[Acoustics] Compass calibration routine error.
Kevin Oberg
kaoberg at usgs.gov
Wed Apr 16 11:34:42 CDT 2008
Tim:
Based on what you have told us and a review of your data, I think
your problems are probably caused by some strong magnetic field at
the site. You are close to a bridge, which may be the source of the
magnetic interference. I you look at the shiptrack plots for
reciprocal transects, the odd numbered transects have primarily
east-west orientation, but if you look at the even numbered
transects, the orientation southeast-northwest orientation. The
Course Made Goods (CMG's) for each transect are as follows:
# CMG WDirection
1 89 (269) 192
2 296 208
3 92 (272) 191
4 297 208
The numbers in parentheses is the heading plus 180 degrees. Based on
this, you can see that the difference between the odd CMG's and the
even CMG's is about 26 degrees. Assuming you were using a tethered
boat (and maybe even a rope/pulley system?), the reciprocal headings
should be approx. 180 degrees apart. The difference between the
water directions for reciprocal courses is approx. 16 degrees. I am
not sure why the differences in the CMG and Water Direction are not
similar, but perhaps there is enough of a moving bed? There is
indication of a slight moving bed based on your stationary moving bed
test (SMBT), and this could cause reciprocal shiptracks to be other
than 180 apart. However, the SMBT only shows a moving bed of .02
ft/s or < 0.5 % of mean flow speed at THAT location.
Perhaps during the compass calibrations the compass is locking onto
some magnetic field that is stronger than magnetic north. It could
be that as you spin your boat, that because it 'locks' onto
something, that it is unable to complete the calibration.
That is the best I can offer at the moment. I assume that you don't
have compass cal problems at other sites/setups?
Kevin
At 4/15/2008 09:13 AM, Lloyd T Brooks wrote:
>We tried doing a compass calibration in WR II 2.01. The ADCP is an
>RDI 1200KHz Rio running firmware version 10.16. While rotating the
>ADCP , the display in the dialog indicated the direction progression
>as follows: N --> NE --> E --> SE --> S --> SE --> E --> NE --> N.
>Eventhough the boat was rotated 360 degrees, the ** never went
>through the western compass bearings. We tried the calibration
>twice again while rotating the boat first clockwise and then
>counter-clockwize to no avail. The "line of sight" between the
>radios may have been during the first test, but the "line of sight"
>was clear during the two subsequent tests. The bridge 20 ft
>upstream has a concrete bed and iron railings.
>Anyone ever see this problem before. The ADCP test indicated no
>errors. I've included all of the files for the measurement.
>Thanks, Tim
>
>
>-------------------------------
>L. Timothy Brooks
>U. S. Geological Survey, WRD
>425 Jordan Rd
>Troy, NY 12180-8349
>Phone# 518/285-5644
>Fax# 518/285/5601
>ltbrooks at usgs.gov
>
>
>
>
>
>
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----------
Kevin Oberg
USGS - Office of Surface Water
Voice: 217.344.0037 extension 3004
Cell: 217.840.9739
http://hydroacoustics.usgs.gov/
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