Tracking Chang’E-1 from Sweden

Sven Grahn
Chang'E-1, China's first lunar probe, was launched at 1005 UT on October 24 on a Chang-Zheng 3A from the Xichang launch site in the Sichuan province. Its initial orbit was 221-50602 km  at 31.0 degrees inclination. On the 25 th the perigee was raised and the orbit became 584-50566 km (1.5 days period), still at the same inclination.  By Oct 26 the orbit had been raised to 594-71317 km (one-day period).

Before launch I had checked the ITU filings of China and found these frequencies  listed for an object called CE-1: 2209.80, 2234.52, 2254.00, 2280.00, 2287.00 MHz. Dick Flagg in Honolulu was the first to pick up Chang'E-1 at 2230 UT on October 24 on 2234.533 MHz.  Shortly thereafter also Grady Whitney in Texas. After that, the following day,  Bob Christy in the UK and Greg Roberts in South Africa also picked up the signal. Dick Flagg observed up to 5 sidebands visible on either side of the 2234.52 MHz carrier.

Strength compared to main carrier as follows:

+/- 33kHz  weak
+/- 66 kHz strong (almost equal to carrier strength)
+/- 99 kHz moderately strong
+/- 132 kHz very weak
+/- 165 kHz weak

Chang'e-1 increased its Earth orbital apogee to 120000 km (2-day period) on October 29 at 1001 UTC, and again to a 380000 km apogee at 0915 UTC on Oct 31. It entered a 210-8600 km polar lunar orbit at 0337 UT on November 5.

27 October 2007

When I switched on the receiving system in the morning  (local time) on 27 October 2007 I had little hope of picking up anything from the Chinese lunar probe Chang’E. The elevation above my horizon would barely touch the 30 degree lower limit of my helix coverage. But I immediately picked up  a weak carrier and set the SDR-14 to move really slowly so that I could get a good overview of the Doppler curve. I had to do gardening work (cutting a high hedge) during the hours before noon, but when I came into the house at 0920 UT after finishing the hedge work I saw that there had been a frequency shift, a Doppler maximum and Loss-Of-Signal. I had run predictions for Doppler shift and elevation angle using the original element set for Chang’E (actually a secondary object but it had roughly the same orbit as the moon probe) and those predictions showed no Doppler maximum or setting below my horizon before 1000 UT. Clearly, if this was Chang’E, the orbit must have changed. I logged into Space-Track and found that this was indeed so. A simulation with the new orbital elements confirmed that I had indeed seen Chang’E (read more here).

The Doppler maximum occurred at about 0850 UT. What happened? Somebody manipulated the uplink and a second maximum appeared! The signal strength was low, because the spacecraft was near the lower elevation limit of my 3.5 turn helix – 30 degrees.


Here are the elevation and Doppler plots for the morning hours (UT) of 27 October 2007 using the element sets after the first big orbital maneuver. But the frequency was much higher at Doppler maximum than predicted by the software for non-transponder operation, the observed value was 2234.58 MHz. The Doppler shift without transponder action would have been +28 kHz and with transponder mode about twice that, which corresponds roughly to the frequency observed.

28 October 2007

I went on watch at 0609 UT, and found a carrier present at 2234.554 MHz.  I Set SDR-14 at 10 kHz filter bandwidth and 4 secs/update..... FFT Average=2, skip N=2 updates. Carrier/Noise (C/N) then came out nicely at 8 dB.
 The spectrogram on the right shows the frequency change at ground station handover and the Doppler maximum. 

29 October 2007 - Chang'E picked up just before another raising of the apogee

I switched on the tracking gear as soon as I had finished breakfast. The signal from Chang’E was there at 0652 UT on 2234.558 MHz, very weak coming out of the noise. It kept fading in and out but finally near 0900 UT the signal strength improved and the Doppler curve showed signs of approaching a maximum.As the morning passed, I thought I had lost it, but finally the signal strength grew and the Doppler reached a maximum:

The maximum occurred at 0916.48 UT at the frequency 2234.5871 MHz, about 6 minutes later than the day before. The signal rapidly faded after that at disappeared at 0917.30 UT, almost exactly as the day before...so still no maneuver. Chinese media announced that a maneuver to a 48-hr orbit with apogee at 120000 km was planned and it was reported to have occurred at 1001 UT, 43 minutes after I lost the signal. Would I be able to pick it up when it comes back to earth in two days?
 
Bob Christy found out that the maneuver to the 48-hr, 120,000 km apogee orbit took place at 1001 UT (See map below). He also found a Chinese paper in which the equivalent isotropic power of the transmitter is 20 Watts with a possible increase by 5 dB in lunar orbit. Some simple arithmetic shows that a 20 dB dish should be sufficient to see the carrier in lunar orbit.
 

31 October - swinging by Earth

Chang'E-1 came back to Earth's vicinity from the 120000 km apogee early on 31 October 2007. I gambled on the possibility of picking up signals and did so on 2234.559 MHz immediately when I went on watch at 0542 UT. The carrier-to-noise ratio was then 0 dB. The frequency and signal level was increasing. Here is my log:
I "dropped track" at 0705 UT. Just over two hours later another rocket impulse was made to bring the apogee of the spacecraft out to 380000 km.

The map below shows where the spacecraft was when I picked it up in Stockholm on 31 October 2007. The graph below the map shows the elevation plot in Sollentuna. Obviously the transmitter was very strong, because I picked it up with my vertically-pointing 3.5-turn helix at an elevation of 10 degrees, well below the normal 30 degree elevation cut-off of my antenna.
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