IEEE COMMUNICATIONS MAGAZINE
Low Probability of Intercept
A.B. GLENN
Satellite communications systems’ performance at EHF and SHF.
LOW Probability of Intercept (LPI) analysis shows
the effect of scenario-dependent parameters and
detectability-threshold factors in jamming and
nonjamming environments. The most significant improvement in LPI performance may be obtained by operating at
Extremely High Frequency (EHF) and by maximizing the
effective spread-spectrum processing gain and the communicator’s antenna discrimination to the jamming signal, and
by minimizing the number of symbols in the message. T o
illustrate this effect, the LPI performance between a n airborne
command post and two advanced satellite-communications
systems were analyzed in this paper.The two satellite
systems analyzed operate at Super High Frequency (SHF)
(8/7 GHz) and EHF (44/20 GHz).
Although this analysis
was
made
for the airborne
command post/satellite scenario, it is directly applicable to
otherscenariossuch
as surface ships communicating to
airborne relays or submarines communicating to satellites.
The basic purpose of an LPI capability fpr a communications system is to preventthe enemy fromlocatingour
communications systems, which will decrease the effect of
both electronic attack (jamming) and physical attack. In an
electronic attack, LPI makes it difficult for the jammer to
locate the communications channel;in a physical attack, the
effectiveness of antiradiation missiles (ARM’S) may be
significantly reduced.
Additionally,
the
effectiveness of
intelligence functions, such as electronic support measures
(ESM), will be significantly reduced by LPI capability.
Critical communications links for command and control
(C*) are likely to be supported by either satellitecommunications or extended line-of-sight (ELOS) relayed communications. These links would be used tosupport
essential
information exchanges and would therefore be likely targets
for enemy ESM interceptors.
0163-6804/83/0700-0026 $01.00 0 1983 IEEE
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r
JULY 1983
..
.
'
. . .... ...,
...
I".
, ,,
Fig. 2.
,
..
i
..
Model for LPI scenario-2. .'','i-:?;'.'
likely carry self-protection jammers, signal-intercept
receivers, andARM'S, in addition to antiship weapons. These
and other situationsare described by the same basic intercept
range relationships which will be derived for the satellite/
airborne command post scenario illustrated in Fig. 1.
LPI performance is related to the probability of detection
(P,) and the probability of false alarm (P,) for some
intercept range ( R , ) from the emitter (such as the airborne
command post). This range
will be shown to be a function of
scenario-dependentparametersand
detectability-threshold
factors. Tradeoffs and bounds for these parameters can be
chosen to show their effects on the detection range of the
interceptor.
Preventing the enemy from locating high-valueunits, such
as the Fleet Ballistic Missile submarinesoranairborne
command post, are significant examples of LPI. When the
locations of these units become known to an adversary,their
capabilities are threatened. Therefore, the probability of an
adversary intercepting any communications from these units
should be minimized.
An important factor affecting LPI is the location of the
interceptorrelative to highly-valued units at the time of
communications. For communications
above
the high
30 MHz), signal propagation is
frequency (HF) band (above
essentially line-of-sight (LOS) limited between the transmitter
and receiver. For example, if a platform such as a submarine
is to be located by signalintercept,themost
effective
interceptor platform would bean
aircraft operating at
sufficiently high altitude to have a very wide view of the
earth's surface.
Satellite communications play a very important role in both
strategic and long-haul communications.The most important
function of a strategic communication system is to maintain
connectivity between the National Command Authority
(NCA) and nuclear-capable forces. Long-haul communications arecharacterized by beyond line-of-sight (BLOS)
distances. Figure 1 shows a n example of the relative positions
of the airbornecommand post,satellite,interceptor,
and
jammer. Figure 2 shows another operational situation, which
consists of an airborne relay, ship, and attacking
aircraft.
Figure 3 illustrates the submarine, satellite, interceptor, and
shipborne jamming scenario. The attacking
aircraft will most
LPI Analysis
The LPI analysis will be performed for the airborne
command post scenario illustrated in Fig. 1 using advanced
SHF (8/7 CHz) and EHF (44/20 GHz) satellite communications systems [ 1,2,3].
