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Originally Posted by 330toSRT8
I agree with you, but I would be really impressed if you could write a dissertation factoring in all the variables such as the number of satellites, nadir angle, rotation of the satellites and the earth, etc. just off the top of your head.  |
I knew someone would ask.....
Imagine a satellite right on the horizon exactly west of your location. Now imagine another directly opposite the first, directly east of your location. You are now in a straight line between these two. The difference in the amount of time it takes for the signal to reach your GPS will tell you exactly how far east/west you are. They can't tell you anything about your altitude, because changes in altitude would affect both satellite signals the same. Further, they can't tell you anything about your latitude (north/south) because any changes would again affect both satellite signals the same.
These two satellites are in the best possible configuration to tell your east/west position. Any thing less than a 180 degree line, and the accuracy drops. (it gets made up somewhat by using multple satellites - more on that in a moment)
Now - do the same drill with a satellite on the horizon directly north and another directly south. This gives the best possible latitude, but can't tell you anything about longitude or altitude. (same reasons as above)
Imagine 10 satellites, all directly on the horizon. No matter what your elevation, the distance to all of the satelites will be constant, thus no data about elevation is possible. You do have VERY accurate readings for lat/lon though.
To get the best possible altitude, one needs a satellite directly overhead, and one directly below - through the earth. Obviously not possible. By the time we can "see" a satellite on the horizon, it has moved from 180 degrees opposed to our perfectly overhead satellite, all the way the horizon - a mere 90 degrees from the overhead satellite. Much of the potential accuracy is already gone.
Just like a bunch of satellites on the horizon, if we had 10 satellites, all 30 degrees up in the sky, we still wouldn't have any altitude data. We have to derive altitude from satellites at diferent degrees above the horizon. Because the earth is in the way, our best possible is 90 degrees, but typically only 60-70 degrees. This compares with lat/lon, where we have a maximum of 180 degrees (perfect) and can usually get a difference of 150 degrees or more.
Phew - that's the simplest version I can draft. Beyond that I have to draw charts. And I ain't gonna!!
