Your Questions Answered

GNSS/GPS Vulnerabilities

What is GPS encryption?

What is GPS interference?

What causes GPS interference?

How do I test for GPS interference?

How is GPS vulnerable?

How can I counteract GPS vulnerability?

What is GPS encryption?

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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What is GPS interference?

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?

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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:

  • Atmospheric conditions
  • Solar activity
  • Natural impediments e.g. in a valley or mountain range
  • Artificial impediments e.g. urban canyon
  • Presence of electromagnetic fields e.g. under power lines
  • Jamming
  • Spoofing

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How do I test for GPS interference?

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.

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How is GPS vulnerable?

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.

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How can I counteract GPS vulnerability?

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:

  • Selecting a receiver – trialing different receivers using a range of signals from a simulator. The better the receiver, the less vulnerable it is to signal drop-off.
  • Assessing the environment – whilst a simulator can model the satellite signals received in any environment, it cannot predict the artificial threats in the area of use. Primarily for stationary receivers (or receivers that perform their key function in a single location) a user can deploy a detector, which will record and analyse all artificial interference.
  • Lab testing – altering the position of the aerial, or the location on the board, can affect the robustness of the signal received, and lab testing using a simulator enables the necessary repeatable test procedures to refine the design.
  • Software testing – whilst a software algorithm will not prevent a spoofing attack, it is possible for it to make the user aware that it is being spoofed. Testing this will offer assurance of effective functionality.
  • Testing back-up systems – in applications where position is safety critical it is important to have back-up systems that can supplement GNSS when signal is lost/jammed/inaccurate. Assessing how additional sources can augment or replace the PNT data can be performed in the lab, again by use of a simulator.

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