Random glitches and problems in the upper bins
6.2.5 Apply Editing Results to the Database
The nal task of editing is applying the results to the ADCP database. At this stage, we assume that you have gone through the entire editing session and have the *.asc les (Section 6.2). The rst program to run is
badbinpath to database badbin.asc
where path to the database could look like ../adcpdb/ademo, for example. This program simply sets the PROFILE_FLAGS variable in the database to indicate that particular bins have been tagged as bad. The original velocity values remain intact.
During later data access with CODAS programsadcpsector profstat, the user can specify whether to consider these tags during access by using theFLAG_MASKoption.
By default, this option is set to ALL_BITS, meaning only data for bins at which
PROFILE_FLAGS are zero will be used. Use showdb, option 16 (GET DATA), variable ID 34 (PROFILE_FLAGS), to see the eects of running badbin on the database.
If there are entire pro les that need to get thrown out you need to run:
dbupdatepath to database badprf.asc
51
0 100 200 300 400 500 600 700
-60 -50 -40 -30 -20 -10 0
Bin Number
../adcpdb/a9212 AMPLITUDE(offset = 10)
Time: 20.71 to 20.89 B.P.: 47,70 to 48,47 Lon.: -75.01 to -74.17
Lat.: -53.07 to -52.86
Figure 18: Bottom reection (*) depicted by sharp maximumin the Amplitudesignal.
Note how the signal strength remains high at the maximum amplitude as the bottom deepens.
52
0 100 200 300 400 500 600 700
-60 -50 -40 -30 -20 -10 0
Bin Number
../adcpdb/a9212 AMPLITUDE(offset = 10)
Time: 13.86 to 14.03 B.P.: 40,285 to 41,22 Lon.: -87.95 to -87.96
Lat.: -61.97 to -62.35
Figure 19: Scattering layer (*) depicted by sharp maximum in the Amplitude signal.
This is a particularly dense layer. The ship was on station and peaks could be seen in the U and V velocities at about the same level. Since the ship was in deep water and since the U and V signals look OK below this level, it is determined to be a scattering layer. Another indication is that the strength of the amplitude maximum varies.
53
0 50 100 150 200 250 300 350 400
-60 -50 -40 -30 -20 -10 0
Bin Number
../adcpdb/a9212 AMPLITUDE(offset = 10)
Time: 21.41 to 21.46 B.P.: 48,197 to 48,211 Lon.: -70.82 to -70.91
Lat.: -53.29 to -53.17
Figure 20: Bottom reection (*) depicted by sharp maximumin the Amplitude signal as the ship enters port, but when the depth becomes very shallow (less than about 50 m), the amplitude signal gets very smooth. In this case, the entire pro les in the shallow water were edited out as indicated with the x's.
54
This sets the ACCESS_VARIABLES.1_last_good_bin to -1 to indicate that the entire pro le is not to be used during subsequent access.
The next group of programs are used for establishing the bottom. First run:
dbupdatepath to database bottom.asc
This sets the database variable ANCILLARY_2.max_amp_bin to the bin indicated in the le. Note that it does NOT set ACCESS_VARIABLES. last_good_bin, which access programs likeadcpsectuse during retrieval to determine the \good" portion of a pro le (in addition to other editing criteria, of course, like percent good).
And to spread the bottom signal out among adjacent pro les, run:
botmpas3path to database
Again, use showdb option 16, variable ID 38 and 39 to see the eect of botm-pas3 on the database. The program updates ANCILLARY_2.last_good_bin and
ACCESS_VARIABLES.last_good_binas needed.
Finally, the last step, last_85, in bottom agging is optional. During plotting, you should have been able to determine whether you want to do this or not. This program raises the bin of maximum amplitude (i.e. the bottom) by 15% to account for an error source due to the geometry of the beams relative to the bottom. To run this program, enter:
last_85path to database
If you care to check all the eects of this editing at this point, you can run through the stagger plots one more time, this time setting all agging criteria in setup.mto
], except for the access criteriaPGOOD_THRESHOLD and USE_LGB.SetPLOTBAD=1and
EDIT_MODE=0. Just plot uvfrom beginning to end to check that you haven't missed anything.
Discussion of editing of xes will be presented in the next Chapter.
7 Calibration
The basic methods of calibration are explained by 3] and 4]. In summary, there are two methods: 1) comparing the ship's displacement measured from bottom tracking with that determinedfrom navigation data, and 2) comparing the acceleration relative to the water, measured with the ADCP, to the acceleration over the ground, calculated from navigation. These two methods should give identical results for the transducer orientation (relative to the gyro compass), but they can give scale factors that dier by something like 0.5%. Therefore the bottom track method is a useful supplement to the water track method, but cannot replace it. For both methods, the quality of the
55 satellite xes and the gyrocompass should be examined, and if necessary, corrected prior to the calibration process.
Dierent authors have chosen various de nitions of the calibration parameters.
We have selected a convention which we nd relatively easy to visualize and use.
Express the velocity vector (uv) as a complex number U = u+iv and let subscripts u and c indicate uncorrected and corrected velocities, respectively. The calibration correction is then
Uc =Aei( =180)Uu:
A is the amplitude factor, a dimensionless number typically in the range 0.98{1.02, and is the counterclockwise angle in degrees (or misalignment angle) from the gyro-compass forward axis (which should be aligned with the ship's keel) to the transducer forward axis. When the ship is underway, errors in A and cause errors in the cal-culated absolute velocity that are proportional to the ship's speed. Expressed as a percentage of the ship's speed, a 0.6 error in causes a 1% error in the athwartships component of velocity, and a 1% error inA causes an error of the same magnitude in the forward component.
In the ideal case, calibration is done in two steps. First, apply a correction to the gyrocompass using GPS heading data. Then use water track and/or bottom track methods to calculate the net transducer oset.
In general, we start o with trying to calculate the transducer oset directly, mainly for benchmark purposes. If the cruise track was not especially complicated (not too many turns no north-south speed changes), and the gyrocompass compen-sators were kept up-to-date, one may be able to nd a good enough estimate of the transducer oset at this point, and not have to perform the gyro correction.