Introduce an alternative source for magnetic variation, MV, into the ECRMC sentence
Make it possible to get MV value from either GPS NMEA
sentence GPRMC, as of today, or the WMM module’s value if present.
A switch button for manually choice of a source would be present.
ECRMC sentence is built within OpenCPN and is broadcasted when a “navigate to” route or point is active.
Most autopilots use the RMC’s MV value to correct the course to steer. Without a correct MV the steered course can drift causing later big corrections and an agitated control.
The ECRMC’s MV value can be false due to old data base in the GPS
The ECRMC’s MV value can be absent if GP-- data source is an AIS
which often won’t supply a MV value.
If the WMM module with a due data base is present a correct MV value is then existing inside OpenCPN.
The user can and has to evaluate the confidentiality before change of MV source.
The switch button for choice of source could preferably be present in the WMM preferences’ window or in the OpenCPN “data connection” tab.
It is only field #10 in the ECRMC sentence that shall have different source opportunities. The rest of the sentence shall be built as of today’s coding.
Default source ought to be the GPS’ GPRMC as of today’s code. That’s probably that GPRMC sentence having the highest priority?
This request is already present in thread “OpenCPN Beta Version 3.1.1319 Released”
but I suppose it fits better here.