GPS was originally developed as a tool for the military. To that end the more accurate pseudo-random code (P-Code) can be encrypted. When encrypted it becomes Y-Code and can only be interpreted by receivers with the encryption key.
The purpose of encryption is to keep the more powerful frequency available at all times, and make it harder to spoof. As long as the encryption key is kept unavailable to the general public it is near impossible to intercept/replicate/interfere with authorized user signals.
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GPS interference is a degradation or disruption in the signals being received by GNSS modules. To individuals with personal navigation devices this will be manifested as an inability to pick up a signal, or the positioning on the display being inaccurate.
In the same way as the signal received by an analog car radio is not constant, satellite radio frequencies can be affected. A simple example would be losing radio sound in favour of static when driving through a tunnel or into a covered car park – the RF signals cannot penetrate the concrete/land surface, and this is the same for satellite signals.
Interference can be categorized as natural or artificial, as well as defining whether it is avoidable, manageable, or disruptive. For the primary causes of GPS interference, see What causes GPS interference?
GPS interference can be caused by a variety of influencers. These are partially dependent on the type of device that the receiver is integrated into. For instance, foliage can have a significant impact on the GNSS functionality of a mobile phone or a fitness tracker. However, it will not often be a factor for static receiver modules. Some causes of interference are:
There are several different methods to test GPS interference. This can refer to assessing jamming and spoofing threats, examining the environment for regular interference, and testing your equipment for its response to different types of interference.
Using a detector is an effective way to assess threats such as jamming and spoofing. For static receivers performing critical functions (around airports, for example) this is important. The detector will record the profiles of jammers and spoofing and can be used to trace the source.
For mobile devices a good method to test interference would be to use a record and playback device. This will record the signals received in any given location and give the user the ability to play back the recording in the lab. This enables the user to configure their device optimally.
GPS is vulnerable in two different ways. Primarily, the satellite signals have to travel a great distance to ground receivers. Coupled with the limited number of satellites this means GPS stability (from user perspective) is vulnerable to natural impediments and conditions, as well as artificial impediments.
The second vulnerability of GPS is to cyberattacks. Initially because of the strategic value of satellite PNT data, and now for a number of reasons – commercial and federal – jamming has become an industry in its own right. Whilst illegal in many countries, jammers are not uncommon, and are not out of the price range of a private individual. The (probably) more dangerous GPS cyberattack is spoofing a receiver. This involves convincing a GPS receiver to accept a false PNT signal as the real thing. This is possible because satellite signals are so low-power it is not difficult for hackers to generate a stronger signal.
With regards to the natural vulnerabilities of GPS signal, it is possible to gain the best possible protection via rigorous testing. This would involve several stages: