# Tag Info

## Hot answers tagged ephemeris

13

I want to extract the Chebyshev coefficients for the positions of the Moon and Sun relative to the Earth from a SPICE kernel (mainly because DE430 is too large ...) You've described almost half (44.8%) of DE430 with just those three items, the positions of the Moon and Sun relative to the Earth. This is not why DE430 is so large. The primary reason DE430 is ...

13

The deviations are real and directly related to the size of valleys on the Moon. The simple calculation you do here does only work for a point-like light source that projects the shape of the Moon onto Earth. In case of an eclipse, the light source has almost the same apparent size as the object. For this eclipse, the magnitude is given as 1.03. This is the ...

12

You can parse and use those ascii JPL ephemeris files. Warning: This is an exercise for people who like to torture themselves. If you aren't into self-flagellation, you should do what Mark Adler wrote and use the SPICE toolkit. I went through this self-flagellation over a decade ago, when SPICE was closed and wasn't what it is now. Now people are thinking ...

12

No, the first step is to download the SPICE Toolkit. It will read the ephemeris and generate the data you need for you, as well as do much, much more. There are many formats used in SPICE kernel files, not just the particular one you're looking at, which is probably a Chebyshev position-only array (type 2). There are 16 types. The values can be stored ...

10

The list in your question is a decent starting list, but there are more to add, and I'd go so far as to say they should be ranked by total usage. My list is: CCSDS OEM SP3 - Heavily used for GPS (NGA and IGS) SPK - Often for interplanetary/deep space, but a lot of use STK - Internal format for AGI, but widely used, and read by other tools JSpOC - JSpOC ...

9

Using the SPICE toolkit and a recent ephemeris file like DE430 (a 120 MB download). You will also need a recent leap seconds kernel. The toolkit provides interfaces for Fortran, C, IDL, and Matlab, but not Mathematica. You can use the C version of the toolkit, and Mathematica's MathLink or LibraryLink to access the toolkit. There are many, many functions ...

8

CelesTrack has Mir ephemerides (as TLE, two-line element sets) in its NORAD Two-Line Element Sets Historical Archives. It's a 755 KB in size ZIP archive (direct link) packing a text file with 22,333 TLE spanning time period from February 19, 1986 to March 23, 2001 when it was deorbited. If these don't go far enough back in time for your needs, you can ...

8

What's wrong with including some of the SPICE code? It is a library, so it would only link in what you need. SPICE can be used freely in any products, including commercial, for-profit use. Ideally you would still use the SPICE code to get states from the kernel. Then you can just edit down the kernel to include only the bodies and time spans of interest. ...

7

The current accuracy of the JPL Developmental Ephemerides released in September of 2013 is given by its authors in their article The Planetary and Lunar Ephemerides DE430 and DE431, quoted below: The present-day lunar orbit is known to submeter accuracy through fitting lunar laser ranging data with an updated lunar gravity field from the Gravity ...

7

This answer is based on a book called "The Design and Engineering of Curiosity" https://doi.org/10.1007/978-3-319-68146-7 On chapter 6 it is mention that The Hazcams and Navcams are flight spares or build-to-print copies of the engineering cameras of the same names on the Mars Exploration Rovers; this not only saved money in hardware, but made it ...

6

I'll add this to the other answers, which do a very good job of explaining why this very irregular shape is not inconsistent with the Moon's original much "rounder" shape. This could be done with some geometry and math, but I was lazy and did a rendering in Blender. I made a spheroid and added some bumps, then added a Sun lamp above and screen far below. I ...

6

Aside from numerical issues, "With Sun as centre" may be part of your problem. Get all the data from Horizons relative to the Solar System Barycenter, not the Sun, which moves relative to the barycenter. That barycenter is an inertial frame of reference, whereas the center of the Sun is not. Also make sure that you are putting in the initial position and ...

6

Not seeing that. I downloaded de438.bsp, and it in fact uses only Чебышёв position polynomials. (Or Chebyshev, Chebychev, Chebysheff, Chebychov, Chebyshov, Chebycheff, Chebyschev, Chebyschef, Chebyscheff, Tchebyshev, Tchebychev, Tchebysheff, Tchebychov, Tchebyshov, Tchebycheff, Tchebyschev, Tchebyschef, Tchebyscheff, Tschebyshev, Tschebychev, Tschebysheff, ...

6

The $a_i$ are the coefficients for the Chebyshev approximation. As you say, NASA gives us those. That is what you find in the DE files, e.g. de430.bsp. (Don't click on that unless you want to download a >100MB file.) NASA/JPL needed a way to provide high resolution and high accuracy functions of time for the positions of the planets, and the most compact way ...

