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Comment by gorgoiler | Hacker News Reader

Space navigation is amazing. If I throw a rock into the lake I can barely see where it is until it hits the water. When we fling these craft into the void how do we know where they are and where they are going?

I can think of three ways:

(1) get the direction right in the first place and just assume the craft is where we calculate it should be;

(2) watch it from afar somehow; or

(3) have it tell us where it is.

These seem like three quite different options and I would love to know how it’s actually done.

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Votager 1 hasn't changed its trajectory much since 1980, so just modeling its trajectory from there would be fairly accurate.

However, they also calculate range precisely by sending it pings and timing its response. They calculate velocity precisely by looking at the Doppler shift of that return signal (https://geekswipe.net/technology/aerospace/how-nasa-calculat...). Measuring those over time provide a fairly precise location.

Thanks for engaging! Do the pings return a location, or are these range and velocity (and orientation from onboard sensors?) values used for dead reckoning, with Earth as the starting point?

When external forces impart a force on the craft (eg exciting new moons!) how is that taken into account?

I've read about determining the "state vector" on the Apollo missions, and I would imagine it's similar for voyager. The ping delay and Doppler shift should be enough to determine 3D position and velocity with respect to some coordinate sysem. I know JPL ( and/or perhaps other agencies like ESA) have a computer model of the solar system where you can run a spacecraft's trajectory forward through time. It can also account for gravitational anomalies if the distance to a planet/moon is close enough where it matters. Assuming they have gravitational anomalies modeled for that body. I'm pretty sure at least Jupiter has a decent anomaly map. In voyagers case, using the sun as a point mass, combined with its state vector at any known time, any undergrad student who knows the vis viva equation can determine voyager's position at any time.

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