WHAT
I want you to imagine a new kind of radio - a radio where the station is selected by its geographic location rather than its radio frequency:
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| Traditional Radio Tuning |
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| Location-Based Radio Console |
A location-based radio is "tuned" by selecting the location of the station of interest. In the figure below, a terrestrial radio station labeled X is at one location in the spectrum. A second signal Y is a celestial source from a different spectral slot.
Another kind of tuning is possible - a radio station can be located by first clicking on its position in the spectrum and then observing its physical location.
HOW
There are several ways we might achieve a location-based radio. Naively, we might index a list of radio stations by their latitude, longitude and altitude. We might then create a program that would look up the location we selected and open that data stream, like for example, iTunes™ does:
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| iTunes Radio Listing |
Such a user-interface, though convenient and even useful, misses a major conceptual opportunity. I propose a more fundamental and far-reaching implementation of location-based radio. One that is considerably more versatile, entertaining and enabling of discovery.
MULTILATERATION
The idea for a location-based radio was spawned by a very simple desire. I wanted a radio that would tell me whether a signal was of terrestrial or celestial origin. It takes several radios listening simultaneously to answer this question. If a radio signal is common to four or more receivers it can be uniquely located in three dimensional space using the Time Difference Of Arrival (TDOA) of the signals.
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| Image Courtesy Agilent Technologies |
However, this required four radios instead of just one. That was bad. Perhaps if three other people could share the cost of one radio each, all could benefit. That was good. Running this idea to its logical conclusion results in the creation of a location-based radio network. We know that networks are powerful and enabling things.
In a location-based radio network, four is the minimum number of receivers, but in general more radios are better. More signals can be heard, more common signals can be found and more people can benefit. The effect compounds quickly.
For example, if a signal is of terrestrial origin, one might listen to it, or in the case of amateur radio, respond to it with a transmission. If the signal is of celestial origin, one might want to analyze it, discovering its location and structure. The former is entertainment, the latter is science. Science can be entertaining, and entertainment scientific.
SOFTWARE-DEFINED RADIO
Preceding my desire for a location-based radio was the advent of software-defined radio. Software-define radios eliminate some of the traditional hardware of a conventional radio. A portion of the physical radio is replaced by software in the form of digital signal processing algorithms. A “soft radio” is a hybrid device, spanning analog and digital worlds, hardware and software. This introduces complexity but produces a payoff - radio signals can be digitized and routed like other web-based media as TCP/IP packets. Such packets can be exchanged by constellations of location-based radios over the internet for comparison and analysis.
If these packets are time-stamped very accurately, they can be used to locate the signal. Time stamps do not occupy much space, so transmitting them from soft radio to soft radio does not create a lot of overhead. In fact, the original RF can be stripped away and just the audio portion of the signal retained - as long as the time stamps are accurately registered against the signal samples. This reduces transmission time and overhead significantly.
Digital signal processing, in the “cloud”, makes location-based radio possible. Software-defined radio enables large swaths of spectrum to be simultaneously processed and shared. So hundreds of signals can be observed and analyzed, instead of just one at a time, as in conventional radio. Location-based radio is to conventional radio like Google is to a filing cabinet.
BACKGROUND
During the second world war, there was a race to develop and use radio techniques to locate aircraft and naval vessels. The British innovated with a technique called multilateration. However it was the advent of GPS that makes location-based radio possible at reasonable cost.
CONSTRAINTS
One constraint on any radio is the band that it listens to. Software-defined radio now routinely spans AM frequencies of 530 kHz on the low end to 1.3 GHz plus on the high end. The radio spectrum from 1 MHz to 1 GHz spans four decades of frequency. Such bandwidth is within reach of soft radios currently on the market.
WHATS OUT THERE
Several companies already manufacture software-define radios that can be used as a starting point for a location-based radio network. A standard software defined radio (SDR) can be equipped with a GPS, so that its internal oscillator is “GPS disciplined”. This GPS discipline guarantees that digitized chunks of spectrum can be accurately time-stamped to within a few nanoseconds. Light travels a foot per nanosecond. So if a signal common to four radios is time-stamped accurate to the nanosecond, the location could theoretically be good to a foot. At this writing, fifty nanoseconds is an achievable goal and more than enough to provide utility. With larger networks of radios, techniques such as differential geodesy can be used to refine the location of the signal by observing common signals for longer periods of time.
IMPLEMENTATION
With just a few location-based radio nodes, one could resolve light-dimmers, microwave ovens, FM stations, satellites,etc. Knowing the location of a radio source is one of the most important ways of determining its nature. And knowing the location of the multitude of radio signals impressing themselves upon us every moment of every day, is not only entertaining, it is interesting science. It is also democratizing.
A simple design could be produced and distributed would enable a location-based radio capability to be grown from the grass-roots level. It could be based on existing SDR designs of which there are several to choose from.
CONCLUSION
Imagine being able to locate every radio signal that was impinging upon your existence, by working in concert with a network of similar web-based soft radios. This has far-reaching ramifications some of which, like the internet, may be difficult to predict or anticipate. One example is the ability to track meteors, to in effect, "Catch a Falling Star".
ACKNOWLEDGMENTS
This work is dedicated to my best friend and technology partner, Marilyn Fulper, who life on earth was cut short when a car ran a red light and struck her on her bicycle.
REFERENCES
Multilateration : Locating an Object by TDOA
L. Van Warren - A Blazing Fast Introduction to SDR
L. Van Warren - To Catch A Falling Star
L. Van Warren - A Short Trek to DNA-Cutter M87
Gerald Youngblood - Four part series on SDR
