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Is it possible theoretically for a sensor to detect Nadir? This could actually be of extreme value as generally spacecraft attempting to land need to maintain certain heading w.r.t Nadir as a function of altitude and down range.

Closest I can think of is gravity gradient sensors. The idea here would be to detect gravity direction which should be for all intent and purposes aligned to Nadir(or not?). Also, I have an intuition that these gravity gradient sensors will also work even if the thrusters are firing. Am I correct? Is there any better way to detect Nadir directly by sensor/sensors?

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Yes, nadir detection is possible.

The best technique depends on how high you are and how accurate you need to be.

If you have enough altitude to see the entire horizon-to-horizon disc of the planet, a simple approach is to capture an image of it and compute the center.

Using magnetometers is another approach.

A star tracker could also work, although accuracy would depend on knowing the time and your location.

Closer to the ground, I would probably use LIDAR. Scan the area below, looking for the closest point, which would be nadir.

A production version of nadir-sensing would almost certainly involve several different instruments, with readings combined in an attitude measurement control system. Different instruments may work better at different altitudes; the control system would know this, and change usage or weights as the mission progresses.

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  • $\begingroup$ Horizon is a good idea and works anywhere on Earth as long as you are very hight, but when you get closer to the ground (as in the landing attempt mentioned in the question) mountains, landmarks, buildings, even trees will cause a problem. The other two would require you know roughly where you are landing (latitude, longitude) and they have other problems. Even a 3D vector magnetometer + vector magnetic map of the Earth + lat/lon leaves 1 axis of ambiguity, you can spin one axis the field line and since the other two would read zero, they'd never know it. $\endgroup$ – uhoh Oct 7 '18 at 8:37
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    $\begingroup$ star trackers work in the daytime @uhoh, and have been able to do since since at least 1958 $\endgroup$ – JCRM Oct 7 '18 at 9:42
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    $\begingroup$ Close to the ground requires a different approach. Offhand, I'd say gyros should work well, synced while still well above the clouds. Haven't done it myself, though. $\endgroup$ – RickNZ Oct 7 '18 at 10:19
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    $\begingroup$ but by 1958, @uhoh, atmospheric vehicles were flying with star trackers that could see stars during the day. It would be pretty pointless to have mentioned it otherwise. $\endgroup$ – JCRM Oct 7 '18 at 18:19
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    $\begingroup$ @uhoh the idea is that you first determine nadir while the rocket is still in space, then use an attitude measurement control system to propagate that model as you descend. If you want or need data close to the surface, then use a LIDAR. I've updated my answer accordingly. $\endgroup$ – RickNZ Oct 7 '18 at 21:09

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