SADCP Report (Lisa Beal)

A new Shipboard Acoustic Doppler Current Profiler (SADCP) was installed on NOAA Ship Ronald H. Brown during the last dry dock period in July. Installation was completed by RDInstruments field engineer Daniel Doherty (cell: 858 414 2891, rdifs@rdinstruments.com). The instrument is a 75 kHz Ocean Surveyor (OS75), and communication/configuration is through RDInstruments software interface VmDAS. The Survey Technician aboard (Jonathan Shannahoff) was not sure of the existing set up of the instrument, including fundamentals such as serial inputs to the deck unit and acquisition computer, and was not familiar with VmDAS. Hence, the configuration was determined (with the help of information from Jules Hummon at the University of Hawaii via email) by examining physical feeds at the back of the deck unit, as well as data output files *.N1R and *.N2R and log file *.VMO. It was found that gyrocompass was configured as a direct snychro feed, therefore acting as primary heading device, as desired. In addition there were two serial data inputs: GPS and Seapath (attitude) data, for navigation and secondary heading, respectively. 

Given the Schuler oscillations and heading-related errors of a gyrocompass, it is established practice to obtain additional, more accurate heading data in order to produce underway SADCP ocean velocities of good quality (+/- 2 cm/s). Such heading data is often obtained from a GPS antenna array, such as Seapath or Ashtech. However, the Seapath installation on the Ronald H Brown has a multi-year history of spikey and intermittent measurements (J. Hummon, pers. comm.). Bad heading causes uncertain calibrations of the system, as well as unrealistic ocean velocities, such as strong cross-track jets. A new gyro-based heading device (MAHRS) with accelerometers to remove oscillatory behaviours is now available aboard the Ronald H Brown. The quality of the MAHRS data was untested prior to our cruise. 

Engineering paperwork from the initial installation (provided by J. Shannahoff) showed a preliminary heading misalignment of 22.7 degrees. VmDAS was set up for the cruise according to University of Hawaii (UH) recommendations in the online file: onetime_setup.txt.  Format of output data was set at 60 second-averaging for STA files and 300 second-averaging for LTA files. About one hour out of port the SADCP was deployed in bottom-tracking mode by loading command file: beal_rb_bt.txt (see Appendix) under the "ADCP Setup" tab  in VmDAS and specifying an ensemble time of 3 seconds . For bottom-tracking mode broadband pings are optimum, with bin lengths of 8 m. Once the sea bottom reached more than 1000 m depth acquisition was halted and the SADCP redeployed in water-track mode, using command file beal_rb_nb16.txt and 2 second ensembles. For water-track profiles maximum range is preferential, so the instrument was deployed in narrowband mode with 16 m bin lengths.

Over the next 24 hours the instrument was halted and redeployed a number of times after it was noticed that the VmDAS real time display was showing large cross-track velocities whilst underway: an obvious sign of a bad misalignment angle. After calculating that the phase error was order 20 degrees (approximately 5 knots of cross-track current whilst steaming at 13 knots) various remedies were tried, such as setting the misalignment angle to 45 degrees, then zero degrees - neither of which made any difference. Finally, it was deduced that in VmDAS under tab "Transform" BOTH Tilt Correction AND Heading Correction must be disabled, in order that the misalignment angle is read from the command file (EA02270) and not from the VmDAS gui (even if the latter is left blank). In order to avoid confusion or ambiguity in the future, it was decided to enter the heading correction in BOTH the VmDAS gui and the command file.

CM examined the first 24 hours or so of heading data, making a statistical comparison between gyro, Seapath and the new MAHRS. The preliminary results suggested that the MAHRS was producing continuous headings of comparable accuracy to Seapath. Therefore, it was decided to swap the secondary heading input used for SADCP processing from Seapath to MAHRS (in light of past problems with Seapath). This was achieved by switching the serial port for RPH Setup under VmDAS tab "Communications" from COM5 (Seapath) to COM4 (MAHRS). Note also, COM1 is ADCP, COM3 is navigation data, and gyrocompass is set as a synchro-feed directly to the deck unit (for real-time ocean currents of moderate quality). A more complete analysis of heading data was made during the cruise and results can be seen in Section: Heading Comparison Report.

