wooslehunter
Active member
Re: could you explain a bit further
Here's another way to think of it. Think in terms of distance off. If you're doing this on a chart, by using something like dipping distance. If you have one point, you know you're somewhere on a circle around the point. Two points, and you have two intersecting circles so two possible solutions. In reality we can generally ignore one of them so we have a fix. Divers use this all the time. It's tri-lateration rather than tri-angulation.
Extend this to 3-D space & assume we don't know about the earth's surface. Distance from one satellite gives us a position somewhere on a sphere. Two distances give two intersecting spheres. i.e. a circle. 3 distances give three intersecting spheres which result in 3 circles and a 3-D fix. As someone already said, time is the killer & we need the fourth satellite to calculate that. Hence 4 satellites for a 3-D fix.
I've two GPSs on board: ones a really old Apelco & the other is a Garmin GPS72 handheld that installed permanently. As far as accuracy is concirned, they rarely disagree by much. So small in fact I've never worked it out. I have logged the Garmin while sitting on a window sill at home and generally get a "wander" of around 8m & that's with 50% of the sky obscured by the house!!!
Fot most practical purposes, even with old wobble turned on GPS was far more accurate that really required by leisure boaters. What happens if it does go down or more likely if your own kit goes down. If you have to rely on that kind of accuracy, you're screwed anyway. Try doing a compass fix on a chart & see how big the cocked hat is.
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Here's another way to think of it. Think in terms of distance off. If you're doing this on a chart, by using something like dipping distance. If you have one point, you know you're somewhere on a circle around the point. Two points, and you have two intersecting circles so two possible solutions. In reality we can generally ignore one of them so we have a fix. Divers use this all the time. It's tri-lateration rather than tri-angulation.
Extend this to 3-D space & assume we don't know about the earth's surface. Distance from one satellite gives us a position somewhere on a sphere. Two distances give two intersecting spheres. i.e. a circle. 3 distances give three intersecting spheres which result in 3 circles and a 3-D fix. As someone already said, time is the killer & we need the fourth satellite to calculate that. Hence 4 satellites for a 3-D fix.
I've two GPSs on board: ones a really old Apelco & the other is a Garmin GPS72 handheld that installed permanently. As far as accuracy is concirned, they rarely disagree by much. So small in fact I've never worked it out. I have logged the Garmin while sitting on a window sill at home and generally get a "wander" of around 8m & that's with 50% of the sky obscured by the house!!!
Fot most practical purposes, even with old wobble turned on GPS was far more accurate that really required by leisure boaters. What happens if it does go down or more likely if your own kit goes down. If you have to rely on that kind of accuracy, you're screwed anyway. Try doing a compass fix on a chart & see how big the cocked hat is.
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