17k views

### What are these orientations called in orbit?

Let's say a spacecraft is in an orbit like this one: If the red arrows point to prograde and retrograde, and the blue arrows point to normal and antinormal, what do the green arrows point to? In ...
9k views

### How is the altitude of a satellite defined, given that the Earth is not spherical?

The altitude of a satellite is the distance between the Earth's surface and the satellite, but the Earth itself is not spherical. At the equator the Earth's radius is 21 km more than at the poles, and ...
4k views

### What are quaternions and how are they used to represent spacecraft dynamics?

The title says it all. Quaternions are widely used to represent the orientation of a spacecraft. Why is that, and how do quaternions compare to other alternatives?
967 views

### Does the ISS have a rotational motion in addition to its translational motion?

We all know ISS is orbiting the earth so it has translational motion, but, does it rotate on its own axis as well? Thanks
2k views

### What precisely is downrange distance - how is it defined mathematically?

Wikipedia: Downrange is the horizontal distance traveled by a spacecraft, or the spacecraft's horizontal distance from the launch site. Spacecraft don't travel horizontally. I don't even know how ...
1k views

### How were Frenet frames and Gram–Schmidt orthonormalization used in spacecraft orbit calculations?

I haven't seen the film Hidden Figures yet, but I have read and listened to some dialogue clips in this NPR Review. In it they mention a Frenet frame and the Gram–Schmidt process for ...
275 views

### Do Curiosity's reported measurements of Mars' surface gravity (~3.717 m/s^2) include centrifugal effects?

In this answer I discuss the first results in the 47th Lunar and Planetary Science Conference (2016) conference paper First Gravity Traverse on the Martian Surface from the Curiosity Rover and a more ...
2k views

### How to calculate the angle between the satellite and ground station?

I have the latitude/longitude/altitude of a satellite and a ground station. Also, I have their Cartesian coordinates in Earth-centered-Earth-fixed frame (ECEF). How to calculate the elevation angle ...
122 views

### What's the difference between zenith and radial?

When it comes to orbital mechanics, are zenith and radial the same? Or to put it another way, are nadir and anti-radial the same?
106 views

### If a spacecraft is orbiting around the Moon, what direction would the nadir axis be?

Nadir for a craft orbiting the Earth would be radially in, is nadir always pointed towards the Earth or is it pointed towards the celestial body that the craft is orbiting?