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GNSS Open Signals Interference
Issues and Countermeasures
DU XIAODONG
Aerospace Long March Launch Vehicle Technology Co. Ltd
WANG FEIXUE and NIE JUNWEI
National University of Defense Technology
BACKGROUND
Global Navigation Satellite Systems (GNSS) can provide users with accurate
PNT information services. It has already been used in various fields of national
economy and peoples’ lives, such as communication, traffic, electric power,
finance, security, dangerous goods management, and so on. These applications
bring forward a critical demand on the continuity and accuracy of GNSS.
GNSS open signals have low power when arriving at the ground, low chip
rate, and short period of PN codes, so they are easily interfered with by various
electromagnetic signals. Therefore, we should pay attention to the interference
issues of GNSS civil signals.
BeiDou-2 is planning to provide regional regional navigation satellite services
in 2012 and will face the same issues. Figure 1 shows the BD-2 system space
components.
CIVIL SIGNALS INTERFERENCE TYPES AND SOURCES
Interference Types
Civil signals will face two kinds of interference—intentional interference and
unintentional interference. The intentional interference consists of jamming
and spoofing. Figure 2 shows the types of interference.
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FIGURE 1 BD-2 system space components.
Du-Wang_Fig1.eps
bitmap
FIGURE 2 Interference types.
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GNSS OPEN SIGNALS INTERFERENCE ISSUES AND COUNTERMEASURES
Interference Sources
Unintentional Interference Sources
Unintentional interference sources consist of three main components:
1. Other systems in the same frequency band (including other navigation
systems with spectrum overlap, transmitters occupying the navigation
frequency band illegally).
2. The harmonic, intermodulation, and dispersion components of the sig -
nals produced by radio broadcasting and communication emitters.
3. The leakage of electromagnetic radiation in navigation frequency band
produced by electronic equipment.
Harmonics may interfere with a variety of civilian GPS band signals as shown
in Table 1.
Intentional Interference Sources
The intentional interference usually comes from man-made jammers. At pres-
ent, civil band jammers can be divided into two types: (1) a suppressing jammer,
TABLE 1 Harmonics May Interfere with GPS L1 Band
TABLE 1 Harmonics May Interfere with GPS L1 Band
Source: Landry and Renard, 1997. Reprinted with permission.
Source: Landry and Renard, 1997. Reprinted with permission.
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156 GLOBAL NAVIGATION SATELLITE SYSTEMS
which makes the receiver unable to output the PNT results, and (2) a spoofer,
which induces the receiver to give false results.
Suppressing jammers’ detailed schemes are freely available from the Internet,
and they are low-cost and simple designs. Therefore, the majority of jammers on
the market are suppressing jammers. The detailed scheme of a 0.5 W GPS L1 sup-
pressing jammer from the Internet is shown in Figure 3; the vehicle GPS jammer
and GPS/ GLObal NAvigation Satellite System (GLONASS) jammer are shown in
Figures 4 and 5. Figures 6 and 7 show the low-power and high-power GPS mobile
integrated jammers.
Spoofers are more complex and expensive. Common products on the market
are the frequency-sweep traction spoofers. The production of spoofers is not easy
because of technical difficulties and high cost. Three kinds of spoofers are now
being studied (Humphreys et al., 2008). Because the implementation costs are
different, the effects of spoofers are not alike. A GPS signal generator is shown in
Figure 8 that can be used as the source of interference, but the price is very high.
Because the intentional interference is deliberately caused by human factors,
its location, transmission power, and boot time are more variable and difficult to
examine.
CIVIL SIGNAL INTERFERENCE TYPICAL CASES
Recently, the news media has reported many civil GPS signal interference
cases, and the construction of the BD system has encountered interference. The
following will give some relevant cases.
It turns out that the second harmonic of the carrier of a U.S. National Time
Service Center Channel 66 television transmitter falls right in the middle of
the 1575.42 MHz GPS Band.1 Although Channel 66 has reduced the coverage,
reduced the receiver power to 1,000 watts, and made the harmonic suppression
to meet the requirements of the Federal Communications Commission, the GPS
receiver is still unable to work within several miles of the transmitter.
In late 2009, engineers noticed that satellite-positioning receivers at Newark
airport in New Jersey were suffering brief daily breaks in reception. Something
was interfering with the signals from orbiting GPS satellites. It took two months
for investigators from the Federal Aviation Administration to track down the
problem: a driver who passed by on the nearby New Jersey Turnpike each day
had a cheap GPS jammer in his truck.
In late 2009, researchers found that the B2 band of BD system Monitor
Station reference receivers had been interfered with and was unable to obtain
complete dual-frequency observations. After monitoring, the radio administra -
tion bureau found that a company warehouse nearby installed wireless video
1Comment on TV Channel 66, “Transmitters Jamming GPS Signals” [EB/OL], at http://blockyourid.
com/~gbpprorg/mil/gps/gps_jammer2.txt.
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FIGURE 3 Detailed scheme of a 0.5 W GPS L1 suppressing jammer from the Internet, at http://servv89pn0aj.sn.sourcedns.com/~gbpprorg/mil/
gps/gps_jammer.png. Du-Wang_Fig3.eps
bitmap, landscape
157
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158 GLOBAL NAVIGATION SATELLITE SYSTEMS
FIGURE 4 Vehicle GPS jammer.
Du-Wang_Fig4.eps
bitmap
Du-Wang_Fig5.eps
FIGURE 5 GPS/GLONASS jammer.
bitmap
FIGURE 6 Low-power GPS mobile FIGURE 7 High-power GPS mobile
Du-Wang_Fig7.eps
Du-Wang_Fig6.eps
integrated jammer. integrated jammer.
bitmap
bitmap
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GNSS OPEN SIGNALS INTERFERENCE ISSUES AND COUNTERMEASURES
Du-Wang_Fig9.eps
FIGURE 8 Portable GPS signal generator.
bitmap
surveillance equipment illegally, and the frequency of the emitter occupied the
B2 working frequency band.
During 2009–2010, researchers found that the BD system Monitor Station
reference receivers had been jammed occasionally in some areas and could not
easily determine the interference sources. The receivers recovered after engineers
started the built-in adaptive anti-jamming algorithm. Figure 9 displays the jammed
power spectrum of B1 band.
As the GNSS has a wide range of applications, the interference would have
serious implications. In the fields of traffic, power, communication, and finance,
the interference will cause socio-economic disorder. It also poses a threat to
armored cars, dangerous goods vehicles, and large equipment. Because of the
impact of interference, the receiver cannot locate the position correctly or identify
output error. Therefore, the civil aviation landing systems, which rely on GNSS,
may lead to fatal catastrophic incidents.
CIVIL SIGNAL INTERFERENCE COUNTERMEASURES
Strengthen Navigation Frequency Spectrum Monitoring
Within the Framework of Law
In China, the usage and protection of radio spectrum resources and the investi-
gation, production, distribution, and importation of the radio equipment have clear
legal provisions. Relevant laws and regulations are: Real Right Law of the People’s
Republic of China, Criminal Law of the People’s Republic of China, Radio Regula-
tions of the People’s Republic of China, and Radio Station License Regulations.
The protection of navigation signal spectrum resources relies on both the
support of technology and the effective implementation of management measures.
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FIGURE 9 Jammed power spectrum of B1 band.
Du-Wang_Fig10.eps
bitmap
The China Bureau of Radio Regulation of the Chinese Ministry of Industry and
Information has overall responsibility for administration and coordination of
civil radio management. Its responsibilities are frequency spectrum monitoring,
interference detection and investigation, matters of electromagnetic interference
coordination, and transmission of radio waves in the air maintenance. Figures 10
and 11 show some radio monitoring equipment.
In the field of radio monitoring, the detection and location of high-power
jammers has been a mature technology and has been widely used in engineering.
At present, China possesses the ability of radio frequency spectrum detection and
interference location investigation. But the low-power interference, which has
serious impact on the navigation signals, can only be detected in limited range.
The location and detection of a weak jammer takes a lot of manpower, material
resources, and time. To enhance the navigation frequency monitoring and the
detection of interference sources, the main issues include (1) high-sensitivity
interference detection and (2) rapid location of the interference. However, the
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GNSS OPEN SIGNALS INTERFERENCE ISSUES AND COUNTERMEASURES
Du-Wang_Fig11.eps
FIGURE 10 Fixed radio monitoring stations. Source: Beijing Radio Administration Bureau,
at http://www.bjrab.gov.cn/jcjc/jcs/twzs/20101221/368.shtml.
bitmap
FIGURE 11 Radio monitoring vehicles. Source: Beijing Radio Administration Bureau, at
Du-Wang_Fig12.eps
http://www.bjrab.gov.cn/jcjc/jcs/twzs/20101221/364.shtml.
bitmap
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162 GLOBAL NAVIGATION SATELLITE SYSTEMS
method of location and removal cannot meet users’ requirement of continuous
services. Thus receivers with higher interference tolerance are demanded.
Enhance the Receivers’ Interference Tolerance
Fully Exploit Anti-jamming Performance of Modernized Navigation Signals
The modernized navigation signals enhance the civil signals’ interference
tolerance. The anti-jamming potential of modern navigation signals is based on
the following aspects.
1. Integrated application of multiple frequencies to avoid possible
navigation unintentional interference. In the future, the civilian
frequencies that can be used include BD-B1, B2, B3, GPS-L1, L2, L5,
and GALILEO-E1, E5, E6, etc. Applying multiple navigation frequen -
cies can effectively avoid the impact of unintentional interference,
meaning that interference with one frequency signal does not affect the
other frequency signals. This is equivalent to increased tolerance. The
deficiency of this method is that it is only effective on unintentional
interference.
