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The GPS can easily wobble by 50ns back and forth as the constellation changes. That's a lot! And, it is not random on a short time scale. Folks often think "Oh, +/- 50ns, 20ns RMS, easy to filter...", but that's totally wrong. The GPS will report -30ns from stable for minutes on end, then slew to +10ns, then -5ns, etc. Any high-precision oscillator (such as for radar) that's being jerked around like that isn't going to be as stable as high performance needs. Even for just handoff of handsets at 2.2-2.3GHz, having the radio network (aka cell towers) all locked to an oven-controlled oscillator that was aligned-to, but far smoother-than, GPS, made a huge difference. Now, improvements to GPS/GNSS that track 12 satellites instead of 6, and across multiple constellations, can result in more stable radio-based time. But then you get into urban canyons, and can only see 5 instead of 12, and you're right back into the jumpy situation. |