CARL COLENA ON SIGID WIKI AND KNOWING SIGNALS
The interview with Carl Colena was recorded and transcribed. On this page, we provide a brief summary. You can read the full transcript here, or listen to the recordings below each answer.
1 - BACKGROUND
SELENA SAVIC: The Signal Identification Guide wiki is an organized database of information on radio signals. It contains data on signals’ characteristics such as frequency and bandwidth, modulation type, as well as short descriptions, audio samples and waterfall plots. Could you give us a bit of background, how was this project started and who supported it?
CARL COLENA: Carl Laufer, the owner of the RTL-SDR blog, started the Signal Identification Guide (SIGID) wiki project in 2014. There was previously no centralized database describing these signals. The SIGID wiki website started as a collection of all the information about radio signals that was held among a community of amateurs and enthusiasts who wanted to explore radio space and understand what they see and hear.
2 - KNOWN / UNKNOWN
SS: The distinction on two major categories caught my attention: known and unknown. The first ones are those that serve identification. This means that I could receive and record a signal using a TV dongle, then go and compare it to those in the database of known signals and hopefully find one that matches. Or, if I do not find a match, I could upload my data to the wiki as “unknown” and wait to have it identified by someone else. How does this work in practice?
CC: Indeed, when someone records a signal, they listen to it and then go to the SIGID website searching for a match. If no match is found, most people would create a page in the unidentified section, and upload whatever traits that they have recorded to help identify it. We provide a form to do this, and accept submissions via email too. The more information that you provide the easier it will be for others to identify a new signal.
SS: This suggests that signals in the database are most probably “identifiable”, just not yet identified. How do those in unknown category pass to the known one? Conversely, how can one challenge a signal as incorrectly identified?
CC: For the most part, identification is a community effort. Someone could look through unknown signals and recognize a signal they know. Or, one could find out that a signal in the unidentified section is the same as a newly identified signal on another website, and simply move it to the identified category. In order to identify signals, the more samples you have, the better. Most signals do not have a static representation, especially if they are transmitting data: they may have different modes, phases, and these are temporal in description. Some signal samples in the unidentified section might therefore be just in a snippet or form of a known signals that we have not seen before.
SS: Which signals fall under the category of Requested signals?
CC: I created that category for a growing list of signals that did not have samples associated with them, even though there was some information on them. Community of wiki users sometimes just wants to have a sample, such as Bluetooth, so that they could analyse the signal and possibly identify it.
SS: Is the aim (or one of the aims) of the Wiki to resolve unknown signals’ identity?
CC: Yes, that is certainly one of the aims. At the same time, The SIGID wiki serves as an archive for all of the signals that have existed, but also what could be natural emissions, like lightning spherics. It acts both as a holistic guide, and a sort of a museum of different radio signals that have existed over the years, how we have transitioned from lower band signals that are not able to carry as much information, to much more complex signals that can carry television and what not.
3 - KNOWING SIGNALS
SS: The archival techniques of tagging, categorization and metadata are important information infrastructures – they are key to access digital information. How do conventions on these on these emerge in the Wiki?
CC: Categories emerged out of a need for organization of the signals we had collected. Categories can be based on listener communities that emerged around certain types of transmissions, with groups that are specifically dedicated to satellite reception, or to trunked radio channels. One category that is hard to pin down is the digital. Signals are inherently not digital - they are continuous waves propagating in space - but they can be used to communicate digital information, like cellular communication.
SS: On the Wiki pages, there is a lot of information on signals that is not quantitative, such as descriptions, application, history of use, and so on. Who writes these descriptions and other information, who checks if it is correct? How does one know what to write? Which resources are commonly used?
CC: In the descriptions, it is typically about answering high-level questions about application and users of a signal, its history, a unique identifying trait. It is similar to a Wikipedia article about fish. My knowledge is based on years of writing about signals. The best source would be a government or a technical white paper, but they do not always exist. Some information, especially about commercial and military signals, tends to be quite opaque. Companies, such as Wavecom can be a source of relatively reliable information. Each government typically has their own regulatory agency, and you can use it to get information about anything that is wireless.
SS: How would you describe what does one know about signals? Which properties do you look at?
CC: For each kind of signal there are different ways of finding out features. It can be about signal structure or symbol rate. I look for its general characteristics: location, modulation type. Then, narrowing down, any unique characteristic that stands out can be useful to characterize a signal, and potentially identify it.
SS: Which methods and tools do you use in signal analysis?
CC: The first thing I always do is a visual and acoustic inspection, establishing a connection between what I hear and what I see. I mostly use free software tools for visual inspection, like SDR#, or HDSDR. I look at how this signal looks at different Fast Fourier Transform (FFT) bin sizes. You have to look at both ends, at high frequency and high temporal resolution to fully understand the signal. Certain signals are simple enough that you can imagine how they sound. But others are hard to tell, from just looking at it from the frequency or time domain.
SS: What do radio signals do outside of transmitting information: for example, they can be used for motion detection, by informing on changes in other conditions of the environment. Can you tell us more on that?
CC: Similar to the way bats use the echo in a cave to orient themselves, radio waves can be used to understand the magnetic field of the earth, by transmitting into the atmosphere to get the sounding of the ionosphere. Receivers located at different locations pick up the reflections from these signals broadcast into the sky. Radio waves are used here purely as energy as opposed to carrying information. Certain amateur radio operators use low energy transmissions in a similar way, almost like an Olympic sport, trying to send a signal at the lowest energy possible, the furthest distance away.
SS: How about motion detection and other kinds of information that are still somehow mappable on the space of radio propagation?
CC: Research in this context is typically interested in non-intentional emissions. These are emissions that exist as a consequence of other processes (i.e supplying energy to devices). This is typically used in the security field, both for information and physical security.
4 – NATURAL RADIO
SS: I was fascinated by the possibility that some of the unknown sounds might actually come from natural radio: lightning, whistlers, ionospheric sounds. Do you think any of the signals in the SIGID wiki could have come from such natural sources?
CC: In the unidentified category, certain traits are unmistakeable signs that it is not a natural transmission. Such signals have strong wave patterns or complex structure. Anything that has a pretty repetitive pattern is probably not natural, but it could be.