The need for the use of another surveillance system when radar cannot be used is the reason for the development of the Multilateration (MLT) Systems. However, there are many systems that operate in the L-Band (960-1215MHz) that could produce interference between systems. At airports, some interference has been detected between transmissions of MLT systems (1030MHz and 1090MHz) and Distance Measuring Equipment (DME) (960-1215MHz)

Alonso Alarcon, Jose Felix

García-Heras Carretero, Javier

Saez Nieto, Francisco Javier

Archivo Digital UPM

Criteria for Positioning Active Multilateration Stations Located Close to Distance Measuring Equipment

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

1 
 
Criteria for Positioning Active Multilateration Stations Located Close to 
Distance Measuring Equipment 
Javier García-Heras Carretero, Francisco Javier Sáez Nieto, José Félix Alonso Alarcón  
Air Navigation Research Group Escuela Técnica Superior de Ingenieros Aeronáuticos,  
Universidad Politécnica de Madrid, Plaza Cardenal Cisneros nº3, 28040, Madrid, Spain 
E-mail: javier.garciaheras@upm.es 
Tel: +34 91 336 3605 
 
The need for the use of another surveillance system when radar cannot be used is the 
reason for the development of the Multilateration (MLT) Systems. However, there are 
many systems that operate in the L-Band (960-1215MHz) that could produce interference 
between systems. At airports, some interference has been detected between transmissions 
of MLT systems (1030MHz and 1090MHz) and Distance Measuring Equipment (DME) (960-
1215MHz). 
 
1. INTRODUCTION 
 
It is therefore necessary to study MLT and DME Systems and investigate the possibility of 
interferences existing between them. This research involves two main phases: 
• Static Analysis: This calculates the interference signal level available at the receiver in 
terms of distance. It aims to establish the received power as a function of the distance 
between multilateration and DME antennas, determining at the received nominal 
frequency when the interference signal level available is below the sensitivity of the 
receiver.  
• Dynamic Analysis: This is to show the behavior of the receiver when interferences are 
introduced and how under this situation it acquires the received signal. By using this 
simulated signals and receiver (specifically modeled), the system is analyze an evaluated 
by taking characteristics of the detections as functions of the channel frequency of the 
DME and the distance between antennas. 
 
1.1 MLT 
 
MLT is a surveillance system that calculates the aircraft position by means of measurement of 
time difference of arrival (TDOA) of the transponder squitter, for which there is a need for a 
minimum of 3 station receiver. 
 
The operating principle of this system is based on the principle of secondary radar (SSR): 
ground interrogator excite the aircraft transponder (1030MHz), which in turn emits the 
squitter (1090MHz) that is received at the ground station receiver. The TDOA is processed by a 
central unit to determinate the aircraft position. [3] 
 
1.2 DME 
 
The DME provides the aircraft with an indication of the slant distance between the onboard 
equipment and ground system, in line of sight. This slant distance measurement is the first 
error to consider; this error increases as the aircraft get closed to the ground station. 
 
The operating principle of the DME is that the onboard equipment emits an interrogation 
signal formed by a pair of pulses with nominal value of 3.5μs. Initially these interrogations are 
made in search mode between 20 and 150pps. This interrogation is captured by ground system 
which replies with the same sequence but delays the signal for some time (systematic delay) 
depending on the mode used (Mode X or Y), with a constant rate of 2700pps useful cycle.[1][2] 
2 
 
2. STATIC ANALYSIS 
2.1. Propagation models 
 
These models attempt to predict the propagation losses based on optical geometry.  Two 
types of models are considered: 
• Free space: It considers isotropic antennas located in a perfect, homogeneous and 
isotropic dielectric and an ideal medium without obstacles, maintaining the distance 
between antennas.  
( ) ( ) ( ) ( )tf bf t rL dB L dB G dB G dB= − −  
Equation 1: Transmission losses in the free space 
Where: 
 ( ) 420 logbf dL dB piλ
 