Final set-up was achieved on 13th September at 20:37 GMT (files AB0509_009_***.LTA), many hours before we reached our working Section. Water-track data was collected continuously (and successfully) for the remainder of the cruise. Processing and calibration of this data will occur after the cruise.

Appendix

(1)	OS75 Bottom-Track Command File:  beal_rb_bt.txt
Based on os75_bt.txt at http://moli.soest.hawaii.edu/docs/VmDAS_ops/

;-----------------------------------------------------------------------------\
; ADCP Command File for use with VmDas software.
; 
; ADCP type:     75 Khz Ocean Surveyor
; Setup name:    RonaldHBrown
; Setup type:    High Resolution, short range profile(broadband)
; 
; NOTE:  Any line beginning with a semicolon in the first 
;        column is treated as a comment and is ignored by 
;        the VmDas software.
; 
; Modified Last: 11Sept2005 by Lisa Beal (lbeal@rsmas.miami.edu)
;----------------------------------------------------------------------------/

; Restore factory default settings in the ADCP
cr1

; set the data collection baud rate to 38400 bps, 
; no parity, one stop bit, 8 data bits
; NOTE:  VmDas sends baud rate change command after all other commands in 
; this file, so that it is not made permanent by a CK command.
cb611

; Set for broadband single-ping profile mode (WP), forty (WN) 8 meter bins (WS), 
; 8 meter blanking distance (WF), 390 cm/s ambiguity vel (WV)
NP0
WP00001
WN0080
WS800
WF0800
WV390

; Disable single-ping bottom track (BP), 
; Set maximum bottom search depth to 1200 meters (BX)
BP001
BX12000

; output velocity, correlation, echo intensity, percent good
WD111100000

; One and a half seconds between bottom and water pings
TP000150

; Two seconds between ensembles
; Since VmDas uses manual pinging, TE is ignored by the ADCP.
; You must set the time between ensemble in the VmDas Communication options
TE00000300

; Set to calculate speed-of-sound, no depth sensor, external synchro heading 
; sensor, no pitch or roll being used, no salinity sensor, use internal transducer 
; temperature sensor
EZ1020001

; Output beam data (rotations are done in software)
EX00000

; Set transducer misalignment (hundredths of degrees)
EA02270

; Set transducer depth (decimeters)
ED00054

; Set Salinity (ppt)
ES35

; save this setup to non-volatile memory in the ADCP
CK

(2)	OS75 Water-Track Command File: beal_rb_nb16.txt
Based on os75_nb16.txt at http://moli.soest.hawaii.edu/VmDAS_ops/

;-----------------------------------------------------------------------------\
; ADCP Command File for use with VmDas software.
; 
; ADCP type:     75 Khz Ocean Surveyor
; Setup name:    Lisa Beal's RonaldHBrown Deep Default (> 1000 m)
; Setup type:    Low Resolution, long range profile (narrowband)
; 
; NOTE:  Any line beginning with a semicolon in the first 
;        column is treated as a comment and is ignored by 
;        the VmDas software.
;
; Modified Last: 11Sept2005 by Lisa Beal (lbeal@rsmas.miami.edu)
;----------------------------------------------------------------------------/

; Restore factory default settings in the ADCP
cr1

; set the data collection baud rate to 38400 bps, 
; no parity, one stop bit, 8 data bits
; NOTE:  VmDas sends baud rate change command after all other commands in 
; this file, so that it is not made permanent by a CK command.
cb611

; Set for narrowband single-ping profile mode (NP), fifty (NN) 16 meter bins (NS), 
; 8 meter blanking distance (NF), 390 cm/s ambiguity vel (WV)
NP00001
WP0
NN050
NS1600
NF0800
;WV390

; Disable single-ping bottom track (BP), 
; Set maximum bottom search depth to 1200 meters (BX)
BP000
BX12000

; output velocity, correlation, echo intensity, percent good
ND111100000

; One and a half seconds between bottom and water pings
TP000150

; Seconds between ensembles
; Since VmDas uses manual pinging, TE is ignored by the ADCP.
; You must set the time between ensemble in the VmDas Communication options
TE00000150

