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

[Alan Henney]

>From one of our colleagues in regard to Andrew's question....

A posting on SCAN-DC addressed signals in 420-450 MHz.

This has always been shared with DOD radar and signals of some origin
are being heard throughout several states (MD,NJ, and VA as far west as
Shenandoah National Park) in this area at times. It does not quite fit
his description but the description is vague and hard to compare.  I suspect
it is a DOD radar and perhaps airborne tests.  A sample sound file
received in USB mode is attached-not the bandwidth is much wider than
the bandpass.


Note that Patuxent River may be testing radars on aircraft and that
Wallop Is. NASA is also a USN radar test site.  Certainly not a radar at
Dulles - I know about signals there having monitored the NEXRAD/WSR-88D
there("Sterling" on 2950 MHz), ATC radar ASR-9 (2810 MHz) there, and the
TDWR (Terminal Doppler Weather Radar) (5605 MHz)radar operated by them
up near Leesburg).  those radars can only be monitored from the ground
when close and of course outside the coverage of most receivers (I used
a HackRF 1 and Signal Hound).

The use of this band is restricted in the National Quiet Zone around
Green Bank and is also restricted at white Sands which I did not know
until driving there on several work trips where signs advise amateur
radio operators as they approach.


On Fri, Jan 1, 2021 at 12:05 PM Andrew Clegg <andrew_w_clegg at hotmail.com>
wrote:

> 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
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