1. An Introduction to Interference in GNSS-bands J. Samson (ESA) jaron
An Introduction to Interference in GNSS-bands J. Samson (ESA) ENC2014, Rotterdam, The Netherlands
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2. Table of Content
The Electromagnetic Spectrum, GNSS-bands, and interference
Impact of interference
Unintentional interference
Intentional interference
Interference from Space-borne Systems
Interference mitigation
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3. The Electromagnetic Spectrum, GNSS-bands, and Interference
Section 1
The Electromagnetic Spectrum,
GNSS-bands, and Interference
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4. The Electromagnetic Spectrum (1/2)
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5. The Electromagnetic Spectrum (2/2)
Lower L-band
Upper L-band
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6. GNSS-bands
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7. Definitions of interference
The effect of unwanted energy […] upon reception in a radiocommunication system
Interference:
Any RF transmission
which is degrading GNSS-performance
Galileo/EGNOS user
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8. RF Transmissions in GNSS-bands
Other GNSS
Other GNSS-satellites
GPS
GLONASS
Compass
SDCM …
WAAS
EGNOS
Galileo/EGNOS user
Galileo
≥-157 dBW
≥-161 dBW
ALOS
Non-GNSS satellites …
Radar
DME
Radio Amateur …
Unintentional Terrestrial Interference
≤~ 70 dBW
≤~10 dBW
≤~40 dBW
Jammer
Spoofer
Intentional Terrestrial Interference
“GNSS-like”
? dBW
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9. Section 2 Impact of Interference
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10. Impact of interference
Impact on GNSS receiver:
GNSS-user would notice:
Degradation of C/N0
Loss of tracking
Lower availability observables
Cycle Slips
Higher noise on code and phase observables
Degradation of accuracy
Longer Acquisition Time
Longer Time-To-First-Fix   
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11. Spectral Separation Coefficient
Spectral Separation Coefficient (SSC):
Normalized power spectral density of GNSS-signal
Normalized power spectral density of interfering-signal
Receiver band-width
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12. Impact of continuous interference (1/2)
Effective C/N0:
Power of GNSS-signal
Power of interfering-signal
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13. Impact of continuous interference (2/2)
Below this threshold:
No new satellites acquired
Satellites no longer tracked
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14. Impact of pulsed interference (1/2)
Pulse Blanker:
Receiver technology to blank signal completely, whenever Total In-band Power exceeds threshold
Impact of Pulsed interference depends not only on:
Power interferer
Frequency interferer
Bandwidth interferer
, but also on:
Pulse Duration
Pulse Duty cycle
Receiver Blanking threshold
Receiver Blanker recovery time
Degradation C/N0 (dB-Hz)
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[Source: L. Musumeci, Politecnico di Torino]

15. Impact of pulsed interference (2/2)
Example (Service Volume Simulation): Impact of pulsed interference on receivers with blankers, in an area
Region where blanker
is activated
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16. Section 3 Unintentional Terrestrial Interference
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17. Unintentional terrestrial interference: Examples
Terrestrial Source
E5/L5 E6 L2
E1/L1
DME/TACAN *
JTIDS/MIDS
Primary Surveillance Radar
Wind Profiler Radar
Amateur Radio
Harmonics (out-of-band sources )
(*)
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18. DME (1/3) Distance Measuring Equipment (DME) :
Navigation-system for aviation, introduced more than 50 years ago
DME principles:
Aircraft sends DME-signal
DME ground-station responds (known frequency-offset and known delay)
Aircraft receives response
Aircraft computes range to DME ground-station
Aircraft computes position
TACAN: military version of DME, principles almost identical
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19. DME (2/3) DME-signal: Double pulse (3.5 µs pulse width)
Max DME duty cycle ≈ (2*3.5-6) / (1/2700) ≈ 2% (per ground station)
1/ s
12 µs
3,5 µs
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20. DME (3/3)
DME LOCATION
Impact of DME mainly depends on number of DME-stations “in view”
Simulation results:
Worst impact at “DME Hotspot”
Worst results at high altitude
Degradation C/N0 up to ±10 dB
Source: L. Musumeci, Politecnico di Torino
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21. Radar Primary Surveillance Radars:
measures the position of aircrafts (i.e. range and bearing)
Spectrum overlaps with E6 and L2
Wind Profiler Radars:
Doppler radar for measuring wind from the ground
Spectrum overlaps with E6
“Upward-looking” antenna, impact on terrestrial GNSS-users limited
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22. Amateur Radio
Amateur Radio is the use of RF spectrum for private recreation
Examples:
Voice
Image (Amateur TV)
Text and data
“Moon bouncing”
Etc.
