Aquadopp Real Time Clock

Is the offset between the RTC in the Aquadopp and the computer clock saved in a log file anywhere, or is it just displayed when you stop a deployment?  I have some data for which I would very much like that difference.  If the offset is not saved, is it possible to set up future deployments to save it?

Thanks,

Peter

Previously Sven Nylund wrote:

A customer asked:

Quote

I am analyzing the data we gathered last may under a rip current project and I have a question about the ouput files. In the output files the time is given up to seconds, where these vary from 0-59 seconds (Hz=1 in my experiments). I need to know what the accuracy is with this. Is the instrument measuring and recording at every second or rounds up to the nearest second? Can I get a better accuracy than the +/- 0.5 seconds I have right now? I need this to compare timelags between aquadopps deployed at the same time at different locations. We started recording while still conected to the computer (to set the internal clocks with GPS time) for each aquadopp so I don't know if the instrument averages at every second or at that time that the recorder got started which could not be an exact second.

The Real Time Clock (RTC) in the Aquadopp has a resolution of 1 second. The time stamp in the data is taken at the beginning of each measurement. This means that with 1Hz data the middle point in your data point is offset 0.5 second from the reported time.

When setting the RTC in the Aquadopp, the software sets it to the nearest second when you set it to the PC clock. Hence, the clock may be up to 0.5 second off. This is an offset though, so it is possible to correct for it. To find the offset value you need to use the stop deployment command. Before the actual deployment, start a deployment that you stop directly after. The software will then report the offset between the Aquadopp RTC and the PC clock in milliseconds. You can then use this value to offset your measurement data.

When you retrieve the instrument, the software will report the time difference again. If you make sure the PC clock to GPS time, you will then see how much the Aquadopp clock has drifted (typical value is 1 second/week). A first order correction of the drift in the Aquadopp would be to distribute the drift evenly throughout the measurement period. Provided that the temperature of the Aquadopp does not change too much during the measurement, the final time scale of you measurement will probably be quite good. " />'>

Best regards,

Sven Nylund
 

Previously Sven Nylund wrote:

A customer asked:

Quote

I am analyzing the data we gathered last may under a rip current project and I have a question about the ouput files. In the output files the time is given up to seconds, where these vary from 0-59 seconds (Hz=1 in my experiments). I need to know what the accuracy is with this. Is the instrument measuring and recording at every second or rounds up to the nearest second? Can I get a better accuracy than the +/- 0.5 seconds I have right now? I need this to compare timelags between aquadopps deployed at the same time at different locations. We started recording while still conected to the computer (to set the internal clocks with GPS time) for each aquadopp so I don't know if the instrument averages at every second or at that time that the recorder got started which could not be an exact second.

The Real Time Clock (RTC) in the Aquadopp has a resolution of 1 second. The time stamp in the data is taken at the beginning of each measurement. This means that with 1Hz data the middle point in your data point is offset 0.5 second from the reported time.

When setting the RTC in the Aquadopp, the software sets it to the nearest second when you set it to the PC clock. Hence, the clock may be up to 0.5 second off. This is an offset though, so it is possible to correct for it. To find the offset value you need to use the stop deployment command. Before the actual deployment, start a deployment that you stop directly after. The software will then report the offset between the Aquadopp RTC and the PC clock in milliseconds. You can then use this value to offset your measurement data.

When you retrieve the instrument, the software will report the time difference again. If you make sure the PC clock to GPS time, you will then see how much the Aquadopp clock has drifted (typical value is 1 second/week). A first order correction of the drift in the Aquadopp would be to distribute the drift evenly throughout the measurement period. Provided that the temperature of the Aquadopp does not change too much during the measurement, the final time scale of you measurement will probably be quite good. " />'>

Best regards,

Sven Nylund

Dear Sven,

I cannot get this method to work. Here is what happens:

After starting the vector (setting vector to PC time)  and then stopping the deployment (before it starts collecting data) I get the offset between the PC clock and the vector.
If I then start the vector again without resetting the clock,  and then stop it again before sampling starts it more often that not reports a different value for the offset. Surely the offset should be constant at least over the short period I do these tests.

Iain

Dear Iain,

I do agree. What are the numbers you get?

Best regards

Jonas Røstad