# When going to Mars, how do you measure exact distances to determine course corrections?

When probes fly to Mars, they must adjust their trajectories multiple times during transit to ensure correct orbit insertion. They fire their small onboard rockets a little bit to do this.

But how do probes know exactly by how much to reduce their velocity and by how many degrees to deviate to ensure they hit the right position in the Martian atmosphere for precise entry? Especially since both Mars and the probes are moving very fast. And since controllers usually have a specific location on Mars they want to target.

There's obviously no GPS in space, and looking at the stars might be a good way to understand your general position in space but I doubt it would provide a precise measurement of distance down to a few hundred kilometers.

So how are precise course corrections performed in space?

• Would it be fair to say that you're asking how guidance, navigation, and control works for space probes? Sep 30, 2019 at 14:04
• @OrganicMarble Yes, rather true, but I'd like an answer with a focus on how the navigation is done so precisely given the large bodies and distances involved and lack of GPS. Sep 30, 2019 at 14:11
• If the probe is able to receive and transmit to ground stations on Earth, it is possible to send a special signal to the probe and echo it back to measure the distance and speed relative to the ground station very precisely. This method has been used for many decades since the WWII rocket V-2.
– Uwe
Sep 30, 2019 at 14:16
• @Uwe But then how do you know the precise distance and speed of Mars? Distance is relative, right? Sep 30, 2019 at 14:18
• This is a good question but I think it has been answered before here. The short answer is that Earth stations monitor the spacecraft throughout the journey using precise timing of returned signals to measure distances and multiple ground stations to triangulate positions. Using those data plus precise orbital calculations (we know where all the big masses are in the solar system at all times) it's possible to calculate not only where a spacecraft is now but to approximate where it's going to be. It's a complicated process, but basically everything is done on Earth with computers and models.
– uhoh
Sep 30, 2019 at 14:23

But how do probes know exactly by how much to reduce their velocity and by how many degrees to deviate to ensure they hit the right position in the Martian atmosphere for precise entry?

Most of JPL's interplanetary probes don't "know" where they are. What they do "know" what time it is and where they are pointing. It's combination of people and equipment here on Earth that determine where JPL's interplanetary probes are. JPL personnel command the probes to point themselves in such-and-such direction and fire their engines starting at such-and-such a time until a specified delta V has been accomplished.

• To the untrained eye, it would seem ludicrous that the probes don’t know where they are. Would it be correct to say that it is designed this way to conserve payload mass? Or is this done to reduce complexity of the system?
– Paul
Oct 1, 2019 at 16:46
• @Paul that's a good new question but it might have been asked before. Until recently now that we have Tesla autopilot and self-navigating quadcopters, it wouldn't have seemed ludicrous.
– uhoh
Oct 1, 2019 at 17:59
• @uhoh - The question asks about "course corrections performed in space". Entry, descent, and landing, which is what I think you're hinting at, is a rather different question. Oct 1, 2019 at 18:24
• @Paul - The technology doesn't exist yet to do this in space. JPL uses huge antenna, cryogenic receivers and amplifiers, and sometimes uses multiple antennae around the Earth to yield an antenna that is essentially a good chunk of an Earth diameter across. Oct 1, 2019 at 18:27
• Good answer. It should also be noted that the burn programs on the Apollo Guidance Computer did mostly the same thing. Tracking was done on Earth; the spacecraft itself only knew its attitude and time. Computers in Houston calculated the parameters of the burn, and the on-board computer executed a burn with those parameters. Oct 2, 2019 at 20:53