I am trying to design something that tracks sea level height at a buoy location.
I would like to have an enclosed box attach to a buoy and have the internals allow the recording of the vertical displacement of the buoy. If the vertical coordinate is Z then I do not, for example, care to record or track X or Y displacement although the buoy may make slight movements in these X, Y directions. I only need to track Up/Down movement of buoy to give me an indication of water level.
Can someone advise what the best way to do this would be? Can I accomplish this with an IMU? How many degrees of freedom would I need. Or a force sensor something?
Yeah I was wondering if the low frequency / slow movement would be an issue. Sonar is an option, but I would like to try without so that the device could be applied also to ground level change if needed. Ie seasonal ground heave, which would have the device sit directly on the ground.
Is there a way with IMU or accelerometer to get away from the error accumulation? Multiple accelerometers and averaging?
How tightly is the buoy tied to the bottom ? If the water level rises, does the buoy have enough slack to rise with it or does the water rise around the buoy ?
Start by searching use of IMUs and read up on to techniques used and issues with accumulated errors.
The basic issue is measurement from an accelerometer are taken periodically. One then uses the acceleration of that measurement and ‘assumes’ it was constant since the last measurement. This is ‘integrated’ (calculus operation) to obtain a displacement since the last measurement and added (+ or -) to the last position.
The problem is the assumption that the acceleration is constant between measurements. If it is not (very likely) then there is a position error on each measurement that accumulates.
I don’t see how any IMU you can afford is going to work. You mentioned seasonal changes, implying months of measurement time ? RTK GPS would be 1 method, sonar or a dropping line (think old time mariners) would be 2 more.
Another thought is to put a pressure sensor on the bottom under the buoy. Then, by wire or acoustics, relay bottom pressure to a recording box on the buoy. Depth can be known from (bottom pressure-atmospheric pressure)/density of seawater. How deep is the sea going to be by the buoy(s) ?
That is a detailed document showing the design of an MRU (motion reference unit), specifically the measurement of heave. Its a sensor specifically designed to measure vertical displacement with high resolution. It also costs $30,000…
The mass on the end of a line might work for a week before marine life would build up. I have seen anchor chain so built up with grown as to render it stiff.
GPS is out. Vertical height is notoriously bad.
Baro pressure is out as day to day changes would be orders of magnitude larger than the signal.
An IMU would be near hopeless for long duration measurements. Any IMU has a drift rate due to time and temperature you would have to zero out. A MEMS based IMU may keep its accuracy for a few minutes. A ring laser based IMU perhaps for an hour. But no IMU that I am aware of could measure to a few feet over days of operation. So you would need a way to reset the zero every hour in the best case.
Sea bed floor pressure has a chance. Problem here is cabling to the surface. You could run it on batteries and send the data up accousticaly. When the batteries get low, have it cut it’s own anchor and float to the surface to be replaced. Copper cabling has all the problems of marine grown and servicing problems.
Sonar/laser might work IF the sea bed floor was very flat. If not, as the buoy sifted horizontal position, it would report different heights.
How far out to sea is this buoy?
Could you use and automated transit system to survey the altitude from land?
I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. [Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.](U.S. Geological Survey - Global Positioning Application and Practice)
Plain GPS is indeed very poor in height even with excelent horizontal accuracy. And I knew that differential GPS was even better in the horizontal plane. What I did not realize was that they have been able to use DGPS to overcome the altitude problem. Now the page you linked to seems to indicate that you can only get those centimeter accuracies if your base station is close by (makes sense). If the OP is close enough to a base station for the accuracy needed, then that would be a solution. Of course, it does come with a price tag!
lyndon:
I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. [Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.[/quote]
I think you mean RTK GPS (real time kinematic; Google that). Surveyors use it. Sub-cm accuracy with long integration times. But not that good on elevation.
Differential GPS (DGPS) which is in-built in most modern GPS such as the Garmin OEM GPS 18 products (hocky-puck sized) … Differential correction data is sent by geo-sync satellites and received by the DGPS receivers. DGPS corrects mainly for atmospheric errors (ionosphere). Coorrection data is also sent via the Internet and other radio means. Again, the elevation accuracy is not as good as x, y which can be less than 5m RMS at about 2 sigma.](U.S. Geological Survey - Global Positioning Application and Practice)
RTK GPS is a [type of differential GPS. I haven’t used it, but the table I linked to shows centimeter altitude accuracy for roving RTK and sub-centimeter for stationary RTK. However, I should have been more specific because there are other forms of DGPS that aren’t as accurate. I think what you are describing on the GPS18 is [WAAS, although I just learned that there is a type of DGPS that’s not as accurate as RTK which uses satellite-transmitted correction signals.
I wish WAAS was as good as DGPS because I’m trying to find the lines on my property and I lost my copy of the surveying document that would let me do it by hand. With my WAAS receiver I can get close, but nowhere near as close as I need to be to find the property markers.
stevech:
lyndon:
I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. [Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.[/quote]
I think you mean RTK GPS (real time kinematic; Google that). Surveyors use it. Sub-cm accuracy with long integration times. But not that good on elevation.
Differential GPS (DGPS) which is in-built in most modern GPS such as the Garmin OEM GPS 18 products (hocky-puck sized) … Differential correction data is sent by geo-sync satellites and received by the DGPS receivers. DGPS corrects mainly for atmospheric errors (ionosphere). Coorrection data is also sent via the Internet and other radio means. Again, the elevation accuracy is not as good as x, y which can be less than 5m RMS at about 2 sigma.[/quote]](U.S. Geological Survey - Global Positioning Application and Practice)
lyndon:
I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. [Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.[/quote] low cm and sub-cm accuracy with GPS requires RTK GPS, not DGPS.