= ⋅  
 
: Basic losses in the free space. [4] 
o d: Antennas distance (m). 
o λ: Wavelength (m) is equal to c/f, where c is the speed of light 
(3·10
8
m/s) and f is the frequency. 
 Gt: Transmitter gain. 
 Gr: Receiver gain. 
• Free space modified by ground effect: This is used in the case of Very High Frequency 
(VHF) band and higher. The hypothesis of this model is based on the ground plane, 
homogeneous environment, small incidence angle ( 1ψ << ). And also the modulus of 
the reflection coefficient is approximately equal to one ( 1R ≅ ) and has a phase 
approximately equal to pi ( β pi≅ ). 
( )
22 4
2
4
4b tx rx tx rx
d d dl
h h h h
pi λ
λ pi
 ⋅ ⋅ 
= =  
⋅ ⋅  ⋅ 
 
Equation 2: Basic loses 
Where: 
 htx: height of transmitter antenna. 
 hrx: height of receiver antenna. 
 d: Distance between antennas (m). 
It is important to note that this model is valid at distances higher than ( )12 tx rxh h λ⋅ ⋅ , 
because is the region in which all the hypothesis are applicable. 
 
2.2. Calculation of received power at multilateration ground beacon receiver 
 
The objective of this subsection is to determine the power of the signal from the DME/N 
ground transmitter received at the multilateration ground beacon receiver. This is expressed 
by the signal level as a function of the range using the following expression: 
2
24 4
eT T
dR eR i R
p gp A g
d
λϕ
pi pi
⋅
= ⋅ =
⋅
 
Equation 3: Friis Equation 
Where: 
• AeR is the effective area. 
• iϕ is the power density produced by the transmitter antenna in the place of the 
receiver antenna. 
• PeT is the power level signal transmitted. 
3 
 
 
Equation 4: Power available at the receiver with corrected field 
Where: 
• Gtx(DME)=12dB. Transmitter gain (DME ground transmitter). 
• Grx(MLT)=0dB. Receiver gain (MLT ground receiver). 
• peT has to be equal to 100W in a DME/N in Terminal areas and 1000W for DME/N en 
Route. 
• c=3·10
8
m/s. Speed of light. 
• htx(DME)=3.5m. Height of transmitter antenna (DME ground transmitter). 
• hrx(MLT)=3m. Height of receiver antenna (MLT ground receiver). 
• f=frequency.  There are two cases that is important to analyze: 
• Nominal transmitter frequency: This is the DME transmitter channel frequency 
(962-1024MHz and 1151-1213MHz), which gives the signal with more power. The 
most interesting studies, for this case, are realized when using the frequency close 
to the nominal receiver frequency (1090MHz) which is the signal that might result 
in interferences.  
• Nominal receiver frequency: This is the MLT reception frequency (1090MHz), 
which might create more interference. In this case, it is necessary make use of 
spectrum of the DME/N, which permits the application of correspondent 
attenuation of the power signal (see Figure 1). 
 
 
Figure 1: DME spectrum envelope shape 
 
Figure 2 and Figure 3 shows an example of a nominal DME frequency (interference signal) of 
1024MHz. In these diagrams there are five curves to consider:    
• The solid red line depict the computed DME signal power level entering MLT ground 
beacon receiver at antenna output port making use of free-space propagation mode.  
• The dotted red line represent the same as the solid red line but using free-space 
propagation modified by ground effect at the transmitter DME frequency. 
• The black solid line represents the same as the solid red line but at the nominal 
receiver frequency. 
• The black dotted line denotes the same as the previous curve but with free-space 
modified by ground effects. 
• The violet line correspond to sensitivity value, -85dBm to MLT. 
( )
4
2
rxtx
rxtxeTdR d
hhggpp ⋅⋅⋅=
4 
 
 
 
Figure 2:  Received power at multilateration ground station by DME/N ground transmitter, with 
Pt=100W and f=1024MHz 
 
 
 
Figure 3: Received power at multilateration ground station by DME/N ground transmitter, with 
Pt=1000W and f=1024MHz 
 
The goal of this study is to calculate the distance which the receiver sensitivity is above 
interference signal power level. The most interesting case in this analysis is when the 
frequency of the interference is equal to the nominal receiver frequency because the receiver 
filter might delete the other interferences. For example: using a Butterworth filter of 6
th
 order 
and 22MHz of bandwidth, at 60MHz (minimum difference between nominal frequency 
received and transmitted), the signal is attenuated around -110dB. 
 
