Interference resistance in Modern GNSS receivers

Interference resistance in Modern
GNSS receivers
Rockwell Collins
Guy Buesnel
Okko F. Bleeker
© 2011 Rockwell Collins, Inc. All rights reserved.
Overview
•
•
Introduction
GNSS position/time determination
– Principle, implementation schemes
•
Sensitivities to other signals
– Interference
– [un-]intentional jamming
– Spoofing
•
•
Some real example scenarios
The need for interference resistance
– versus other solutions to preserving "accurate" position/time estimate
•
•
Requirements analysis
Interference mitigation techniques
– Original techniques (e.g. jumping into trench)
– Modern techniques: use of security module, filtering, hardware, software (direct
acquisition etc) and software defined architectures
•
Summary and conclusions
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GNSS (GPS) principle of operation
Satellite unique ‘Pseudo
random noise’ (PRN)
Satellite unique ‘Pseudo
random noise’ (PRN)
Correlator: ranging
User terminal
generates replica PRN
User terminal
(receiver)
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Tracking;
clock synchronization
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GNSS interference types
1. Saturating the user terminal
front end may preclude PRN
detection
– Satellite PRN is a very weak signal
– Well inside noise
2. Transmitting a PRN at untrue
phase
– Will cause the user terminal to
correlate an “untrue” range
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Anti jamming - principle
Antenna with
directional gain
Interferer
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Anti spoofing - principle
Shared key – privileged users
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Interference vs Spoofing
• Interference (intentional, un-intentional)
– Affects the GNSS Receiver’s ability to acquire and track a sufficient number
of Satellites to provide a reliable navigation solution
• Jamming
• Spoofing
• Meaconing
• We are inclined to think such interference as intentional
– But un-intentional interference is probably predominant
1. Jamming
– Denotes the operation of drowning the navigational signals in high power
signals to cause loss of lock and to prevent re-acquisition
2. Spoofing
– The operation of generating and emitting legitimate-appearing false signals
• to shift the computed position solution of a user.
– Meaconing
• Form of spoofing; the original signal is re-broadcast at untrue phase
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Interference mitigation principles
• To mitigate effects of jamming
– Receiver electronics
– Controlled Reception Pattern Antenna (CRPA) in front of the receiver
• To mitigate effects of spoofing
– Render the original signal unique
– By adding an encryption function (requiring decryption key)
– Enabling privileged users to receive protection
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8/21
Scenarios
• GNSS Receiver performance against jamming and spoofing
– What does it mean if someone tells you one receiver has “better” AJ than
another?
– Or has “better anti-spoofing” than another receiver?
• Both meaningless statements in isolation
• Scenarios have to be well defined and realistic!
–
–
–
–
Number of interfering source(s)
Type of interference (broadband, CW, pulsed)
Power of interferer(s)
User environment
Some scenarios can be more challenging
than others…….
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9/21
Real Examples (1/3)
“FMG/14 noted information provided on a case of interference recently experienced in Germany by a GPS
repeater operated in a hangar in Hannover. The interference resulted in an alert of the Enhanced Ground
Proximity Warning System providing the messages "pull-up" and "FMS/GPS Position disagree" during
taxing and departure. With an EIRP of the GPS repeater in the order of -60 dBm (to be confirmed) the
interference range was several hundred meters. The operation of the GPS-repeater has been suspended
until the end of the investigation. It was noted that it remains to be seen whether the EIRP limit of -77
dBm as stipulated in the draft ECC recommendation ECC/REC/(10)02 would have ensured sufficient
protection”
–
ICAO Information Paper ACP-WGF23/IP-21
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10/21
Real Examples (2/3)
“FAA investigation was sparked while the FAA was installing a new GPS-based landing system for aircraft
at Newark International Airport.
This new system uses GPS receivers on the ground to aid GPS receivers in the approaching aircraft. This
technique allows the use of all runways during restricted visibility conditions. During system test, the FAA
noticed that the GPS ground receivers suffered one or two breaks in reception on many days.
PDDs (sic) were identified as the cause of the continuity breaks after an investigation that lasted several
months.”
–
National PNT Advisory Board comments on Jamming the Global Positioning System - A National
Security Threat
•
Recent Events and Potential Cures, November 4, 2010
“In July 2003, it was reported to the Federal Aviation Administration (FAA) that a cellular phone when
turned on simultaneously interfered with three different aircraft GPS receivers, causing complete signal
loss.
The three GPS receivers were using three separate antennas, and were installed on a small aircraft. The
phone was on, however, calls were not made during the incidents and subsequent tests.