The two basictechniques
of signal processing which
provideboth antijam(AJ) and LPI are direct-sequence
frequency
pseudonoise (DSPN) spread spectrum and
hopping (FH). Psrudonoise
(PN)
spread
spectrum is
accomplished by using a very-high-rate PN code to phase
modulatethe
information signal. The transmitted signal
spectrum is significantly increased and the signal appears
noise-like. Thus, the major advantages of DSPN are its AJ
and excellent LPI characteristics. The primary disadvantage
27
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IEEE COMMUNICATIONSMAGAZINE
0
Fig. 3.
The two types of radiometric systems which may be used
as interceptors are wideband and chip. Both systems utilize
multiple-beam antennas with one receiving system per
antenna beam. This technique is effective in providing high
antenna gain with wide anglecoverageand
effective
direction-finding capability. In the wideband radiometer
system, a radiometricreceivertuned
to the full spreadspectrum bandwidthis connected to each antenna beam. The
chip radiometer system uses multiple radiometric receiverson
each beam, and each
receiver is tuned to a different part of the
total spread-spectrum bandwidth. The wideband radiometer
system is most effective, against pseudonoise spreadspectrum modulation, whereas the chip radiometer system is
most effective against FH spread-spectrum signals.
The following are someof the communications parameters
affecting the performance of the interceptor systems: signal
power radiated in the direction of the interceptor, durationand
frequency of transmission, number of symbols in the
message, spread-spectrumbandwidth, jamming threat, interceptor bandwidth, andratio of antenna gain to system noise
temperature of the interceptor receiving system. Since the
radiometer is an energy detector,it is responsive to the total
energy content of the message. Therefore, a message which
contains a small number of symbols will be more difficult to
detect.
For a chip radiometer system, a large spread-spectrum
bandwidth requires that the system use a large number of
filters. It is necessary that the detection threshold be set,for
high signal-to-noise ratios (SNR's), such that the P,cA is low
due to the noise in the filter not containing the signal. False
alarm probabilities of 10-loand detection probabilities of 0.9
are typical. For noncoherent energy detection, these conditions require an SNR of about 9 db [5].
As previously discussed, a hybrid FH/PN spreadspectrum signal is optimum for high AJ and low LPI. This
technique will increase the noise power in each one of the
filters of the radiometer, as eachfilter must accommodate the
spread bandwidth of each hop. This condition will most likely
reduce the interceptorrange, to meet the requirementsof false
alarm and detection probabilities.
In the LPI analysis, the following assumptions were made:
Model for LPI scenario 3.
of DSPN is that the AJ processing gain, which is defined as
the ratio of the transmission bandwidth W to the information
data rate R D , is limited to the clock rate ofthe PN generator,
which limits the spread-spectrum bandwidth W (presently
about 100 MHz). It is necessary to use error-correction
coding and symbol interleaving to mitigate against partial
time jamming.
FH is atechniquewherethe
center frequency of the
narrowband transmitted signal is pseudorandomly hopped
over a large transmission bandwidth W. The major
advantage of FH is that it can attain the highest AJ because
the only limitation to W is the channel-bandwidthavailability.
For example, at44 CHz the available channelbandwidth is 2
GHz. It is necessary to usefrequencydiversity,symbol
interleaving, and error correction coding to combat partialband and other types
of jamming threats (suchas multi-tone),
and thus force the jammer to use broadband noise.
The major disadvantage of FH is that it is not a very
effective LPI signaling technique, since a narrowband highpower signal is being transmitted at each hop. Therefore, a
properly-designed intercept receiver can more readily detect
the FH signal than the DSPN noise-like signal. It follows from
the previous discussion that, in order to provide the optimum
AJ/LPI characteristics, a hybrid FH/DSPN signaling technique should be used.