The results of this analysis is made from conservative point of view because of the spurious 
attenuation is at 65dB (DME/N En route) and at 55dB (DME/N Terminal areas). The result 
indicates the need to separate the interfering antenna and the receiver antenna by 650m for 
both types of DME (en routes and Terminal areas). It is important to observe that the DME/N 
transmit the 90% of the signal power level at 2MHz, thus is reasonable that at 60MHz to the 
nominal transmitted frequency the level of the signal will be below -91dBm (MLT sensitivity). 
-160
-110
-60
-10
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Po
w
er
 
R
ec
ei
v
ed
 
(d
B
m
) 
Distance (m)
DME-MLT
DME-MLT with corrected field
DME/N Termial, f=1090MHz Corrected Field
DME/N Terminal f=1090MHz
MLT Sensitivity
-160
-110
-60
-10
40
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Po
w
er
 
R
ec
ei
v
ed
 
(d
B
m
)
Distance (m)
DME-MLT
DME-MLT Corrected Field
DME/N En Route-MLT, f=1090MHz Corrected Field
DME/N-MLT En Route, f=1090MHz
MLT Sensitivity
5 
 
 
2.3. Calculation of received power at DME/N ground beacon receiver 
 
This makes use of Equation 3 and Equation 4 like in the section 2.2, but in this case with the 
following values: 
• Gtx(MLT)=7.5dB. Transmitter gain (MLT ground transmitter). 
• Grx(DME)=8dB. Receiver gain (DME ground receiver). 
• htx(MLT)=3m. Height of transmitter antenna (MLT ground transmitter). 
• hrx(DME)=3.5m. Height of receiver antenna (DME ground receiver). 
• peT has to be equal to 1200W. 
• f=frequency.  There are two cases that is important to analyze: 
o Nominal transmitter frequency: This is the MLT transmitted frequency 
(1030MHz) that has a signal with more power.  
o Nominal receiver frequency: This is the DME/N ground reception frequency 
(1025-1087MHz and 1088-1150MHz {1025-1040MHz Reserved}) that has the 
signal that might create more interference. In this section, it is necessary to 
make use of spectrum of the MLT ground transmitter (see Figure 4), which 
permits the application of correspondent attenuation of the signal power 
level. 
• In the analysis with free-space modified by ground effect propagation model, 2dB of 
cable losses and 2dB of signal losses have been added. 
 
 
Figure 4: MLT transmitter spectrum envelope shape [3] 
 
In the graph shown in the Figure 5, an example for a nominal MLT transmitted frequency 
(interference signal) of 1030MHz can be seen. In this diagram, there are six curves:    
• The solid red line depict the computed MLT signal power level entering DME ground 
beacon receiver at antenna output port making use of free-space propagation mode.  
• The dotted red line represent the same as the solid red line but using free-space 
propagation modified by ground effect at the transmitter DME frequency. 
• The black solid line represents the same as the solid red line but at the nominal 
receiver frequency. 
• The black dotted line denotes the same as the previous curve but with free-space 
modified by ground effects. 
• The light violet line correspond to sensitivity value, -81dBm to DME/N terminal areas. 
6 
 
• The dark violet line correspond to sensitivity value, -91dBm to DME/N en route. 
 
 
Figure 5: Received power at DME/N ground station by MLT ground transmitter, with Pt=1200W and 
f=1030MHz 
 
This analysis was performed with every possible value of nominal receiver frequency in 
conjunction with the Equation 4 because this expression is more realistic. Results can observe in 
the Table 1. 
 
Table 1: Distance between DME/N-MLT ground antennas to prevent interferences  
fDME (MHz) 
|fDME-fMLT| 
(MHz) 
MLT  Attenuation envelope 
(dB) 
Antennas Distances with Corrected Field (m) 
Receiver Terminal DME/N  Receiver en Route DME/N  
1041-1050 11 -19 7380 13120 
1051-1060 21 -31 3700 6580 
1061-1070 31 -38 2480 4400 
1071-1080 41 -43 1860 3300 
1081-1087 51 -47 1480 2620 
 
1088-1090 58 -47 1480 2620 
Above 1091 63 -50 1240 2210 
 
 
3. DINAMIC ANALYSIS 
3.1. DME/N ground receptor design 
 
Figure 6 shows the schematic of the DME/N receiver. It begins with the signal passing through 
the receiving antenna, a bandpass filter and amplifier as first step. This process has not been 
simulated due to the inability to model the bandpass filter, since the cutoff frequencies are in 
the GHz band. The purpose of this filter is to avoid the inclusion of interference signals from 
other parts of the spectrum, for example, it limits the bandwidth of the noise that enters the 
receiver. 
 
