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?