When used with “relative” (apparent) wind data source, PolarCOM provides two modes of “true” wind calculation. These modes are labeled “STW” (Speed Through Water) and SOG in the configuration. STW is a “traditional” method, used by most hardware wind instruments. It needs no GPS input and requires STW and HDG (your vessel heading) to calculate “true” wind. It also results in an approximation of “true” wind.
SOG mode provdes more precise “true” wind calculation, but it also requres COG as well as HDG. That means you still need a heading sensor to calculate “true” wind – having only GPS input without heading is not sufficient.
If you are interested in explanation as to why that is, read on:
Let’s say you have the following –
1. Relative apparent wind (from VWR/MWV NMEA sentence) as delivered by windvane
2. GPS information including COG/SOG.
3. nothing else
Consider this – your vessel is heading in some direction which is most likely NOT aligned with COG. Relative wind is a vector that you can calculate based on a coordinate system that has boat heading as one axis (and another axis perpendicular to it).
On the other hand your SOG (and SOG induced “wind”) is a vector that by itself can only be considered in a system with one axis along the COG (and another perpendicular).
Without heading there is nothing to reconcile these two systems and no way to do any reasonable operations on these vectors. You need heading to convert one of these vectors to the coordinate system of the other.
Here is a practical example using PolarCOM. The first set of dials (img 1) shows COG and heading that differ by 20 degrees. The true wind is calculated here based on SOG (and using both COG and HDG to adjust vectors). You can see true wind both relative to the boat (second dial from the left) and as an absolute direction (last dial on right). There is no STW (as the heading dial shows).
On the second image (img 2) I adjusted heading to match COG. This is what would effectively happen if you used SOG for true wind calculation but did not have an adjustment angle (and assumed direction of travel matching heading).
It is quite clear that true wind resulting from such calculation is not the same as the one that results from properly accounting for COG/HDG difference. And the difference is not insignificant – 30 degrees and about 40% stronger.
Incidentally, third picture (img 3) shows calculations using STW and heading. I set STW to be equal to SOG, which is not necessarily true, but it helps make an example simpler. As you can see it is also incorrect, but the result is essentially the same as when using SOG alone without proper heading. I.e. there is no gain in calculation precision.