March 10, 2019
How can we get more use out of the radio spectrum? One way is by sharing radio bands between users who have never shared before. Consider radio frequencies near 3.5 GHz. Until recently, that part of the spectrum was used almost entirely by U.S. government radars, many of them being on Navy aircraft carriers. These radars do the same air traffic control jobs for aircraft carriers as radars on land do at airports.
The new 3.5 GHz spectrum arrivals will be communication systems like cellular phones. Operating on land, the new radios will be sharing frequencies with the Navy radars at sea. It might seem like there would be a lot of distance between shipborne radars and the land-based radios, but it turns out that there can be interference between the new land-based systems and the existing (and future) sea-based radars if the new sharing isn’t done properly.
We know this because years ago NTIA helped to blaze the trail to this kind of spectrum sharing when it did studies on exactly what it takes to share frequencies between regular radios and radars. [NTIA-TR-06-444] NTIA discovered that radar receivers are so sensitive that they start missing aircraft if interference power is just one quarter of the power level of their own internal, self-generated receiver noise. That’s really low!
Now NTIA’s has applied that knowledge to the new 3.5 GHz spectrum sharing. Industry and the government, including NTIA, have developed a strategy to share 3.5 GHz without any interference from the new shore-based radios to the Navy’s aircraft carrier traffic control radars.
Here’s how it will work: When industry builds out a new 3.5 GHz network on shore, it will also build a network of shoreline radar detectors designed to see the Navy radar’s signal. When one of the Navy’s radars sails within about 120 miles of a detector, the station will see the radar’s signal, note its frequency, and alert the local on-shore communication network to immediately vacate that frequency.
So, how does the government know that these industry-designed and industry-built radar detectors will work? First of all, the detectors have been designed to meet the government requirement ¼ of radar receiver noise level for detection sensitivity.
Second, the government has tested the radar detectors itself. For this job, ITS teamed up with several companies who plan to build and operate the radar detectors. Late last fall, ITS engineers worked with industry engineers for several weeks in Boulder to test the new radar detector designs. For these tests, ITS designed its own automated system that exposed each radar detector to thousands of different sorts of radar pulses at the low detection level required to protect the Navy’s current and future radars.
The ITS-industry radar detector testing was technically grueling. Not only did each radar detector have to see thousands of low-power radar pulses successfully, but the detectors also had to keep functioning in the presence of high-power radar pulses that were equivalent to what you would see if you were just 6 miles from a radar transmitting a billion watts of power.
In the end, the radar detectors passed all of their tests. ITS gave the companies test reports showing the successful results. The companies will send those reports to the FCC along with their requests to use these radar detectors all along all of America’s coasts. With the detectors in place and running as trip wires to prevent harmful interference to the Navy’s radars, Americans will be able to use the 3.5 GHz spectrum on-shore for their new communications, while US Navy aviators off-shore will continue to land on aircraft carriers with assurance that their controller’s radars are following them every inch of their way.