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OceanGlidersCommunity / DepthAverageCurrents_SOP

Depth Average Currents (DACs) Standard Operating Procedure (SOP)
https://oceangliderscommunity.github.io/DepthAverageCurrents_SOP/README.html
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Estimated position and DACs for yoyo diving #83

Open LewisDrysdale opened 2 years ago

LewisDrysdale commented 2 years ago

Seagliders: Reliable Lat/Lon position estimates require DACs, which are not calculated for yoyo dives hence the glider track is not realistic for these dives where there are strong currents.

rtodd-WHOI commented 2 years ago

This is not unique to a particular platform. DAC estimates can only be computed between successive GPS fixes. For multiple dive/climb cycles between surfacings, there can be only one DAC estimate.

A reasonable practice is to assign the same DAC estimate to each dive/climb cycle between surfacings. Any position estimate between those surfacings must be interpolated. Without other information about the vertical/temporal variability in currents between surfacings, a reasonable best practice would be assume that the current is constant between surfacings.

As the quantity we refer to as a 'depth-average' current estimate is truly a 'time- and path-average' current estimate between surfacings, special care must be taken when interpreting DAC estimates that result from successive dive/climb cycles over different depth ranges or with differences in glider behavior (e.g., faster/slower flight). There is language about this in the DMQC chapter that could be included in this longer best practices document.

LewisDrysdale commented 2 years ago

We follow this method by generating evenly spaced values between GPS fixes to correct the dive surface positions and then do the same between those positions to generate new estimates of subsurface position. We then calculate the depth average current across the YOYO dive sequence using the new surface fixes (i.e. same current speed for all dives in that sequence). A lot of our YOYO dives are across sloping seabed and also occur when the glider reaches a waypoint and the heading changes by 180 degrees, so I'm not sure how we are going to deal with them.

AnthonyBosse commented 2 years ago

I am reactivating this section, at the moment I believe there is no DAC real-time QC. From this discussion it clearly appears that checking whether the glider had a spend an evenly distributed time between the surface and the max diving depth. This could be a first RT test to flag bad DAC that cannot be actually considered as depth-average currents... For W yoyos dive, we loose precision as the integration time is longer. Also, importantly, more than assigning a point in space and time to DAC measurements, one should considered giving the underwater segment and diving time. It would be interesting to propose something along this line in the SOP.