RE: Analysis from a JHU CS Prof

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

In modern (non-military) atc systems, this info is relayed by the
transponder to atc.

Source: "How to become a Pilot" Bantnam Press.

If true, this is a weird practice. In Sweden, there are several civilian
radars that are true radars with transponder receivers mounted on them as
a compliment.

Having served in the Swedish Airforce and actually having access to
information provided by both civilian and military radars, my experience
is that as long as you're flying fairly high (ie several thousand feet)
even the civilian radars are going to see you fairly far away. Yes, your
transponder is visable long before that, but if you turn it off you'll
still be visable. We received information from both the transponder and
actual civilian radar on our screens and most of the time the civilian
traffic including small props were visable as both transponder
position/hight and radar echo.

What might be the case is that the true radar echo information is not
relayed to the individual ATC because even the civilian high altitude
radar gets cluttered by false echos.

The military low-altitude costal radars are the ones really cluttered.
Man, if it was like in the movies with a beep each time the radar swept an
echo, all military radar officers would be deaf :slight_smile:

I can't speak for the US, but in Canada we definitely have an abundance of
"true radar" installations in most non-military control towers. I don't
know the number of times I've heard tower indicating that they have an
aircraft out in the practice area--keep an eye out. Altitude is unknown.
That's just the standard advisory control tower's radar - and not the large
area control centre radar. Canada has 7 area control centres for Canada and
the Territories, and one "advisory" tower for each ATC centre at each
airport. Edmonton for example, has two "advisory" towers - one for the
Municipal airport, one for the International airport slightly to the south
of the city.

Ejay, you did make me wonder about your comments. I've sent off an Email to
Nav Canada. If I get a response as to how the radar in Canada operates,
I'll share it with you.

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: