INTERNATIONAL TELECOMMUNICATION UNION
RADIOCOMMUNICATION
STUDY GROUPS
Delayed Contribution
Document 4-7-8/55-E
8 May 2002
English only
Received: 8 May 2002
France
PROPOSALS OF REGULATORY PROVISIONS TO PROTECT AIRBORNE
RADARS FROM FSS EARTH STATIONS IN THE 13.75-14 MHz BAND
1
Introduction
Resolution 733 (WRC-2000) invites the ITU-R to conduct studies, as a matter of urgency and in
time for consideration by WRC-03, on the sharing conditions indicated in Nos. 5.502 and 5.503,
with a view to reviewing the constraints in No. 5.502 regarding the minimum antenna diameter of
GSO FSS earth stations and the constraints on the e.i.r.p of the radiolocation service. Therefore, it is
necessary to study the way any relaxation in these constraints would affect the interference situation
from GSO FSS earth stations to radiolocation receivers in the band 13.75-14 GHz.
Today, FSS earth stations are compliant with 5.502 of the current regulation if their antenna
diameter is larger than 4.5 m. The interference caused by such stations is thus accepted, and will be
use as a reference value.
In this study, we will try to assess regulatory provisions, in term of EIRP mask to be respected by
FSS VSAT stations, to make sure that the use of these stations will not cause more interference to
airborne radars than the currently accepted interference, under 5.502.
2
Methodology
2.1
Assessment of the interference
Recommendation ITU-R M.1461 recommends the use of I/N ratio to assess the impact of a noise
like interference into radars. The interference reduces in particular the probability of detection.
The theoretical approach, presented in Annex A, allows linking this impact to the loss of range of
detection. Therefore, the impact of interference can be assessed in a simple way as a range loss,
in percentage, in every azimuth.
Figure 1 shows an example of representation of this range degradation, in percentage of range lost,
per azimuth.
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FIGURE 1
Representation of interference per azimuth
This representation is equivalent to the following one, used in previous Document 4-7-8/36 of
January 2002, where interference is shown around the location of the airborne radar (48 N, 7 E):
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2.2
Definition of a measurement to assess the interference
In order to compare the interference in different cases, we propose to use as measurement of this
interference the “relative surface loss”, defined as follows:
Without interference, the surface of detection is .R , where R is the detection range.
2
With an interference, this surface becomes
2
Surf   0.5(k()R) d
2
0
Where k( ) is the percentage of remaining range.
In this paper, we propose to use as a measurement of the interference the relative surface lost.
This measurement is calculated this way:
The loss of surface of detection due to the interference is:
2
2
loss .R   0.5(k().R) .d R .(   0.5(k()) .d)
2
2
2
0
2
0
Related to the maximal surface covered, the relative loss is :
2
 0.5(k().R) .d)
rel _loss loss /.R 1. 0
2
3
2

Simulations
The assumptions used in the following simulations are given in Annex B. These assumptions were
already used in previous papers.
In a first step, we will calculate the relative loss for the following cases:
–
30 stations of 4,50 m diameter transmitting 25 dBW in a 10 MHz bandwidth (currently
authorized, under current regulation);
–
150 stations of 1,20 m diameter (2 MHz bandwidth);
–
several numbers of VSAT stations of 1,20 m diameter sharing the 10 MHz bandwidth.
The assumptions regarding the VSAT FSS earth stations, the radar radiation pattern, and the general
condition of the simulation are described in Annex B.
4
Results
First case
Figure 3 gives a result for 30 stations of 4.5 m diameter.
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FIGURE 3
Example of loss of range for 30 FSS earth stations of 4.5 m
For 100 simulations, the results in measurement of the interference are given in Figure 4.
FIGURE 4
Surface of range lost
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From this result, we can conclude that an average of 25% of range loss is already accepted under
the current regulation.
Second case
Figure 5 shows the interference resulting of 150 stations of 1.2 m diameter (black curve). In this
figure, we added the curve obtained for 150*32 stations of 1.2m diameters using each a 64 kHz
bandwidth (red curve).
FIGURE 5
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The range reduction calculated for 100 simulations for 150 stations are shown in Figure 6.
FIGURE 6
Range loss for 100 simulations
This range loss is around 35%.
Third case: evolution of the interference with the number of VSAT stations
Interference measurements were made for different number of VSAT stations (sharing the same
frequency band, that is to say with a power adequately reduced for each station). The results are
shown in Figure 7.
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FIGURE 7
Interference measurements per number of VSAT stations
Conclusion
When the number of stations increases, the interference measurement converges. With the current
assumptions, this limit is around 36%.
This result can be explained by the fact that, to share the same spectral resource, the increase of the
number of stations is balanced by the decrease of the power of each station. Therefore, for
a sufficient number of earth stations, the “interfering power density” remains constant and
independent of the precise location of each station.
5
Synthesis
The results in term of relative loss are shown in Table 1:
TABLE 1
Number of stations Antenna diameter
Bandwidth per
earth station
Transmit Power
(Maximum
Recommended by
ITU-R)
Relative surface
loss
30
4,5 m
10 MHz
25 dBW
25 %
150
1.2m
2 MHz
15 dBW
35%
150*K
1.2m
2 MHz/K
15-10*LOG10(K)
36%
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From the above results, we can see that the interference from VSAT stations is more important that
the one coming from FSS earth stations of 4.5 m diameter, for the same spectral resource used
(corresponding to the same data rate available).
In the following part, we will try to define constraints in term of EIRP mask of the VSAT to allow
the same 25% of relative loss that the current regulation.
6
Regulatory provisions to accept VSAT stations
6.1
Impact of several EIRP masks
A set of simulations, with the same assumptions that the previous ones, were made with a decrease
of the EIRP mask of 10 dB, 5 dB and 3 dB. The results for 150 VSAT earth stations are shown in
Figure 8.
FIGURE 8
Comparative range loss for several EIRP masks
From these results, we can conclude that an EIRP decreased of about 4 dB allows the same
interference than the currently accepted one.
6.2
EIRP mask proposal
From previous results, and taking a bandwidth reference of 1 MHz to assess the impact of
interference into radar receivers, the EIRP mask allowing being compliant with current accepted
interference could be the following one:
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Angle off-axis

Maximum e.i.r.p. in any 1 MHz band
2   7
43 – 25 log  dBW
7  9.2
22 dBW
9.2  48
46 – 25 log  dBW

4 dBW
 
FIGURE 9
Current (black) and proposed (red) EIRP mask for VSAT to protect radiolocation
7
“En route” simulations
In the previous part of this paper, the airborne radar was located in a fixed position, close to the
border: 48° N and 7° E.
In this last part, we will present interference calculated for an airplane route as shown in Figure 10,
for 30 FSS earth stations of 4.5 m diameter, for 150 VSAT stations with current EIRP mask, and for
150 VSAT earth stations with proposed EIRP mask.
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FIGURE 10
Route followed by the simulated airplane
The interference measured for 30 FSS earth stations is the following one (for numerous simulations,
and with the position of the airplane on the above airway, spotted by its latitude):
FIGURE 11
Interference caused by 30 FSS earth stations of 4.5 m diameter
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Figure 12 shows results for 150 FSS with current EIRP mask (black), and for proposed EIRP mask
(blue).
FIGURE 12
Interference for 150 VSAT stations
Conclusions
These simulations, comparing Figures 11 and 12, confirm that the interference with VSAT using the
current EIRP mask in Recommendation ITU-R S.728, is more important that the currently accepted
one, but that it may be brought to the same level by a 4 dB reduction from the current EIRP mask in
this Recommendation.
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ANNEX A
Assessment of the interference in term of loss of range
A.1
Interference criterion and operational impact
Recommendation (RAD-CHARZ) recommends using the ratio between the interference power and
the noise (I/N) of –6 dB as protection criterion.
In order to assess the operational impact of the interference power on the airborne radar, we propose
to translate the I/N in term of range decrease.
A.1.1
Performance parameters for radars
To assess the performance of a radar, the two following parameters are usually taken into account,
for specified targets and environment:
–
The probability of false alarm: Pfa, which must be as low as possible.
–
The probability of detection: Pd, which must be as high as possible.
Unfortunately, for a given power, the improvement of one of these characteristics leads to the
deterioration of the other one. Then, a radar must be a compromise. The usual way to conceive it is
to fix a probability of false alarm (typically Pfa = 10-5), and to choose the emitted power to obtain
the required probability of detection (Neymann - Pearson criterion). In the case of a white noise, the
formulae to calculate these two values are given in the appendix of this annex.
These two elements are linked to the power and the noise received by the radar receiver. In this
document, we will assume that the FSS signal is processed by the radar receiver as a noise like
signal. Then, the interference produced will result in an increase of the power spectral density of the
noise, from the N0 value, corresponding to the thermal noise, to a value of N = N0+I, where I is the
increase of the spectral density power due to the interfering signal.
Most of radars use false alarm control circuits, in order to make the false alarm probability
constants (Constant False Alarm Rate (CFAR) circuits). With these radars, the detection threshold
is adapted to keep the Pfa constant when the noise increases, then, the only result of interference is
that the probability of detection decreases. Therefore, in the following paragraphs, we will study the
impact of the interference power, through the increase of I, on the probabilities Pd.
A.1.2
Interpretation of the interference in term of range decrease
This probability of detection depends of the ratio between the energy received by the radar receiver
and the noise spectral density. The impact on the Pd, for a given target, of an increase of noise can
be interpreted, for better understanding, as a variation of the distance of a target to be received with
the same probability by the radar.
In the radar theory, the link between received and emitted powers is given by the following
formula:
Pr  Pe
G ² ²
(4) 3 D 4 
where:
Pe = emitted power
Pr = received power
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G = antenna gain
D = distance between radar and target
 = wavelength
 = radar equivalent surface
l = loss between receiver, emitter and antenna.
The detection probability is function of the ratio between the received energy and the spectral noise
density. In our case, it can be reduced in the signal to noise ratio. If the noise increases in a ratio
N2/N1, the detection probability will be the same if the received power increases in the same ratio.
This will be the case for a distance variation of D2/D1 if D24/D14= N1/N2.
The relation between the distance decreasing and the variation of noise is given by the formula:
D2
N
( 1)
D1
N2
1/ 4
For N2 = N+I, the relation can be written function of I/N and becomes:
D2
N
(
)
D1
NI
1/ 4
I
 (1  )
N
1 / 4
diminution de distance
100
90
80
70
%
60
50
40
30
20
10
0
-20
-10
0
10
20
30
40
I/N
FIGURE A.1
Range decrease (%) function of I/N
From this analysis, the impact of interference on the probability of detection can be translated in
term of range decrease.
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APPENDIX TO ANNEX A
Probability of false alarm and of detection for radars with a gaussian noise
In case of a white gaussian noise, the probability of false alarm for one impulsion is:
Pfa  e

