As a theoretical physicist, there is a question that I have been asking myself for a while: I would like to know how bearing and distance are precisely calculated by the GPS.
That is also my experience: Great Circle Routes. Every GPS I've seen allows you to select a geodetic model other than WGS84. DON'T! There are still a few areas where the charts aren't based on WGS84, but unless you know what you're doing, never deselect WGS84. Whenever crew have unsupervised access to my GPS, I always check to make sure they didn't change the geodetic system. Bad things can happen if you don't notice that change (the young people these days seem to like to twiddle every setting on everything when they're bored on deck watch.)Ive flown transport category aircraft for decades using both INS and GPS nav systems. I feel like I should know the answer to the OPs question. Im sure Ive glanced over it in reading the minutia of a particular nav system. Im fairly certain (most?) GPS uses the WGS84 model and will always plot great circle routes for course and heading.
I agree, one should really see how the software works and talk to someone who knows about the GPS engineering. CrispyCringle's answer is along these lines, I like it.Think we need someone who knows more about the engineering of gps use to answer this question. It remains a good question. How does the software work? Understand how you get a position from it but don’t understand how the software uses that.
Cool answer, thank you.Ive flown transport category aircraft for decades using both INS and GPS nav systems. I feel like I should know the answer to the OPs question. Im sure Ive glanced over it in reading the minutia of a particular nav system. Im fairly certain (most?) GPS uses the WGS84 model and will always plot great circle routes for course and heading.
This confuses me...Most people find great circles difficult to steer so the default is generally rhumb line. If you are going longer distance, you switch to great circle and find a way to steer the great circle, traditionally it was done by squaring off the curve, but I am guessing newer autopilts can do it on their own.
Perfect explanation and completely understand and agree!Here is an article that explains it. It describes the squaring off I was describing. I was taught the same way the article describes. Change course once daily. So on a 30 day crossing you could make 30 course alterations, once per day. So during a given 24 hour period you are steering a straight line, but the resultant course on a mercator projection chart, is curved.
Some one specifically used the New York to London example because its a higher latitude, long distance run where the difference between the great circle and rhumb line is more pronounced.
Why do people sail rhumb lines? Because they allow for constant compas headings over a distance and and most of us have traditionally used a compas course relative to north to get to where we are going, rhumb lines appear as straight lines on mercator projection charts, which is what a lot of us use a lot of the time. Its simple, its easy.Why would anyone trying to get someplace sail a rhumb line? Makes no sense. Sure, a great circle will probably have a heading change. But its insignificant. Especially at sailboat speeds. Changing winds, currents, trim changes, will make you adjust the autopilot or helm way more than great circle heading change.
Unless you tweak your GPS, its always going to give great circle. There is no "short range" gps solutions and "long range" transitions. Also, GPS is not using a mercator projection. Its using the WGS084 model, which is WAY more accurate that mercator.
Don't APs need a compass? Mine does and does not accept GPS steering info.If autopilot is talking to a compas, whether magnetic, flux gate or a gyro, its following a rhumb line. If autopilot is programmed to follow a GPS it may be following a great circle.
Bread crumbs are neither great circle nor rhumb line. They are an indication of where you have been and include variables including lee way and manual course alterations for weather.
If you want know whether your gps is following a rhumb line or a great circle and its not described in the owners manual, just look at the course it plots over a long distance. You have recently sailed to the carribean? Was it a straight line on a mercator projection? Thats a rhumb line. Was it curvy? Thats a great circle.
On a mercator projection chart the great circle would have had you do more easting further north then curved to the south. Rhumb line would have been straight.
How could I have? The term bread crumbs comes from Hansel and Gretel leaving a trail of bread crumbs to find their way back out of the forest. In chartplotter terms, that's your track, not your route.A shouldn’t have said bread crumbs as that implies what we left behind. Actually it’s a colored line of where we’d like to go. Guess you didn’t pick up on what I was saying.