The transmitter power for some required bit error rate
(BER) can be reduced
in an FH system by using a high-order
frequency-shift-keyed modulation such as 8-ary FSK plus
error-correction coding. On-board satellite signal processing
may be used to optimize the up-link and down-link signal
designs and to prevent capture of the satellite transmitter
power by the jamming signal [ 1,2,3].
Therefore, an advanced satellite communications system
is assumed to include the following capabilities:
0
0
0
0
high-order noncoherent frequency-shift-keyed modulation (8-ary NCFSK).
0
0
0
satellite signal processing,
large transmission bandwidths,
FH/DSPN spread-spectrum modulation,
large antenna discrimination against a jamming signal
(adaptive nulling and low sidelobe response), and
0
0
The SHF and EHF up-link bands were operating with
frequencies of 8 and 44 GHz, respectively.
The transmission bandwidths of SHF and EHF are 500
MHzand 2 GHz, respectively. Therefore, the communicator utilizes FH/DSPN spread-spectrum modulation in order to make use of these large bandwidths.
Transmission is from an airborne commandpost with a
parabolic antenna whose diameter is 3 ft.
The airborneinterceptor is located such thatthe received
signal comes fromthe low sidelobes of the airborne
command post's transmit antenna.
The interceptor range will be determined for a P D of
90% and a false alarm rate of lo-''.
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JULY 1983
0
0
0
0
The satellite antennahasnarrow-beamcoverage
(beamwidth approximately 1 degree). Therefore, the
up-link landbased or shipborne
jamming signalwill enter
the sidelobes of the satellite antenna.
The interceptoruses a chipradiometersystem
with
multibeam antenna beamwidths which are equivalent to
a parabolic dish of 112 ft in diameter.
The altitudes of the airborne interceptor and the airborne
command post are approximately 60,000ft and 40,000
ft, respectively. The LOS range is approximately 550
nautical miles (nmi).
Itis
assumedthatthe
transmitted power from the
airborne command post is varied such that the BER at
the satellite or receiving ground
terminal
remains
LPI performance will be
essentially constant.The
analyzed for thenonjamming
and up-link jamming
environments.
Ebs/No’
= bit energy
to
noise-power spectral
density at the satellite receiver
RD
ND
= message
rate
data
= number of message bits (or
symbols)
= frequency oftransmitted
the
signal
f
SNR in the
dT
= effective post-detection
interceptor receiver
WI
= bandwidth of interceptor receiver
= total transmission bandwidth
W
<W
If G S T is a parabolic antenna, its gain may be expressed
as in [6] :
C S T = 5.9 D f T
f2
(2)
where,
DST
i
The analysis given in Appendix A shows that the
interceptor range between the airborne command post and
the interceptor in a nonjamming environment is given by
(A-10)as
= diameter of antenna inft
= frequdncy in GHz
Therefore, ( 1 ) becomes
Assume the following:
where,
RS’
IT
IR
CST
SR
5
LS
LI
TSR
M
= Airborne command post to satellite
CSR
3 ft (see [4])
dB at 8 CHz
= 50 dB at 44 GHz
= 5 dBat 8 GHz
= 10 dB at 44 CHz
= 33 dB at 44 GHz
= 20 dBat 8 GHz
= 46 dB at 44 GHz and 32 dB at 8 GHz
Ls
M
= L/
= 6 dB
5
RS
TSR
TIR
=1
= 10 dB
= 75 b/s
= 21,500 nmi
= 1800OK
= 435OK
ND
= 40
DsT
=
C ~ T
= 35
CST
range
= gain of airborne
command
post’s transmitting antenna in the direction of the
interceptor
= gain of the
interceptor’s receive antenna
in the direction of the airborne command
post
= gain of airborne
command post’s transmitting antenna in the direction of the
satellite
= gain of the satellite’s receive antenna
to
the desired signal
= correction
factor
in using Gaussian
statistics in the output of the energy
(square law) detector when it should be
chi-square statistics (see [5] for further
discussion)
= additional (above
free-space loss) on the
up-link from the airborne commandpost
to the satellite channel loss (such as rain,
atmospheric losses)
= additional channel loss between the
airborne commandpost and interceptor
and TIR = system noise temperature of the satellite
and interceptor, respectively
= airborne
command
post to satellite link
margin
IT
CIT
IR
IR
Ebs/No
RD
From [5],
dT =
8 for a PD = 0.9 and
PFA =
lo-’’
.
