Comparisons between 2015 and 2016

The NOAA ship Nancy Foster has 150kHz Ocean Surveyor ("OS150"), which was injured in the shipyard. We hypothesize that water has gotten into the transducer through the hole in the urethane or into the connector, resulting in a very weak outgoing signal, hence not much coming back. This creates or exacerbates an issue in narrowband mode, where the top 3 bins are giving biased data. The recent degradation of the temperature signal is ominous. It seems there is a trend towards failure.

The table below highlights a summary of differences between 2015 and 2016 OS150 narrowband mode; the comparison is in the same region of the ocean (Caribbean), similar time of year, same instrument settings (8m bins).

Watertrack Calibration

Automated calibrations are performed during CODAS processing of the data. Ocean velocities are calculated by adding ship velocity (speed and direction) to the horizontal measured velocities in earth coordinates. These are all 2-dimensional vectors. Watertrack calibration results in two numbers that are to be applied to the measured velocities -- phase (degrees; rotate the measured velocities by this amount), and amplitude (a scale factor, multiply the measured velocities by this number). When applied, these calibration values represent the phase and scale factor that must be applied to get the best consistency in ocean velocities when the shipis manuvering (undergone some acceleration, i.e. turning, or stop/start). Watertrack calibration is a noisy calculation and works best when the ocean currents are weak. In regions of strong currents, the spread is greater than in quiescent regions.

Interpretation:

Amplitude:

• if perfect, should be 1.00
• Amplitude for Ocean Surveyors is almost always slightly greater than 1.000:
  • a range of 1.003-1.004 is typical, but low
  • a range of 1.005-1.006 is not uncommon
  • 1.007 is a little high
  • 1.008-1.009 is uncommon
  • 1.01 and larger are alarming, usually indicating a problem

Phase:

• if perfect, should be 0.00

• water track calibration only works if the transducer angle is already close to correct (phase correction is within a few degrees of zero)

• to have confidence:

  • there should be at least 20 points
  • mean and median should agree
  • standar deviation should be low

• if the heading device is accurate and the water is quiet, a solid phase calibration will have a standard deviation of under 0.4;

• the phase should agree throughout the water column; it is a diagnostic that relates to the accuracy of the heading device and should be constant at all depths

Watertrack calibration summary:

Comments:

• Amplitude: Processed data show a very different calibrations between the top 3 bins and deeper water, with the shallow calibration of 1.027 being very large. The value in deeper water, 1.007-1.0077, is a little high but not a big concern.
• Phase: Processed data show a very different calibrations between the top 3 bins and deeper water (top = -0.35; deeper = -0.1). The values themselves are not alarming, but the disagreement of over 0.3 degrees is alarming. In addition, a high standard deviation in the deep water is a concern, but the higher value in shallow water indicates a problem.

Overall Summary:

We think the data below the top 3 bins is probably OK for NF1601-NF1602. The data in the top 3 bins should probably be discarded; applying the calibration values to those bins might work, but would probably not yield useful scientific data . The instrument should be sent in for repair as soon as practical, but until there is a replacement, all we can do is:

• watch for more signs of failure
• assess the data after the cruise to determine what might be trustworthy

In order to really monitor the instrument over the course of the season, (i.e. not just say "OK it's dead"), we need big chunks of data (eg. minimum of 12 hours at a time; "days" would be better) not just 2 hours when the ship leaves port. Monitoring the progress of this degradation will require multiple periods with 12+ hours of data collected when the ship is underway. If possible, collecting data when the ship is stopped and moving, manuvering, or otherwise turning, will increase the number of watertrack calibration values and increase our ability discern failure. If the ship is in shallow water, i.e. under 100m and steaming, then it is probably a good idea to turn on bottom tracking.

The following figures illustrate the scenario.

(1) Damage in shipyard:

(2) Signal return and range:

Two chunks of data are plotted (two hours from 2015 and 2016) from a similar region in the Caribbean. In the upper panel, the averaged RSSI (signal return) is plotted for each beam. Solid is 2015, dotted is 2016. The 2016 signal return is clearly much weaker. In the lower pair of panels, beam velocity from one beam is plotted to crudely indicate range.

(3) Temperature fails:

Data up through May 30 showed plausible temperatures. The ship was in port for 2 days, during which time no data were collected. When they left port, the temperatures were all over the map, with a variety of unrealistic values. These values are not pegged to a particular value however, but do vary.

(4) Compare os150bb and os150nb (early):

Data from this cruise were colelcted with 2-meter bins in broadband mode and 8-m bins in narrowband mode. This figure shows eastward ocean velocity ("u") for broadband and narrowband mode, in the top two panels. The third panel is the difference. The fourth and fifth panels are the northward ("v") velocity for broadband and narrowband mode, and the last panel is their difference.

Of note:

• top 3 bins of narrowband mode are "brighter" (darker colors), indicating higher values. This is most obvious in "v".
• the difference between broadband and narrowband mode is significant
• broadband only barely has the range to get below the top 3 bins of narrowband data, but there are hints that the biased upper 3 bins in narrowband mode may not be biased in broadband mode.

(5) Compare os150bb and os150nb (1mo. later):

This figure is the same as the previous one, but occured a month later. Now the range of broadband mode is small enough that we cannot compare the two modes below the bad part of the narrowband mode. The bad data in narrowband mode is even more obvious now, and the third bin is actually missing data.

(6) Bin 3 damage:

This figure shows the beam velocities for OS150 narrowband mode for the first fraction of the data in the previous figure. Note the missing bins in all beams, concentrated at bin 3.

(6) Survey of Eddy: top 3 bins (bad)

This figure shows the swirl of an eddy which the scientists were trying to sample. If the water really did what is depicted here, it would illustrate the currents in a sucking vortext, perhaps part of the Bermuda Triangle? This masks the real eddy (below).

(7) Survey of Eddy: deeper bins (better/good)

This looks more plausible.

Links to three raw data files