; Set to calculate speed-of-sound, no depth sensor, external synchro heading 
; sensor, no pitch or roll being used, no salinity sensor, use internal transducer 
; temperature sensor
EZ1020001

; Output beam data (rotations are done in software)
EX00000

; Set transducer misalignment (hundredths of degrees)
EA02270

; Set transducer depth (decimeters)
ED00054

; Set Salinity (ppt)
ES35

; save this setup to non-volatile memory in the ADCP
CK

LADCP Report (Lisa Beal, Jon Molina, and Humberto Guarin)

SUMMARY

In brief, for the Deep Western Boundary Current (DWBC) section, LADCP set up and operations were conducted as during last year's MOCHA2 cruise on the Knorr in May 2005. That is, a "hybrid" system, consisting of an up-looking 300 kHz Workhorse ADCP (WH300) and a down-looking 150 kHz Broadband ADCP (BB150) running on one battery pack. Subsequently, a dual 300 kHz system was used for the North West Providence Channel and the two Florida Current sections, owing to failure of the broadband instrument on station 26. The timing of the failure at least was deemed fortuitous, since a dual 300 kHz system is capable of collecting good data down to depths of about 1500 m and the maximum depth over the last three sections is no more than 800 m. Lacking the BB150 data in the DWBC would have been a much more serious set back. Instead, we consider the LADCP data set quality minimally effected by the instrument failure.

SET-UP, PROCESSING, and PROBLEMS

The LADCP acquisition computer and power supply was set up in the aft wet lab, where it was only a short distance to run out the sea cables (one for power, and two for communication with each instrument) to the package. During set up it was noted that where the BB150 communications cable has been spliced with non-sea cable and that cable has been exposed to sunshine and salt water on deck, the cable has become brittle. Recommend that a new sea cable is purchased. In addition, the BB150 communication end-cap or dummy plug was found to be leaking badly on every cast. After station 4 a spare dummy was located and the old one was discarded. Recommend purchase of a new back-up sea-cable dummy (eight pin).

First pass processing WITH navigation data was carried out on the data acquisition PC in the wet lab immediately following every cast. Ten second GPS data was logged directly onto the PC via network cable. Operators used the LADCP-derived depth, plus the engineering plots, and warning messages to assess the quality of the data download before erasing the LADCP recorders.

On stations 3 and 4 a warning message "Large compass deviation" was observed after first-pass processing. LMB had not seen such an error before, but JM had seen the message on a previous Windward Passage cruise. On looking at the heading differences between instruments (Figure 6) it was clear that some spurious heading data is present on each of these two casts, with differences as large as 48 degrees between up-looker and down-looker compasses. It is not clear what the errors are attributable to and the heading differences returned to normal for the remainder of the cruise. Stations 3 and 4 were processed twice: once using only up-looker heading, and again using only down-looker, in order to examine the effect of heading error on the super ensemble. The differences between the profiles were small, within velocity error margins, and top-to-bottom velocities were not altered.

On stations 24 and 25, at the western end of the DWBC section, beam 2 of the BB150 was weakening rapidly, until it was reported broken on station 26 and 27. It was not deployed on station 28, and finally it was taken off the package and replaced with a down-looking WH300 during the transit to NW Providence Channel. All remaining stations were occupied with a dual 300 kHz system. The 300 kHz down-looker showed signs of a weak beam on stations 38, and 39. Up to 30% of the velocities on these stations were made using 3-beam solutions. This instrument should be further investigated at the lab.

Version 9 of Visbeck's software (edits, modifications, and wrappers by Andreas Thurnherr, released 2004) was used to process all the LADCP data following the first pass. Some advantages with the latest version of the software are: previous ping editing for 150 kHz data, separate weightings of first bin for down- and up-looker, and better loading and merging of CTD and LADCP data. Discrepancies between v9 and v8 solutions were investigated prior to this cruise by LMB: default v9 solutions tend to reduce velocity maxima and minima and increase high-wavenumber shear. To reproduce profiles with similar high-wavenumber energy to v8 solutions without reducing low-wavenumber shear, v9 is run with ps.smoofac = 2 and ps.smallfac=[1,0]. 