In an e-mail message to the FAA, the company who owned the airplane reported the subsequent tests
taken to prove a clear and convincing direct relationship between the phone being in ON-mode, and
interference with the three onboard GPS systems.”
–
NASA/TM-2004-213001, Evaluation of a Mobile Phone for Aircraft GPS Interference
•
Truong X. Nguyen, Langley Research Center, Hampton, Virginia
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11/21
Real Examples (3/3)
“S. Korea Blames North for GPS, Phone Jamming
The local Munwha Broadcasting Corporation, or MBC, reported the General Bureau of Surveillance of the
North Korean People's Army, blamed for the sinking of South Korea's Cheonan warship last year, was
behind the latest attempt to block South Korean communication devices.
Col. Yoon Won-shik at the public affairs office of South Korea's Ministry of National Defense declined to
comment on the report, and whether or not the GPS-jamming signals were sent by the North.
Yoon said, however, his ministry "has already recognized the North's intent to develop its technology used
in blocking GPS signals in the South.“
- Defense News, 6 Mar 2011
War scenario - Deliberate interference
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12/21
Need for protection (1/3)
• What sort of threat? What sort of protection?
• One technique: to shield the receiver antenna from the jammer
physically
– “in a trench” – It can really can work – to an extent...
• Antenna Electronics added to system
– Null-steering
– Beamforming
• Build robustness into GNSS Receiver
– Software and Hardware techniques
• Redundancy
– Multi-constellation/multi-frequency
– Non-satellite based positioning systems
• Aircraft can use DME, IRS, other
• Detect and eliminate jammers quickly
– Military can use weapons to take out jammers if necessary
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13/21
Need for protection (2/3)
• GNSS Receivers may be critical in several applications
– Accurate position/time information is vital
•
4 combinations of resilience are possible
Interference Protection
Anti-Spoofing
No
No
No
Yes
Yes
No
Yes
Yes
• Most modern GNSS receivers have a degree of interference
protection built in
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14/21
Need for protection (3/3)
• Having identified a real need to improve receiver resistance
how can we make GNSS receivers more robust?
– Scenario-driven approach to setting requirements needed
Users
Knowledge of
environment and
threats “need to
have…”
CONOPS
User
Requirements
Scenarios defined
Including threshold and other
requirements for jamming/
spoofing resistance
System
Requirements
Product
technology
(or upgrade)
GNSS receiver
requirements defined– what
measures are needed?
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15/21
Anti-Interference techniques - Rockwell Collins
Digital anti-jam
electronics
GPS AJ
GNSS receiver
Adaptive
Filtering
Antennabased AJ
Receiver
signal processing
Frequency
domain
Spatial
techniques
Lengthened
Pre-detection
Time
Domain
Polarization
techniques
Lengthened
Post-detection
Amplitude
Domain
Aiding-based
techniques
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Example Mitigation Techniques (1/4)
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17/21
Example Mitigation Techniques (2/4)
Stand-alone GPS AJ System
(Integrated Architecture)
GEM VI AJ Accessory
(Federated Architecture)
GPS
Receiver
16 Beam
Interface
•
•
•
Standard AE-1 / GAS-1 / ADAP Form Factor
Low power consumption (<45 W)
24-channel GPS
•
•
Full dual frequency operation
7 element CRPA interface
•
Uses existing Antenna Electronics wiring
GEM VI
– Can operate with Embedded SAASM or Digital Beam Interface to GEM VI-D GPS
Receiver
– 14 RF Channels (7xL1, 7xL2) for true dual-frequency operation
– 16 Simultaneous Steered Beams
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Example Mitigation Techniques (3/4)
“NavStorm+”
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Example Mitigation Techniques (4/4)
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20/21
Conclusions
• Can’t talk about “GPS jamming” or “spoofing” in general terms
– Need to talk about particular scenarios
• Integrated anti-spoof and anti-jam in receiver
– Can either put receiver into AJ Electronics box
• the receiver is small enough
– Miniaturize the AJ electronics and the receiver onto 1-2 chips
• Multi-constellation Receivers are a big advantage
– More satellites that the receiver can “see” make it harder for interferers
– Including advantages of potemtially un-equal spectrum
• New frequencies – GPS L5 signal
– more powerful signal than current GPS L1/L2
• Selective Availability/Anti Spoofing (SAASM) continues to have
a role
– SA may not be practiced today
• but in wartime L1 C/A code will be the first to be jammed (NAVWAR)
• GPS M-Code for military applications on the horizon
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