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􀂄 As a bias between GS clocks will imply a measurement error
􀂄 This can be achieved by several means :
􀂄 transmission of a calibration signal
􀂄 use of an universal common time reference signal (regional time
signal transmitter, GPS)
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Thales ATM Do not reproduce without permission
2. Aircraft Signal
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L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
Cooperative target
Unequipped aircraft will not be seen by the MLAT system.
Only cooperating targets will be detected.
For civil aviation, the signal transmitted by aircraft can be:
􀂄 either a Mode A/C or Mode S reply to any interrogator in the
neighbourhood (e.g. Radar, ACAS)
􀂄 the Short Squitter (acquisition squitter for ACAS) transmitted once per
second for aircraft equipped with a Mode S Transponder
􀂄 In the next future, the Extended Squitter transmitted twice per second
for ADS-B equipped aircraft.
In case the aircraft are not equipped with Mode S transponders, and no
MSSR are available in the neighbourhood, a specific interrogator must be
implemented to trigger Mode A/C replies.
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Thales ATM Do not reproduce without permission
Aircraft Signal which can be used by multilateration
Implementation
is just starting
24bit aircraft
address and the
rest is variable
Various rates up ADS-B
ADS-B to 2 per second
Extended Squitter
Widespread due
to ACAS
mandate
24bit aircraft
address and
transponder
capability
Once per second ACAS
Acquisition
squitter
« short squitter »
Expanding (few
ground Mode S
interrogation)
Mode S replies
widespread due
to ACAS
mandate
24bit aircraft
address. The
rest depends on
interrogation
Ground ATC
surveillance and
ACAS
Sent in response
to interrogation
Mode S reply
Mode A or Mode Very widespread
C code
depending on
interrogation
Ground ATC
surveillance and
ACAS
Sent in response
to interrogation Mode A/C reply
Transponder When sent Original purpose Data contents Use today
transmission
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L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
Difference between processing Mode S and
Mode A/C aircraft
With Mode S signals, each ground station receives a signal which is
uniquely identified by the ICAO 24 bits address; this allows the MLAT
system to unambiguously associate the various messages as
belonging to the same aircraft
For Mode A/C signals, the association is easy if the signal is a Mode A
signal, but if it is a Mode C signal, the MLAT system must maintain a
table of all aircraft in the service area before being sure to associate
the replies received by ground stations as belonging to the same
aircraft.
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Thales ATM Do not reproduce without permission
Identification of aircraft (1)
In civil applications, identity of the aircraft is required
MLAT extracts aircraft identity information from the
transmitted signal (also used to measure aircraft position)
This is obtained by Mode A information when the signal is a
reply to MSSR or Mode S interrogation
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Thales ATM Do not reproduce without permission
Identification of aircraft (2)
The identification is not straightforward in case no Mode S or MSSR
radars are implemented in the neighbourhood.
If the aircraft is equipped with a Mode S transponder it transmits the
short squitter, including the 24 bits ICAO address of the aircraft
􀂄 The 24 bits ICAO address is currently not included in Flight Plans
=> does not allow to correlate the signal with aircraft ID
􀂄 The MLAT must then interrogate the aircraft to obtain Mode A information
􀂄 This will no longer be true with Extended Squitter as the Call Sign is
transmitted by Extended Squitter
If the aircraft is equipped with a MSSR transponder, the MLAT system
must interrogate the aircraft to obtain a Mode A reply.
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Aircraft Altitude
In the same manner as for identity, aircraft barometric
altitude will be obtained by using Mode C.
In the case of 3D MLAT system, only the geometric altitude
of the aircraft is measured :
􀂄 Not used in “normal” surveillance operation
􀂄 Used in monitoring of the performance of aircraft altimeters,
　
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