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GNSS Interference: How? Why? And What Can Be Done?

HOW ? 

The American GPS constellation of satellites, along with its rival counterparts; Russia's GLONASS, China's BeiDou and Europe's Galileo (all collectively known as Global Navigation Satellite Systems or GNSS) all share a common trait.


Being in Medium Earth Orbit (or MEO) they are all extremely far away, around 20,200 km (or 12,550 miles, depending what number you find to be more impressive) and all are extremely low power compared to many other radio systems. The satellites themselves are only transmitting approximately 50 Watts, less than a standard household light bulb, and a common analogy is that the GPS power level being received here on the surface of the Earth is akin to standing in London and seeing a car headlight turned on in Australia! GPS Receivers need to be extremely sensitive to overcome this.


This low power means that GNSS reception can be easily overwhelmed by external signal sources or errors introduced by environmental factors.

WHY ? 

There are several ways that GNSS signals can be interfered with, either purposefully or by chance. Nearby large radio or cellular transmitters should keep to their own frequency... in theory... 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. 


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.

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.

It is not just nearby antennas that could cause issues for a GNSS antenna though. It could be the genuine signal itself. 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. 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. Calculating the length of this time is the fundamental way in which a GNSS receiver can tell you where and when it is.

The receiver knows when each signal was sent, and (approximately) when it arrived, so it can calculate its length very accurately. 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. Once the distances to each visible satellite position are calculated, the receiver will multilaterate its position.

Imagine the RF signal from each GNSS satellite to a receiver as a piece of string. The longer the string, the more time it has taken to reach you. 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 signal has taken longer to reach you and the Pseudorange will be longer than it should be.

A receiver in these conditions can find it difficult to lock, or to maintain lock. 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.

AND THEN WE COME TO JAMMING  

Jamming is a far more threatening concept to accidental or environmental issues. Jammers are relatively simple electronic devices that can cover the whole RF spectrum but are particularly damaging to the weak GNSS signals. They work by basically producing a random noise on and around the target frequency. 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.


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

There are several uses cases for jamming, and in the UK at least, none of them legal. 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.

Spoofing is similar to jamming, though again more involved and insidious. Instead of random noise, a simulated copy of the real radio signal is generated and broadcast locally. 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.


WHAT CAN BE DONE 

The first action can be made at the antenna. Higher quality antennas include filters, limiting what frequencies can be received, and blocking extraneous signals from nearby transmitters. This will help in some simple cases but will not help targeted attacks such as jamming. 

Controlled Radiation Pattern Array antennas (or CRPA Antennas) are multiple antennas built into a single housing, with intelligence built into to limit and defeat jamming. They do this by detecting the direction of the jamming (using the signal time of arrival to each individual element) and then effectively ignore that portion of the sky, while boosting reception to the remaining sky-view.
CRPA antennas are typically much larger, more advanced, and far, far more expensive than a standard antenna, and so are typically reserved for military applications, though the technology has become less expensive and more obtainable over the last few years.

Proper antenna placement can make a huge impact also. Having been an installer for these antennas in the past, I have seen many instances of poorly installed systems, with antennas clustered together, in the shadow of a nearby building or structure, or even halfway down a wall. Beyond cutting off a view of the sky and limiting the satellites the antennas can see, poor placement can invite multipath, causing the signal from any satellites the antenna can see to be potentially erroneous.

The receiver itself can aid greatly in mitigating interference. Spectrum analysers built into in high-end receivers can monitor and alarm, or even eliminate the effects of interference altogether.
Power levels of GNSS frequencies can be monitored, with sudden peaks or drops acted upon automatically. Algorithms can be used to monitor one constellation against the others, identifying if a spoofer is transmitting false signal nearby transmitting false signal nearby.

Protection against interference can also be installed between the antenna and the receiver.
A "GNSS Firewall" is a combination of high-end receiver and GNSS simulator, acting in a similar way to a standard network firewall, stopping potentially harmful data efficiently, whilst still allowing correct information to pass through.

Once interference is detected, a GNSS Firewall has two remedies; the GNSS signal can be switched off to devices downstream (stopping older, legacy equipment becoming spoofed) or a synthetic GNSS signal can be generated, of the correct location and time, and fed down to the receivers. As far as a standard GNSS receiver is concerned, the antenna is still connected and there is no issue. Monitoring software will be alerted by the GNSS Firewall, letting engineers or security personnel know there is an issue, but GNSS receivers will continue ticking on.

The best way of dealing with interference can often be to first identify it. Beyond the automatic mitigations mentioned above, detecting the source can allow then inform how to best act on the issue.

Using a GPS Jamming Detector & Locator can help you find a faulty antenna too close to another. Often, large datacentre sites will have dozens of antennas, each servicing a unique customer. If one becomes faulty, it can be logistically problematic to just start unplugging antennas to find the suspect, potentially creating more issues. Precisely locating the single radiating antennas allows you to rectify the issue for all users, without causing further problems.

Intermittent faults can often be a sign of a jammer being used nearby by someone on a regular schedule. This could be delivery drivers, utilities workers, or taxis, for example. Often jammer users in cases like these might not realise that parking near GNSS antennas in infrastructure can lead to large issues. Investigating the times of alarm logs and deploying a GPS Jamming Detector can ensure that a jammer such as this can be reported, and its use halted.

GPS Jamming Detectors might not just be being used by workers, but by a criminal instead. If equipped with a GPS jamming detector, routine patrols made by police or security teams can often come across jammers being used out and about. The act of jamming itself is already an offence, but detection can provide a vital lead to a more serious breach of the law.

Over the years, we have come across many forms of GNSS interference here at Chronos, and helped telecoms operators and law enforcement agencies all over the world to identify threats, mitigate against their effects and toughen their systems/procedures against future issues. We're always eager to hear more stories of interference use and how it impacts people in their personal or professional life.

If you have ever had an incident with GNSS interference or would like to know more about how to protect yourself or your infrastructure, please get in touch and let us know.
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