[Acoustics] Delay in GPS data processing

[McBride, Daniel J MVR]

Francois--

We experienced this issue and found it to be related to our serial port
adapter.  We use a Socket Ruggedized Dual-Serial PCMCIA adapter (series #
8010-00077 C).  We now use our laptop's built-in serial port for GPS (GGA),
and one of the PCMCIA ports for the ADCP.  There doesn't seem to be a problem
with the ADCP data through the card--just with the GPS data.  I presume any
other external device (depth sounder, external heading, etc) coming in
through the PCMCIA card would show the same problem.

Dave Mueller has documented this phenomenon with newer Socket cards as well.
I should mention that we did not experience this problem with the cards until
recently--we used this setup in Windows 2000 without any problems for a few
years.  I suspect (but haven't bothered to test it) that Windows XP is the
source of the problem.

If anyone has drivers that get around this problem, please post them.

--Dan

p.s.  Below is Dave Mueller's original post on the topic (from 2/2/2005):

"I recently spent a day in the field trying to track down a problem several
people had reported to me. The problem manifests itself as an extreme delay
in GPS data used in WinRiver (up to 110 sec old). This causes the beginning
of transects to go in the wrong direction (based on GPS) and not following
bottom track at all. It can also result in short DMG. The discharges are
obviously wrong.

After testing on two computers (one Win98 and on XP Pro) and using three
different serial ports (internal, new Socket card, Keyspan 4-port USB RS-232
adapter) I reached the conclusion that there is an incompatibility between
the new Socket cards and WinRiver.

By splitting the data coming from the GPS unit and monitoring it with a
second computer, it was clear that the GPS unit (Ag132) was providing correct
data but that the data were being buffered in the computer running WinRiver.
The problem grows between transects. It appears that between transects when
you are not recording data the GPS data gets buffered and when you start
another transect the buffer is not flushed and you start using old GPS data.
This occurred using 5 Hz update rate and 9600 baud rate.
However, it only occurred when the GPS was fed through the Socket card. The
problem was minimized (delay of about 2 seconds) going to a higher baud rate
19.2K. This problem occurred on both computers and only occurred when the GPS
data was fed into the Socket card. Reversing the ADCP and GPS ports appeared
to fix the problem, as did using the Keyspan.

I also tried an earlier version of WinRiver but that did not eliminate the
problem.

I would like to know if those of you who have experienced this problem are
using a newer Socket card and if so, does the problem go away by switching
ports are using a different external serial interface adapter.

The new Socket cards have a bar code on the back with the numbers:
8510-00073 J
I do not have the version number of the driver used.

I am aware of other software with incompatibilities with the new Socket cards
and expect that card is the cause of many of the GPS issues that have
appeared in the last 6 months.

Thanks for your help.

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"



-----Original Message-----
From: acoustics-bounces at simon.er.usgs.gov
[mailto:acoustics-bounces at simon.er.usgs.gov] On Behalf Of Rainville,François
[CIS-WSB]
Sent: Monday, April 03, 2006 1:53 PM
To: acoustics at simon.er.usgs.gov
Subject: [Acoustics] Delay in GPS data processing

Hello all!

We have observed that GPS data is sometimes delayed and lagging with respect
to the ADCP data collected.  This can be observed as a strange path that does
not correspond well to the shape obtained by bottom track but which in fact
corresponds to events (velocities) of moments before. This delay is sometimes
quite small (by 1 second) and sometimes bigger (up to 15 s).  We have not yet
been able to determine any trend in the type of computer and communication
systems causing this to happen.  But this is definitly a problem we would
like to resolve.

- Did any of you also witness such problems while using GPS?

- If so, can you provide a brief description of the type of computer used,
communication ports (USB, PCMCIA, etc.), GPS receiver, output frequencies,
type of modem radio link if any used?



Many thanks for helping us with this issue!

Francois Rainville
Water Survey of Canada
Environment Canada
373 Sussex Drive, Ottawa, Ontario;  K1A 0H3
Phone: 613-992-1673;  Fax: 613-992-4288


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