<John wrote: I'm curious why you say, "the dome for that is mounted high above everything else at about 38' elevation to make it work well," when conventional wisdom is to mount them as low as possible to avoid "swing."> I think this is why the GMDSS requires, or a least seems to, a gyro stabilized reference. The GPS can give a very accurate output but needs to be above other EMF outputting devices..i.e. it needs several sq meters of clear space to see the birds. You can only get this on most boats our size by going up as we can't get 100 ft back or forward ;-) This tends to cause the reference to jump or skitter quite a bit and given the algorithm to compute all the information we see that a very small change yields a scattering of the computed results. We have a flux gate which is stabilized by a solid state gyro referenced by a GPS input. This yields a very stable heading reference for our CE. Unfortunately the Class B units don't have an input for this reference so we are stuck with only using the embedded GPS in the AIS transponder. Perhaps a bad call on the initial design specs for the class b units, IMO. Our Raymarine has its own fluxgate and gyro for the MARPA which also gives it a much more stable display/computation than the normal fluxgate alone. However, it is not as good as the flux gate/gyro/gps that we have for the autopilot and what we tap for CE's input. We may find in the end that we'll want to go to a Class A unit and then we can use this more stable input and thus have a better transmitted heading/course for others to receive. Dave & Nancy Swan Song Roughwater 58 Caribbean Cruise '07