Finally picking up Selene in Stockholm

Sven Grahn

Selene AOS 23 Nov 2007 1917 UT

November 23, 2007 - three passes across the visible face of the Moon

When I picked up my wife at the train station tonight I noticed that the Moon was very high in the sky and figured there was a chance to pick up the japanese spacecraft Selene that is orbiting the Moon. I turned on the waterfall (see the article "Finally on the air on S-band") at 2 sec/update at 50 kHz bandwidth (FFT bins 16384) and centered at 2263.602 MHz. At about 1917.50 UT a very weak, but definitely visible carrier appeared on 2263.614 MHz. A few minutes after acquisition-of-signal (AOS) the Doppler shift reached a maximum and shortly thereafter the frequency jumped up 4 kHz and then again 8 kHz up. The ground station locked up on to the receiver. There was Doppler minimum at about 2023 UT at 2263.587 MHz. a few minutes later (2027.30 UT) the frequency jumped up 11 kHz and at 2029.30 UT there was loss-of-signal (LOS). I guess this whole reception period represented a pass across the lunar disk. The signal was weak, but considering that my 3.5-turn helix points straight up, I guess it was OK. The signal could only be seen on the waterfall, not on the spectrum display.
So, I joined the select club that have tracked Selene. Thanks to the Moon for climbing so high in the sky.

On the next pass across the face of the Moon I certainly picked it up again. his time the signals strength was much higher. The Moon looked as if it was 65-70 degrees above the horizon. The pass lasted from 2115-2228.30 UT. The maximum frequency was 2263.625 MHz and the minimum was 2263.587 MHz. Just after that the frequency suddenly jumped up 11 kHz and the the probe slipped around the edge of the Moon as the signal disappeared. The figure on the right shows the initial part of the pass. Here it is also evident how much spurious signals and interference that is visible on the waterfall display. (By the way: 10.700 MHz  corresponds to 2263.602 MHz).

The Moon was still high in the sky so the next pass around the Moon came in loud and clear as well at 2314-0026 UT. Just as before the frequency jumped up about 13 kHz just before LOS.

November 24, 2007 - now it is routine!

The high elevation of the Moon made it impossible to stay away from receiving Selene again the following evening. The Selene signal appeared faithfully again and the same behaviour with a frequency jump just before LOS  was repeated. At times the C/N was 10 dB, which is amazing, but depedent on the 3.4 Hz bandwidth used. The reception times were:

1850-2004 UT
2045-2202.00 UT
2241-0000 UT

Estimating the orbital period and altitude from observations

By analyzing these reception times one can see that the probe came 27.8, 25.5, 30, 26.5, 33, 26 minutes earlier on the 24 th compared to the 23 rd . I think the"outliers" should be eliminated. The average  of the remaining numbers is -26.45 minutes, which corresponds to 117.8 minutes orbital period. This is equivalent to an average altude of 99.5 km, close to the announced figure of 100 km. By simple trigonometry the time that the probe is visible from eart on each orbit can be estimared. to be 71.3 minutes. The observed reception times are: 71.7, 73.5, 72, 74, 67, 69 minutes. I think the reception times that are much longer than 71.3 minutes are inaccurate because  I estimated  the times from "eyeballing" the waterfall display.

The reason for the frequency jumps - a hypothesis

Why does the frequency suddenly jump just before loss-of-signal? The velocity relative to earth when the probe disappears behind the moon is about 1.54 km/s.  Of course, at the equator the circumferential speed is about 0.45 km/s, but at 59 N and with the Moon at 70 degrees elevation, let us disregard this effect for the moment. So, if the Selene transmitter were not locked to an uplink the carrier would be shifted downwards by 11.6 kHz. If the donwlink were locked to a fixed-frequency uplink the frequency would be shifted downwards by another 10.7 kHz (one has to take the turn-around ratio 240/221 into account), again neglecting the effect of the earth's rotation. So, just before LOS the uplink is turned off and the Selene transponder returns to its free-running transmit frequency and the uplink downward shift disappears. That is why the frequency jumps up by 10-13 kHz. I think the fact that it is not alwyas abou + 11 kHz may be related to which antenna is providing the uplink and where it s in relation to the Moon.

But why do we not see simular big jumps near AOS he observed frequency jumps are much smaller, often just a few kHz- I tink that the uplink transmit frequency is adjusted so that the downlink frequency does not jump very uch. This will simplify receiver lock-on on the ground.

A complete Doppler cycle during a pass across the Moon

In the waterfall picture below I have edited out all the interference (while they are still visible in the spectrum display) and hand -edited every pixel where the carrier was visible by making it yellow. Whe running the display this slowly (12 seconds between updates) and the signla is varying very much close to the nise level, the display catches the carrier rather seldom. So, overview and signal clarity do not quite match when the signal is this weak. Anyway the 13 kHz frequency jump just before loss-of-signal is clearly seen.

Reconstructed Selene pass

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