What modern technique significantly improved accuracy supporting the fixed AU definition?

Answer

Radar ranging to Venus and Mars.

The shift to a highly precise, fixed definition for the Astronomical Unit was made possible by significant advancements in measurement technology, surpassing the limits of pure optical observation. Modern techniques, most notably radar ranging directed at nearby planets like Venus and Mars, allow scientists to bounce radio signals off these celestial bodies and measure the time it takes for the signal to return with extraordinary accuracy. This precision in measuring interplanetary distances provides a highly accurate empirical basis for confirming the fixed numerical value of the AU, ensuring that navigation calculations for probes traveling millions of kilometers rely on a foundation of known, verifiable distance measurements.

What modern technique significantly improved accuracy supporting the fixed AU definition?

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