RFI Impact on Ground Based Augmentation Systems (GBAS) Nadia Sokolova SINTEF ICT, Dept. Communication Systems SINTEF ICT 1
GBAS: General Concept - 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. Ground Subsystem: 4 Reference receivers GBAS Ground Facility/ processing unit VHF data broadcast (VDB) transmitter SINTEF ICT 2
GBAS: Implementation Status Operational S-CAT1 Pre-operational GBAS Experimental GBAS Planned GBAS (source: www.flygls.net) SINTEF ICT 3
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. SINTEF ICT 4
Commercial GNSS Jammers ("Personal Privacy Devices") Objective: partial or complete destruction of GNSS signals in the vehicle it is operated in. 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). [1]. SINTEF ICT 5
Receiver Requirements Interference Mask RTCA MOPS RFI mask (GBAS and WAAS receivers). 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. RTCA Radio Technical Commission for Aeronautics MOPS - Minimum Operational Performance Standards SINTEF ICT 6
Transmitted jammer power [dbw] GNSS Jammers: Interference Range 0-10 -20-30 -40-50 -60-70 Broadband interference Jammer output signal power: - 37 dbw Antenna gains (G T = 1 db, G R = 3 db) Tracking (with car attenuation) Acquisition (with car attenuation) Tracking Acquisition -80 10 0 10 1 10 2 10 3 10 4 10 5 10 6 Effective jamming range [m] 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. SINTEF ICT 7
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 8
GNSS Repeaters Basic GNSS retransmission system architecture. - 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. Applications: provision of a signal for testbench 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. SINTEF ICT 9
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. SINTEF ICT 10
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. Running average error- area inside the multipath envelope, delimited and normalized by the signal delay. The individual pseudorange error is different for each satellite being tracked depending on the repeaterreceiver-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. SINTEF ICT 11
RFI Detection Based on Common Receiver Observables (1/2) 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). Signal Power Test: PRN8 [4]. SINTEF ICT 12
References [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. SINTEF ICT 13