X Marks the Orbit?

Radio amateurs Edgar Kaiser, DF2MZ and Paul Marsh, M0EYT received a radio signal today from the Moon consistent with frequencies published by the ITU for the Chinese lunar exploration program.  However, there’s a catch.  The data does not fit the orbit that it should be in.

Day 2 of Chang’e 4 in Orbit?

Edgar Kaiser, DF2MZ posted to twitter a very interesting plot showing a sinusoidal signal on 8470MHz on X-band.  This is a co-ordinated frequency by China with the ITU for Lunar Lander.

edgar tweet

There are a few interesting things to note in this image:

  1. The frequency reversal timings closely correlate to those of the object on 2210.81MHz we are also monitoring. [UPDATE] – The period of the observed orbit is ~134 minutes.
  2. With the jump in frequency we are not seeing a ‘stretching’ of Doppler rate.
  3. The signal is visible for the entire orbital period.

Based on what we know about the state vector at injection the orbital plane suggested by this data is 90 degrees out.

Qualitative Analysis of the Data

As we all know the Moon orbits the Earth.  Its rotation is tidally locked to the orbital period of its rotation around Earth so the same face of the Moon faces us all the time.

When you place a satellite in lunar orbit the satellite will rotate around the moon and the moon will turn on its axis under the orbital plane of the satellite.  Something to understand clearly is the plane of the orbit of the satellite around the Moon is fixed in respect to the stars.  So as the Moon goes around the Earth we are presented with different perspectives of the satellites orbit.

If the satellite is in a polar orbit like LRO and now Chang’e 4, then the plane of that orbit can be seen face on at times and then a fortnight 7 days later we will see it edge on.  Imagine looking a the edge of dinner plate lined out so you can just see the edge…  Now slowly rotate it 90 degrees over 14 days and you’ll note you are looking at the face of the dinner plate and the cycle repeats every fortnight 7 days.

Here’s a VIDEO showing the relationship using NASA’s GMAT software loaded with my best guess of Chang’e 4’s orbit.

A few days ago a graphic of the Chang’e 3 lunar transfer and injection orbit was shared on Twitter.  This clearly shows an energy efficient injection into a polar lunar orbit this the plane initially edge on as one would expect.


As you can see from this plot the object arrives at the Moon over the south pole and performs its Lunar Orbit Injection burn.  From there it enters a polar orbit with its plane pretty much edge on to Earth.  The Perilune will be somewhere over the south pole where the LOI burn occurred.


This fact means that the spacecraft will be eclipsed by the Moon for a period of time for each orbit and should produce a Doppler plot like the following sample of data from Chang’e 5T-1 when the orbital plane is seen edge on from Earth and goes behind the Moon each orbit.



As you can see from this plot below with the plane of the orbit rotated 90 degrees to what was expected we can see how the geometry of this special case could allow the orbital plane to be revealed entirely to Earth.  This geometry could explain the data observed by amateurs today.



The fact that the data describes a special case in the orbit where the spacecraft’s plane is aligned in such a manner as to be visible from Earth for its entire orbit is unlikely for two reasons.

  1. They would have needed a very large deltaV to change the orbit from the injection trajectory.
  2. The period of time a low altitude orbit like this would present the entire plane to Earth would be brief and measured in minutes to a few brief hours.

The fact the timings of the frequency reversals appear to correlate to the reversals of the signal on 2210.81MHz remains a mystery.

Also investigation of the behaviour of Queqiao should be begun to see if there are correlations between frequency shifts and its frequency behaviour.

[UPDATE] As the Doppler characteristics of Queqiao are profoundly different from the signal observed we can rule out it as the source.  It too would also see Chang’e 4 pass through eclipse with the Moon every orbit so the same logic applies as the Earth facing argument.

This all suggests we need to develop a better model of the observations and expected outcomes.  So we are presently working to develop a full model in NASA’s GMAT software to investigate these questions quantitatively.  If you are interested on contributing observations, insight or analysis please feel free to reach out.

One thing’s for sure, this is certainly a Riddle in the Sky!



Author: Scott Tilley

Amateur visual and radio astronomer, radio amateur VE7TIL

2 thoughts on “X Marks the Orbit?”

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