Chang’e 4 in Lunar Orbit

After a few days of head scratching the data being received from the Moon by amateurs clearly shows Chang’e 4 in lunar orbit, just not the expected orbit.  The plane of the Chang’e 4 appears to be about 90 degree off of what was expected based on the Chang’e 3 trajectory assuming we understand it correctly.  Communication activity appears limited to direct communication with Earth on X-band as there have been no signs of activity from Queqiao or on S-band from Chang’e 4.  Chang’e 3 has made some brief appearances apparently being told to keep quiet while Chang’e 4 gets ready to land.

I knew I should’a taken that left turn at Abeir-Toril

After the LOI burn we didn’t hear anything from Chang’e 4 for a day or so when amateurs Edgar Kaiser DF2Mz and Paul Marsh M0EYT detected signals from the direction of the Moon that revealed an object apparently in low lunar orbit that was not being eclipsed by the Moon as it orbited.

sinceloi

The above plot shows a model of the estimated current orbit of Chang’e 4 with the data from DF2MZ and M0EYT.  The blue line is the modeled Doppler behaviour if the spacecraft was not eclipsing.  The yellow marks indicate the expected Doppler and signal copy from M0EYT’s station. The green and red are data points shared with me by the respective stations.

While at first the data may not appear to fit the orbit well one needs to reflect on the fact that the Earth has been in communication with Chang’e 4 during some of the windows and this will double the observed expected Doppler frequency.  When you look closer the model really does fit the data fairly well and only needed tweaks to True Anomaly to get a decent orbital model. As the fitting process is tedious with my limited tools I’m waiting to get some more data so the refinement will be better.

Therefore, Chang’e 4 entered lunar orbit with  it plane face onto Earth which allowed for constant view of the spacecraft from Earth for the first approximately two days in orbit around of the Moon.

Hello, operator, get me the Moon…

As we’ve noted here the Chinese have placed a relay satellite, Queqiao, in a halo orbit around the Earth Moon L2 point to allow for communications with the lander once on the far side of the Moon.  To date based on limited observations we have seen no evidence Queqiao being involved in communications with Chang’e 4.  Communication appears to be direct with Earth.

queqiao_dec16

The Queqiao curve above shows little ground locking activity while Chang’e 4 is over North America.  Somewhat unexpected considering we know that the Chinese have a shiny new tracking station in Argentina.

After discussing the use of ITI allocations with radio amateur F4DAV he was kind enough to put together a map of the allocations for the Chang’e 4 communication plan as co-coordinated with the ITU.

lunar comms

Credit: F4DAV, twitter @F4DAV

Uplinking signals were reported by way of Earth-Moon-Earth reflection by Paul Marsh M0EYT. This communications chatter likely explains the signal from Chang’e 4 having twice the amplitude in frequency for it’s Doppler characteristic for most of the communications M0EYT noted on Dec 16th.

eme.png

Over the past day or so Chang’e 4 has shifted its frequency from 8470MHz to 8479MHz as apparently the Chinese commanded Chang’e 3’s lander to remain silent so as not to interfere with Chang’e 4 operations. Some activity was noted from the Chang’e 3 lander on 8470MHz while Chang’e 4 remained active on 8479MHz.

Moving Forward with Observations

As the mission evolves we are planning to pay much more attention to any correlation between the operation of Queqiao and Chang’e 4.  Also I would suspect at some point orbit changes to be made.  As we gather more data over the coming week I hope to refine the orbital model and be able to track these changes and perhaps even get an idea of when they can attempt to land at their chosen destination.

 

 

Author: Scott Tilley

Amateur visual and radio astronomer, radio amateur VE7TIL

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