The intercept range RI versus the ratio of the interceptor
receiver bandwidth WI to the total available bandwidth W is
plotted in Fig. 4 for transmission frequencies of 8 and 44
GHz. These curves, which are for the nonjamming case,
show, for example that,for WI/W = 1, the intercept range is
8 nmi at 44 GHz and 36 nmi at 8 GHz. As indicated by ( l ) ,
the intercept range R l is primarily affected by the transmitted
frequency, which in turndetermines the spread-spectrum
processinggain (WIR,), the antenna gains of the com29
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IEEE COMMUNICATIONS MAGAZINE
.
I
municator’s transmitter (CST) ,and receiver ( C S R ) , and the
transmitter antenna gain in the direction of the interceptor
(Y
= satellite-antennadiscrimination to the jamming
(CIT).
Y
=
In an up-link jamming environment, the analysis given in
Appendix A, (A-18), shows that the- interceptor’s range is
- .
given by
RJ
SJ
signal = GSR/CSJ
R J / R ~% 1 for the satellite system
and
a
ground or airborne jammer
= distance between jammerand communication
receiver
= satellite antennagain to thejamming signal
General guidelines for maximizing theLPI performance by
minimizing theinterceptrange
Rl (which increasesthe
physical vulnerability of the interceptor) are the following:
where
= velocity of light = 3 X lo8 meters per second
0
or 186,000 miles per second
= bit energy to jamming-power spectral density
at the satellite receiver
= J/W
= jamming power at satellite receiver
= Effective Isotropic RadiatedPower
(EIRP)
from the jammer = ElRPJ
=
The single most important factor is to use a large signal
transmission frequency ( 1 ) .
Use the maximum signal transmission bandwidth ( W )
and minimum data rate (R,,), or maximize the spread
spectrum processing gain (-
0
0
kTsR
= additional loss in the jamming channel
. ..
= Boltzmann’s constant
0
W
> > 1).
RD
Minimize the message length or thenumber of symbols
(N,) in the message, which will minimize the intercept
integration time ( T ) .
Use a high-gain (or narrow-beam) transmitting antenna
with low sidelobe gain, so that the EIRP ( C I T P T ) in the
direction of the interceptor is minimized.
Use receiving antennas with high-gain discrimination
30
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JULY 1983
against jammingsignals ( a > > 1). This requires
antennas with adaptive nulling and/or low sidelobe gain.
Useerror-correctioncoding
and/or high-order rn-ary
modulation to minimize E,JN, and thus the transmitter
power for some specified BER.
the available bandwidth W(or W//W = 1). As seen from
equation (4) or (6), the intercept range R I is primarily
influenced by the transmitted frequency f, which affects the
available bandwidth W or the spread-spectrum processing
gain (W/R,); the satellite antenna capability to discriminate
against the jamming signal a ; and the satellite antenna gain
CsT. Of course, a very important factoraffecting the intercept
range in a jamming environment is the ElRP of the jammer
As the intercept range R I decreases, the physical
vulnerability of the interceptor increases.
It is seen from the curves in Fig. 5 that, for the assumed
system parameters, the interceptrange at8 GHz is limited by
the line-of-sight range (in our case, 550 miles) for jamming
threats exceeding 85 dBW. For this threat, R,= 20 nmi at 44
GHz. Ata jamming threatof 100 dBW, RI is about 110nmi
at 44 GHz, whereasRI is limited to about 550 nmi at 8 GHz.
Above a jamming threat of about 115 dBW, R , is limited to
the LOS range of 550 nmi at 44 GHz.