Then there is a local oscillator fixed at the on-board DME channel frequency, to carry out the 
multiplication of the signal coming out of the local oscillator with the sum of the input signals 
received (DME interrogation, MLT interference, CW interference, Gaussian white noise). The 
purpose of this step is to acquire the baseband signal, in other words, the envelope of the 
interrogation signal, which is the DME pulse signal to be reviewed. The logarithmic amplifier is 
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Po
te
n
ci
a 
re
ci
bi
da
 
(d
B
m
)
Distancia (m)
MLT-DME MLT-DME C. Field
MLT-DME f=1041MHz C. Field DME/N Terminal areas Sensitivity
DME/N en Route Sensitivity MLT-DME f=1041MHz
7 
 
necessary to regulate the DME signal power level. Finally the video signal is evaluated in order 
to check if the pair pulse is within tolerances (12±0.1μs distance between pulses and 3.5±0,5μs 
width of the pulse). 
 
 
 
Equation 5: MLT signal model 
 
 
 
Equation 6: DME signal model 
 
10
máx
XY log K
X
 
= ⋅ 
 
 
Equation 7: Logarithmic Amplifier model 
 
 
Figure 6: DME/N Receiver Design 
 
 
3.2. Results 
 
In the case of analysing Noise and Continuous Wave (CW) interference, there were right 
detections except when the received DME frequency overlapped with CW frequency and the 
CW signal power level was higher than -75dBm. 
 
In the case of Noise and MLT interference without consider spurious, there were right 
detections in all conditions. In the case of Noise and MLT interference considering spurious, 
there were 6 cases: 
• fDME-fMLT between 10 and 20MHz, Is necessary a distance between antennas of 8500m. 
• fDME-fMLT between 20 and 30MHz, d≥4500m. 
∑
+∞
−∞=
+−⋅⋅−⋅=
k
kMLTkP1P2P6TMLT ))t(tfcos(2π)t(tSP(t)S ϕ
)∆TM(t
T
∆Tt
w
T
∆Tt
w
T
t
w(t)S 16
6
16
2
12
1
P1P2P6 −⋅





−
+





−
+





=
∑
+∞
−∞=
+⋅⋅⋅−⋅=
k
DMEkPPTDME tfttSPtS )2cos()()( ϕpi
2
PP
2 )∆tα(ttα
PP ee(t)S −−⋅− +=
8 
 
• fDME-fMLT between 30 and 40MHz, d≥3000m. 
• fDME-fMLT between 40 and 50MHz, d≥2500m.  
• fDME-fMLT between 50 and 60MHz, d≥2000m. 
• fDME-fMLT >60MHz, d≥1500m.  
 
Finally, if it includes every kind of interferences (CW, MLT considering spurious and Noise) we 
obtained: 
• For PdrCW≥-75dBm (case of CW). 
• For PdrCW≤-120dBm (case of MLT considering spurious). 
• For the rest, a mixture of the two cases. 
 
3.3. CONCLUSION 
 
It is necessary to separate the antennas of the MLT ground station transmitter and the 
antennas of the DME/N ground receiver according to: 
• The values obtain in accordance to de frequency of DME channel. 
• Te power level of the DME signal from the DME receiver. 
 
The transmissions of the DME ground station do not interfere with the process of the MLT 
ground station receiver. 
  
REFERENCES 
 
[1]. Convention on International Civil Aviation, Radio Navigation Aids, Annex 10, Vol. I, pp. 
26-39, 2006. 
[2]. EUROCAE, Minimum operational performance specification for DME ground 
equipment, 1992. 
[3]. Convention on International Civil Aviation, Surveillance Radar and collision Avoidance 
Systems, Annex 10, Vol. IV, pp. 1.1-3.69, 2007. 
[4]. Hernando Rábanos J.M., Transmisión por Radio, 1995. 
 


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Criteria for Positioning Active Multilateration Stations Located Close to Distance Measuring Equipment

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