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Re: Analysis from a JHU CS Prof

  • From: Owen DeLong
  • Date: Thu Sep 13 19:54:33 2001

This is long, and off topic.  If you don't want to know about air
control, delete it now.

Most ATC services are not provided by towers.  Generally, the tower
a 5 statute mile radius around some arbitrary point on the airfield
the middle) up to 2,500' AGL.  Some towers have more airspace, some have
(For example, Hayward tower has less altitude, and is cut short to the
and north by airspace controlled by Oakland tower and Bay Approach).

Most towers don't have RADAR at all.  Of the towers that do have radar,
usually  they receive a feed from the closest TRACON or ARTCC (see my
previous message).  The RADAR information presented to controllers in
TRACON and ARTCCs is the result of a computer compositing several
RADAR transcievers to produce a digital image presented to the

Among the things that can be presented to controllers on these displays
are the following:

	Primary Target*
	Digital Map
	Data Fields***
		Beacon Code
		Ground Speed
		Vertical Change Status (Climb/Descent/Level)****
		Heavy Marker(If heavy) *****
	Secondary Target***

*Some facilities do not have Primary target capability.  There is talk
of decomissioning this altogether, although it has met with substantial
opposition in the aviation community.

**Only in the facilities with the oldest and newest equipment.  The
equipment displays weather whether the controller wants to see it or
and does a very poor job of it.  This is the result of poor filtering
technology in some of the oldest equipment still in use.  This equipment
will probably disapper within 5 years.  The newest equipment has the
capability to integrate information from the NexRAD weather radar sites
onto the display.  This is a significant improvement.

***Data fields only appear if the target has an active transponder
operating.  The same is true for secondary target.

****Altitude information only appears if the transponder is active
in mode C or mode S (altitude encoding, or Data-Link modes).

*****This information is only present if it has been entered by a
controller with an ATC flight plan. This is done in order to assign
a particular aircraft a unique beacon code for specific tracking.

Here are what the terms above mean...

	Primary Target
		The depiction on the scope of the actual reflection of
		radio energy back from something some distance and bearing
		from the radar.

	Digital Map
		This is a terrain and aviation facility map that is encoded
		into the computer system.  It includes things like landmass
		borders, intersections (aviation waypoints, not roads),
		navaids, airports, approach gates, etc.  It is a depiction
		of the area to help the controller remain oriented to
		the traffic flows and guide aircraft to their destinations.

	Beacon Code
		The 4-digit octal number programmed into the targets

		The vertical distance between the aircraft and mean sea level.

	Ground Speed
		The speed of the aircraft over the ground.

		The flight number or tail number of the aircraft. Examples
		would be things like:

		UAL563	United 563
		N1254M	November 1254 Mike
		AAL952	American 952

		The type of aircraft.

		The intended destination of the aircraft

	Vertical Change Status
		Provides a blank for level, upwards pointing arrow for climb,
		or downwards arrow for descent.  Represents any altitude change
		of 100 feet or more since last interrogation.

	Secondary Target
		The active reply to the radar interrogation of the transponder.
		This is generally represented as a single letter which is used
		to indicate which controller is talking to the specific aircraft
		at the particular time.

		Radar can (generally) only measure liquid water in the air.  As
		such, some forms of clouds and precipitation can be displayed
		on some facilities radar units.

	Tour of Palo Alto Tower (BRITE Radar equipped)
	Tour of San Francisco Tower (Equipped with multiple types of tower
				radar, including BRITE, SMGCS, etc.)
	Tour of Bay Approach (TRACON)
	Tour of Socal Approach (TRACON)
	Tour of Oakland Flight Service Station (AFSS)
	Tour of Oakland Center (ARTCC)
	Discussions with multiple working Air Traffic Controllers in
		towers, TRACONS, and ARTCCs.
	Airmans Information Manual
	ATC Handbook (7110.65J, 1997 edition)

Hope that helps clarify things for everyone.  If anyone wants to know
about how these components of the system interrelate, let me know off