SHF and EHF transmitter tubesmost likely to be available
in the next 10 to 20 years which are capable of high-power,
continuous-duty, broadband transmission of the jammer are
the traveling-wave-tube amplifiers (TWTA's). For frequenciesexceeding 2 GHz, thesetubes exhibit a power
output which varies inversely with the square of the
frequency [ 11.
The two major factorswhich limit the antenna gain of.SHF
and EHF earth terminals are: the deviations of the surface
reflector from the idealsmooth surface,which tend to defocus
the antenna beam; and beam
pointing errors or tracking
inaccuracies which can result in significant gain reductions in
the desired direction [l]. For large fixed terminals,the
parabolic antenna gainincreases
as the square of the
7 GHz. Above 7 GHz, the maximum
frequency, upto about
antenna gain is approximately constantwith frequency and is
limited to about 78 dB [ 11.
Since ElRP equals the product of transmitter power and
antenna gain,the variation of ElRP for large fixed terminals
at frequencies' exceedingapproximately 7 GHz will vary
inversely with the square of the frequency. Therefore, the
ElRP of large jamming terminals is
(f'JcJ).
Conclusion
The LPI analysis showsthe
effect of the scenariodependent parameters and detectability-threshold factors in
where
a,,
= a proportionality factor whichis a function of the
TWTA's and antenna characteristics.
Then, using C ~ =T
(5),
and
(4) becomes
Assume the following additional parameters:
.
a
a
PJCJ= 85- 130 dBW
= 20 dB at 8 GHz
= 30.dB at 44 GHz
= 2 GHz at 44 GHz
W
W = 500 MHzat 8 GHz
TIR = 435K
Ls
= L,
L,
=OdB
k
= 1.38 X
= 1
y
J/K
The intercept range versus the jamming threatis plotted in
Fig. 5 for transmission frequencies of 8 and 44 GHz. The
interceptor receiver bandwidth W , is assumed to be equal to
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IEEE COMMUNICATIONS MAGAZINE
jamming andnonjamming environments.The results indicate
that themost significant improvement in LPI performance in
a jamming environment may be obtained by the following:
0
0
0
0
Inserting the relationship of P T from (A-3) into (A-4) gives
operating at EHF,
maximizingtheeffective.. spread-spectrum processing
gain,
minimizing the number of symbols in the message,
maximizing the communicator's antenna discrimination
to the jamming signal, and
minimizing the communicator's transmitter power in
areas which do not include the
communications
receivers.
The effective post-detection SNR in the chip radiometeris
shown in [5] to be
shown
that, for frequencies
exceeding
It has been
approximately 7 GHz, the interceptrange variesinversely as
thecube
of thetransmitted
frequency in a jamming
environment with large-terminal jammers. This effect is
especially evident in the communications link operating in a
jamming environment between an airborne command post
and an advanced 'satellite system operating at SHF (7/8
CHz) and at EHF (20/44 CHz).
It is shown in this example, that the airborne interceptor
range will most likely be limited by maximum LOS range
(about 550 nmi) at 8 CHz for jamming threats exceeding
approximately 85 dBW, andat 44 CHz for jamming threats
exceeding approximately 1 1 5 dBW.
Although this analysis was made for a scenario which
included a n airborne command post and a satellite system, it
is directly applicable to other scenarios such
as surface ships
communicating to airborne relays or submarines communicating to satellites.
APPENDIX
where
5
1 for WIT products greaterthan 10
T = total time of the message transmission
The message time may be expressed as
(A-7)
The communications signal will be detectable for some
probability of detection P D and false alarm probability PFA, if
the interceptor's SNR exceeds some value d T [ 5 ] .Thus, the
detectability criterion then becomes
A - LPI Analysis
Without Jamming
The LPI SatelliteCommunication (SATCOM) system
model which will be analyzed is illustrated in Fig. 1. The
received carrier power C S to noise power spectral density No
ratio at the satellite receiver in a nonjamming environment is
or
r
.
where
PT
A
No
k
1
Scenario Dependent
Factors
= airborne command post transmitter power (watts)
= wavelength of the transmittedsignal
= thermal noise power spectral density = kTs,
= Boltzmann's constant = 1.38 X
(J/k).
