# Tag Info

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### Is 75 meters an exceptionally close distance for two satellites to pass at >6,000 m/s?

No, 75 meters is not exceptionally close — several satellites show predicted passages that close or closer every single week of the year. But it is close enough to potentially appear in the top 10 ...

### Is 75 meters an exceptionally close distance for two satellites to pass at >6,000 m/s?

Exceptionally rare? No. Uncommon, well, yes. From what I can recall, these happened about once every 10 years per satellite for the constellation of satellites I worked with, at least, prior to doing ...
• 121k

### How can I plot a satellite's orbit in 3D from a TLE using Python and Skyfield?

SGP4 is the standard procedure that TLEs are intended to work with. All of the following are extremely helpful, but the most important point would be use a standard, recent SGP4 package that is ...
• 148k

### Is 75 meters an exceptionally close distance for two satellites to pass at >6,000 m/s?

75 meters isn't all that rare - for the full public catalog there are 75 meter predictions about 2-3 times per day. What you have to consider is the uncertainty in that number. For TLE data, that's ...
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### Was the time of Schiaparelli's landing chosen specifically so the Giant Meter Wave Radiotelescope could listen?

No. The landing time is determined by the interplanetary trajectory, largely fixed by launch energy constraints, and then by the landing site location, mainly the longitude. This paper identifies the ...
• 58.2k
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### Does it even make sense to talk about Mercury's triangular libration points (L4, L5)?

You're right, the Sun-Mercury libration points (all five of them) are merely mathematical curiosities of a hypothetical two-body system. As you've calculated, the actual gravitational effects of ...
• 1,756
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### How can I plot a satellite's orbit in 3D from a TLE using Python and Skyfield?

Code updated for python3* ...
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### How can I calculate if a satellite is currently in sunlight or eclipse using pyEphem or Skyfield (preferred)?

As a new user I cannot comment or tweak the original answer, so I'll try my own. Happy update per any recommendations that come up. @uhoh's answer is close, but a few things to note. The TLE tells ...
• 441

### How can I plot satellite's trajectory from three different TLEs to detect any deviation on path with time?

You're plotting the orbit of a geosynchronous satellite in active commercial use. At the scale you've drawn it, it has to overlap exactly, because variation smaller than you can see at this ...
• 8,057
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### Has Skyfield or any other SGP4-capable open-source Python package yet wrapped/implemented the new, improved SGP4 (version 8+)?

The new SGP4 library comes with a Python wrapper in the same zip archive file (also Java, Matlab, and Visual Basic, and documentation for the C API), but it's not at all idiomatically Pythonic, since ...
• 8,057
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### What exactly is the interaction that blocked Juno's data downlink near solar conjunction?

This article claims that it's the charged particles emitted by the sun that actually block communication with Juno. https://www.space.com/38668-juno-8th-jupiter-science-flyby-success.html This ...
• 2,294
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### Skyfield, how to use https:// instead of ftp://?

Looks like this was an issue that was fixed and closed recently so try updating your copy of Skyfield to at least version 1.31.
• 2,506

### How big is Mars as seen from Phobos?

We can start by noticing that Mars, Phobos, and the horizon forms a right-angled triangle. By trigonometry, the angular radius ($\alpha$) is then given by: \alpha = \sin^{-1}\left(\frac{r_{parent}}{...
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### satellite ground track calculation

Possibly a bit late, but it seems like you're 99% of the way there. All you need is a loop of some kind to feed timescales into the satellite.at method. But, if you ...
• 5,544

### Local observer coordinate system at satellite panel (LVLH coordinate system)

So after help from @uhoh, digging into this post and the discussion here, I managed to produce this minimal working example. Comments appreciated. ...
• 161

### Skyfield, how to use https:// instead of ftp://?

For those who found this question because they wanted the IERS data, from one of the main sources of space data in the world, here are the lower-level details about how to get ...
• 761
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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 ...
• 441
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### JPL Horizons original reference orbit data for Juno - how to retrieve now that it's been updated?

Comments from @oefe pointed me to the solution! I'll post it here. Since I'm not an expert I'll keep the explanations minimal to avoid saying anything misleading. A good reference is the SPK required ...
• 148k
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### Need help getting the true ICRF coordinates of SOHO using Horizons

Change the "Table Settings". You want to select reference plane: Earth mean equator and equinox of reference epoch reference system: ICRF/J2000.0
• 75.3k

### How to obtain UTC of the epoch time in a satellite TLE (two line element)?

Celestrak states that time is UT. "https://celestrak.org/columns/v04n03/#FAQ03" UT is astronomical (more precisely rotational) time. It is determined by observations dependent on earth's ...
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### What is the difference between GCRS and J2000 frames?

Second question first: About the difference between GCRS and ITRS: is GCRS an ECI frame, and ITRS is an ECEF frame? Correct. What is the difference between GCRS and J2000 frames? Which is ...
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### How can I plot a satellite's orbit in 3D from a TLE using Python and Skyfield?

The answer given before seems to work great, I am just here to give a different answer if you're more interested in learning about the software that goes into numerically propagation trajectories and ...
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### Ephemeris output from Skyfield

Skyfield, like most other tools that claim the ability to process TLE data, depends on a bunch of people's educated but aged guesses about what SGP4 might have been doing some time ago, rather than ...
• 8,057

### Python - Beyond Orbit raises TleParseError for valid TLE

It appears that beyond python package considers a TLE that has a zero in character 3 of line 1 in the TLE to be an invalid TLE. According to the beyond package, ...
• 5,544
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### What are the product terms shown here representing an orbit?

Those numbers are the altitudes of perigee (lowest) and apogee (highest). They are given as heights, $h_p$ and $h_a\$, above Earth's surface. This requires you to assume a value for Earth's radius, ...
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### Does it even make sense to talk about Mercury's triangular libration points (L4, L5)?

Reference 1, citing Reference 2, reports that hypothetical trojan-type asteroids are invariably unstable at the proposed Mercury L4 and L5 points, whereas stable Lagrange-point librations at least ...
• 8,535
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### What does it mean to ask for a date beyond epoch time?

A TLE describes the orbit of a body at a single reference point in time, which is called the epoch. It is straightforward to extrapolate the position of the object for some time forward or backward ...
• 170k

### How to calculate cone angle between two satellites given their look angles?

Computing the angle between two vectors is going to be difficult if you first transform their x, y, and z coordinates into angles, because you'll then need to dive in the formulae of spherical ...
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### A dataset for Earth satellite size information?

JSpOC - now 18th SPCS - uses following values: Payloads and platforms (5meters), rocketbodies and unknownobjects (3meters), debris(1meter) The catagorisation of each object can be found in the NORAD-...
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