Audiovisual performance - 2025
Antenna, radio amplifier, software define radio, computer, audio fx.
For over 20 years, the three NOAA satellites have been crossing our sky at over 20,000 km/h, continuously scanning the Earth's clouds and radiant energies.
An Eye from above is a performance that attempts to receive a live signal from one of these satellites, using an adapted antenna and a radio tuned to 137 MHz. As the satellite rises above our horizon, its signal slowly emerges from the surrounding noise, giving rise to a characteristic pulsation. The antenna acts as a revealer, making perceptible the electrical energy that passes through our bodies and our environment amidst a crowd of other artificial signals.
Poetry of analog protocols: this pulsation can be decoded into an image. Slowly, line by line, a nephanalysis of the 2000 km that surround us appears. Clouds and infrared radiation from the ground are displayed, offering a shift from our point of view to that of a space object.
Context
The NOAA 15, NOAA 18 and NOAA 191 satellites were put into orbit between 1998 and 2009 by the USA meteorological agency. Their purpose is to provide data to help build weather forecasts. To do this, they continuously scan the Earth in the visible and infrared domains, and transmit the images obtained by radio waves to the ground surface. Their orbits are known, and it is possible to pick up their transmissions by being in the right place at the right time, with the right equipment.
One of the signals sent is a vertical scan of the satellite twice a second2. This analog signal can be received with a simple antenna tuned to 137 MHz and then demodulated into sound or visual shape in a software radio. The sound produced is a distinctive 120 BPM pulse, while the video is a horizontal photo of the Earth, displayed line by line as the satellite moves.
The other members of this NOAA-POES family of satellites are no longer functional, having become zombie satellites or been deliberately disintegrated in orbit. And with the increasing complexity of orbital telecommunications, these infrastructures, which amateurs can easily receive, are living their last hours.
NOAA satellite scan diagram3.
1 : NOAA for National Oceanic and Atmospheric Administration.
2 : Signal APT - Automatic Picture Transmission, see Sig ID wiki for more details.
3 : from "Satellite Remote Sensing of Sea Surface Temperatures", by P.J. Minnett, 2001.
Proposal
The speeds and distances involved in satellite orbits are so great that they call for a change of scale: from the human to the Earth globe. Radio waves are the only way to communicate with these objects.
In An eye from above, I work with three satellites that share interesting characteristics for an artistic performance:
- They are well documented4 and emit a signal to Earth that is relatively easy to receive with inexpensive equipment (~50€ and a computer).
- This signal is a real-time photo of the Earth as seen from the satellite, which can also be decoded into sound, producing a characteristic rhythmic pulsation.
This image/sound duality is a property of analog transmissions that disappears in more modern digital transmissions. As well as being aesthetically pleasing, the sound heard by the audience during the performance has semantic properties: by listening to it, we project ourselves towards the satellite passing in the sky, we look for it with our eyes, we imagine it in operation, we shift towards its point of view. In order to achieve this evocative power, a little introduction is needed before the performance to explain the strength of the setup, to charge the sound we hear, while leaving room for the senses and the individual imagination.
Unlike other artistic projects based around these satellites, An eye from above is a live performance in which we listen to and visualize the signal in real time, reinforcing our imagination. The satellite is currently passing overhead, not as a metaphor: the signal is a wave passing through us now, and the image we see corresponds to the current state of the sky.
It's also a way of evoking the complexity of the radio spectrum. What we decode here is only a tiny part of the radio landscape at a given moment... what do the other waves contain?
4 : See for example the clear explanations of Jacopo Cassinis or the work of the Open Weather Collective.
Performance
The audience gathers outdoors at night at a specific time. Ten minutes before the satellite passes over, the performance begins with a short introduction: presentation of the satellite, its role and orbit, explanation of the device and presentation of the equipment, estimated time of appearance in the sky.
Radio equipment switched ON. Noise appears in the speakers and on the screen, and the wait for the satellite begins.
After a few minutes, the satellite appears on the horizon and a signal emerges through the noise.
After a few beeps, the signal becomes clearer and the noise diminishes. The sound pulses twice a second. The image becomes clearer, and with each pulse a line appears on the screen, representing a horizontal photo taken directly beneath the satellite.
With minutes passing, the sound evolves as the satellite moves towards us, passes over our vertical, then moves away. The artist plays slightly with the live sound, sculpting its timbre. The image is completed line by line, displaying a photo of the sky over the 2,000 km surrounding the performance site, which becomes recognizable.
After fifteen minutes, the satellite disappears behind the horizon, the signal blurs and noise returns to the screen and sound.
Radio equipment switched OFF.
Tech note
Radio reception
- V-shaped dipole antenna precisely tuned in-situ to the satellite frequency.
- Receiver chain: Filter around 137 MHz, LNA (Low Noise Amplifier).
- Digital acquisition: RTL-SDR v3.
Software processing
- rtl-sdr library.
- Sound demodulation: SDRAngel.
- Video demodulation: Pure Data patch + GEM.
Audio processing
- Elektron Analog Heat.
Crédits
Color pictures by Raúl Goycoolea.