Reception of signals on 183.54 MHz from the Luna 20 return spacecraft in Stockholm

Sven Grahn, Sollentuna, Sweden

Table of contents

Luna 16 return vehicle signals heard by Jodrell Bank in 1970

During the flight of Luna 16 it was announced that the return spacecraft carrying the lunar samples transmitted signals on 183.6 MHz. It was reported in the media that the Jodrell Bank tracking station picked up signals on that frequency up until 12 minutes before the craft landed in Kazakhstan. (The picture on the left has been obtained from the old web site of the Lavochkin Association).

The equipment

Encouraged by these reports I set up to receive on this frequency in Oct.1970. The equipment was located on the balcony of my flat on the 10 th floor of a high-rise apartment building in Taby outside Stockholm (59o.515 N, 18o.080 E). The equipment consisted of a Hirschmann 9-element Yagi for European TV-channel 10 with a 10 dB gain. The Yagi fed a Vanguard Labs Model 407 converter for 183-184 MHz through a RG 58C/U coax cable. The converter had an output at 0.6-1.6 MHz which was tuned on a Lafayette HE-30 short-wave receiver.

Luna 20 lands on the Moon

The Soviet spacecraft Luna 20 landed on the Moon on Feb. 21,1972 and took off towards the Earth after sampling the lunar surface at 2258 UT on February 22, 1972

My tracking activities

February 23,1972

Tracking attempts started on Feb.23 but the exact frequency or signal character was not known, so there was no success despite the fact that the Moon was clearly visible high in the southern sky in the evening and could be used as a reference for pointing the aerial. However, the performance of the tracking system was not considered sufficient to ensure a S/N greater than 1 until the craft was within 100 000 km. The noise figure of the converter is 2.5 dB and the reception bandwidth is 5 kHz. I asked Geoff Perry in Kettering, England to find out the exact transmission frequency of Luna 20.

February 24,1972

I phoned Geoff Perry and learned that Jodrell Bank had not tracked Luna 20 at that time but that Luna 16 used 183.538 MHz and that the signal was FSK. No signals picked up on the 24 th.

February 25, 1972 - I find the signal from the Luna 20 return craft

The return craft came into view from my tracking post at 1500 UT on 25 February. No signals were picked up then but at 1640 UT I made a search of nearby frequencies and found the signal of Luna 20 slightly off the dial position where I had expected it to be. This was probably due to a slight error in the local oscillator frequency of the converter.

Signal character

When the signal was found it was at S=2 on the HE-30 S-meter. Noise level was at S=1. The signal was frequency-shift-keyed very slowly. When using the BFO the signal had basically this pattern: A steady tone which was interrupted every 10-20 seconds by a frequency shifted interval which lasted 8 seconds. The signal could easily be heard to shift frequency slowly and disappear out of the audible range and later return in the same manner.

Below you can see a spectrogram of a part of the frequency-shift keying (FSK) pattern where the shifted period was 8.0 seconds and where the slow frequency shift is clearly seen. The 1700 Hz tone is generated by beating the incoming RF signal with a local oscillator, i.e. the tone is the difference between the received RF frequency and the frequency of the beat oscillator. When the frequency shifts, it shifts more than the audio tone frequency (here 1700 Hz) and therefore the audio tone goes through "zero beat" and then increases again. But after going through "zero beat", the RF signal has moved out of the pass-band of the IF strip of the receiver and therefore, as we can see in the figure below, is much attenuated. However, we can see that the audio tone, albeit very weak, reaches about 2300-2400 Hz, so the total frequency shift is 1700+2300=4000 Hz. (The picture below was obtained from the output of the Cool Edit software package).

Luna-20 signal on 183.54 MHz

You can hear this part of the transmission (31 kB mp3).

Sometimes the interval between the frequency-shifts was 40 seconds. There were intervals with no keying and also periods with more complicated sounds.

