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There's no way for any probe that is sent to orbit/land on a distant body to get there only on precalculated flight parameters alone - minuscule errors in the beginning of flight would convert to huge differences in speed and trajectory near its end. Of course these errors must be corrected during the flight as they become apparent (and before they escalate to uncorrectable levels), but first - they need to become apparent. And while in the initial phase the probes would be able to measure their speed and location quite precisely using the GPS satellites, this is not viable, say, halfway to Saturn, and good few AU away from any near bodies.

So, what methods do the probes use to measure their own flight parameters respective to planned ones?

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Interplanetary probes generally don't have enough equipment on board to calculate their own position. So the location of a probe is measured by ground control, using data from the probe.

  • Distance and speed along the Earth-spacecraft axis is done by sending signals to the probe, having the probe transmit them back immediately and measuring the time elapsed plus Doppler shift. See Mark Adler's answer to a related question.
  • The remaining parameters are derived from the above with some calculation, plus data from the spacecraft. Navigation images can be used, for example: these show the stars and planets from the perspective of the spacecraft, this can be compared to the view from Earth.

For New Horizons' Pluto flyby, this process was complicated by the fact that Pluto's orbit wasn't precisely known. So the NH team used several methods (*) to measure Pluto's position accurately so they could refine NH's trajectory in turn.

*: the NH team went through Tombaugh's 1930s photographic plates and digitized them to get data from as far back as possible.

There are projects to design a method for space probes to measure their position independently, but none of those are anywhere near 'ready to use'.

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