bqmassey
Line Up and Wait
In an attempt to satisfy a simple curiosity about how GPS receiver's determine speed, I started doing a little research about the math behind it all. As usual, I ended up with more questions than answers. I was wondering if some of you intelligent folks could help with a few of them.
First off, how do GPS receivers (GPSr's) determine distance between two points? When I started reading about the Great Circle formulas, I realized that the distance between two lat/long pairs (which are more a description of angle than they are distance) would differ depending on how far away from the center of the earth you are. For instance, at sea level the distance between two lat/long pairs is smaller than the distance between those same pairs at 40k feet above sea level. In the same way, two points that are tens of thousands of miles apart at seal level would only be inches apart if the radius of the earth was that of a basketball. So... Does a GPSr determine distance based on the Great Circle formula? If so, does it use sea level as the radius, the terrain level, or your altitude? If it's not based on Great Circle, does it instead convert a straight line distance (which would actually go through the earth if you're standing on the ground)?
My assumption is that it uses Great Circle at MSL based on some particular datum (WGS84 perhaps), but that's simply an assumption.
I have more, but I'll stop there for now.
Anyone knowledgeable on this stuff?
First off, how do GPS receivers (GPSr's) determine distance between two points? When I started reading about the Great Circle formulas, I realized that the distance between two lat/long pairs (which are more a description of angle than they are distance) would differ depending on how far away from the center of the earth you are. For instance, at sea level the distance between two lat/long pairs is smaller than the distance between those same pairs at 40k feet above sea level. In the same way, two points that are tens of thousands of miles apart at seal level would only be inches apart if the radius of the earth was that of a basketball. So... Does a GPSr determine distance based on the Great Circle formula? If so, does it use sea level as the radius, the terrain level, or your altitude? If it's not based on Great Circle, does it instead convert a straight line distance (which would actually go through the earth if you're standing on the ground)?
My assumption is that it uses Great Circle at MSL based on some particular datum (WGS84 perhaps), but that's simply an assumption.
I have more, but I'll stop there for now.
Anyone knowledgeable on this stuff?
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