
Interference in global navigation satellite systems
This article was written due to the fact that in the Russian-language segment of the Internet it is very difficult to find such information, and in the English-language it should be collected bit by bit. I work in the development of navigation receivers, and, in particular, noise immunity. This article contains a small educational program about navigation as such, about the typical construction of receivers, as well as about the main interference and their influence. If this topic turns out to be interesting to the habrasociety, a continuation will be written about various methods of combating interference and their effectiveness.
Part 2

Global navigation satellite systems - complex radio systems designed to determine location, synchronize time scales, mapping tasks, accurate positioning, construction, agricultural tasks and many others. GNSS consists of a subsystem of spacecraft (SC), a subsystem of control and a subsystem of consumers.
At the moment, two GNSSs are fully operational - GPS (Global Positioning System) and GLONASS (Global Navigation Satellite System). Systems from the EU, China, and India are also in the deployment phase.
A feature of GNSS signals is a very low power on the earth's surface, less noise. This is due to the low power of the signal emitted from the spacecraft (about 60 W) and long range (about 20,000 km). However, due to the use of broadband signals, using correlation processing, signals from spacecraft can be detected and captured.

If you don’t go into details (and the article is for informational purposes only), then the receiver can be depicted as in the picture above: antenna, analog and digital paths, as well as a certain interface and power system. The tasks of the antenna path include the primary amplification of the signal and the allocation of the necessary frequency ranges. In the analog paths, further multiple amplification, selection (frequency sampling), and frequency transfer occur.
In the digital path, the signal is digitized, with further multi-channel signal processing, where each satellite signal has its own channel. In modern receivers of such channels, there are already about two hundred for the possibility of simultaneous operation with all existing and promising GNSS and correction systems. It should be mentioned right away that most often, 1-2 bits are used when digitizing a signal. Thus, the signal-to-noise ratio is slightly reduced, but correlators are greatly simplified.
After the satellites are detected, tracking begins with the subsequent solution of the navigation problem, i.e. finding the coordinates and offset of the clock of the receiver.
Interference can be either of natural origin or obtained as a result of human activity. The second is divided into unintentional and organized. The following brief classification can be given:
1. Interference aimed at destroying the navigation message
2. Spoofing (imitation of the constellation of satellites)
In addition, the former are subdivided by the bandwidth of the interference into broadband (the frequency band is comparable to the band of the attacked navigation signal), as well as narrowband.
Methods of dealing with these interference vary significantly and for the reliable operation of the equipment in the design should be borne in mind the possibility of exposure to all three.

Above is the simplest narrowband jammer, which can be ordered from China for 2000r. Such a device can easily degrade the performance of receivers in a radius of ~ 50 km, as well as completely disarm them in a radius of ~ 5 km. This is due to how exactly the interference affects the receiver:
1. The appearance of harmonic distortion in the analog path of the receiver. This leads to the fact that instead of one interference in the frequency band appears several;
2. When applying 1-2 bits when digitizing a signal, the narrow-band noise is cut off in amplitude to a square wave (rectangular pulses), followed by saturation of the spectrum with harmonics by noise.
3. There is a breakdown in tracking satellites in the loops of tracking the code delay and tracking the phase of the carrier wave.
4. In the end, this leads to an increase in error when decoding the navigation message.
In general, in the absence of measures to combat interference, this leads to the following effects:
1. Loss of satellite tracking;
2. Increase in error with code changes;
3. The increase in error during phase measurements;
4. Reducing the signal-to-noise ratio;
In the first case, the harm from interference is obvious - the user cannot determine his coordinates. The following factors significantly impair the accuracy of measuring pseudo-delays and pseudo-Doppler frequency shifts and, accordingly, the receiver’s own coordinates. Thus, ordinary household receivers are not intended for use in complex jamming environments.
The right question. Due to the availability of simple jammers, as in the picture above, they are becoming more and more common. There are many well-known cases, for example, when the suppressor on the ship of the US Navy was incorrectly aimed and paralyzed the life of the entire city of San Diego, disrupting not only navigation for end users, but also the time synchronization system of cell towers, ATMs, etc.
However, for us, as end-users, it’s the miniature jammers who are presented as devices for maintaining privacy (foil cap) Or, for example, very often car hijackers use combined GNSS and mobile communications suppressors to combat smart alarms that send a car’s position and distress signal to the dispatcher and, for the former owner of the car.
I will talk about this in the next part, if this is of interest. Thanks for attention.
Part 2
What is GNSS?

