Tracking satellites with a solar radiospectrograph

Sven Grahn, Sollentuna, Sweden The Soviet manned space program has always been a focal point of the space tracking activities of the Kettering Group. Of course much of the group's fame has come from monitoring the Soviet/Russian reconnaissance satellites program, but in my personal view, this is something we did while waiting for the manned flights! In the beginning of the manned program the Soviet Union published radio frequencies of the craft so that the world could listen in and verify the flights. As they became more and more routine this practice disappeared. When new elements in the manned spacecraft fleet appeared we were often at a loss as to which frequency to listen to. This was a problem we discussed constantly. In the mid- to late seventies several new craft appeared, like the Salyut 3 and 5 space stations (Almaz) and the other craft of the Almaz system like Kosmos 929 (TKS). Of course, at that time the relationship between the two Salyut categories and the strange craft represented by Kosmos 929 was not known.

Dick Flagg comes up with the idea

So, finding new frequencies was a priority! When Richard "Dick" Flagg (you can read about our exploits in tracking Apollo 17 in the piece "Tracking Apollo 17 from Florida ") visited me in Stockholm, Sweden in July 1978 he had brought a new idea of how to find frequencies of Soviet spacecraft. At that time was a radio astronomer at the University of Florida in Gainesville. He built radio systems for radio astroniomy and was going to Zürich to install some equipment at the radio astronomy obervatory of the ETH Technical Univerisity. He had become aware of the fact that the Swiss were operating a solar radiospectrograph at Zürich operating in the range 100-1000 MHz and recording the signal on film. (As the receiver swept through the frequency range an oscilloscope beam swept perpendicular to the motion of the film and the beam was intensity-modulated with the received signal strength). The 5 meter dish antenna of this instrument was kept pointing at the Sun by means of motor drive. The receiver that swept through 100-1000 MHz had a bandwidth of 1 MHz. The feed was a log-periodic antenna for circular polarization feed with 7.5 dB of gain. So, Dick said to me, "why could we not try to use this instrument to find signals from Soviet spacecraft?"


The beamwidth of a 5 meter dish is about 12 degrees, quite narrow,a nd for the satellite to stay longer in the beam, it would help if the elevation of the anetnna was low. However, this of course also meant that the slant range was large. Also, at low elevations the feed to could pick up signals at high angles off its boresight axis as the satellite passed overhead. Also, we wanted the dish to point east, towards any possiblem overlap in coverage with ground stations in the Soviet Union. This meant early morning passes. If these occured in the summer, the antenna would be pointing slightly to the north (to see the sun) near sunrise. This would also help in getting a good overlap with Soviet ground stations.

So, there we had the specification:

  1. Overhead passes in Zürich
  2. going slightly north of east into the Soviet Union
  3. shortly after sunrise in the summer.

Salyut 5 - the first try

We were naturally very keen on finding such frequencies as the Salyut downlink TV frequency, but also to try to find UHF frequencies for the "miltary Salyuts", i.e. Salyut 3 and 5 (a.k.a OPS Almaz). We wanted to find good passes over Europe at times when we knew that teher was a TV transmisison going on. very often this could be discerned from listening to the downlink voice. After Dick had gone back to the U.S. I started to scrutinize my log-books to find such an oportunity, that also matched conditions 1-3 above. I finally found a good Salyut 5 pass on August 3, 1976. Dick got the Salyut 5 spectrogram from Zürich on Oct 4 , 1978 and he immediately noticed a signal at 185 ± 5 MHz (which of course we knew was 181.0 MHz PPM-AM telemetry). Later we were also able to detect voice on 143.6 MHz.
Signal Spectrogramme My log
Voice on 143.6 MHz 0438.57-0440.22 UT 0439.05-0443.40 UT
Telemetry on 181.0 MHz 0437.49-0440.22 UT 0437.45-0443.40 UT

We were disappointed not to find any other, unknown, signals in the spectrogramme, but delighted that the idea of using the radiospectrograph really worked. In hindsight, the lack of any signal that could have been the TV link is understandable, . The TV signal probably would have been very wide-band, perhaps 10 or 20 MHz wide. Since the bandwidth of the spectrograph was only 1 MHz, it would pick up maybe less than 10% of the signal power of the TV transmitter. This is probably the reason why we did not see any other signals than 143.6 and 181.0 MHz.

More than two years later - radiospectrogrammes of Kosmos 929

In any case the succesful detection of signals from Salyut 5 had, in a sense, qualified the method and it was now time to try this method on more "targets". So, in December 1978 I made a list of "targets", in order of descending priority, for which we would ask the Swiss for spectrogrammes: For various reasons (Dick switched jobs, the Swiss radio observatory got a new boss...) it took until January 1981 before we got a new batch of spectrogrammes. From the list above only the two passes of Kosmos 929 in August 1977 were available! On 16 January 1981 the Kosmos 929 spectrogrammes finally arrived at my home and a day alter I departed on a business trip to the U.S. On the way back from Seattle I travelled to Dick's home in Melbourne, Florida we analyzed the spectrogrammes on January 25.

The ground track of Kosmos 929 across Europe 
in the early morning of 6 August, 1977

So, the spectrogramme had to be carefully examined for a period from 0447 to 0454 UT that morning. As can be seen from the spectrogramme below, there certainly was a trace on the film during that period (See the trace between the two red arrows).

Click on the picture to see an enlarged version

If one plots the time when the signal appears in this spectrogram against time in an elevation-time plot, one can see that the signal is stronger before and slightly after Closest Approach (CA).

Dick and I spent several hours analyzing the spectrogramme both for 6 August, 1977 but also for the following day (see spectrogramm below).We had frequency data for the "fixed interference sources" at Zürich and we finally came up with the frequency 250 ± 5 MHz for the trace that coincided with the pass of Kosmos 929.

Click on the picture to see an enlarged version

Since I knew that Soviet satellites (actually the Meteor 2-series) used 248 MHz, we concluded that Kosmos 929 transmitted on or near this frequency. We only had to wait for the next such spacecraft, but it had been 3½ years since Kosmos 929. Was this flight just a single, freak, event? Unknown to us the next flight of this type of spacecraft was very near!

Kosmos 1267 - confirmation!

On 25 April 1981 Kosmos 1267 was launched. This craft was almost a copy of Kosmos 929, a TKS spacecraft , which was part of the original Almaz space station concept developed by the Chelomei design bureau. On the second day in orbit I picked up wide-band FM telemetry on 247.0 MHz, neatly confirming the spectrogramme analysis. A detailed account of the radio signals from the TKS craft can be read in the article entitled "Radio Observations of TKS and Related Flights".

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