FM Radio: a good radar?

In an age dominated by the Internet, FM radio is still making an impact in modern science, based on the work of Dr. John Sahr.

The ionosphere, the upper portion of the atmosphere ranging from about 50 km to 600 km above sea level, is difficult to study, as it lies above the reach of aircraft and below the orbit of satellites. However, Dr. John D. Sahr, from the University of Washington, has found that FM radiowaves can act as a radar and help scientists discover more about this remote region of the atmosphere.

Sahr and his team originally thought that the ionosphere would be invisible to the radiowaves at frequencies above 30 MHz, but further testing showed there was backscatter at very high and ultra high frequencies. This backscattering was a result of supersonic columns of plasma in the ionosphere.

To refine his study, Sahr wanted to look at the ionosphere at 100 MHz. However, 100 MHz is within the frequency range assigned to FM radio transmissions. Thus, rather than try to get an FCC license to operate a radar station in the midst of the FM spectrum, Sahr decided to use the existing FM transmissions as the radar.

Currently, using FM radio stations as a radar creates data accurate to approximately one kilometer, but Dr. Sahr is convinced that new technology could help improve its accuracy.

The echoes were “quite concentrated in space. It’s… like, when you drop a rock in a lake, you expect the ripples to go in all directions, more or less equally. But that’s not at all what we observe… The azimuthal extent of the echoes is as small as we can measure; in other words, [the actual concentration of some areas] could be even smaller than we can currently measure.”

Sahr compared his FM radar to other digital radars. “The new ‘digital receivers’ are enormously powerful and flexible, but they have a ‘million’ knobs to turn… and a lot of sophistication is required to operate them. I understand why it’s hard, but there’s a lot of engineering needed to take away the pain of running these new receivers.”

On the other hand, Sahr’s simpler FM doppler proved it could compare with modern receivers. “Instead of providing ‘okay’ data,” said Dr. Sahr, “the [FM radiowave] radar produces the best range, doppler, and time resolution of any VHF coherent radar.” Scientifically, it is a powerful radar with the correct wavelength and a useful radar waveform. Economically, it is free, safe, cheap, and does not require a FCC license.

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