What are the two or three most accurate technologies used to determine interplanetary spacecraft distance (e.g. Doppler / radiometric tracking...), and what accuracy is currently obtained from such methods? Do they rely on long integration time or on Earth signals?

Links appreciated.


OK, here's the Link from NASA.

Spacecraft velocity is determined by measuring the Doppler shift of a coherent downlink to determine its earth-relative velocity.

Spacecraft distance is measured by sending a uniquely coded ranging pulse to the spacecraft (including the transmission time of the pulse) and then measuring the time it takes for the spacecraft to return this pulse. The time it takes for the spacecraft to process the transmission is calibrated and measured before launch.

Spacecraft position in the sky can be very accurately measured by observing the spacecraft's signal and doppler shift from two different locations widely separated on the Earth. The signals are synchronized using Quasar measurements. More details are on the link I referenced above.

Spacecraft that are equipped with imaging instruments also use them to observe its destination planet or other body against the background starfield.

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  • 2
    $\begingroup$ That last one seems like the most accurate - ask the spaceship where it is! $\endgroup$ – corsiKa Aug 5 '15 at 22:49
  • $\begingroup$ @corsiKa: that's harder than you think. Spacecraft can generally tell you their attitude but not their position. $\endgroup$ – Hobbes Aug 6 '15 at 7:15
  • $\begingroup$ @corsiKa Actually my impression reading the NASA document is that the opposite is true. You can calculate position very precisely by triangulation of doppler measurements with quasar syncing. "Asking the spacecraft" relies on the resolution of the imaging instruments. $\endgroup$ – ghoppe Aug 6 '15 at 16:07
  • $\begingroup$ So you're saying it's more likely we're telling the craft where it is as opposed to it telling us? $\endgroup$ – corsiKa Aug 6 '15 at 16:24
  • $\begingroup$ Well, technically, it's telling us because of how we detect and interpret its return communications signal. I'm just saying whatever imaging instruments the spacecraft has is a very minor component. $\endgroup$ – ghoppe Aug 6 '15 at 16:28

I might go look for some links, but just off hand Doppler velocity is good to less than 1 mm/s (in the line-of-sight dimension), can be as good as 0.1 mm/s, ranging is good to better than a meter (also in line-of-sight), and delta-DOR position in one dimension, in the plane connecting the two DSN complexes and the spacecraft, is good to about 1 nanoradian.

The first two are used all the time. delta-DOR less so, since it requires engaging two stations at different complexes at the same time.

Yes, they rely on long integration times, if minutes to hours is long. Doppler requires a very accurate clock on Earth sending a frequency to the spacecraft, which is turned around and sent back. Similarly, ranging requires a pseudo-noise signal from Earth which is turned around on the spacecraft with a well-characterized latency.

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