T
N0
where
–
N0 is the power spectral density of the noise;
–
T is the value of the decision threshold. In a radar design, the choice of a pfa value gives the
threshold to be used to decide the presence of an echo: T = –N0.log(Pfa);
and the probability of detection is then:

Pd  
T
1  l  A 2 A.l
)dl
e N0 I 0 (
N0
N0
where A is the energy of the signal received;
and

1  x cos()
d
e
0
I 0( x )  
In the case of radar functioning in Pfa constant, when the noise changes, the threshold becomes
T = –Nlog(pfa)
Pd 

1  l  A 2 A.l
)dl
e N I0 (
N
 N log(PFa) N

with a variable change: L = l/N0, dL = dl/N0
Pd 

e
( L 
 log( pfa)
A
)
N 0
I (2
A.L
)dL
N
Pd is function of the A/N ratio. When the noise increases, Pd remains similar if A increase in the
same proportion.
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ANNEX B
Assumptions used in the simulations
B.1
VSATs characteristics
B.1.1
Radiation pattern
Recommendation ITU-R S.1428 gives radiation patterns for FSS earth stations. The parameters to
be used for different VSAT diameters are calculated in Table 1:
For 25 
D

 100
 D 2

 
G()
 Gmax – 2.5  10–3 
G()
 G1
G()

29 – 25 log 
G )

–9

G)


G)

for
0
   m
for
m
    95
for
95
dBi
for
33.1    80
–4
dBi
for
80
   120
–9
dBi
for
120
   180
dBi
dBi

D



D

   33.1
where:
D = antenna diameter 
 expressed in the same unit ( D is the equivalent diameter for non- symmetric antennas)

 = wavelength

 : off-axis angle of the antenna (degrees)
D
Gmax  20 log    7.7




D
G1  29 – 25 log  95

For
m 
D

20 
D
dBi

Gmax  G1
degrees
 100 (for GSO and non-GSO earth stations):
 D 2

 
G()
 Gmax – 2.5  10–3 
G()
 G1
G()
 29 – 25 log 
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dBi
dBi
for
0
   m
for
m
   r
for
r
   10
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G()

34 – 30 log 
G()
 –12
G()
 –7
G()
 –12
dBi
dBi
dBi
dBi
   34.1
for
10
for
34.1    80
for
80
   120
for
120
   180
where:
D
Gmax  20 log    8.4
dBi

