Brian, you have selected "last known bearing" now, and since that is what everyone else seems to agree is more logical, stick with that, but if I may I will answer a point from earlier in the thread. Certainly in a calm a wind vane remains in its previous position because there is no wind to move it into a different direction, thus for weather stations using a vane, last known bearing seems appropriate.
Steepleian wrote: I am not sure what is meteorologically correct however.
The answer (according to the World Meteorological Office) Ian is Zero. If you look at aviation weather reports (METAR), or most output from official Meteorological organisations, Calm is always reported with both speed and direction as zero.
Many professional automatic stations do not use moving parts to reduce maintenance demands, and for sonic detectors (and similar instrumentation) zero is the more logical response.
You can see why Steve originally chose defaulting to zero, and then made it into an option (9 years ago) at