Author Topic: LIVE: TG-2 Tiangong-2 - CZ-2F/T2 - JSLC - September 15, 2016 (14:04 UTC)  (Read 203619 times)

Offline linxiaoyi

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During April 4-6 the orbit of Tiangong 2 was raised from 92.12 min, 374-382 km to 92.26 minutes, 379-391 km which is slightly above the 46-circuits repeating orbit period.

Presumably this is in preparation for the Tianzhou 1 launch - still scheduled for April 23?

According to speculation here, in 23 April 19:40
http://www.spaceflightfans.cn/wp-content/uploads/2017/03/148990140.jpg
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Offline Phillip Clark

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Leonard David is reporting that the second refuelling test from Tianzhou 1 to tiangong 2 has been completed:

http://www.leonarddavid.com/china-in-orbit-auto-refueling-second-test/
I've always been crazy but it's kept me from going insane - WJ.

Offline Galactic Penguin SST

Copying from the TZ-1 thread:

While all eyes were trying to find that missing comsat, TZ-1 and TG-2 flew under the radar and completed their 2nd docking test earlier today.

The two spacecraft undocked at 01:37 UTC, afterward TZ-1 retreated to the 5 km holding point for 90 minutes, then made a flyaround to the front of TG-2. After the two spacecraft yawed 180 degrees back to heads on position, TZ-1 re-approached and re-docked at 06:55 UTC.

Source

News report with video
Astronomy & spaceflight geek penguin. In a relationship w/ Space Shuttle Discovery. Current Priority: Chasing the Chinese Spaceflight Wonder Egg & A Certain Chinese Mars Rover

Offline Phillip Clark

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I am attaching a Chinese illustration which shows Tianzhou 1 docked with Tiangong 2.   Could someone please translate the captions for me?   Since the text is embedded in the illustration I cannot copy-and-paste it to Google Translate.   Many thanks!
I've always been crazy but it's kept me from going insane - WJ.

Offline omegaomega

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There are a few ways to translate text embedded in picture.
Method 1: Upload the image to Google drive, then right click it and select Open in Google Doc. Google Doc will automatically detect text and language and extract it into text format
Method 2: Use Google Translate Mobile APP, aim your phone camera at picture, it translates in real time. Or you can also upload picture to Google Translate APP
Method 3: Insert picture into Microsoft Onenote, right click the picture and select text. OneNote will try to extract text

I don't know any space flight terms, but I will try to translate your picture literally shortly.

Offline Phillip Clark

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There are a few ways to translate text embedded in picture.
Method 1: Upload the image to Google drive, then right click it and select Open in Google Doc. Google Doc will automatically detect text and language and extract it into text format
Method 2: Use Google Translate Mobile APP, aim your phone camera at picture, it translates in real time. Or you can also upload picture to Google Translate APP
Method 3: Insert picture into Microsoft Onenote, right click the picture and select text. OneNote will try to extract text
I don't know any space flight terms, but I will try to translate your picture literally shortly.

Thank you for these tips.   Since I don't have one of these modern phone thingies (:) ) I will investigate 1 and 3.
I've always been crazy but it's kept me from going insane - WJ.

Offline omegaomega

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I could be totally wrong on following translating. I am a news reader only.

天舟一號完成繞飛和第二次交會對接試驗
Tianzhou-1 completed circle flight and 2nd rendezvous test
(Note: Don’t know how to translate 繞飛, it is like two dancers circling each other and both make 180 degree turn)

6月19日9時37分 在地面決策指導 下,天舟一號繞 飛試驗開始實施
At 9:37AM, June 19, Instructed by ground control, Tianzhou-1 circle flight test started

地面首先發送分離指令,天宫二號停控,對接機構解鎖,兩航天器分離
Ground control sent separation instructions, Tiangong-2 停(stop)控(control), locking system unlocked, two devices separated.
(Note: Not sure the meaning of 停控, it could be stopping control of Tianzhou-1 by Tiangong-2, or something else)

天舟一號按程序逐步撤退至後向5千米,并保持約90分鐘
Tianzhou-1 retreated behind 5KM as programmed, kept it there for 90 minutes.

