[Acoustics] Compass calibration routine error.

Fri Apr 18 12:35:49 CDT 2008

I was able to do a compass calibration in the parking lot and the display 
went through the entire 360 degree rotation.  So, it looks the bridge 
railing is indeed the culprit.  The calibration indicated 3.1 degree of 
error and the subsequent evaluation yielded 1.7 degree of error.  I'll do 
an in-water compass check during the next 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

Lloyd T Brooks <ltbrooks at usgs.gov> 
Sent by: acoustics-bounces at simon.er.usgs.gov
04/17/2008 07:46 AM

To
Kevin Oberg <kaoberg at usgs.gov>
cc
acoustics-bounces at simon.er.usgs.gov, acoustics at simon.er.usgs.gov
Subject
Re: [Acoustics] Compass calibration routine error.

Kevin, Thanks for responding.  We tried the compass calibration after the 
moving bed test and before the transects.  It's definetly possible that 
the bridge railing is a greater magnetic force the the geologic pole.  The 
railing consists of 5-6 horizontal rails and is about 6.5 ft tall.  I plan 
to do a compass calibration/check by rotating the ADCP on a non-metalic 
cart int the parking lot.  At least that should tell me if the calibration 
routine will go through the entire 360 degree of arc.  To the best of my 
knowledge, I'm the first person to do a compass calibration with this 
ADCP. 
The station where we measured has a VERY stable control and the 
measurement ended up plotting about +0.2 off the rating.  Previous ADCP 
and current-meter measurements at this location all plotted on the rating. 

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 

Kevin Oberg <kaoberg at usgs.gov> 
Sent by: acoustics-bounces at simon.er.usgs.gov 
04/16/2008 12:34 PM 

To
acoustics at simon.er.usgs.gov 
cc

Subject
Re: [Acoustics] Compass calibration routine error.

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
>
>
>
>
>
>
>_______________________________________________
>Acoustics mailing list
>Acoustics at simon.er.usgs.gov
>http://simon.er.usgs.gov/mailman/listinfo/acoustics

----------

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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