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* Secondary radars, which involve sending a radio pulse to a transponder that then shoots back a
response, remain in widespread use. One of the important uses is in the form of a commercial
aircraft navigation tool, as part of the "VOR/DME" stations generally found at commercial

VOR/DME is actually two more or less independent systems that make up a single beacon, with
the two subsystems using different radio bands. VOR stands for "VHF Omni-Range", and it is a
type of "radio-compass" unit, broadcasting a directional signal that not only acts as a beacon but
provides coded information that gives the compass angle of the signal between the aircraft and
the ground station.

The DME or "Distance Measuring Equipment" component is the secondary radar. The DME unit
in the aircraft shoots out a radio pulse to the DME transponder in the ground station, which
immediately sends back an amplified response. The DME unit measures the total time delay to
determine the distance to the ground station.

The military has a "Tactical Air Navigation (TACAN)" system that is very much like
VOR/DME. In fact, TACAN uses the same DME scheme, the main difference being that
TACAN's VOR equivalent uses the same radio band as DME instead of a different band. Some
stations provide both VOR/DME and TACAN, and are known as "VORTACS".

* The old IFF technology has evolved in the decades since World War II, leading to the current
"Mark X/XII" IFF standard, and is an important element in both military and commercial
service. An airport IFF interrogation unit operates in one of two modes, querying the IFF
transponder in an aircraft to either obtain the identity of the aircraft or its altitude. The
interrogator will send out two pulses modulated on a 1.03 GHz carrier. To obtain the aircraft
identity, the interrogator performs a "Mode 3/A" interrogation, with the pulses 8 microseconds
apart. To obtain the aircraft altitude, the interrogator performs a "Mode C" interrogation, with the
pulses 21 microseconds apart.

The aircraft transponder replies on 1.09 GHz with 12 pulses giving the requested information,
with each pulse providing a "0" or "1" bit giving a total of 4,096 possibilities. The aircraft
identity code is assigned when the aircraft departs from an airport, with the code entered by the
flight crew. Light civil aircraft operating under daylight flight rules will always respond with a
"1200" code. There are also three reserved reply codes, including "we have an emergency", "we
have been hijacked", and "our radio is broken". The altitude is given in multiples of 100 feet (30

The 3/A and C modes are common to both civil and military aircraft, but three modes are
reserved for military use:
      In "Mode 1", two pulses are sent by the interrogator 3 microseconds apart, and the IFF
       transponder responds with an 8-bit code that gives the type of aircraft and the mission
       that it is on.
      In "Mode 2", two pulses are sent 5 microseconds apart, and the aircraft provides a 12 bit
       reply, giving a "serial number" for the specific aircraft.
      None of the modes described so far actually determines if an intruder is a friend or a foe,
       which was what IFF was invented to do, and so the military uses a "secure" mode
       designated "Mode 4" to sort friendlies from hostiles. Instead of sending two pulses, the
       interrogator sends a long "word" of encrypted bits. The "key" for enciphering and
       deciphering this word is handed out on a periodic basis, usually daily. The transponder
       decrypts the challenge with the same key, and returns a sequence of replies depending on
       the format of the challenge. Using a sequence makes the probability of an enemy
       guessing the correct response very unlikely. If the transponder doesn't respond correctly,
       the aircraft is marked as an unknown or "bogey", or in a "hot war" theater as a "bandit", a
       hostile to be engaged.

* The current commercial IFF modes are basically manual in operation, and so the US Federal
Aviation Administration (FAA) has approved a new "Mode S" that will eventually replace Mode
3/A and Mode C. Mode S uses the same frequencies as the old modes, but the challenge and
response formats are much more elaborate. In particular, each aircraft will have its own unique,
permanently assigned IFF code, with more than 16 million possibilities available. The response
will also include altitude and other relevant data. The Mode S scheme is much more convenient
for automated systems.

The military has also investigated advanced IFF modes providing a level of capability along the
lines of Mode S, with a higher level of security by using "low probability of intercept" features
such as wideband spread spectrum communications. Work on an advanced "Mark XV" IFF
bogged down and was abandoned, but efforts continue, focusing on more modest enhancements
of the Mark X/XII technology.

* With Mode S, IFF begins to seem much more like a communications technology than a radar
technology. However, IFF is based on radar technology and can still be used very much as a
radar technology. A number of large airports use "multilateration" or "multistatic dependent
surveillance" systems to track aircraft on the runways; such multilateration systems consist of a
network of ground-based sensors that triangulate the position of aircraft by comparing the time
of arrival of signals from aircraft IFF transponders. Multilateration is less complicated and less
expensive than radar, consumes less power, and is easier to maintain because it doesn't require
rotating antennas. The basic idea is far from new, but it wasn't practical until low-cost computing
hardware that could perform the triangulations became available.

"Wide area multilateration (WAM)" systems that track traffic in flight are in increasing use as
well. For example, the airport at Innsbruck, Austria, installed a WAM system to track incoming
and outgoing flights. The airport is small and couldn't afford an expensive radar system, but the
approach to the airstrip is bordered by mountains and very hazardous. Innsbruck controllers had
relied on radar tracking from Munich, 100 kilometers (62 miles) away, but for final approach the
local controllers were effectively blind. Only one aircraft could be brought in at a time, with the
others remaining in line of sight of Munich.

The Innsbruck system features two transmitters and eight antennas, with three receive & transmit
antennas and five receive-only antennas. Six of the antennas are sited in the surrounding
mountains and two are sited at the airport. It can provide three-dimensional locations with
accuracies of 30 meters (100 feet) or better, with position updates once a second. It can be used
with any aircraft with a standard IFF transponder, and can also be used to keep track of ground
vehicles fitted with transponders to prevent runway collisions.

Since the installation of the Innsbruck system, dozens of other small airports have also obtained
WAM systems, and they have been used for tracking air traffic over military firing ranges.
However, WAM has little or no use for combat operations since it requires that the aircraft being
tracked cooperate with the scheme.

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