Ah, that rings a bell now. You can vary a parameter they call "beta" to allow for more convoluted routes, and I think a larger value gives a little leeway for less frequent fixes.

Agreed, the log scale is really important to avoid arithmetic underflow =] I believe OSRM and Graphhopper both do it that way. In my implementation I've flipped from thinking of measurement/transition "probabilities" to "disparities", and I choose the final route that has the least disparity from the trail. It seems to handle trails with around a 30-60s frequency over a 5-10hr period with decent accuracy.