2. Use of wider bandwidth signal to get greater gain of the spread spec-
trum. In the modernized GNSS open signals, there are signals with wider
bandwidth, such as BD-B2 and GPS-L5. The higher chip rate increases
the processing gain and anti-jam performance. So it can improve the
ability of anti-jamming. Such a measure is both effective on intentional
and unintentional interference.
3. Pilot signals improving tracking capability. Pilot signal is modulated
by long-period PRN code without navigation message. Receivers can
extend the coherent integration time as long as possible to improve
signal-to-noise ratio and eliminate the impact of square loss, thus the
sensitivity can be significantly improved. So it can enhance the robust-
ness of the equipment to the interference condition. Such a measure is
both effective in intentional and unintentional interference.
4. New message error correction coding scheme. Modernized GNSS
signals adopt new-style forward error correction coding schemes. High-
gain, soft-decision decoding can correct the navigation message error
and improve the continuity of navigation in various kinds of interfer-
ence. Such a measure is both available in intentional and unintentional
interference.
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GNSS OPEN SIGNALS INTERFERENCE ISSUES AND COUNTERMEASURES
Improving Anti-jamming Capability through Auxiliary Information
Currently, satellite navigation has been closely integrated with inertial naviga-
tion and mobile communication. In the future, integration with other systems will
become a trend. Full use of auxiliary information of other systems or equipment
will greatly improve the anti-jamming capability of satellite navigation receivers.
At present, the method can be used in the following aspects:
1. For static timing users, the receivers’ interference tolerance can be
enhanced by setting precise coordinates and using high-stability clock.
2. For vehicles, aircraft, and other dynamic users, GNSS/inertial navigation
system integration can improve anti-jamming capability.
3. With messages and other necessary information broadcast by 2G or
3G mobile communication networks, pilot positioning can be achieved
directly and anti-jamming capability can be improved.
Active Anti-jamming Measures
Adding active anti-jamming measures to receivers can upgrade the ability
of the anti-jam. At present, the methods that can be used in active anti-jamming
include:
1. Time and frequency domain anti-jamming. Time and frequency
domain processing can suppress various narrowband interference effec -
tively. Anti-jamming capability can be improved by 40–70 dB based
on the architecture of hardware and processing algorithms. The main
advantages of the approach are that the instruments are low cost and eas-
ily integrated; however, its disadvantage is it is ineffective to wideband
interference. Figure 12 presents the power spectrum comparison before
and after anti-jam.
2. Spatial anti-jamming processing. Spatial processing is mainly using
the antenna array disposal techniques from the direction of signal to dis-
tinguish the interference. Adaptive space-frequency and space-time fil -
tering technology can effectively suppress various types of interference.
According to the hardware scale and processing algorithms, we can get
30–60 dB or higher anti-jam improvement. Spatial processing can sup -
press multiple wideband and narrowband interference at the same time.
But it has the disadvantages of complex hardware, large scale, and high
cost, and the number of anti-wideband interference is directly related to
the scale of hardware and software. Figure 13 presents the directional
map of antenna array in spatial processing.
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FIGURE 12 Power spectrum comparison before and after anti-jam.
Du-Wang_Fig13.eps
bitmap
FIGURE 13 Directional map of antenna array in spatial processing.
Du-Wang_Fig14.eps
bitmap
In order to increase the active anti-jamming measures, we have to increase the
complexity, size, power consumption, and implementation costs of the receivers.
However, engineering has to considerer cost-effectiveness. For the individual
user equipment, because of its constraints on size and power, we propose to use
time-domain to suppress narrowband interference. For the critical civilian infra -
structure equipment, we can appropriately increase the hardware scale and use
the spatial-domain or polarization to enhance the anti-jamming capability and
protect the safety of infrastructure.
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GNSS OPEN SIGNALS INTERFERENCE ISSUES AND COUNTERMEASURES
Developing the Auto-integrity Technology in the Background of Interference
Using the RAIM (receiver autonomous integrity monitoring) technology,
receivers can monitor abnormal navigation signals and mitigate errors of PNT
calculation caused by false signals.
SUMMING UP
• Governments have a responsibility to protect the navigation frequency
band against illegal interference.
• Technical measures can greatly improve the receiver’s anti-jamming
capability. Good PNT services can be obtained by adopting proper anti-
jamming techniques according to the role of GNSS in specific applications.
• Active anti-jamming technology will affect the accuracy of measure
precision. Therefore, in the mapping and other precision application
fields, active anti-jam technology is limited.
• Interference-free environments are the goal pursued by all satellite navi-
gation users.
REFERENCES
Landry, R. Jr., and A. Renard. 1997. Analysis of Potential Interference Sources and Assessment of
Present Solutions for GPS/GNSS Receivers[C]. 4th Saint-Petersburg on INS, May 26–28.
Humphreys, T.E., B.M. Ledvina, M.L. Psiaki, B.W. O’Hanlon, and P.M. Kintner Jr. 2008. Assessing
the Spoofing Threat: Development of a Portable GPS Civilian Spoofer[C]. ION GNSS Confer-
ence Savanna, Georgia, September 16–19.
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