MDS技术资料(Thales)

航空

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航空

Multilateration & MAGS
L. Gonzales
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L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
Outline
1. Principle of multilateration
2. Aircraft signal
3. Multilateration on airports
4. Conclusion
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1. Principle of multilateration
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Basic Principle of Multilateration
Multilateration (MLAT) is a technique initially developed for military
applications, which allows to passively locate co-operative targets by
multistatic measurements.
􀂄 Passive: no interrogation from the surveillance system are
required (i.e. receive only), provided the aircraft transmits a
signal
􀂄 Co-operative: the principle requires appropriate onboard
equipment (e.g. a transponder)
􀂄 Multistatic: The same signal needs to be received
simultaneously by several ground stations
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Comparison with other Surveillance Principles
Data measured by
surveillance system?
Interrog.
required?
Onboard equipment
Surveillance Principle required?
No
dependent surveillance
No
passive
Yes
ADS-B co-operative
Yes (partly)
partly independent
surveillance
No
passive
Yes
Mode S Multilateration co-operative
Yes (partly)
partly independent
surveillance
Yes
active
Yes
Mode A/C Multilateration co-operative
Yes (partly)
partly independent
surveillance
Yes
active
Yes
Secondary Surveillance Radar co-operative
Yes
independent surveillance
Yes
active
No
Primary Radar non-co-operative
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Principle of Multilateration Systems (1)
Time of Arrival in A: TOA1
Time of Arrival in B: TOA2
A and B are a pair of Ground
Stations receiving both a
signal from an aircraft.
The Time of Arrival TOA of the
signal is measured by each
Ground Station.
The time difference
TOA1-TOA2 corresponds to the
distance difference
X2 - X1 = c • (TOA2 – TOA1)
X1 = c • TOA1
X2 = c • TOA2
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Principle of Multilateration Systems (2)
Time of Arrival in A: TOA1
Time of Arrival in B: TOA2
At a given time, the Aircraft is on
the locus of points having the
distance X2 - X1 constant:
X2 - X1 = c • (TOA2 - TOA1)
This is a hyperbola (curve in
blue)
=> Two ground stations allow to
determine one hyperbola where
the aircraft is located
X1 = c • TOA1 X2 = c • TOA2
X2 - X1 = c • (TOA2 - TOA1)
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Principle of Multilateration Systems (3)
Time of Arrival in A: TOA1
Time of Arrival in B: TOA2
A third station in C gives
two more differences
X2 - X1 = c • (TOA2 - TOA1)
X2 - X3 = c • (TOA2 - TOA3)
X1 - X3 = c • (TOA1 - TOA3)
and thus allows to determine
two more hyperbolas
=> The aircraft is located
at the intersection of the three
hyperbolas
X3 = c • TOA3
Time of Arrival
in C: TOA3
X2 = c • TOA2 X1 = c • TOA1
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Multilateration Principle Summary
Ground stations determine the precise time of arrival
(TOA) of received signals
Intersection of several hyperbolas is the target
position
Knowing the speed of wave propagation,
a hyperbolic line of position results
TOA difference is calculated for each pair of
ground stations
Signal transmitted by aircraft transponder is received by several ground
stations (a minimum of 3 for 2D position) in the vicinity
This principle can be extended to measure 3D positions : a 4th ground station
is then required
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Generic System Architecture
To implement the principle of multilateration system, the generic
system architecture consists of:
􀂄 A sufficient Number of Ground Stations (GS) capable of:
􀂄 receiving the signal(s) from aircraft located in the service area,
􀂄 measuring the time of arrival and forwarding the TOA to a central
station,
􀂄 being synced to the same timebase
􀂄 A Central Processing Station (CPS):
􀂄 to receive the TOAs from the Ground Stations and
􀂄 to compute the aircraft position from the set of measurement.
􀂄 In addition the CS has to manage the fact that several aircraft
can be located in the service area,
􀂄 A communication network to link all the GS to the CS
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Constraints related to the principle of multilateration systems
The measurement of time of arrival must be very accurate
􀂄 As an inaccurate measurement will degrade the accuracy of the
position calculation