"Hire, Ejay" wrote:
> Most ATC towers do not have true radar.  I.e. the ability to detect flying
> objects above altitude x by bouncing radio waves off of the object and
> computing the time vs. Doppler shift vs. inclination to determine
> altitude/heading/speed.
> In modern (non-military) atc systems, this info is relayed by the
> transponder to atc.
> Source:  "How to become a Pilot" Bantnam Press.
> -----Original Message-----
> From: Leigh Anne Chisholm [mailto:[email protected]]
> Sent: Thursday, September 13, 2001 5:11 PM
> To: Borger, Ben; [email protected]; Hire, Ejay
> Subject: RE: Analysis from a JHU CS Prof
> I despise posting off-topic, but I want to say two things...
> 1.  If a transponder is turned off, it doesn't mean that you don't show up
> on radar--a blip appears on the radar screen as long as you're high enough
> to be detected.  If however you fly low enough, you can fly below the
> radar's detection capability.  I don't offhand recall what height that
> is--it's been years since I was active as a pilot and prospective Air
> Traffic Controller.
> 2.  What's the point of having transponder codes for hijacking if they're so
> well published everyone is aware of them?  The purpose of the codes was so
> that the pilot could communicate this information without the hijacker
> becoming aware of what was happening.  I have always REALLY DISLIKED the now
> common practice of advertising this information.  You're taking away one of
> the pilot's best tools...
>   -- Leigh Anne
> > -----Original Message-----
> > From: [email protected] [mailto:[email protected]]On Behalf Of
> > Hire, Ejay
> > Sent: Thursday, September 13, 2001 2:35 PM
> > To: 'Borger, Ben'; '[email protected]'
> > Subject: RE: Analysis from a JHU CS Prof
> >
> >
> >
> > The transponders, like most avionics, has a handy-dandy off switch.
> >
> > -----Original Message-----
> > From: [email protected] [mailto:[email protected]]On Behalf Of
> > Borger, Ben
> > Sent: Thursday, September 13, 2001 2:32 PM
> > To: '[email protected]'
> > Subject: FW: Analysis from a JHU CS Prof
> >
> >
> >
> > >>At 06:05 PM 9/12/2001, you wrote:
> > >>
> > >>
> > >>Quite more interesting is why nobody noticed that 4 airliners where
> > hijacked
> > >>almost the same time.
> > >
> > >Not surprising.  Aircraft are "flight followed" by a series of control
> > >centers across the nation, each responsible for a given chunk of
> > >airspace.  Something happening in an area controlled by Center "A", for
> > >example, wouldn't be passed on to Center "B" (which has it's own
> > problems
> > >to work) unless it impacted Center "B".  Furthermore, unless
> > someone TELLS
> > >Center they're being hijacked, there's no way for a controller -
> > looking at
> >
> > >a blip - to know what's up.  And any controller can tell you
> > that pilots do
> >
> >
> > Somehow the people who did this managed to turn off the transponders on
> > these planes.  Normally a plane flying in controlled airspace squawks a
> > unique id and altitude which is decoded by their radar and associated with
> > each blip.  Sometimes low cost homebuilts/ultralights fly with no
> > transponder, but Boeings <sarcasm>usually</sarcasm> do.  If you set a
> > transponder to XXXXX, it means you're being hijacked.
> >
> > BTW if you see your friend Jack at the airport, be sure to say,
> > "What's up,
> > Jack!" instead of "Hi Jack!"
> >
> > So how do you deal with this?  Blowing up a whole country?  I
> > wonder if the
> > US should adopt a 'fire w/ fire' approach and invest in
> > intelligence, covert
> > ops and assassinations.  It would seem that it is open season on terrorism
> > by every democratic nation, I expect to see very conspicuous
> > Samuel Jackson
> > style ass whoopins on whiny extremist groups to satiate America's anger.
> > Terrorize the terrorists.
> >
> > Oh yeah, obviously Echelon should probably have MacOS loaded on it.
> >
> > -b

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