Wednesday, January 8, 2020

Localizing source of QRM to weather radar

I met two FCC technicians today at an FAA weather radar site to assist them in tracking down two sources of interference to the radar. See the back story here. This was a "first" for me and I was very much looking forward to learning what equipment was used and how they go about it.

The techs have a full time job as QRM detectors (not their official title) - they were familiar with ham radio and have experience in tracking down pirate FM stations. We had an interesting conversation as we waited for the clock to hit 1400Z - the time I had coordinated the shutdown of the radar.

Antenna input removed from receiver; spec-an now attached to the N-cable

I had imagined that, with the radar down, they would bring out some sort of receiver with a directional antenna and find the direction to the source of the QRM. I had assumed that the source was more local than it turned out to be and that finding it would be accomplished with the equipment they'd brought with them.

And it was, but the first step in the procedure left me with a "why didn't I think of that?" feeling.

Once the radar transmitter was shut down, the FCC techs told me that they needed to attach their spectrum analyzer (an H600 RFHawk, an analyzer made specifically for tracking down illegal transmissions) to the radar's parabolic antenna. I patched them in to the proper place and they set the spec-an to sweep a 30 MHz wide swath of frequencies centered on the radar's operating frequency.

The result was essentially a C-band receiver with a panadapter/waterfall.

Radar's antenna can be controlled via this terminal

Per their request, I commanded the antenna to slowly rotate, half a degree/second. We watched the spec-an's display as the antenna approached the two bearings of interest and, sure enough, there were the pips that indicated reception of the devices operating on our frequency.

The QRM (yellow)

I stopped the antenna on each bearing and the FCC guys replaced the spec-an with a data analyzer that decoded the SSID and other info.

And off they go...

They then set out with a 5GHz feedhorn mounted to the top of their SUV and, two hours later, had located the offending transmitter - 47 miles away. The device has an output of only 50mW but our antenna has 50 dB gain and an antenna beamwidth of only half a degree. Compared to return echoes normally received by radars, this produced a potent signal.

All in all, a fun day.


If you've read this far, you may be interested in further info on this topic found here.


Costas SV1XV said...

Very good article, thanks for sharing your experience.

73 Costas SV1XV

John AE5X said...

Thank you Costas. I wish I could have taken more photos of the rest of the equipment - they had some impressive toys.
73 - John

John M. said...

Enquiring minds want to know: What was the transmitter causing the emissions?

John AE5X said...

It was one of these:

Anonymous said...

FAA weather radar? Do you mean NWS? FAA does not maintain Doppler in the lower 48.

John AE5X said...

I'm in Texas.
I'm in the FAA.
I maintain two Doppler wx radars in Houston.
Ergo, the FAA maintains Doppler wx radars in the lower 48.

JimV said...

Keep an eye on this page to find out:

John AE5X said...

FAA wx radars were put into place around the country in the late 1990's. Unlike NWS wx radars, Terminal Doppler Wx Radars (TDWR) are not intended to provide wx forecast data but rather to alert air traffic controllers to wind shear conditions.

But we do provide a real-time feed of our data to the NWS to be used in conjunction with their Doppler radars (WSR-88). Each agency's radars have different characteristics - having both sets of return data available is akin to "diversity reception" of wx data in the coverage area.

You may be interested to know that the TDWR provides return echoes of wind movement even when no precipitation is occurring, due to return echoes from the water vapor; wind shear can be detected regardless of precip conditions. Air traffic controllers can then inform pilots of the conditions. That's why we don't like these radars to be QRM'ed.

Andreas Nilsen said...

Fascinating story. I love reading stuff like this :)

Dave New said...

I thought this was familiar:

"Philip D. Estridge, who was largely responsible for the move by I.B.M. to enter the personal computer business eight years ago, was killed on Aug. 2, 1985, when he was a passenger aboard a Delta Air Lines flight that crashed at Dallas-Fort Worth Airport."

I was working for IBM Lexington when this happened, on the Electronic Keyboard project, which formed the basis for the IBM PC keyboard when it was introduced in 1982.

IIRC, the cause of the crash was wind shear experienced by the plane during takeoff in a clear sky.

John AE5X said...

Yes, I remember that too. When was the last time you heard of an aircraft accident due to wind shear? They are few and far between nowadays. The TDWR alerts (audibly and visually) on wind shear, microbursts and other potentially catastrophic (to aircraft) wx scenarios. We test it often with various pre-recorded false wx inputs to the system to make sure it alerts when it should.

Maybe I'll do a write-up on that next time I do it - the level of interest in this topic (based on hit count) has surprised me.

73 - John

John AE5X said...

A recent news story on the topic: