DSC DOH!, the M.821 gotcha

Above, in a scene from my DSC testing, the Northstar NS100 has just received a position requested from another radio and the E120 has automatically picked up the message and is offering to place a waypoint and even make it a “go to”. Very cool, except that when you look at the bigger picture, you’ll notice that the position received is an overly tidy 44 13’.000N by 069 04’.000W. It’s damn hard to get yourself that geopositionally neat, and in fact I’m pretty sure this had to do with an ugly little DSC detail known as the M821 expansion. Apparently the original DSC spec had radios sending position in whole minutes, probably to save bandwidth. Later ITU recommendation M821 lets manufacturers optionally broadcast to the accuracy of the attached GPS. That means that the radios have to ask for and send the extra digits of lat and long as another message. DOH! 

Actually all the radios I tried (Icom 504, Uniden 625, and Standard Horizon GX3000S, besides the Northstar) seemed to handle the extra precision fine. I suspect that the zeros above, which I didn’t notice until I looked at the pictures, were the result of  the second message somehow getting lost. Still, I’m told that there are plenty of “minutes only” radios out there, so if you ever see such a neat DSC position, realize that you’re looking at the corner, bottom right in North America, of a location box that’s one minute square (which good map heads will know is always a nautical mile high but only a mile wide if you’re on the equator).

Finally, note that what you see above is all that a Raymarine E or C series will do with a DSC position, whereas the Garmin 3120 below (bigger here) and some other plotters will let you attach names to MMSI numbers and will keep lists of received calls. I asked Raymarine about this and a product manager said that while he thinks the naming/listing features are cool, he’s not sure they worth the programming resources because so few boaters are really using DSC yet. Point taken!