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2/22/2017 3:25 am  #1


Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

As of now in Expedition (as it looks to me), SET and DRIFT are averaged separately as elements of a current vector of length and angle (Polar coordinate system).
I think it would be better to average current in a Cartesian coordinate system (as Current-x, Current-y) and then present the result as in the usual polar way.
One advantage would be that when averaging in the cartesian system you can average out measurement noise to 0.

Example:
You sail an area with 0.1 kn DRIFT, 180° SET real current. You have a current measurement noise (eg. due to GPS position noise) vector of 0,2kn average in random directions. When averaging as presently in a polar system, you get a measured current of something like DRIFT 0,21 kn and 180° SET.
If you do averaging in a cartesian system (and assuming that you can average over a longer time than the time constant of the noise, which should be possible for current) you get a correct reading of 0.1 DRIFT and 180° SET.

I assume you would still present and log current as SET and DRIFT. In the user interface, the only change could be that it would only make sense to state one damping time, valid in both x and y directions. Under the Setting>Channels  one could replace the "Current, Drift" and "Current Set" damping options with a "Current" damping option. Or keep "Current, Drift" and "Current Set" on the Channels page but state that "Current Drift" damping is valid for all currect calculations and Current Set has no effect. (if this is easier to implement).
 

 

2/23/2017 1:39 am  #2


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

That is probably a good idea. They are currently damped separately.

 

10/31/2017 5:22 pm  #3


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

Averaging just one component of a 'velocity vector' doesn't work as you have noticed. This applies to [set;drift], [SOG;COG], [TWS,TWD], and more. You may reduce the apparent noise a little by averaging your two cartesian velocity components independently as you suggest but the answer will continue to be incorrect.

Another important question is whether we should be working with 'velocity vectors' at all. After all it's the 'position vectors' that contain the noise in the first place.

And have you ever noticed that our GPS delivers both position coordinates (which bounce around a little even when we are not moving) as well as our instantaneous [SOG;COG] vector based on position changes from one measurement to the next. Some higher end GPS units apparently incorporate accelerometers, magnetometers and gyros with sophistocated predictive filters to give much better estimates of position and velocity.

Ideally I would like to see some of this sort of functionality available within Expedition.

 

10/31/2017 6:37 pm  #4


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

The averaging in the cartesian system has been implemented and for me has ment a significant improvement to the SET and DRIFT measurements. My feeling is that the remaining errors originates primarily from the BSP and HDG sensors and their general accuracy and calibration/deviation

Suggestion for further improvement:
It would be helpful to have the cartesian averaging done also when using stripchart. For example averaging SET/DRIFT between two wands. It appears that here the "polar averaging" is still used. 

     Thread Starter
 

10/31/2017 11:20 pm  #5


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

Tnx for the valuable insights!
John, you say that it is implemented.  But in the Exp 10.7.6 channel settings I still see separate damping for set and drift.   How does it work?

 

11/01/2017 1:36 am  #6


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

It is really an artefact of the channels being different - there is no single channel called 'current'. Drift is still damped as a vector, but you can have different damping values for each channel. I can't imagine a reason to make them different though.

 

11/09/2017 8:23 pm  #7


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

I thought I responded to this earlier but now I get "Bad request. The link you followed is incorrect or outdated." when I follow the link in the notification email. Hmmm. So, let's try again ...

Shouldn't all 2D vector quantities be damped/averaged/filtered as 2D vectors? Damping the magnitude and angular components independently simply makes no sense.

For instance, 'current' is a vector comprised of [set,drift]. Similarly, 'true wind', 'apparent wind' and 'ground wind' are vectors are based on [TWS,TWA], [AWS,AWA], and [GWS,GWD] respectively. [SOG,COG] could also be considered a vector but both are received independently from the GPS so the characteristics of the GPS (incl. internal damping/aveaging, sampling rates, etc) need to be considered here.

Ideally, all of the basic corrections/compensations/etc should be completed before the data is presented to Expedition in the first place but this is often not possible. But that said, true vector-based smoothing/averaging/filtering would be a great improvement over what we have available today.

 

11/09/2017 11:48 pm  #8


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

Can you tell us precisely which damping value is actually used for the 'vector damping' and how the other one is used (or not used). A comprehensive answer would be appreciated.

Also, what about the other 'vector' quantities I mentioned?

 

11/10/2017 12:25 am  #9


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

In the case of measuring SET and DRIFT, the position (LAT,LON,SOG,COG) noise from the GPS can be of the same magnitude as the actual SET and DRIFT. Hence here cartesian filtering is of significant advantage.

When filtering other data this might not be the case and cartesian or polar filtering typically doesn't make much difference.

On the other hand, look at the case where you would like to know the average windspeed GWS (e.g. over a year) at a venue. Assuning a balanced windpattern cartesian averaging would possibly render an average windspeed of 0 whereas polar averaging would give you th typical windspeed of that venue which is probably what one would be looking for.

As for me, I see no reason to switch to cartesian averaging for the other measurements.

     Thread Starter
 

11/12/2017 4:24 pm  #10


Re: Damping/averaging of SET and DRIFT. Polar vs. Cartesian system

JohnA: The same can apply to any vector quantity, not just 'current' [drift,set].

Consider 'apparent wind' in a light wind situation with let's say a true 4kt easterly [4.00,090] where you are travelling at 2kt to the NE [2.00,045]. Your apparent wind in this situation would be [5.44,073].

But what if there is a bit of wave action causing the boat to roll back and forth? If the top of the mast passes through vertical at around 4m/s perpendicular to the direction of travel (ie. 045) then that's around 7.8kt of apparent wind max  it would experience or around 5.5kt RMS.

So, what does this mean. Compared with a vector average, the wind speed would read around 50% high and the direction would be incorrect by around 12 degrees.
 

 

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