After having done some literature research I have come to the startling conclusion that the frequency sweeps from Luna 20 are classical examples of ranging. This method was used at Cape Canaveral under the name of MISTRAM. The ground station transmits a carrier to the spacecraft and the spacecraft returns this carrier on another frequency. The ground station sweeps the uplink carrier and the phase shift of the downlink carrier is measured (counted) while it is being swept. The round-trip delay time can be shown to be T=(delta-phi)/(delta-f) ; where delta-f is the frequency shift (here ~4000 Hz) and delta-phi the measured phase shift in radians. Suppose T=2 sec (~lunar distance) then delta-phi=8000 radians, i.e. (8000*180)/Pi . Assume also that the phase can be measured with an accuracy of 1 deg, i.e. means that the range can be determined with a precision of (600000*1*Pi)/(2*8000*180)=0.33 km. In all probability there was an additional carrier quite near the one described above that remained fixed in frequency and used as a phase reference. That carrier and the two frequencies (that the sweep changed between) were probably generated as multiples of the same basic oscillator frequency. In this way all signals would have a fixed phase relationship. This is the way it was done in MISTRAM.

A Russian description of the development of this ranging system describes how it was used on all the early Luna probes as well as later Luna probes (15-24) and missions to Mars and Venus.

In (1) the wavelengths 1.6 m and 2.5 m are mentioned for the Luna series. 1.6 meters probably refers to 183.6 MHz (which is precisely 1.634 m). The wavelength 2.5 meters corresponds to 120 MHz. But, in connection with the flight of Luna 15, the Soviet Union told NASA (2) that the frequency 115 MHz was used. It is not clear of this was the up- or downlink even though (2) indicates that it is the downlink...

The rod antennas on the return craft bus were measured by enterprising persons at an aerospace exhibition where the Luna 20 return craft was displayed. The figure below shows the result of these measurements.


Let assume that the antennas are quarter-wave rods fed part of the way up from the "ground" end. If that is so, then antenna AA' is resonant at the frequency fA=300/(4 * 0.36)=208 MHz. The corresponding frequency for the BB' antenna rods is f´B=300/(4 * 0.57)=131.6 MHz. It is probably safe to assume that the antenna system is operating a lower frequency than calculated here. Let us assume that antenna AA' operates at 183 MHz, then one would have to apply a correction factor to the frequency of antenna BB' equal to the ratio (183/208)=0.88. If this is a correct way of reasoning then the BB' antenna worked on 0.88*131.6=115.8 MHz. The picture below I took at the kaluga Cosmonautics Museum in 2008. It shows a view of the return vehicle and the antenna pair. The ratio between the lengths of the antenna rods is 1.697. This would lead to an uplink frequency of 108 MHz.

However, in 2006, at the new web site of the Lavochkin association, it was revealed that the uplink frequency was 101.965 MHz and the downlink 183.537 MHz !

The signal keeps growing in strength

As the craft approached the Earth the signal strength steadily grew:
 
  • 1700 UT: S = 2.5
  • 1815 UT: S = 3.5
  • 1825 UT: S = 4 
  • 1835 UT: S = 5 
  • 1844 UT: S = 6
  • 1846 UT: S = 6.5. 
    		•

    Changes in signal character as the craft approaches earth

    In my log I made notes about the character of the signal that I received and the frequency sweeps described above were a typical ingredient of what I receievd, but they were not continuous. Here is a translation of my log notes: .

    Loss-of-signal and landing

    At 1850 UT reception was troubled by car ignition interference from the nearby parking lot and this continued up to LOS (loss-of-signal). LOS took place at 1900.10 UT ± 15 seconds. Radio Moscow announced the landing time as 1912 UT on Feb.25. This means that the signal was lost 12 minutes before touchdown. The same thing happened at Jodrell Bank while tracking Luna 16. The recovery of the capsule took place at in the morning of Feb. 26 some 40 km NW of Djeskazghan in Kazakhstan.

    It should be noted that the landing capsule separates from the return craft bus at 50,000 km:s from the Earth and that the signals that I had were received after the separation. Did I track the bus or the re-entry vehicle? Probably the bus! However, the two pieces follow almost similar paths and tracking during the last hours of flight is probably used to pinpoint the landing spot.

    from the Lavochkin web site we now know that the recovery beacons of the capsule operated on 121.5 MHz (the international distress frequency) and 114.167 MHz.

    References

    1. S.P Korolev (edited by M Keldysh), "The Creative Legacy of Academician S.P. Korolev", Nauka, Moscow 1980, p.402.
    2. Aviation Week & Space Technology, 28 July 1969 ("Soviet Luna 15 hard-lands in Sea of Crises")

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