􀂄 This can be achieved by high frequency sampling of incoming
signals
The clocks of the ground stations must be very well synchronised
􀂄 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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2. Aircraft Signal
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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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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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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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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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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,
for example in the case of RVSM implementation. In this case
a modelling of the variation of atmospheric pressure with
altitude must be established
ADS-B provides barometic and geometric Altitude
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3 - Conclusion
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Airport Multilateration Summary
Strengths
􀂄High performance
􀂄For airports, it exceeds present
SMR performance
􀂄No additional aircraft equipage
required
􀂄Aircraft widely equipped with SSR
transponders
􀂄More and more aircraft are
equipped with Mode S
Transponders
􀂄Lifecycle cost lower than Radar
􀂄no rotating machinery,
essentially maintenance-free
Weaknesses
􀂄erformance affected by
ground effects (multipath,
shadowing, etc)
􀂄Change in installations and
procedures may be required
􀂄So transponder is not disabled
on the ground
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Key Features
􀂃Automatic aircraft labelling/
Identification
􀂃assive aircraft location
􀂃Growth potential to receive and
forward ADS-B reporting
􀂃 Easily adaptable to airport layout
􀂃 Integration into STREAMS (Thales
ATM’s SMGCS)
Co-operative Multilateration System
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MAGS Target Location Methods
Passive multilateration
􀂉 Using all Mode S downlink formats
received
Active multilateration
􀂉 Includes a low power interrogator
(100 W) for less covered areas and
approach
Capable of growth towards reception
and processing of ADS-B / extended
squitter
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MAGS Adaptability
􀂉 Scaleable number of Ground Stations
􀂉 Adjustable antenna coverage
􀂉 Processing algorithms individually
adaptable for each airport area
􀂉 Full local and remote control
􀂉 Easy integration into STREAMS
􀂉 Wide range of Commercial Off The Shelf
(COTS) network equipment
􀂉 Industry standard interfaces and protocols
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Coping with Multipath
􀂉 Antenna patterns and special MAGS signal processing design allow
to reduce multipath influence
􀂉 It is essential to optimise ground stations placement
􀂉 A trade-off must be carried out between station geometry ,
multipath avoidance, and operational constraints
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MAGS Technical Data
TOA resolution: 128 MHz (7 ns / 2.4 m)
Mean accuracy: < 7 m
Detection probability: >95%
>99% in restricted areas
(e.g. runways)
Mean update rate: 1/s
Localisation capacity: 300 plots/s max.
Interrogation capacity: 200/s
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MAGS-GSR (outdoor Version)
􀂉 Off-the-Shelf Cabinet
􀂉 Heat Exchanger between Twin Walls
􀂉 300 W Heater
To be mounted to a wall, to a mast or
standing on Ground (together with
plinth as shown)
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MAGS at Köln/Bonn Airport
• Focus on one area
(Apron, TWY A/B/E, RWY14L)
• Five Ground Stations
(1 GST 4 GSR)
• Uses all valid Mode S
downlink formats
• Raw data shown,
i.e. no tracking or filtering
• Remote control, diagnosis
and configuration
over ISDN/SNMP
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ACRONYMS
􀂃 ADS-B : Automatic Dependence
Surveillance Broadcast
􀂃 COTS : Commercial Off The
Shelf
􀂃 CS : Central Station
􀂃 DPX : Duplexer
􀂃 GSR : Receive only Ground
Station
􀂃 GST : Transmit only Ground
Station
􀂃 HDOP : Horizontal Dilution of
Precision
􀂄 ISDN : Integrated Service Digital Network
􀂄 MAGS : Mode S Airport Ground Sensor
􀂄 NTA : Network Terminal Adapter
􀂄 RWY : Runways
􀂄 RXU : Receiver Unit
􀂄 SNMP : Simple Network Management
Protocol
􀂄 SPB : System Processing Board
􀂄 SPC : System Processing Computer
􀂄 SSR :Secondary Surveillance Radar
􀂄 TOA : Time Of Arrival
􀂄 TWY : Taxiway
􀂄 TXU : Transceiver Unit

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