Velocity

2013

NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

Issue link: http://velocitymagazine.epubxp.com/i/164724

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anti-jamming Figure 2: In figure at left, a jamming signal overpowers a GPS receiver GPS Rx mit all possible GNSS signals simultaneously to spoof the target receiver. An additional measure of protection can be added by aiding the navigation solution with an inertial measurement unit (IMU), as an adversary cannot spoof the Earth's gravitational field or vehicle dynamics and cause the inertial unit to think it has moved in a way that it hasn't. jamming Low received signal power makes all CDMA signals, not just GNSS, susceptible to accidental interference and intentional interference (jamming). Once the interference level passes a certain limit, the GNSS signal will be lost within the interfering signal. (See Figure 2.) The fact that the Y-code is transmitted at 10 times the code rate of the C/A code provides an inherent improvement in jamming performance. This arises from the "spreading" of P/Ycode power over 10 times the frequency range, allowing Y-code receivers to handle 10 times the jamming power of C/A code receivers. The same improvement in jamming performance can be seen with some of the newer and faster civilian signals such as GPS L5, Galileo E5a, and Galileo E5b. But if the interference level is high enough, both low–code rate and high– code rate signals will eventually be overpowered; so, even SAASM receivers can be jammed. How can we protect against jamming? Luckily, we have multiple mitigation strategies to help us overcome interference: For more Solutions visit http:/ /www.novatel.com GPS Rx 1. Filtering in the receiver. The first line of defense for interference of any type in any RF system is to filter out as much of the interference as possible as soon as it reaches the receiver. This is especially effective for what we call out-of-band signals, or signals that are not directly in the GNSS frequencies that we are trying to receive. Unfortunately, if a signal falls directly in-band it may still overpower the receiver. 2. Aid the receiver with an IMU. IMUs are impervious to radio frequency interference and can provide a navigation solution to bridge gaps of seconds to a few minutes in GNSS performance. 3. Null the interfering signal by using an adaptive antenna array. By using multiple antenna elements spaced a known distance apart, signal-processing techniques can be employed to discern the direction from which an interfering signal is arriving and then adaptively change the apparent receiving strength of the antenna array, creating lower gain ("nulls") in the antenna receiving pattern. By pointing these nulls towards the source of interference, the receiver can be protected from interference arriving from that direction. (See Figure 3.) Controlled reception pattern antennas, or CRPAs such as NovAtel's GAJT® anti-jam antenna (Figure 4.), are extremely effective at mitigating all types of interference, even if that interference falls within the GNSS frequency band. Figure 3: In figure at right, steering a null in the direction of the jammer as a means to mitigate interfering power Figure 4: Multiple antenna elements used in NovAtel's GAJT-700ML CRPA 2013 velocity 29

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