# Questions tagged [eci]

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### ECI position and velocity from angular velocity

I need help with an orbital mechanics issue that I cannot figure out. I have the initial ECI position and velocity ($r_{vec},v_{vec}$) of a certain satellite. Through them, the angular velocity, the ...
1 vote
127 views

### Perifocal to ECI using Argument of Latitude

Suppose a satellite is orbiting Earth in a circular orbit. I know the following orbital elements: Inclination i, RAAN (Ω), Semi-major axis a (or orbit radius given that it is a circular orbit) ...
310 views

### Calculate velocity vector of elliptical orbit given the position vector at that point, the instantaneous speed, the true anomaly, and the inclination?

Sorry if this is obvious, but I am an amateur at orbital mechanics and trigonometry was never my strong suit. I would like to calculate the velocity vector at a point in an orbit given: position ...
127 views

### (In ECI) How can I compute the RAAN of an orbit given the eccentricity vector, the declination of the orbiter at apoapsis, and the inclination?

I am working on a personal project, and I want to write a program to obtain the RAAN given the angle of the eccentricity vector to the X-axis (ECI) along the XY plane, the declination (angle from XY ...
1 vote
66 views

### Why ECEF does not coincide with ECI on January 1, 2000 at 12.00? [duplicate]

I assumed that ECEF frame coincides with ECI frame on January 1, 2000 at 12.00. To see that, on MATLAB, I used the dcmeci2ecef() function as: ...
638 views

### Transformation ECI to ECEF acceleration and forces

I know that for velocity conversions between ECI and ECEF there is an $\omega \times r_{ECI}$ term, such that the overall transformation is $v_{ECEF}=v_{ECI}-\omega \times r_{ECI}$. In my belief ...
709 views

### In which direction does the x axis of the ECI at 1.1.2000 at 12:00 (J2000) go and where is the 0 deg meridian?

I am currently getting a bit lost in reference frames and coordinate systems. From my point of view, I am currently in the ICRS frame. I know that the ECI from my point is only moving but not rotating,...
1 vote
39 views

### Calculating angular deviation from LEO magnetic field

The situation I need to calculate pointing errors for a satellite that points along Earth's magnetic field. The satellite's position and orientation are given to me w.r.t ECI J2000. The orientation is ...
2k views

### How to find ECI coordinates for a circular orbit

Suppose a satellite is orbiting Earth in a circular orbit, of which I know the inclination $i$, the right ascension of the ascending node $\Omega$, the semi-major axis $a$ (or orbit radius given that ...
704 views

### How to solve the two-body problem in the ECI frame through numerical integration?

I need to know how to solve two-body problem by solving a system of first order equation derived from the equation below. $$\ddot{\mathbf{r}} = -\frac{\mu}{r^3}\mathbf{r}$$ How do I go about this, and ...
5k views

### Are ECI and ECEF both frames and/or coordinate systems? Is there a difference?

I don't know really know the difference between a frame and a coordinate system. I'd proposed ECEF and ECI tags, do we need them? Frames seems to be the standard in meta but there is concern so any ...
308 views

### J2000 to current ECI transformation matrix

I want to find the transformation matrix that converts an ECI frame at some time 't' (lets say t = 1st January 2020) to J2000 ECI frame (that is ECI frame at noon of 1st January 2000). From what I ...
108 views

### Could the orbit of a satellite place it on the z-axis of the ECI coordinate frame (x=y=0)?

It is mostly a hypothetical question; I assume the answer is no, but I am looking for any data to contradict my claim that there are none or at most only one satellite at a time which could ever ...
1 vote
1k views

### Accuracy of direct ECI (Earth-Centered Inertial) to LLA (Longitude, Latitude, Altitude) conversion via sub-satellite point?

The process of converting between ECI and LLA is generally known to be complex, as it involves a intermediate conversion to ECEF (Earth-Centered, Earth-Fixed). This, in turn, involves the ...