# Tag Info

31

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 list of satellite conjunctions for the week: http://celestrak.com/cgi-bin/searchSOCRATES.pl?IDENT=NAME&NAME_TEXT1=&NAME_TEXT2=&ORDER=MINRANGE&MAX=...

28

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 a collision avoidance maneuver. That of course included debris/ satellite close approaches. Of course, I note that the two you listed are both live satellites,...

16

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 recommended, do not try to use the elements in a TLE yourself. This is becuase the TLE and the SGP4 package are built for each other. Documentation webpage TLE ...

13

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 75 meters +- several kilometers. The 75 meter miss distance is from the nominal position of each object at closest approach. But TLE data has a significant ...

10

The best guess available about the time scale used in TLE files is from the epic Revisiting Spacetrack Report #3 report from Celestrak, which they have put online here: http://www.celestrak.com/publications/AIAA/2006-6753/ They took the old SGP4 software, collected every single patch and improvement they could find in the dozens of versions online and ...

9

First: the main problem with your technique of subtracting the two positions is that it does not account for light-time travel. In the case of the Moon observation that you set up in your code, the error is only about 38 km, which might be well within your tolerances. The way to ask for a relative position that is properly light-time backdated is through the ...

8

Given: 16031.25992506 The 16 corresponds to 2016. As 1957 was the first year with satellites launched, 57 would be 1957, and in 2057 this might change, as there will be an issue. The 31 means the 31st day of the year (January 31) The .25992506 is the fractional day from midnight. This means 6.2382 hours, 14.292 minutes, 17.52 seconds, basically ...

7

I often have to go read the Skyfield guide to dates and times to keep everything straight, but, briefly: A Julian Date is simply a rival way to name moments, that is simpler than our customary date-plus-time system. Normally, to specify a date and time requires six different numbers — year, month, day, hour, minute, and seconds — and comparing two dates ...

7

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 landing site selection criteria, which were almost entirely about the safety of the landing. And appropriately so for an EDL demonstration mission. The landing ...

7

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 us the number of orbits per day is 15.50995519 (line 2 columns 53–63) $$\frac{24 \frac{hours}{day} * 60 \frac{mins}{hour} }{15.50995519 \frac{orbits}{day}} = ... 5 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 Venus and Earth (both much larger than Mercury, but also farther away) make the two-body approximation "less than useful" for any real world systems. Even the ... 5 The term "Julian day" (or date) has three very distinct meanings. To historians, it typically means a date that is expressed in the Julian calendar, as opposed to the Gregorian calendar. For example, the Julian date 3 September 1752 is the same day as the Gregorian date 14 September 1752. To astronomers, it means the number of days since noon (in some time ... 5 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. 4 Code updated for python3* def makecubelimits(axis, centers=None, hw=None): lims = ax.get_xlim(), ax.get_ylim(), ax.get_zlim() if centers == None: centers = [0.5*sum(pair) for pair in lims] if hw == None: widths = [pair[1] - pair[0] for pair in lims] hw = 0.5*max(widths) ax.set_xlim(centers[0]-hw, centers[0]+... 4 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 article about Mars conjunction cites the same physics and points out more succinctly that it's the command uplink that operators are worried about corrupting. https://... 4 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 reading: http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html I was pointed to the following folder, which has solutions in kernel form as .bsp ... 4 Change the "Table Settings". You want to select reference plane: Earth mean equator and equinox of reference epoch reference system: ICRF/J2000.0 4 So after help from @uhoh, digging into this post and the discussion here, I managed to produce this minimal working example. Comments appreciated. from skyfield.api import Loader, EarthSatellite from skyfield.api import Topos, load from skyfield.timelib import Time import skyfield.functions as sf from sklearn import preprocessing import numpy as np import ... 4 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}}{r_{orbit}}\right) The angular diameter is twice that. Inserting the average radius of Mars of 3,389.5km, and the semi-major axis of Phobos of 9,376km, we have ...

3

JulianDate basically counts the number of days since Jan 1, 4713 BC 12:00 UTC in the Julian calendar. However, that is a bit hard to understand since it is so long ago, uses a different calendar, and you must take the transition form BC to AD into account. So we use Reduced JulianDate, which is another official standard, related by \$\text{Reduced JD} = \...

3

Celestrak states that time is UT. "https://www.celestrak.com/columns/v04n03/#FAQ03" UT is astronomical (more precisely rotational) time. It is determined by observations dependent on earth's rotation and is conventionally related to apparent sidereal time. The conventions are chosen keep UT synchronized with the sun. The United States Naval ...

3

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 the long/lat for a satellite orbit projected onto the Earth. Here's a quick script that shows how to use Skyfield's built in functions with plotting using cartopy to build the map and projections. ...

3

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 commonly used worldwide in satellite communications? The difference is small. The GCRF frame is essentially the ICRF frame, but with a slightly different ...

3

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 from that snapshot, but beyond that, limitations of the propagation model cause errors to grow, and the position of the object cannot be accurately estimated. ...

3

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-SAT-CAT, you can access via Space Track Another database is the ESA-Discos-DB, which bases on real dimensions and not a catagorisation. But you need to apply ...

2

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 trigonometry. Skyfield natively considers all positions to be x,y,z vectors, and it's often easier to compute if you leave them as "position" objects until you're ...

2

note: This answer addresses the question directly: How to calculate cone angle between two satellites given their look angles? If you need to use the look angles, this is a good way to do it. This better answer explains to the OP that if you are using Skyfield, that you should not use the look angles but instead use the coordinates in their original form....

2

Here is manual approach: Setup orthogonal coordinate system: Unit is 1Km (but it does not matter much). Origin is in ground station. x-axis points to 0° azimuth, y-axis to 90°, z-axis straight up. Fact: x-y-plane is tangent plane of Earth. Compute unit vectors pointing of each satellite. Beware of radians/degrees. Unit vector's x-coordinate is cosine of ...

2

MEGA note: My answer is totally wrong and should be unaccepted by any means necessary. Luckily @AllenKummer's answer explains why and gives a revised script. This was posted at about 7 AM local time and must have been pre-coffee; the period shown here is only 40 minutes, seeing that it's wrong would have been a no-brainer, which is indeed the state I'm often ...

1

Not necessarily "outdated" as much as simply exactly what the ever-articulate (see also) Brandon Rhodes has written: "X days from the epoch" (unsigned, could be either direction) because TLEs are sometimes post-dated to the future by as much as several weeks, and PyEphem may feel that it is too early to use that TLE as well as too late. note: pyephem (...

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