[Scan-DC] Federal(?) spread spectrum system in 420-450 MHz in the DC area

[Andrew Clegg]

For several years, there has been a frequency-hopping spread spectrum system operating in the DC area across the entire 420 - 450 MHz band. That band is allocated on a primary basis to the federal government for radiolocation and on a secondary basis to amateur radio (the "440 band"). Given the primary allocation status, and the persistence and coverage of this system, I presume this is a federal operation of some type. Does anyone know what this system is?

>From my QTH in Arlington, it's fairly strong. If I park my receiver on a 440 frequency, the spread spectrum system manifests as constant clicking noises as the system rapidly pops in and out of the frequency I'm tuned to. It's more noticeable in CW/SSB/AM modes than in FM mode. If I run a spectrum analyzer in max hold mode and let it build up for a minute or two, there's clearly five 6-MHz wide channels over which the frequencies hop. I don't know what the instantaneous bandwidth is; the system hops too fast for me to see that.

I have heard the system while mobile driving up 95. I start to hear it ~20 miles south of the city, and it grows in strength as I get closer to my home QTH in Arlington. Other than driving up 95, I have not driven around the DC area to see where the signal is max strength.

The signals complicate reception of weak signals in the 440 band, especially amateur satellite downlinks. I have not noticed any complaints from other local hams, but my antennas are in the clear and up high, so I may suffer more interference than some other hams do. I'm also not in much contact with local ham groups so if they're discussing this topic I may have missed it.

I'm curious about what the system is and what it's used for. Does anyone know where the signal(s) originate from? Who else hears it at their QTH?

73,
W4JE