Global navigation satellite systems - complex radio systems designed to determine location, synchronize time scales, mapping tasks, accurate positioning, construction, agricultural tasks and many others. GNSS consists of a subsystem of spacecraft (SC), a subsystem of control and a subsystem of consumers.
At the moment, two GNSSs are fully operational - GPS (Global Positioning System) and GLONASS (Global Navigation Satellite System). Systems from the EU, China, and India are also in the deployment phase.
A feature of GNSS signals is a very low power on the earth's surface, less noise. This is due to the low power of the signal emitted from the spacecraft (about 60 W) and long range (about 20,000 km). However, due to the use of broadband signals, using correlation processing, signals from spacecraft can be detected and captured.
Typical receiver construction
If you don’t go into details (and the article is for informational purposes only), then the receiver can be depicted as in the picture above: antenna, analog and digital paths, as well as a certain interface and power system. The tasks of the antenna path include the primary amplification of the signal and the allocation of the necessary frequency ranges. In the analog paths, further multiple amplification, selection (frequency sampling), and frequency transfer occur.
In the digital path, the signal is digitized, with further multi-channel signal processing, where each satellite signal has its own channel. In modern receivers of such channels, there are already about two hundred for the possibility of simultaneous operation with all existing and promising GNSS and correction systems. It should be mentioned right away that most often, 1-2 bits are used when digitizing a signal. Thus, the signal-to-noise ratio is slightly reduced, but correlators are greatly simplified.
After the satellites are detected, tracking begins with the subsequent solution of the navigation problem, i.e. finding the coordinates and offset of the clock of the receiver.
What interference is there?
Interference can be either of natural origin or obtained as a result of human activity. The second is divided into unintentional and organized. The following brief classification can be given:
1. Interference aimed at destroying the navigation message
2. Spoofing (imitation of the constellation of satellites)
In addition, the former are subdivided by the bandwidth of the interference into broadband (the frequency band is comparable to the band of the attacked navigation signal), as well as narrowband.
Methods of dealing with these interference vary significantly and for the reliable operation of the equipment in the design should be borne in mind the possibility of exposure to all three.
Interference effect

Above is the simplest narrowband jammer, which can be ordered from China for 2000r. Such a device can easily degrade the performance of receivers in a radius of ~ 50 km, as well as completely disarm them in a radius of ~ 5 km. This is due to how exactly the interference affects the receiver:
1. The appearance of harmonic distortion in the analog path of the receiver. This leads to the fact that instead of one interference in the frequency band appears several;
2. When applying 1-2 bits when digitizing a signal, the narrow-band noise is cut off in amplitude to a square wave (rectangular pulses), followed by saturation of the spectrum with harmonics by noise.
3. There is a breakdown in tracking satellites in the loops of tracking the code delay and tracking the phase of the carrier wave.
4. In the end, this leads to an increase in error when decoding the navigation message.
In general, in the absence of measures to combat interference, this leads to the following effects:
1. Loss of satellite tracking;
2. Increase in error with code changes;
3. The increase in error during phase measurements;
4. Reducing the signal-to-noise ratio;
In the first case, the harm from interference is obvious - the user cannot determine his coordinates. The following factors significantly impair the accuracy of measuring pseudo-delays and pseudo-Doppler frequency shifts and, accordingly, the receiver’s own coordinates. Thus, ordinary household receivers are not intended for use in complex jamming environments.
Well, why do I need this?
The right question. Due to the availability of simple jammers, as in the picture above, they are becoming more and more common. There are many well-known cases, for example, when the suppressor on the ship of the US Navy was incorrectly aimed and paralyzed the life of the entire city of San Diego, disrupting not only navigation for end users, but also the time synchronization system of cell towers, ATMs, etc.
However, for us, as end-users, it’s the miniature jammers who are presented as devices for maintaining privacy (
And how to fight?
I will talk about this in the next part, if this is of interest. Thanks for attention.