12 votes

A spacecraft is travelling at X units per hour. But relative to what exactly? Does it depend on the orbit? How?

Your concerns are all perfectly valid - giving just a number doesn't tell a lot. So, in all proper publications, the reference system has to be mentioned. Typically, the reference is the body the ...
asdfex's user avatar
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12 votes
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Transform ECI to ECEF

The Explanatory Supplement to the Astronomical Almanac has all the equations you need. Take a look at chapters 3 and 4. Keep in mind you need some clear definitions of what you mean by ECEF and ECI. ...
Carlos N's user avatar
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10 votes
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How is space mapped out?

There are many different coordinate systems (X, Y, Z axes as you refer to them). The center of the Earth is often used as the origin "center" of the coordinate system, at least for calculations ...
Organic Marble's user avatar
9 votes

Is it possible to plot the ground track of a satellite with just azimuth & elevation angles without range?

Yes. This is a classical astrodynamics problem of orbit determination. The technique you would use is called Gauss' method. It allows you to determine an approximate orbit from three timed ...
Tristan's user avatar
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8 votes
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Which side of Europa is which?

The standard for any tidally locked body, of which Europa is a member, is to have the 0 longitude be the point at the center of the planet-facing side. That being the case, the middle of the map ...
PearsonArtPhoto's user avatar
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8 votes
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ECI to LVLH conversion

An LVLH frame is easy to construct. One construction: Let $\boldsymbol r$ and $\boldsymbol v$ denote the spacecraft's position and velocity with respect to the center of the planet as expressed in an ...
David Hammen's user avatar
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8 votes
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Orbit Elliptical Fitting

What you are looking for is called orbit determination, and in particular batch least squares orbit determination. To learn about it I can recommend Statistical Orbit Determination by Bob Schutz, ...
yzokras's user avatar
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8 votes
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Is this commonly attributed ECI (Earth-Centered Inertial) to ECEF (Earth-Centered Earth-Fixed) transformation accurate?

This is a low accuracy approximation. You can tell by the bottom row and right column of the transformation matrix. There is no allowance for precession, nutation, or polar motion in this simple model ...
David Hammen's user avatar
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7 votes
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Accuracy of ECI (J2000) to ECEF (WGS84) for a LEO satellite only accounting for Earth Rotation

Are you modeling drag? If you are not, you don't even need to model the Earth's rotation because the J2 effect depends on latitude only. This is a low fidelity simulation (there are lots of effects ...
David Hammen's user avatar
  • 74.8k
7 votes
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Why was this point chosen as the origin for the Saturn V coordinate system?

For the H-1 engines of Saturn I/IB, it appears that at some point in development, the origin was ...almost... at the bottom of the engines, according to this drawing I found linked from AlternateWars: ...
Russell Borogove's user avatar
7 votes
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Is this a typo in the Python Skyfield API? "International Coordinate Reference Frame (ICRF)"?

The quotation correctly (though not completely) describes the International Celestial Reference Frame, so I'm comfortable calling "Coordinate" a typo.
Ryan C's user avatar
  • 7,942
6 votes

TLE SGP4 Coordinate Transformations

While this answer was written for a different question/project, it points out that the Two Line Element sets and the SGP4 propagator work together and each is designed specifically to work only with ...
uhoh's user avatar
  • 149k
6 votes

Can't find ISS position data API that uses the https protocol

I found this one here: https://api.wheretheiss.at/v1/satellites/25544 You can get position of all satelites include ISS. ISS number is 25544
Tores's user avatar
  • 61
6 votes
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Do geodesics close on an ellipsoid?

No, in general, they don't close. (Though, as you say, some geodesics do.) Consider, for example, an oblate ellipsoid of revolution, and take two points on its equator such that the angle $\alpha$ ...
Litho's user avatar
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6 votes
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How is the Celestial Ephemeris Pole (CEP) exactly defined?

Thanks for bringing this up. I am not an expert in this field (Geodesy), and to be honest, this term has given me headaches. Loosely speaking it is closely related to the "instantaneous" ...
Ng Ph's user avatar
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6 votes
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At what point on the Earth's surface does the +X axes of the J2000 coordinate system intersect?