地面確認航天器狀態正常後,發令控制天舟一號開始繞飛,從後向5千米繞 飛至天宮二號前向5千米
Ground control verified that both devices were normal, then sent instructions to Tianzhou-1 to start circle flight test: From behind 5KM, to go around Tiangong-2, to ahead of 5KM

在此期間,天舟一 號完成偏航180° 轉 倒飛,天宫二號完 成偏航180°轉正飛
During this period, Tianzhou-1 changed direction by 180 degree, from forward flight to backward flight
Tiangong-2 also changed direction by 180 degree, from backward flight to forward flight.

6月19日14時55分 天舟一號貨運飛船完成與 天宮二號空間實驗室的繞 飛和第二次交會對接試驗
At 14:55PM, June 19. Tianzhou-1 and Tiangong-2 completed circle flight and 2nd rendezvous test

Offline Star One

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To avoid the no doubt unwanted fuss & attention that surrounded the end of their first space station is it known if the Chinese are planning to use a Tianzhou to shunt this one out of orbit? Would seem they could gather valuable data by doing so?

Offline Phillip Clark

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To avoid the no doubt unwanted fuss & attention that surrounded the end of their first space station is it known if the Chinese are planning to use a Tianzhou to shunt this one out of orbit? Would seem they could gather valuable data by doing so?

Tianzhou 2 is slated as the first cargo freighter to the Tiangong modular space station (NOT called Tiangong 3 by the Chinese!), to be launched after Tianhe 1 and before Shenzhou 12 in 2020.

So, the Chinese plan is to let Tiangong 2 use its own propulsion system to bring itself out of orbit, as should have happened with Tiangong 1.   Maybe they simply won't keep TG 2 in orbit as long as they were doing with TG 1?
I've always been crazy but it's kept me from going insane - WJ.

Offline Star One

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To avoid the no doubt unwanted fuss & attention that surrounded the end of their first space station is it known if the Chinese are planning to use a Tianzhou to shunt this one out of orbit? Would seem they could gather valuable data by doing so?

Tianzhou 2 is slated as the first cargo freighter to the Tiangong modular space station (NOT called Tiangong 3 by the Chinese!), to be launched after Tianhe 1 and before Shenzhou 12 in 2020.

So, the Chinese plan is to let Tiangong 2 use its own propulsion system to bring itself out of orbit, as should have happened with Tiangong 1.   Maybe they simply won't keep TG 2 in orbit as long as they were doing with TG 1?

The clarification is much appreciated.

Offline Phillip Clark

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The last altitude manoeuvre by Tiangong 2 was during June 20-21, 2017 when the altitude was raised from 92.280 minutes, 379-393 km to 92.432 minutes, 391-397 km while it was still docked with Tianzhou 1.

The orbit has now decayed somewhat and the period is now 92.256 minutes which compares with 92.249 minutes for the 46-circuits repeating orbit.

Of course, with no further spacecraft visits planned within the Tiangong 2 programme there is no reason for the laboratory to keep raising its orbit.   The altitude is roughly the same as that of Tiangong 1 when control with that station was lost, and therefore if there are no further orbital manoeuvres one would expect natural decay to be due in about 24-30 months time.   Of course, one hopes that Tiangong 2 will perform a controlled de-orbit manoeuvre at some time before this happens.
I've always been crazy but it's kept me from going insane - WJ.

Offline Phillip Clark

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To follow on from my posting on April 9, the orbital period of Tiangong 2 fell below the 46-circuits repeating orbit period on April 19.

At the risk of inspiring the "nutter"/"doom and disaster" brigade, if Tiangong 2 were to perform no further manoeuvres and the Chinese leave it in orbit until natural decay then comparing with the orbit of Tiangong 1 one would expect natural decay to come around September 2019 - plus/minus a couple of months.   However, I stress, there is NO evidence that there is anything wrong with Tiangong 2, this result is just a "thought experiment"!
« Last Edit: 04/20/2018 11:48 am by Phillip Clark »
I've always been crazy but it's kept me from going insane - WJ.

Offline Cristiano

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If we take a look at the decay rate of the two stations, we see that the prediction cannot be based on a comparison between the two stations, because the decay rate are much different:



[original size]

The graph shows the mean radius vector of the two stations obtained from the TLEs (950 TLEs for the Tiangong-1 and 913 for the Tiangong-2). The axis scales are exactly the same for both the stations.
The linear regression gives about 31 m/day for the Tiangong-2 and about 95 m/day for the Tiangong-1, so the reentry should take much more time for the Tiangong-2.

