A 21 minute video? Surly you jest. How about a summary?
Well, it’s the usual historical revisionism that you often get from ketogenic diet fans with a hint of cholesterol denialism thrown in. In the style of Gary Taubes et. al. (Not the first time that particular YouTube “myth buster” has crossed my radar screen, manifestly )
It’s aimed really at folk who like the message that “The Food Pyramid made you fat…aided and abetted by Ancel Keys”
A 27+ minute video jest for you…
If you can’t explain it in terms a 6th grader can understand, you probably don’t understand it, either.
So GPS - Here goes my test. I’m probably going to have to dabble in simple high school geometry, so if that’s a fail, I’m going to fail.
If you have 3 points, they all share a single plane - a single flat surface, like a sheet of paper.
Now if you can measure the distance from any other point in space to each of those three points, you can uniquely define that point. No other point will have the same distance from each of those three points.
GPS works by measuring the distance from the GPS reader to three (or more) satellites in orbit around the earth. That then defines a unique spot on earth.
The measurement is done by calculating the time it takes for a radio signal to get from the satellite to the receiver. The more accurately you can measure that time, the more accurately you can locate the receiver. You can also improve accuracy by measuring the distance to more than 3 satellites.
There’s also a bit of complexity is knowing exactly where the satellite is. After all, it is orbiting the earth at around 17,000 miles per hour (or 27,000 kilometers per hour if you prefer). The exact orbit of each GPS satellite needs to be tracked and updated, because orbits change slightly over time and can be adjusted with thrusters on the satellite. You can do this by running things in reverse. Have a few fixed locations on earth send signals to the satellite so it can measure it’s own distance from those locations, then report that data back to ground so any difference between the expected location and the reported location can be calculated and future expected locations can be adjusted.
The satellites and receivers also need to have a synchronized time system. The better you can keep the time in sync, the more accurate your GPS will be.
Finally, we need to give a nod to Einstein and relativity. Because the satellites are moving faster than the GPS receiver, their clocks will drift out of sync with the ground based receiver. So that has to be adjusted for as well.
So how does GPS accuracy improve over time? A combination of better satellite tracking, better time synchronization, and better calculation of the signal arrival time (and hence, distance) at the GPS receiver.
There you go. A 2 to 3 minute read summarizing a 27 minute video. Obviously imperfect, as any summary is going to be. But enough info to reasonably decide if you want to watch the full video.
Now, how about summarizing the 21 minute video you posted. It’s not that hard if you truly understand the subject.
You need the 4th satellite if you are going to locate the receiver in a vertical (height above sea level) space. Three triangulates the horizontal plane, the 4th will give you vertical. Not usually necessary, obviously, but if you want to use it to control missiles, airplanes, dirigibles, etc. then there you go.