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This question asks if Voyager suddenly encountered unexpected gravity from an unseen object, how would it be detected, and there are several answers.

For the farthest probes - those now farther from the sun than Neptune - how are (or have) precision trajectory measurements actually been made? I'm looking for a concise answer with links. Of course more info is always welcome, but I need one or a few "punch lines." For example "ranging by means of timing of a coded pulse train transmitted from earth, received by spacecraft and rebroadcast on a different frequency" (question and answer) or "doppler shift by means of same" or "apparent position by VLBI".

I'm not looking for "well for example you could..." type of answers in this case, just how actually it is, or has, been done.

With velocity anomalies measured via doppler on the scale of milli-Hz mentioned here by @JohannesD, I'm guessing the doppler measurements are not based on an internal, high-precision clock in the spacecraft because the remote timebase stability could always be called into question, but instead rely on retransmission by each spacecraft of a ground-based signal, with some clever way possibly to change frequency/band in a precise way.

For reference only, here are some related hypothetical items: visible imaging, and infrared thermal imaging are ruled out quantitatively for this question. Ranging is mentioned here and here as well.

From the image below (from here) I count five spacecraft now past Neptune, four of which are using 1970's technology.

solar system probe spacecraft

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  • $\begingroup$ The Pioneer spacecraft are no longer tracked. So there are only three. $\endgroup$
    – Mark Adler
    Apr 7, 2016 at 5:28
  • $\begingroup$ @MarkAdler double check the tense used - I've tried to be inclusive of measurements before tracking was stopped. For exampe: "... how are (or have) precision trajectory measurements actually been made? " or "... just how actually it is, or has, been done. " $\endgroup$
    – uhoh
    Apr 7, 2016 at 10:08
  • $\begingroup$ Have "removed" instances of "actually". $\endgroup$
    – uhoh
    Apr 26, 2017 at 7:56

1 Answer 1

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Doppler and ranging are used routinely. They are both two-way, with the Doppler turning the frequency around and the ranging turning around a pseudo-noise signal. This is complicated, only for these spacecraft, due to the transmitting station on Earth rotating out of view (and in, and out again!) by the time the return signal hits Earth, being then received by an antenna at a different complex. $\Delta$DOR (delta-differential one-way ranging) has been used to get plane-of-sky position measurements (in one dimension). Optical navigation was used by New Horizons, using images of the targets on approach to pin down their location relative to the spacecraft.

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  • $\begingroup$ Great - I was hoping you'd post! Can you elaborate? What is DOR? Also, I think the "turn-around" is on a different band sometimes? How is this frequency shift done without introducing error in the doppler measurement? Where can I read in more depth/detail? $\endgroup$
    – uhoh
    Apr 7, 2016 at 10:13
  • $\begingroup$ For the optical navigation by New Horizons, you mean it was done locally (by the spacecraft itself)? I'm thinking that when a flyby is close, the images may be too late by the time they reach the earth, and without precise time of "shutter snap", difficult to use. $\endgroup$
    – uhoh
    Apr 7, 2016 at 10:15
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    $\begingroup$ No, the optical navigation was done with Earth in the loop. The only case I know of where OpNav was done autonomously on the spacecraft was Deep Impact, which needed it in order to assure impact. $\endgroup$
    – Mark Adler
    Apr 7, 2016 at 14:54
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    $\begingroup$ DOR is measuring the difference in the range (not Doppler) to the spacecraft as measured by two different, widely separated stations on Earth. $\Delta$DOR calibrates that measurement by doing the same thing immediately after with a quasar with a small angular distance from the spacecraft. That allows subtracting out several error sources in the DOR measurement. $\endgroup$
    – Mark Adler
    Apr 7, 2016 at 15:01
  • $\begingroup$ I see! Δ DOR is a little bit like VLBI in that it uses a measurement of the phase difference of some aspect of the signal between two widely spaced ground stations, giving a precise angular position measurement along one direction (per pair of receivers). It's just receive-only, it doesn't require any transmitted beam as part of the measurement. Excellent! The PDF is incredibly clear, thank you! $\endgroup$
    – uhoh
    Apr 7, 2016 at 15:40

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