D
G1  –1  15 log
20 

m 

r  15.85  

D

Gmax  G1
 D  –0 6


dBi
degrees
.
degrees
TABLE B.1
VSAT radiation patterns parameters for Recommendation ITU-R S.1428
Diameter (m)
Frequency (MHz)
1,2
4,5
14 000
14 000
Wavelength (lambda) 0,02142857 0,02142857
D/lambda
210
Gmax (dB)
42,6637605 54,8443859
G1 (dB)
23,2616105 33,8332894
Phim (degree)
1,57313827 0,43655107
Phir (degree)
B.1.2
56
no
0,64076339
EIRP masks
The EIRP of the earth stations must be in conformity with Recommendation ITU-R S.728
excerpt of Recommendation ITU-R S.728-1
recommends
1
that VSAT earth stations operating with geostationary satellites in the 14 GHz frequency
band used by the FSS be designed in such a manner that at any angle  specified below, off the
main-lobe axis of an earth-station antenna, the maximum e.i.r.p. in any direction within 3° of the
GSO should not exceed the following values:
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Angle off-axis

B.1.3
Maximum e.i.r.p. in any 40 kHz band
2  7
33 – 25 log  dBW
7  9.2
12 dBW
9.2  48
36 – 25 log  dBW

–6 dBW
 
Transmitted power
The combination of the two constraints gives the maximal transmitted power. For a station with
a diameter larger than 1 meter, this power is 15 dBW for a 2 MHz bandwidth. For a 4.5 m diameter,
this power is 25 dBW for 10 MHz bandwidth.
B.1.4
Number of VSAT earth stations used in simulations.
With the following assumptions:
–
The radar receiver bandwidth is 10 MHz
–
The “reference VSAT” bandwidth is 2 MHz
–
The bandwidth for FSS earth stations of 4.5 m is 10 MHz
Number of satellites
For an elevation angle above 5°, the geostationary orbit angle visible is 140 degrees. If we assume
a satellite every 3°, the maximal number of satellites in visibility is 47. Nevertheless, the current
satellites networks using the 13.75-14 GHz band in orbital position between 30° W and 40° E are
the following ones:
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Satellite
Orbital position (-=West)
Hispasat-2
–30
Intelsat 8-335.5E
–24.5
NSS-K
–21.55
Intelsat 8-342E
–18
Usasat-14L
–12
Videosat-6
–8
Videosat-7
–5
Intelsat 8-359E
–1
Eutelsat W3
7
Eutelsat W1
10
Eutelsat W2
16
Agrani 1A
29
Astra 2
28.2
Turksat-1B
31
Eutelsat W4
36
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In the simulation, we use these 15 satellites, randomly chosen for each FSS earth station.
With these assumptions, and considering that earth station can use two different polarisations per
satellite, the maximal number of FSS earth stations having their signal spectrum inside the radar
receiver band is
Nes = 2*15*10/2 = 150 VSAT earth stations
Nes = 2*15*10/10 = 30 earth stations of 4.5 m diameter.
For the simulations purposes, the angle of polarisation between the radar and the stations are
randomly chosen (uniformly).
B.2
Airborne radar model
B.2.1
Radar characteristics
Working Party 8B is currently drafting a new Recommendation ITU-R M.[RAD-CHARZ] that
gives several characteristics for airborne radars. We can use the given characteristics to build the
following models:
TABLE B.2
Airborne radars characteristics
Type F
Frequency (MHz)
Wavelength lambda
Noise temperature
0,02142857
290
Thermal noise (per Hz)
–203,97602
Thermal noise for 4 kHz
–167,95542
Noise figure (hypothesis)
4
Noise (4 kHz)
–163,95542
Threshold –6 dB (4 kHz)
–169,95542
Equivalent surface (dB/m2)
–13,9554201
Pfd threshold (4 kHz): given
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14 000
–156
Equivalent surface (m2)
0,04022147
Max antenna gain
1100,17353
Max antenna gain (dB)
30,4146119
Antenna diameter (m)
0,22635712
D/lambda
10,5633324
Lambda/D
0,0946671
Main beam (degree)
6,62669669
Interference power (threshold 2 MHz)
–142,96572
Interference power (threshold 10 MHz)
–135,97602
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B.2.2
Radiation pattern
The radiation pattern used is the following one (radiation pattern of a circular transmitting source):
D
D


2
G ( ) G max .(2.J1(sin(  ). . ) /(sin(  ). . ))
since the level is above –10 dB, and –10 dB elsewhere;
where
 is the angle between the main beam and the considered direction;
J1 Bessel 1 function;
D = antenna diameter.
FIGURE B.1
Radiation pattern of the airborne radar used
(the antenna is horizontally directed and rotates)
B.3
–
–
–
Other assumptions
The airborne radar is assumed to be at altitude 11 000 m.
The radar is a scanning radar, which covers an angle of 360 degrees.
The area taken for the airplane location into account is the France, close to Eastern frontier.
_________________
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