This will help in some simple cases but will not help targeted attacks such as jamming.Ĭontrolled Radiation Pattern Array antennas (or CRPA Antennas) are multiple antennas built into a single housing, with intelligence built into to limit and defeat jamming. Higher quality antennas include filters, limiting what frequencies can be received, and blocking extraneous signals from nearby transmitters. The first action can be made at the antenna. This simulated signal can be of a different place and time, tricking a receiver's location, frustrating anyone trying to track a stolen vehicle, or unlocking geo-fenced trucks, for example. Instead of random noise, a simulated copy of the real radio signal is generated and broadcast locally. Spoofing is similar to jamming, though again more involved and insidious. They can range in seriousness from a worker simply not wanting his boss to know where they had gone for lunch, to a criminal gang hiding stolen vehicles. There are several uses cases for jamming, and in the UK at least, none of them legal. If the GPS Satellite was a teacher at the front of the classroom, and the receiver was a student at the back of the room trying their best to hear the low volume, a jammer would be someone sat near the student shouting as loud as they could to drown out the teacher's voice The civilian GPS signal is centred on 1575.42MHz, and many jammers being crude devices they will create interference in a larger band around this frequency. They work by basically producing a random noise on and around the target frequency. Jammers are relatively simple electronic devices that can cover the whole RF spectrum but are particularly damaging to the weak GNSS signals. Jamming is a far more threatening concept to accidental or environmental issues. Frustratingly, multipath is an effect that can change throughout a day as the satellites travel through space, with their angle in relation to the receiver and nearby surfaces changing. The signal has taken longer to reach you and the Pseudorange will be longer than it should be.Ī receiver in these conditions can find it difficult to lock, or to maintain lock. Now imagine that one of those lengths of string went past you, hooked around a tree and then came back, like it had been reflected off a nearby building. The longer the string, the more time it has taken to reach you. Imagine the RF signal from each GNSS satellite to a receiver as a piece of string. Once the distances to each visible satellite position are calculated, the receiver will multilaterate its position. With visibility of multiple satellites, and an inherent knowledge of where each satellite is in space (from a data set known as an Almanac) the receiver will calculate the distance to each satellite based entirely on the time of arrival (known as the Pseudorange) of each signal. The receiver knows when each signal was sent, and (approximately) when it arrived, so it can calculate its length very accurately.
Calculating the length of this time is the fundamental way in which a GNSS receiver can tell you where and when it is.
This is a significant problem for GNSS signals as multipath will extend the time of arrival for a signal from the satellite to the receiver. Multipath is the term used for receiving the same signal twice (or more times in extreme cases), both from its original source and nearby RF-reflective surfaces, confusing the receiver into not knowing which signal is genuine. It is not just nearby antennas that could cause issues for a GNSS antenna though. This is because most antennas contain some preamplification and under failure conditions this amplifier rebroadcasts the GNSS signal or can oscillate (just like the audio feedback you hear when a microphone can pick up some output signal from somewhere) but in the case of GNSS antennas the oscillation covers the frequency of the GNSS signal itself. Other antennas that can accidently transmit in the GNSS frequency band are GNSS antennas themselves! Though normally GNSS antennas are receive only, if an antenna becomes faulty (physical damage or water ingress, etc.) or is poorly connected it can cause random noise to be transmitted on the exact frequency the antenna is designed to receive. The reality is that RF harmonics, and the much higher power being transmitted means that these signals can easily sweep over the tiny, sensitive GNSS antennas nearby. Nearby large radio or cellular transmitters should keep to their own frequency. There are several ways that GNSS signals can be interfered with, either purposefully or by chance.
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