Expedition Expedition Navigation Software

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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.

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.


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