I started this as a comment, but ran out of room. It doesn't actually answer the question, but explains why the question can't be answered in a meaningful way. I'm sure you're aware that Jan 1 isn't ...
Greg Miller's user avatar
5 votes
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How to calculate the current position of a satellite?

Here is information on an open source Java package that has code for TLE propagation: https://www.orekit.org/forge/projects/orekit/wiki/Tle It also contains classes for all the components you need ...
DuffBeerBaron's user avatar
5 votes

Transform ECI to ECEF

I know this is a old/dead post but I essentially took on the same challenge and when I googled the answer I kept being brought here. I wanted to add some information here so anyone in the future who ...
Dylan's user avatar
  • 301
5 votes
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Coord. system:collection of charts (local homeomorphisms to ℝ𝑛) for differentiable manifold 𝑀, frame:section of tangent vector fiber bundle over 𝑀

I'm sorry, I should have included more explanatory links in what I wrote earlier. When reading what I put together below, please keep in mind that each of these paragraphs is normally an entire grad ...
Ryan C's user avatar
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5 votes
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Coordinate systems for state vectors

Here's a partial answer until you add more information as requested in comments thanks to the numerical imprecision of Python I don't think you are anywhere near the limit of Python's floats. ...
uhoh's user avatar
  • 149k
5 votes

Where do I find ECEF coordinates of a satellite?

Where do I find ECEF coordinates of a satellite? You don't "find it." That information is not published. You'll have to compute it rather than find it. What is published for many satellites ...
David Hammen's user avatar
  • 74.8k
5 votes
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TEME state vector to TLE orbital elements

The problem with your approach is that TLEs just don't work that way. You seem to have made assumptions that sound reasonable in principle, but they are nothing at all like the assumptions that are ...
Ryan C's user avatar
  • 7,942
4 votes

Computing GEO satellite's longitude from elevation/azimuth from a given latitude/longitude?

No, you would need to know the range to the satellite as well. Think of it this way - if you draw a diagram of what you describe above, the vector representing the line of sight from your ground ...
CoAstroGeek's user avatar
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4 votes
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Better way to get approximate ground track for a satellite using Skyfield?

I know this is an old question but for funsies, here's a quick script. This question was asked when the .subpoint() feature wasn't supported in Skyfield to grab ...
Allen Kummer's user avatar
4 votes

ECEF to LLA Test Numbers

There are multiple reference ellipsoids for the Earth. Given a latitude, longitude, and altitude, the corresponding ECEF cartesian coordinates will differ with different ellipsoids. The reverse is ...
David Hammen's user avatar
  • 74.8k
4 votes
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How were the geodetic and geocentric latitudes of the Space Shuttle defined and calculated?

Given either geocentric latitude, longitude, and radius or geodetic latitude, longitude, and altitude, the computation of Earth-centered, Earth-fixed cartesian coordinates is fairly simple. For ...
David Hammen's user avatar
  • 74.8k
4 votes

C/Fortran library for ECI to ECEF conversion

SOFA The IAU’s Standards of Fundamental Astronomy (SOFA) library has a ANSI C and FORTRAN version. See the documentation on the C2T06A function for converting from ECI to ECEF. note: You must ...
cms's user avatar
  • 388
4 votes

LVLH to ECI Conversion

If you have a direct cosine matrix (also called a "rotation matrix") which converts from ECI to LVLH, then the transpose of that matrix will perform the opposite rotation: LVLH to ECI.
ChrisR's user avatar
  • 6,220
4 votes
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Accuracy of ECEF to ECI, using just GAST

You must be using a computer from the 1960s to have even the most precise IAU earth orientation computation take "5-10 seconds". That said, computing the Earth's orientation using an extremely ...
David Hammen's user avatar
  • 74.8k
4 votes
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How can Earth-Centered Inertial (ECI) coordinates be properly called "inertial" if the Earth's orbit is always accelerating towards the Sun?

How can Earth-Centered Inertial (ECI) coordinates be inertial if Earth's orbital motion is always accelerating? It is true that "Earth-Centered Inertial" is a bit of a misnomer. What this means is ...
David Hammen's user avatar
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