Measuring changing sea level precisely enough to say something interesting about it is an extremely difficult task. Many places have tide gauges that have been making measurements for ages, but regional sea level patterns are variable, in part because the coastline itself can be rising or subsiding in some locations.
Satellites sound like the ideal technological solution, but we need to measure the change in distance between the sea surface and a satellite hurtling by over 1,300 kilometers overhead very accurately. Getting accurate enough to detect an average change of about three millimeters over the course of a year has its challenges.
Even if a satellite holds its orbit perfectly, you still have to worry about subtle shifts in the measurements reported by your electronics as they bounce radio waves off the planet. Those shifts are called “bias drifts”—gradually increasing errors that mess with the trend you’re trying to reveal.
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