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Transponders and the ATCRBI system

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  • brad_ramsey
    The ATCRBI (air traffic control radio beacon interrogator) should interrogate your transponder no more than a few times on each rotation of the radar antenna.
    Message 1 of 12 , Jun 23, 2004
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      The ATCRBI (air traffic control radio beacon interrogator) should
      interrogate your transponder no more than a few times on each rotation
      of the radar antenna. Those few times should all occur when the
      antenna is pointed at you, and should only appear as a single pulse on
      the transponder light. The interrogator antenna on most radars is the
      rectangular screen mounted above the primary radar's antenna. The
      interrogator uses an electronic antenna sidelobe suppression mechanism
      (SLS, or ISLS in the ATCBRI-5) to prevent transponders from replying
      to interrogations that are received outside of the main lobe of the
      transmit antenna. All ATCRBI systems are co-channel, using the same
      frequency. They avoid inteference by using different delay (stagger)
      sequences between successive interrogations. This allows multiple
      radars to interrogate the same transponder at the same time. In the
      LA basin, for example, you're interrogated by at least 4 separate
      radars while airborne.

      Airport ASR-9's rotate at about a 4 second rate. The long-range
      ARSR's have about a 10-12 second rotation. The rate is selectable by
      the site and does vary between installations. In many cases the rate
      uniquely identifies a particular radar. If you ever suspect that
      you're getting radio interference from a radar system, the first thing
      to measure is the periodicity. (One quick note -- my experience is
      that the radio emissions from radars are regularly checked for
      compliance. Nearly all interference issues with out-of-band services
      are receiver related.)

      So what can cause the light to be continuously on (discarding failure
      modes)?

      1. The SLS system has been disabled and your trasponder is replying to
      all of the interrogations from the radar throughout it's rotation.
      This would only happen when you are physically close to the radar so
      that the radar's sidelobes would exceed the transponder's receive
      threshold.

      2. Your transponder is responding to other interrogations (TCAS, etc.)

      3. There is a separate interrogator illuminating the area that does
      not rotate.


      --- In coldwarcomms@yahoogroups.com, Bob Stratton <highflight@g...>
      wrote:
      > The rate of flickering will depend on many circumstances. If you're
      in
      > RADAR coverage, it should be blinking whenever either the ground
      RADAR
      > or an aircraft with TCAS interrogates your transponder. Enroute that
      > could be almost all of the time, depending on how the coverage is.
      >
      > The thing that surprised me was that I was on the ground, next to a
      > hill, and the thing was often lit practically solid. I had never
      seen
      > that in any aircraft prior to 9/11.
      >
      >
      > ----- Original Message -----
      > From: Bill Smith <brs.comm@c...>
      > Date: Mon, 21 Jun 2004 17:33:49 -0400
      > Subject: RE: [coldwarcomms] 9/11 Aircraft over DC
      > To: coldwarcomms@yahoogroups.com
      >
      >
      >
      >
      >
      > I occasionally have to take a commuter flight from Hyannis to
      Nantucket as
      >
      > part of my job. The last time I flew over, I was in the right front
      seat and
      >
      > noticed the reply light blinking about every 15 seconds or so.
      What's a
      >
      > normal interrogation rate? It didn't help that it was foggy on the
      return
      >
      > and the pilot missed the approach...
      >
      >
      >
      > -----Original Message-----
      >
      > From: Bob Stratton [mailto:highflight@g...]
      >
      >
      >
      >
      >
      > These days I can tell when the P-3's or AWACS are up because I can
      >
      > watch my reply light blinking like mad even when I'm still on the
      >
      > ground.
      >
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