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equipment (e.g. a transponder)
Multistatic: The same signal needs to be received
simultaneously by several ground stations
5
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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
6
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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
7
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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)
8
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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
9
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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
10
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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
11
L. Gonzales 29/07/05
Thales ATM Do not reproduce without permission
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
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