Here’s a possible reentry trajectory:



[original size]

The graph shows the minimum, average and maximum radius vector obtained from the propagation of the 4 most recent TLEs. Surprisingly, the 4 trajectories almost overlap after 1500 days (it seems that the graph only shows one trajectory).
It’s useless to say that the reentry date cannot be very accurate for a 1500 days prediction and that the solar activity can significantly change the reentry date.
All that said, it’s also useless to say that the reentry trajectories are based on a totally uncontrolled reentry (which is not the case for the Tiangong-2).

Offline Cristiano

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Here’s the first deboost:



[original size]

almost 90 km!
The graph shows the perigee, the semi-major axis and the apogee obtained from the TLEs.

The updated reentry trajectories (9 TLEs used):



[original size]

but it will change again.

The next graph shows the very big deltaV (about 50.4 m/s, the dots are the TLEs):



[original size]

According to the TLEs, the mean radius vector (which is equivalent to the semi-major axis) decreased of about 87.77 km.
« Last Edit: 06/19/2018 08:21 am by Cristiano »

Offline Olaf

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https://twitter.com/planet4589/status/1009492965151117312
Quote
China's Tiangong-2 spacelab made a dual-burn manuever on Jun 13 about 0645-0735 UTC, to lower its orbit from 380 x 386 km to 292 x 298 km

http://spacenews.com/china-appears-to-be-preparing-to-deorbit-its-tiangong-2-space-lab/

China appears to be preparing to deorbit its Tiangong-2 space lab 

Offline Cristiano

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If we want to see the orbit before and after the deboost, we just need to propagate the appropriate TLE with the SGP4 propagator.

Here’s the orbit obtained from the last TLE before the deboost (the dots are the calculated points with step size = 60 s):



[original size]

and here’s the orbit obtained from the first TLE after the deboost:



[original size]

I take the Earth’s mean radius used in geodesy ((2a + b) / 3) as the reference radius; in this case the orbit was 386.3 x 392.2 km before the deboost and 298.8 x 303.6 km after the deboost.
But if we like to use the WGS-72 equatorial radius (6378.135 km, WGS-72 is the reference ellipsoid used by JSpOC), we get: 379.2 x 385.1 km before and 291.6 x 296.5 km after the deboost.

Offline Phillip Clark

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But please remember that the TLEs give altitudes only to an accuracy of +1 km, so including decimal places in altitudes is meaningless.
I've always been crazy but it's kept me from going insane - WJ.

Offline Cristiano

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With the TLEs (to be always used along with the intended propagator: SPG4 or SDP4) we calculate the geocentric XYZ position (radius vector) components and XYZ velocity components in the TEME reference frame.
With an additional numerical algorithm, we can also calculate the altitude, longitude and the geodetic latitude for a given reference ellipsoid or geoid.

The +/-1 km uncertainty is the widely adopted value for the TLE epoch, but only JSpOC knows the actual uncertainty, because they don’t publish the covariance matrix for the position and the velocity components.
For many satellites, the uncertainty is much better than 1 km, while in rare cases the uncertainty for the TLE epoch could be as high as 2 or 3 km.

All that said, if you see that the orbit is 123.456789 x 987.654321 km, it means that those values are the (scaled) magnitude of the perigee and apogee radius vector as obtained from a particular TLE. It’s very important to understand that the magnitude of the radius vector is not the position of the satellite; it’s very important because most of the uncertainty component is in the along-track direction, while the radial component of the uncertainty is very small. It means that if the uncertainty in the position (3D radius vector) of the satellite is 1 km, the uncertainty in the magnitude of the radius vector could be 100 m or less.
For the Tiangong-2 (and many other LEO satellites) it’s very likely that the uncertainty in the position is much better than +/- 1 km.
« Last Edit: 06/21/2018 02:54 pm by Cristiano »

Offline Phillip Clark

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The +1 km error in the TLE altitudes came from Cheyenne Mountain, and I assume that they know what they are talking about.
I've always been crazy but it's kept me from going insane - WJ.

Offline Phillip Clark

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If someone at the NORAD told you that with the TLEs you calculate altitudes, it means that he was simply drunk.
With the TLE alone you calculate absolutely nothing.
With the TLE + the correct propagator (SGP4 or SDP4) you calculate the state of a satellite (position and velocity).
With the state of the satellite you calculate anything else (including its altitude).

They were not drunk, they knew what they were talking about.   There is no need for you to get insulting.   The comment related to whatever software you use to interpret the TLEs.
I've always been crazy but it's kept me from going insane - WJ.

 

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