[Acoustics] GPS/ADCP questions

Wed May 10 16:41:22 CDT 2006

Chris and others who may have these conditions:

A review of the data file that you sent to me indicates that the problem you
describe is caused by rough water-surface conditions, excessive boat speed,
and using GPS.  I observed that you not only have considerable pitch and
roll (+/- 3 degrees), but also rapid changes in pitch and roll. These
changes are likely reflected in both the bottom track data and the GPS data;
however, the effect in the bottom track data is likely more in synch with
the water track and the effects of the pitch and roll on GPS velocities are
not in synch with the water track data. This will result in noisier water
velocities when using GPS, which is what can be seen in the attached file
showing water and boat speed time series plots for DGPS and BT.

When using GPS you need to drive the boat as SLOW as you reasonably can.
Any compass/GPS errors are a direct function of the ratio of boat speed to
water speed. In the data file you sent me, the boat speed is more than
DOUBLE the water speed. It is imperative in these conditions to SLOW DOWN to
improve the data.

Pitch and roll will also affect the performance of the compass, and result
in compass errors which will translate into velocity errors that are a
direct function of the ratio of boat speed to water speed.

I hope this helps.  If you have questions, please give me a call or reply by
email.

David S. Mueller

U.S. Geological Survey

Office of Surface Water

9818 Bluegrass Parkway

Louisville, KY 40299

dmueller at usgs.gov

Voice: 502-493-1935

Fax: 502-493-1909

Mobile:502-558-5907

  _____  

From: acoustics-bounces at simon.er.usgs.gov
[mailto:acoustics-bounces at simon.er.usgs.gov] On Behalf Of Christopher A
Curran
Sent: Wednesday, May 10, 2006 12:52 PM
To: acoustics at simon.er.usgs.gov
Subject: [Acoustics] GPS/ADCP questions

Hello, 

1)  We did some ADCP meas. recently with an Ag132 GPS that shows vertical
banding in the WinRiver velocity magnitude contour plot when GGA is used as
reference, but when Bottom Track is toggled as reference, the bands
dissappear and the transects looks much cleaner. I was using a usb-serial
adapter for GPS, a 5Hz position rate and a 9600 baud on the NEMA output.
The transects were done in saltwater bay during a tidal change and were
long,10-15 min. each. The GPS coordinates at start and finish agreed with a
separate handheld unit. The compass calibration on the 1200KHz RioGrande was
done correctly, but the unit is about 6 yrs old now. Any ideas what's
causing that apparent velocity banding w/ GGA plot? 

2) What's the best ADCP baud rate to use with GPS? 

3) What's the best GPS baud rate into laptop (Ag132, output port A for NEMA,
I think 38.4K is max for the unit)? 

4) Do I want to use the fast output option, 10Hz position rate? 

Thanks for your answers. 

Chris Curran
U.S. Geological Survey
WA Water Science Center
Tacoma, WA 
(253) 428-3600  x2614 
Chris Curran
U.S. Geological Survey
WA Water Science Center
Tacoma, WA 
(253) 428-3600  x2614

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://simon.er.usgs.gov/pipermail/acoustics/attachments/20060510/f91c3db7/attachment-0001.html