5

I recommend searching for the string 1962 in https://www.ucolick.org/~sla/leapsecs/timescales.html to see the list of changes that happened to the way time was reckoned as of 1962-01-01. To save you some time, that culminates in: coordination of UT on 1962-01-01 Following the direction of the IAU, all observatories in the world adopted new longitudes ...

5

You can find a C++ method contained in the source code of Andrew Holme homemade GPS receiver project. The method is called GetXYZ and is the EPHEM (ephemeris) namespace and looks as the following: void EPHEM::GetXYZ(double *x, double *y, double *z, double t) { // Get satellite position at time t // Time from ephemeris reference epoch double t_k = ...

4

Based on this answer in spice-discussion, one must input the time explicitly as TDB, like this: tdb = '2017-09-01 12:05:50 TDB' et = spice.str2et(tdb) Then the results match exactly: CSPICE_N0066 [ 19219587.76713952 97944303.7160171 42842235.3192082 ] [-34.6088113 4.64408183 4.27890913]

4

The outline of the lunar umbra on Earth (shadow of totality) will be defined by those rays of light emanating from the very edge of the solar disk as seen from Earth which just miss the surface features at the lunar terminator (line between day and night on the Moon's surface). Because the Moon is some 500 times smaller than the Sun (by diameter), those rays ...

4

Yes, we have good orbital elements for Sedna.

4

The best and most concise answer is here The "gold standard" is the SPICE toolkit. You can read more about using spice in this excellent answer. The JPL Development Ephemerides are lists of the Chebyshev coefficients you seek, along with some additional information. They can be downloaded as text files or binary. If you are familliar with Python you can ...

4

The associated script, mcc.py, appears to get the reference frame wrong but does get the units correct. An imperial foot and a US customary foot are the same unit. (This is not the case for all imperial vs. US customary units.) What is PROP_MAN 11.0? If 11.0 is a version, are there other versions I might encounter? I can't help you with that. Is M50 ...

3

As Brian Carcich noted, you will need to learn and use SPICE. There is a bit of a learning curve, but it is well worth it. You can find the latest kernel here. The latest CATT_*.BC file in that directory has the detailed orientation and rotation information for the comet. It looks like a new one is uploaded every three or four days. Look at the readme there ...

3

Yes, SPICE is your best bet. The learning curve is steep but worth it. You can start at the SPICE Required Reading. and look at the PCK, FK and SPK required reading files. That will lead you to the other required reading files. You also will need this Frames Kernel (FK) Caveats: That is not much guidance, and assumes you are a self-starter, but this ...

3

I figured out,It was something wrong in the code: The function observer.next_pass(sat), return a vector of 5 values 0 Rise time 1 Rise azimuth 2 Maximum altitude time 3 Maximum altitude 4 Set time 5 Set azimuth So the instruction if Time >= ephem.localtime(st) and Time <= ephem.localtime(rt): I was using a innecesary comparation ...

3

From the point you are, you need to: Calculate the XYZ coordinates of your ground station Find the line in space between the satellite and the ground station Find the XYZ coordinates of the point at the intersection of that line with the ellipsoid defined by all the points with elevations of 300 km Find the latitude and longitude of that point. For (1) you ...

3

The ast343de430.bsp file looks to be fine. There is only one thing of note - all bodies in this file are referenced to the sun (10) but the ephemeris of the sun is not included. (The reason is given here - the author of this file considers the position of the Sun to be better defined than the SSB). For this reason an additional .bsp file which contains the ...

3

The exact format of the SPICE Toolkit SPK files (also called BSP files) is called DAF (Double-precision Array File). It is a binary format made up of consecutive blocks of 1,024 bytes. The first record is always the file record, then it has some blocks serving as a comment area, then it has the array records, and finally a collection of data records. [FILE ...

3

SPICE has multiple tools for constructing and using piecewise Lagrange and Hermite polynomials because those techniques are textbook. In contrast, techniques for constructing piecewise Chebyshev approximations are anything but textbook. SPICE supports one very specific format of using (but not constructing) piecewise Chebyshev polynomial approximations. That ...

3

The required equations and much more are contained in the GPS Interface Control Document (IS-GPS-200L Table 20-IV. Broadcast Navigation User Equations), available on the GPS Website: https://www.gps.gov/technical/icwg/ This is the source of the equations given above by cy8berpunk

Only top voted, non community-wiki answers of a minimum length are eligible