ADCP processing and CODAS processingΒΆ

Note

As of April 2013, this version of the documentation is no longer maintained; however, it is consistent with the last (now retired) version of CODAS processing that used Matlab. We no longer update or fix the Matlab processing code, but we will maintain the Matlab code that reads CODAS Matlab output. Although the notes refer to both Python and Matlab processing, none of the code here will be maintained. This (now retired) documentation and code will remain available for awhile longer.

Python processing code is actively maintained and developed, and CODAS Python processing is documented here.

ADCP processing, in general

There are at least four necessary processing steps for ADCP data which are performed by (or made possible) by the CODAS routines.

First, an ocean reference layer is used to remove the ship’s speed from the measured velocities. By assuming the ocean reference layer is relatively smooth, positions can be nudged to smooth the ship’s velocity, which directly results in the smooth reference layer velocity. (This was more important when fixes were rare or jumpy (such as with LORAN) or dithered (such as SA GPS signals prior to 2001).

Second, a GPS-derived heading source (such as Ashtech, POSMV, or Seapath) may provide a more accurate (though often less reliable) heading source than a gyro. Routines are in place for pingdata and UHDAS data to correct the gyro heading with the GPS-derived heading, using a quality-controlled difference in headings. An example is available for VmDAS data. Gyro headings may be reliable but they can vary with oscillations of several degrees over several hours, thus creating spurious fluctuations in the ocean velocity that resemble “eddies” , but which are soley the result of cross-track velocity errors (from the associated gyro heading errors).

Third, calibration routines are available to estimate the heading misalignment from either “bottom track” or “water track” data. Watertrack calibration routines use sudden accelerations (such as stopping and starting of the ship when doing station-work) to derive an estimate if the heading misalignment. For a ship travelling at 10 kts, a 1-degree heading error results in a 10 cm/s cross-track velocity error. It is critical that the misalignment be accounted for if one is to avoid cross-track biases in the velocities.

Fourth, it is crucial that bad data be edited out prior to use. It is best if the single-ping data can be edited prior to averaging (to screen out interference from other instruments, bubbles, and some kinds of underway bias). Once the data are averaged and the above steps are applied, it is still often necessary to further edit the data (eg. remove in-port data or velocities below the bottom). To some extent this can be automated but for final processing, a person must visually inspect all the averages from a dataset.


The CODAS database

CODAS (Common Ocean Data Access System) is not a heirarchical database; it is a portable, self-descriptive file format (in the spirit of netCDF), that was designed specifically for ADCP (and other oceanographic) data. For historical reasons it is stored as a collection of files. Because it is an organized body of information, it is referred to as a database.

The term CODAS processing refers to a suite of open-source programs for processing ADCP data. The CODAS processing suite of programs consists of C, Python, and Matlab programs that will run on Windows, Linux, SunOS, or Mac OSX, and can process pingdata from a Narrowband ADCP, data collected from a Broadband or Ocean Surveyor data by VmDAS, or data collected by any of those instruments using UHDAS (open source acquisition software that runs RDI ADCPs). Present work is focused on eliminating the need for Matlab.

CODAS processing can be used for data that have already been averaged (eg. pingdata, LTA files) or for single-ping data. In the latter case, routines are employed that extensively screen single-ping data prior to averaging. Under certain conditions, this may be necessary to avoid underway biases caused by bubbles or ice near the transducer, or acoustic interference from other instruments.