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Re: Analysis from a JHU CS Prof
From: Owen DeLong <owen () delong sj ca us>
Date: Thu, 13 Sep 2001 16:51:31 -0700


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

Most ATC services are not provided by towers.  Generally, the tower
controls
a 5 statute mile radius around some arbitrary point on the airfield
(usually
the middle) up to 2,500' AGL.  Some towers have more airspace, some have
less.
(For example, Hayward tower has less altitude, and is cut short to the
west
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
the
TRACON and ARTCCs is the result of a computer compositing several
different
RADAR transcievers to produce a digital image presented to the
controller.

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

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

*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
oldest
equipment displays weather whether the controller wants to see it or
not,
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
                transponder.

        Altitude
                The vertical distance between the aircraft and mean sea level.

        Ground Speed
                The speed of the aircraft over the ground.

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

                UAL563  United 563
                N1254M  November 1254 Mike
                AAL952  American 952

        Type
                The type of aircraft.

        Destination
                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.

        Weather
                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.

Sources:
        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
more
about how these components of the system interrelate, let me know off
list.

Owen

"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:lachisho () tnc com]
Sent: Thursday, September 13, 2001 5:11 PM
To: Borger, Ben; nanog () merit edu; 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: owner-nanog () merit edu [mailto:owner-nanog () merit edu]On Behalf Of
Hire, Ejay
Sent: Thursday, September 13, 2001 2:35 PM
To: 'Borger, Ben'; 'nanog () merit edu'
Subject: RE: Analysis from a JHU CS Prof



The transponders, like most avionics, has a handy-dandy off switch.

-----Original Message-----
From: owner-nanog () merit edu [mailto:owner-nanog () merit edu]On Behalf Of
Borger, Ben
Sent: Thursday, September 13, 2001 2:32 PM
To: 'nanog () merit edu'
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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