From preliminary processing it is noted that, as seen during previous Abaco sections, for casts of over 5000 m depth (particularly stations 6 through 12) the LADCP technique is stretched to its limit by the low-scatter environment, resulting in accumulative low-mode shear errors, and significant bias errors. Shear solutions produce bad X profiles and often run-away shears at depth. Inverse solutions fair much better, usually reproducing realistic bottom velocities without using a bottom-track constraint. But it is noted that changes to smoothing factors smoofac and smallfac can cause significant changes in the structure of the super ensemble, which is of concern. Second pass processing was completed during the cruise, where CTD data is used to refine the speed of sound through the water column and improve depths, and ship drift (GPS) and bottom track (from water track pings, BT) data are used as constraints in the inverse.

RESULTS

The DWBC was found very close to the continental slope, with maximum velocities over 30 cm/s on station 23 at 76.5 W (figure ). Above it, the Antilles Current has a subsurface core at 400 m depth, in 1000 m of water. Offshore, there is evidence of strongly barotropic recirculations or eddies. However, the quality of the LADCP data, particularly between 2000 m and 4500 m depth in this region, should be treated with caution. The dual 300 kHz system produced good data in the Florida Current at 27 N, despite two warnings of beam weakening in the down-looker. The core of the current was seen at the western edge of the section, in waters of depth less than 200 m. There appeared to be the remnant of a more centrally-located velocity core below 50 m depth.

Heading Comparison Report (Lisa Beal and Christopher Meinen)
 
CM conducted a preliminary investigation of the statistical comparisons between gyro, Seapath, and MAHRS headings using 19 hours of ten-second data. This quick analysis was designed to help us determine whether the new MAHRS headings would be sufficiently accurate and reliable to use as the secondary (corrective) heading device to the OS75 SADCP data. It was found that the MAHRS data was continuous with few outliers, and that the standard deviation of MAHRS-gyro was similar to Seapath-gyro. This was thought enough evidence (together with previous experience of bad Seapath) to set up the SADCP to use MAHRS heading and not Seapath.

Subsequent to this initial analysis LMB began collecting 1-second gyro, Seapath, and MAHRS heading, logged with time and position, from the shipboard SCS to make a more statistically robust comparison. It was found that the standard deviation of the difference between Seapath and MAHRS is three times greater over a period with a lot of manoeuvering (e.g. station section), than over a period with constant heading (e.g. during a transit).  This was due to bad headings from the Seapath tending to follow large heading changes (but not always). Over the entire cruise, the Seapath recorded bad data 15 - 20% of the time (higher for higher averaging intervals), with a maximum interval of good data of only 2 hours for 10-second data (up to 12 hours using 5-minute averages). This illustrates how difficult it has been to get a good water-track calibration for the SADCP with Ronald Brown Seapath data - one needs to chop out manoeuvres, which is exactly the kind of data one needs to use for tweaking the transducer misalignment angle. In addition, two hour chunks of data are not enough to produce robust calibrations of either phase (resulting from misalignment) or amplitude (resulting from changes in sound speed). 

Comparing the auto-covariances of MAHRS-gyro, Seapath-gyro, and Seapath-MAHRS illustrates that the Schuler oscillations present in gyro headings (at lags of +/-80 minutes) are damped by the other instruments, but are not non-existent in the MAHRS. Schuler oscillations appear to be about 5 times smaller as a result of the accelerometer-assisted damping in the MAHRS gyro.

In order to compare MAHRS and Seapath data when both are operating, bad Seapath data was removed using a kind of bootstrap method, whereby outliers with over 1 degree difference (Seapath-MAHRS) were initially removed, before calculating a standard deviation. Subsequently, data more than two thirds the standard deviation from the mean difference of this "de-spiked" data were removed and a statistical analysis conducted on the remaining "good" data. The standard deviation of good Seapath-MAHRS is 0.11 degrees (5.6 degrees with no editing), compared to a standard deviation of 2.2 degrees for MAHRS-gyro.

In conclusion, the MAHRS varies very closely with good Seapath, indicating that the two instruments are capable of similar accuracy. The MAHRS shows no heading-dependent error, like the gyro, and Schuler oscillations are heavily damped. The MAHRS is (so far) one hundred percent reliable.