Good
or evil? A summary of the “pros” and “cons”, without too much
detail, based on the developments in Europe.
A
bit of history
Transponders, the airborne part of Secondary
Surveillance Radar (SSR), were developed to enhance primary radar. Primary radar,
which we all know from its impressive antennas seen rotating at airports (for
approach control) or sometimes in the middle of the countryside (for long range
surveillance), was developed during World War II. Strong pulses (in the order
of a megawatt), bundled in a narrow beam, proved to be reflected by aircraft to
give a weak, but sufficiently strong signal in the radar receiver, which is turned
on immediately after each pulse has been transmitted and which obviously uses
the same antenna “to hear better”. The azimuth (geographical direction)
of the antenna and the time interval between transmission of each pulse and the
reception of the reflected energy determine the azimuth and distance of the aircraft.
We are not surprised that this works well with Boeing 747s, but the metal and
wiring in gliders often reflect enough energy to let the glider show on the radar
screen as a “blip” as well.
All this is fair enough, but there are
two important things missing. One is the identity of the aircraft, the other is
its altitude. During World War II, therefore, IFF (Identification Friend or Foe)
was developed and later developed further for civil applications as SSR. A military
version with special functions is still extensively used as IFF.
A
bit of technique
Primary radar is a passive system, because
the weak reflections on the skin and other metal parts of the aircraft are detected
on the ground. SSR transponders actively reply to interrogations with relatively
strong pulses of up to 500W. Apart from the information contained in the reply
pulses, it will be clear that these relatively strong signals give a much better
radar picture. In almost all modern radar systems, the primary and secondary radar
returns are detected, combined and tracked in sophisticated algorithms, so that,
for instance, also the heading can be derived and shown on the screen. Previously,
the controller had to either ask the pilot for his heading or watch the afterglow
of the blips on his screen.
Modes
All
interrogations are made on 1030MHz, all replies on 1090MHz. There are four ways
of interrogating, called Modes. When interrogated in Mode A an aircraft is asked
for his identity, when interrogated in Mode C he is asked for his altitude. The
Modes B and D are not used. The civil Mode A coincides with the military Mode
3. That is why Mode A is often called Mode 3/A.
The replies are called
Codes. The Mode A Code is set in the cockpit in four digits, the Mode C Code automatically
transmits the altitude, with 1013.2hPa as the reference pressure, just like in
Flight Levels.
Coupling a dedicated Mode A Code to a flight plan allows
the controller to see the callsign of a particular aircraft directly on his screen.
Mode C does the same for him with the altitude. Together with the heading, derived
from the tracking algorithm, we have now enormously improved the information available
to the radar controller on his screen. This already explains why transponders
without Mode C can no longer be accepted. We also begin to understand why controllers
don’t like to gi |
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