Jim Donohue wrote: "Don't listen too closely. His understanding of GPS is, in part, deficient. GPS solves for present velocity and heading as well as position. He should know this - and if he does not he is not keeping up to date. " It would appear that Jim is using "heading" to mean "course over ground" or "track". It seems clear to me, based on my limited understanding of all position finding systems from celestial to GPS, that the only way I can know (not estimate) my ship's heading is by reference to a magnetic or gyro compass, or by measuring the relative bearing of the sun or other celestial objects. Heading and track have often diverged significantly at our slow trawler speeds; crossing to the Bahamas would illustrate this well. Jim's post reminds me that I must be careful to frame my comments so that I will not be too embarrassed if my assertions turn out to be wide of the mark. Tony Marshall K39 "Growler" Lying Hilton Head, SC

You are correct - I could defend the use of "heading" in this discusssion but it pretty clearly incorrect in this nautical context. It is however actually a little more complicated than that. We really do not have a nautical term for earth referenced velocity which is a vector quantity - that is it has both a magnitude and a direction. The GPS solves for position but also for dy/dt and dx/dt which are vectors. These vectors are then interpreted by the GPS into whatever system is being utilized to present the information to the user. Garmin calls it your direction of travel (or compass track) and instantaneous speed. Given that it actually the most important of the directional numbers and is now practical to use I suspect we need a new nautical term. There is a popular misconception that a GPS solves for positon and then backs into speed and direction by comparing position points. This is apparently the understanding of our autopilot maven and on this he is incorrect. It is relatively trivial to get heading information via GPS technology. We will likely see such products in the next year or two. Jim > > It would appear that Jim is using "heading" to mean "course over ground" > or "track". It seems clear to me, based on my limited understanding of > all position finding systems from celestial to GPS, that the only way I > can know (not estimate) my ship's heading is by reference to a magnetic > or gyro compass, or by measuring the relative bearing of the sun or > other celestial objects. Heading and track have often diverged > significantly at our slow trawler speeds; crossing to the Bahamas would > illustrate this well. > > Jim's post reminds me that I must be careful to frame my comments so > that I will not be too embarrassed if my assertions turn out to be wide > of the mark. > > Tony Marshall > K39 "Growler" > Lying Hilton Head, SC > > > > >

It still sounds like COG and SOG to me, though these are generally presented as averages over a user-selected interval of time. I suggest that as long as relatively slow-moving boats and aircraft navigate in moving air and water masses, and therefore do not necessarily move in the direction their bows are pointing, we will not derive precise heading information from GPS. Have you ever watched a helicopter or a sailplane in a strong crosswind? Tony ----- Original Message ----- From: "Jim Donohue" <jim_donohue@computer.org> To: "Tony Marshall" <tonymarshall@hargray.com>; <trawler-world-list@samurai.com> Sent: Saturday, April 28, 2001 2:55 PM Subject: RE: GPS & autopilots > You are correct - I could defend the use of "heading" in this discusssion > but it pretty clearly incorrect in this nautical context. It is however > actually a little more complicated than that. We really do not have a > nautical term for earth referenced velocity which is a vector quantity - > that is it has both a magnitude and a direction. The GPS solves for position > but also for dy/dt and dx/dt which are vectors. These vectors are then > interpreted by the GPS into whatever system is being utilized to present the > information to the user. Garmin calls it your direction of travel (or > compass track) and instantaneous speed. Given that it actually the most > important of the directional numbers and is now practical to use I suspect > we need a new nautical term. > > There is a popular misconception that a GPS solves for positon and then > backs into speed and direction by comparing position points. This is > apparently the understanding of our autopilot maven and on this he is > incorrect. > > It is relatively trivial to get heading information via GPS technology. We > will likely see such products in the next year or two. > > Jim >