The bit energy ( E b s )to noise power spectral density (No) at
the satellite receiver is:
E bs
No
-
Detectability
Threshold Factors
C
The interceptor detector range is then given by
NoRD
The received power to noise power spectral density at the
interceptor's receiver is given by
(A-10)
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JULY 1983
With Jamming
I t is assumed that the jammer ElRP is sufficient such that
thethermal noise level at the satellite (see Fig. 1) may be
considered negligible.
The information bit energy ( E b ) to the total noise power
J , ) at the satellite is approximately
spectral density (N,
Solving for the interceptor range R,,
+
J,
> > N,
(A-11)
Since the processing gain W/RD > > 1, the probability
density function of thejamming noise may be considered
Gaussian. Therefore, the BER versus Eb/N, curves for
thermal noise may be used for Eb/Jo.
Thus, the signal to jamming power at the satellite is
References
L. J. Richardi,“Fundamental
performance
characteristics
that
influence
EHF MILSATCOM
systems,” IEEE Trans. Commun. Syst., October 1979.
for future MILSATCOM
[2] D. J . Frediani,“Technologyassessment
systems - the EHF bands,” MITLincoln Loboratory Report DCA-5,
April 12, 1979.
[3] D. J.Frediani, M. L. Stevens,and S. L. Zolnay,“Technology
assessment for future MILSATCOM systems: an updatefor the EHF
bands,” MIT Lincoln Laboratory Report DCA-7, October
1, 1980.
[4] S H F / E H F Dual Band Airborne SATCOM Terminal, Raytheon Co.
brochure, AN/ASC-28.
[5] J. D. Edell, “Wideband,non-coherent,frequency-hoppedwaveforms
and their hybrids in low-probability-of-interceptcommunications,”
Naval Research Laboratory Report 8025, November 8, 1976.
[6] Reference Data for Radio Engineers, Howard W. Sams & Co., Inc.,
Sixth ed., 1977.
[l] W . C. Cummings, P. C.Jain,and
(A- 13)
From (A-11) and (A-13), the following is obtained:
W
where K = -,
RD
the spreadspectrum
Alvin B. Glenn received the B.E.E. degree from the Polytechnic Institute
of Brooklyn, the M S . degree from the Mass., Inst. of Technol., and the
Ph.D.degreefromSyracuse
University in 1938,1941,and1952,
respectively, all in electrical engineering.
of ElectricalEngineering atthe Florida
He is currentlyaprofessor
International Universityin Miami, Florida.His research interest is in the area
of digital communication’systems,with special emphasis on satellite systems,
spread-spectrum communications, and telecommunication systems.
He recently retired from industry where he was employed from 1941 to
1982 atthe Western Electric Co.,G.E. Co., RCA Corp., and, lastly, at the
Mitre Corp. At the MITRE Corporation, he was employed
in the synthesis,
analysis, and design of satellite telecommunication systems and equipment
used to support command, control, and communication networks.
effort
This
was directed towards increased effectiveness and utilization of the Defense
SatelliteCommunicationSystems
a s it evolvesto fulfill nationalgoals
through the 1980’s.
At RCA, he was involved in telecommunication networks, satellite, and
deep space communication systems.
Dr. Glennis a memberof Sigma Xi, Tau Beta Pi, and Eta Kappa
Nu. He is
a Life Member of IEEE
and
associate
technical
editor
of IEEE
Communications
processing gain.
Substituting P T from (A-15) into the CI/N, from (A-4) gives
~~
No
Jo
PJCIM CITCIR
CYK
CST Y2
C2
( 4 ~ R l f )kTIR
~
(A-16)
where
C
= hf = velocity of light.
From (A-6), (A-7),
and (A-16)’;the postdetection SNR in
the interceptor is given by
Magazine.
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