Estimation: 2 million radio amateurs in 2011 (world-wide)
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23. Impact of interference: examples (1/2)
Date:
Sensor Station:
EGNOS RIMS Warsaw
Source:
Unknown
Impact:
RIMS A (L2): ΔC/N0 = -4 dB
RIMS B (L2): ΔC/N0 = -18 dB
Source: J. Wendel, EADS Astrium, O. Nouvel, ESSP
Spectrum Analyser
RIMS A RIMS B
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24. Impact of interference: examples (2/2)
Date:
March 2006
Sensor Station:
ESTEC Giove Station
Source:
ESTEC’s “Amateur Radio Club”
(Packet Radio Repeater, f=1299 MHz, transmission interval: 35 seconds)
-> Transmission was legal: license received from Dutch Authorities
Impact:
Frequent loss of E6 data
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25. Intentional interference
Section 4
Intentional interference
Note: all information in this section is based on open literature, e.g.:
R. Mitch et al., Signal characteristics of Civil GPS Jammers, ION 2011
Humphreys et al., Assessing the spoofing threat, ION 2008
Inside GNSS, July 2013
S. Pullen et al., GNSS Jamming in the name of Privacy, Inside GNSS, March/April 2012
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26. Jamming (1/2) Jamming: RF-Transmission to deny use of GNSS
Jammer 1
Jamming: RF-Transmission to deny use of GNSS
Jammers are cheap: few tens of dollars
Most jammers use a swept tone method
Use of jammers is illegal
Jammers are often used for “Personal Privacy”
Jammer 2
Source: R. Mitch, ION 2011
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27. Jamming (2/2) [Michael Jones, 2011]:
“Theoretically, at least, a 10-milliwatt jammer will prevent a receiver from acquiring the C/A code at a distance of 10 kilometers, and a receiver already tracking the C/A code will lose lock about a kilometer from the jammer.”
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28. Jamming, example Jamming at Newark Airport (USA), [S. Pullen, 2012]:
Four LAAS-antennas, within 200 metre of highway (> vehicles passing each day)
After installation, LAAS went into “Alarm mode”
Investigations: Interference coming from the road, caused by “Personal Privacy Devices”
Even after mitigation actions, still several incidents per week
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29. Spoofing (1/2) Red dots: DLL tracking points
Source: T. Humphreys, ION 2008
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30. Spoofing (2/2) Simplistic attack Intermediate attack
Sophisticated attack
Difficult to synchronise simulator to actual GNSS-signals
Spoofer collects GNSS-signals for synchronisation and position
Coordinated attack
Easy to detect at victim RX
Difficult to detect at victim RX
Very difficult to detect at victim RX
(no Angle-of-Arrival defence)
Source: T. Humphreys, ION 2008
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31. Spoofing, example [Inside GNSS, July 2013]:
“In a startling experiment a research team from the University of Texas successfully spoofed a ship’s GPS-based navigation system sending the 213-foot yacht hundreds of yards off course”
“The ship actually turned and we could all feel it, but the chart display and the crew saw only a straight line”
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32. Spoofing, example ? http://www.dailymail.co.uk (19/12/2011):
US drone lost over Iranian airspace
Drone shown on Iranian TV, apparently intact
“Iranian engineer claimed his country managed to ‘trick’ a US. drone into landing in Iran […] by spoofing its GPS system”
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33. Spoofing, mitigation Defense against spoofing:
Amplitude discrimination
Time of arrival discrimination
Consistency with other sensors (e.g. IMU)
Polarization discrimination
Angle-of-arrival discrimination
Cryptographic authentication ….
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34. Section 5 Interference from other GNSS-systems
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35. Interference from other GNSS-systems (1/3)
Current and planned Navigation Systems [G. Hein]:
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36. Interference from other GNSS-systems (2/3)
Congestion of bands, especially in E1/L1
Compatibility between systems needs to be assured
Source: S. Wallner
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37. Interference from other GNSS-systems (3/3)
Figures of merit
GALILEO
GPS
QZSS
Established
IALT
IINT
IREF
SBAS
New system
Reference system
Other RNSS
IINTEROP …
“Figures of merit” described in ITU-R M.1831 (under revision)
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38. Section 6 Interference mitigation
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39. Interference mitigation (1/4)
Receiver based techniques:
Mitigation in time-domain:
Pulse blanking
(suppress total signal, when total in-band power exceeds a threshold)
Mitigation in frequency domain:
Notch-filter
Mitigation in time-frequency domain:
Signal processing techniques
Example:
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40. Interference mitigation (2/4)
Phase Array antenna
Pro:
Strong suppression of interference
Con:
Complexity receiver -> Costs
Size of antenna
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[J.R.Sklar, 2003]

41. Interference mitigation (3/4)
Local shielding of antenna
-> of interest for Sensor Stations only
Integration with other sensors (e.g. wheel sensors, etc.)
-> of interest for short outages only
Track several GNSS-signals, at multiple frequencies
-> of interest for Unintentional Interference only
Interference reporting (e.g. using a network of sensors)
-> input for authorities, to take action
GNSS-antenna
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42. Interference mitigation (4/4)
… And if all previous mitigation techniques are not sufficient: Use a back-up system !
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