I’ve seen explanations that the GPS altitude is less accurate because it is measuring the height about some math function that approximates the shape of the earth’s sea level. However, is the actual measurement of distance to the satellites less accurate in the Z dimension? I don’t see how it could be, any distortion or timing errors that cause GPS error would logically affect your measurement of X-Y (latitude and longitude)
In addition, isn’t sea level height the same, globally? If you ignore the tides, the level of ocean water throughout the world has to remain the same, or water would flow.
Knowing your elevation above sea level is important because if global warming really does raise ocean heights a few feet, it’s important to know how much margin a particular building has to spare.
GPS measures the time difference between satellite signals. When measuring horizontal position, satellites in near-opposite directions, give the best results. For the vertical position, you can’t take any measurements from the satellites below the horizon. Hence, from a vertical perspective, the time difference is not as precise.
Is “sea level” the same height? Yes and no, depending on your frame of reference. Google “geoid”.
GPS is only accurate to a meter or two. To measure sea level rises, one needs surveying grade instruments.
Remember that the GPS signal you get is a degraded one - the primary purpose of NAV Sat’s is to enable you to kill people far away with great precision.
The militaries which built those systems keep the good stuff for themselves.
We could have sent a cruise missile into bin Laden’s hideout, but the use of live soldiers was a major display of ability.
Which is why there are at least 3 different systems (EU), and I am reasonably certain that the newly-ascendant Chinese have seen the value in having their own.
Don’t be flying IFR through mountains using only your GPS nav unit if the US gets in a serious war - they just might turn off that signal.
SWAG: If you ignore altitude you can use the earth as an additional sphere when performing the GPS calculations and get a fix with fewer satellites.
GPS depends on figuring out distances from multiple satellites and doing the math to identify the intersection of the spheres defined by the distances. Using the earth as an additional sphere would simplify things if signal strength is poor. But in order to effectively use the earth as an extra sphere in the equation, you would need to know the exact radius, either by choosing a nominal radius or possibly using geographic data for decent estimates of altitude in the region around the GPS receiver.
Well, almost. A receiver that incorporated a super-accurate clock (like the ones aboard the GPS satellites) could do this - but such clocks are bulky and expensive. There’s a better way.
A normal GPS receiver actually works by measuring time difference between the arrival of signals from pairs of satellites. In 2 dimensions, a curve of constant difference (e.g. of time, or distance) is a hyperbola; in 3 dimensions, it’s a hyperbloid of revolution. The receiver continuously calculates the points at which sets of these intersect, and thus finds your location in 3 dimensions.
But note that military receivers can use the P code, and thus deliver better accuracy than a normal commercial receiver that has access only to the C/A code.