by a mobile GPS receiver.

The prediction is done using as few as 4 components and time. Each component is phase and amplitude in the X and Y axes. So 16 pre-computed floating-point numbers and a timestamp. You can use more components for better accuracy with an insignificant increase in computation time – which is measured in microseconds per cycle.

Right, so if I’m understanding you correctly, the potential value adds would be

1. more responsive control, because the navigation filter knows about some potentially significant external forces
2. given a reasonably accurate position estimate and tidal currents map, could act as an early warning system to a pilot of strong currents their vehicle may not be able to handle
3. possibly more effective inertial navigation, because the vehicle’s inertial measurements could be used together with known tidal currents and control inputs, and a model of the vehicle’s dynamics and thrust capacity, to try to simulate the vehicle’s motion

Those would come at the cost of needing to take some tidal water speed measurements (where it’s relevant to ask: how many? over what time period? how expensive are the sensors to have, deploy, and use?), and possibly some significant computation for the last point.

Is that about right, or are there other values or costs that I’m missing?

I’m intrigued by this, as it would seem to require one or multiple current sensors on the vehicle, which I understand isn’t particularly standard in the ROV world. I did a quick search and found a somewhat brief discussion about the idea here, but that was seemingly dismissed on the assumption that the currents around the vehicle aren’t known (in which case it’s understandably not a particularly useful sensor to have unless the aim is to measure currents given known vehicle velocity+position).

I’d be interested in how significant these are in practice. While they may average out to be insignificant given enough time, I imagine the majority of vehicle navigation doesn’t occur over such broad time frames. I also understand that the effects of wind and air pressure are most pronounced at shallow depths, since they apply at the water’s surface.

Do you have a particular use-case in mind where you think tidal currents solves a currently unmet need, or is your experience more with the detection and estimation side and you’re hoping to find such a need through this discussion?

The vehicle will not sense any force if it’s just floating. It is at rest with respect to the water inertial reference frame. The prediction is about the relative motion of the water frame to the terrain frame.

The current predictions would be input to the planning stage. Though it would be possible to update the plan over time.

The purpose of tidal current prediction is to take away a known component of the movement of the water frame WRT the terrain frame. This means the INS can spend more effort on modeling the non-tidal effects.

You can’t usually measure current using a sensor on a moving vehicle. However, if you moor it you can measure using sensors and vessel heading – assuming say it’s heading into the current. This is how some current sensors/recorders work. A fish (small torpedo shape) is moored and measures the axial current velocity along its length.

You have significant non-tidal currents well offshore and they can be quite large values – probably much larger than any tidal component. Near the shore, it’s more likely tidal currents are larger. In intermediate depths, you get both.

Offshore non-tidal currents can meander and wax and wane seasonally but in the short term – weeks – will be pretty constant at any location. They can be estimated based on data and added as a ‘DC’ component to the transform relating the water inertial frame to the terrain inertial frame.

The other source of current changes is weather effects which operate on the scale of hours to days. They end up as noise in your transform and degrade its quality.

The purpose of tidal current prediction is to inform the water-terrain transform over periods of tens of minutes upwards to months.

One use case is maintaining station. You monitor the power used and you use the tidal current predictions (and low-frequency regional currents) to calculate how much power you should be using due to tides and known currents only. If you find you are presently using say 30% more power than expected, you run the tidal current prediction a few hours into the future and add 30% to that. If that exceeds your power budget you need to take action now to avert an accident.

This is very similar in concept to storm surge warning systems in coastal areas. The water height is measured and the tidal component is subtracted. You then forecast the tide-only levels in the future and add the residual to them. This gives a good approximation to the storm surge level.

And yes, I’m hoping to find a need, and even better find someone to fund the development of a module that can be used in route planning and/or as an input to an INS.

For general information. Placing a water current sensor in your work area for a period of two weeks, preferably a month, and capturing the speed and direction is sufficient to do quite accurate tidal analysis and prediction; and also extract any local non-tidal current data.

You can do this with a tethered vehicle, a dedicated sensor, or with more effort recording vehicle thrust and location and deducing the data values.

If you have data from adjacent sensors you can interpolate the values of tidal component phase and amplitude to a specific work location. You will also be able to calculate local non-tidal current data.