SINTEF ICT 1

RFI Impact on Ground Based Augmentation Systems (GBAS)

Nadia Sokolova SINTEF ICT, Dept. Communication Systems

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GBAS: General Concept

Ground Subsystem:

•  4 Reference receivers •  GBAS Ground Facility/

processing unit •  VHF data broadcast (VDB)

transmitter

-  improves the accuracy, provides integrity and approach path information for precision landing.

-  45 km operation radius: multiple runway coverage.

-  intended support of ground management operations.

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GBAS: Implementation Status

Operational S-CAT1 Pre-operational GBAS Experimental GBAS Planned GBAS

(source: www.flygls.net)

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GBAS: Current Limitations

Current Status:

•  GPS L1 C/A only

-  on-going standardization of GPS/GLONASS GBAS -  initial design of GPS/Galileo GBAS (potentially all 3

constellations)

System Limitations:

•  Insufficient integrity (poor ability to suppress anomalous ionosphere effects).

•  Limited accuracy and availability (constellation geometry limitations, susceptible to RFI).

Potential interference sources: -  in-car GPS/GNSS jamming devices (personal privacy

devices (PPDs)). -  GPS/GNSS repeaters.

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•  Cheap in-car/handheld GPS jammers -  prevention of being

tracked by a fleet management or theft protection system

-  a fraud attempt against a distance based charging system, etc.

•  Relatively low output power (from -9 dBm to – 30 dBm).

•  Simple brute force jammers (less common CW jammers).

Objective: partial or complete destruction of GNSS signals in the vehicle it is operated in.

Commercial GNSS Jammers ("Personal Privacy Devices")

[1].

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Receiver Requirements – Interference Mask

RTCA MOPS RFI mask (GBAS and WAAS receivers). •  RTCA – Radio Technical Commission for

Aeronautics •  MOPS - Minimum Operational Performance

Standards

Interference levels at which a compliant receiver will still provide nominal performance.

•  Max broadband interference source power level is specified to be –110.5 dBm/ MHz.

•  For CW tones near the L1 carrier frequency, the upper limit is –120.5 dBm.

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100 101 102 103 104 105 106-80

-70

-60

-50

-40

-30

-20

-10

0

Effective jamming range [m]

Tran

smitt

ed ja

mm

er p

ower

[dBW

]

Tracking (with car attenuation)Acquisition (with car attenuation)TrackingAcquisition

Vehicle attenuation: 22 dB [2] -  loss of lock for a single

frequency GBAS GPS receiver at a distance of 300 meters and prevention of signal acquisition at 597 meters.

Vehicle attenuation: 0 dB -  loss of lock at a

distance of 3.76 km and prevention of signal acquisition at nearly 7.5 km.

GNSS Jammers: Interference Range •  Broadband interference •  Jammer output signal power: - 37 dBW •  Antenna gains (GT = 1 dB, GR = 3 dB)

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GNSS Jammers: Potential Impact on GBAS GF Site at Newark Airport [3]: A single powerful interferer can

temporarily deny GBAS service for an entire airport.

PPD with -37 dBW power will jam 3 GBAS receivers at the same.

-  implies allowing the loss of two reference receivers for a short period.

SINTEF ICT 9

GNSS Repeaters

Basic GNSS retransmission system architecture.

Applications:

•  provision of a signal for test-bench testing of GNSS receivers.

•  final verification tests on GNSS receivers on car production lines.

•  positioning indoors. •  avionics maintenance - can

provide on-board avionics with real-time GNSS position information within the maintenance hangar.

-  can only be used by authorized parties - license is required.

-  maximum allowed emission power level –77 dBm (corresponding to a repeater overall gain of 45 dB).

-  max re-radiated signal power 10 dB below the direct signal power 30 m away from the perimeter of the facility.

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Effects on non-participating GNSS Receivers

-  depend mainly on the strength of re-radiated signal

•  Re-radiated signals are stronger than the direct signals from GPS satellites - receiver tracks re-radiated signals instead of the direct satellite signals - position solution of the receiver will represent the phase center of the receiving repeater antenna.

•  Re-radiated signals are equal to the direct signals from GPS satellites – receiver will experience multipath like effects (path of the retransmitted signal is longer than the direct signals from the satellites).

•  Re-radiated signals are weaker than the direct signals from GPS satellites – no effect starting from the 10 dB difference.

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Multipath Effect Due to Repeater Operation

•  Re-radiated signals ≤ direct signals: error magnitude depends on the delay, amplitude and phase of the re-broadcasted signal relative to the direct one.

•  The individual pseudorange error is different for each satellite being tracked depending on the repeater-receiver-satellite geometry.

•  Re-radiated signal of -170 dBW will result in max 0.3 m pseudorange error at a distance of 275 m from the repeater.

•  Running average error- area inside the multipath envelope, delimited and normalized by the signal delay.

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Interference mitigation

•  Antenna level (adaptive antennas, antenna arrays, etc.)

•  Receiver level (RF/IF filtering, code/carrier tracking aiding and enhancement, integration GPS with inertial sensors, etc.)

Interference detection based on common receiver observables

•  Signal-to-Noise Ratio monitor •  Carrier-Acceleration-Step - detects

sudden jumps or rapid accelerations in the carrier phase measurements

•  Automatic Gain Control (AGC) – is used to adjust the input signal gain, reflects the noise level at input. (not commonly accessed by the users).

RFI Detection Based on Common Receiver Observables (1/2)

Signal Power Test: PRN8 [4].

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[1] Kraus T., R. Bauernfeind, B. Eissfeller, "Survey of In-Car Jammers - Analysis and Modeling of the RF Signals and IF Samples (Suitable for Active Signal Cancelation)," Proceedings of ION GNSS 2011, Portland, OR., Sept. 20-23, 2011.

[2] Bauernfeind R., I. Krämer , H. Beckmann, B. Eissfeller and V. Vierroth, Proceedings of 2011 IEEE Forum on Integrated and Sustainable Transportation Systems, Vienna, Austria, June 29 - July 1, 2011.

[3] S. Pullen, G. Gao, C. Tedeschi and J. Warburton, "The Impact of Uninformed RF Interference on GBAS and Potential Mitigations," Proceedings of ION International Technical Meeting, San Diego, CA, 2012.

[4] Normark P., Xie, Akos, Pullen, Luo, Lee, and Enge, "The Next Generation Integrity Monitor Testbed (IMT) for Ground System Development and Validation Testing," Proceedings of ION GNSS 2001, Salt Lake City, UT.

References