[Discuss-gnuradio] Homing in on the mystery of Pulsy McGrooder

[Marcus Leech]

A little while ago, I posted a message talking about some "mysterious" 
pulse-like interference I've been receiving
 with my Gnu Radio/USRP/DBS_RX setup, using my radio-astronomy receiver 
software.  I affectionately
 dubbed this signal "Pulsy McGrooder".   Ok, so I'm a bit strange.

Tonight, I did some hunting around in frequency, looking for where this 
signal was strongest.  I found that I could
 hear it across the entire passband of my front-end filter, which 
covers 995-1600Mhz.  I confirmed that it dropped
 off sharply at the edges of the filters passband (good thing, since I 
paid $75.00 for that filter!!!).

I found that the signal was strongest near 1350Mhz.   I used peak-hold 
in my spectral display, and found that it was
 a fairly broad feature, roughly 20dB out of the noise on peak-hold.  
With averaging turned on, you couldn't
 see it clearly, but peak-hold worked like a champ.

So, I went looking for 1350Mhz as some kind of "magic" frequency.   
Turns out it's a very popular airport
 radar frequency, although I don't think that it's used from passing 
aircraft, but rather as part of the
 ATC radar network.

I looked up that frequency range in Industry Canadas frequency 
allocation search tool, and found that NAV CANADA
 had several radars in Ontario in that frequency range, including two 
at the Ottawa airport--one right at 1350Mhz.
 But, Ottawa is 60Km away from me, I would have thought the signal 
would be too weak for me to see.

Not so, as it turns out.  I ran a path-loss calculation, using the Tx 
Power data in the Industry Canada spectrum
 database.  That radar in Ottawa is at +45dBW (+75dBm).   I assumed a 
20dB gain transmit antenna, 60km
 free-space path, and 30dBi gain for my receiving antenna.  Plugging 
that all in to a path loss calculator, I find
 that the radar signal is arriving at my feed at approximately 
-11dBm.   Which is just a *smoking* strong
 signal, in anybodys book!

So, how is this getting into the passband of the receive chain?   I have 
the DBS_RX configured to low-pass-filter
 the I and Q to 2Mhz, but the signal levels from that radar getting 
into the DBS_RX must be quite high--I have
 about 35dB of gain in front of the DBS_RX.

Given how strong this signal is here, 60Km away, I imagine that Ku-band 
satellite users near the airport must
 suffer quite a bit of interference, even though good RG6 has about 
80dB of isolation!

Is anybody aware of 1350Mhz being used from aircraft themselves, rather 
than as passive reflectors?