Contemplating a mystery

Over the last few weeks, we have returned again to the technical questions related to receiving satellite signals for Signal Tide. (We have also returned, in a more practical sense, to a place that’s quite familiar to us — we are spending a large part of this summer living and working in Ponderosa, outside Berlin).

The work that we are doing involves chipping away at a few different interrelated questions that we will need to answer in order to make Signal Tide work. We’ll be posting about them all in the upcoming weeks. One of these in particular, however, has recently been more central than any of the others — what happened to the dips?


The dips

All the previous times that people have tracked the signals from the LES-1 satellite, since the rediscovery of its signal in 2013, they have received signals that have a ‘gap’ or ‘dip’ approximately every four seconds. We also received this ‘dipping’ signal, in our own previous recordings. Although nobody can say for certain, it seems reasonable to presume that this ‘dipping’ is caused by the satellite spinning around its own axis as it orbits the earth, so the signals are like the light from a lighthouse, disappearing every few seconds.

The ‘dips’ — as viewed in Baudline — from a pass of the LES-1 monitored in September 2016

However, in the last few weeks, as we started to monitor passes of the satellite again, all the signals that we were receiving were completely steady — they had no dip or gap.

No dips, just a line

At first, we were worried that the signal we were getting was coming from some other source. However, everything else about the signal indicated that it was coming from our satellite — it’s at the right frequency, it’s got the same long doppler shift as it crosses the sky overhead, it follows the expected path, and rises and sets at the right time. It’s our satellite.

We were wondering what might be causing this change in the signal. It seems unlikely that the satellite would have simply stopped spinning around its own axis. If so, then it might be the case that, alternatively, the satellite has subtly rotated in such a way that it is giving a constant signal, even if it is still spinning — the radio equivalent of looking straight down on a lighthouse from above, and so always getting a steady signal, instead of having it blink on and off as it rotates.

We monitored passes of the satellite early in the morning and late in the day, in order to see whether or not there would be any change that could be caused by the angle of the sun relative to the satellite. No change.

The dips that exist in the signal from the satellite are central to our plan for the Signal Tide installation. We want to use them, on the one hand, as a practical piece of audio: a radio-derived metronome that can be used for structuring musical compositions. On the other hand, we also want it to be a central conceptual element of the work — a kind of extraterrestrial sonic readymade. So, missing dips are a big problem.

And then, suddenly, they came back.

We have been using a gnuradio application for tracking all recent passes of the satellite. It ain’t the most beautiful way to look at the signal, but it is possible to see the dips in the signal in real-time, as the satellite passes overhead, by looking for the tell-tale ‘dashed’ line that results on-screen.

Screenshot of the signal, as seen in the waterfall spectrogram plot of our gnuradio application — now with dips again!
Full view of the screenshot of the gnuradio application running

Opening up the same data in Baudline, we were able to see that, although the dips are indeed back, they are still pretty faint compared to the large swoops that were present in earlier data — like in the first screenshot above, from last September.

Small dips.

We don’t know why they came back. Nobody has been consistently tracking the signals from the LES-1 satellite, so it’s impossible to know for sure whether or not the disappearance and reappearance of the dips are a regular feature of the signals given by the satellite. But in all recent passes that we have monitored, the gaps are present. Not yet as prominent as they were in our earlier recordings, but, hopefully, slowly returning.

Of course, this will be something that we will need to keep a close eye on, as we organise the other elements of the work. But for now, we can’t really do much else other than to hope that this might have been some kind of temporary aberration in the signal — perhaps caused by some sort of temporary overlap of different causes, but also perhaps a regular recurring phenomenon that simply had not been observed before. We’ll see!