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How to Make Untestable GNSS Interference Testable

For years, government and commercial organisations alike have had to rely on ‘JamFests’ or exercises, often conducted on remote test ranges to test their GPS or other GNSS-reliant equipment against real-world interference. 

In June 2017, the Department of Homeland Security hosted its own Jamming Exercise to ensure first responders were equipped to deal with situations where GNSS positioning, navigation or timing is compromised.

While this kind of event can help organisations test equipment against realistic interference scenarios, it presents a range of problems from the logistics (moving equipment and trained engineers to an often-remote location), to issuing warning notices in the local area and the developing test scenarios.

This post will examine the challenges of GNSS interference testing, and suggest suitable alternatives that avoid the complexity of physical, live-sky testing events.

The issues with live jamming tests

Unlike using live-sky signals to test general GNSS performance, testing RF interference from a GNSS jammer or spoofing device is illegal in most countries. This means you either attend a scheduled ‘JamFest’ style event, or obtain the necessary permits from a government body to test.

Either option requires significant forward planning, and in many cases a government sponsor that will handle your permit request. These events are often carried out by military or governmental customers, making it difficult for commercial receiver developers to participate in these events. At best attending or setting up a testing event is a very complex operation, and at worst it can be completely impossible to do so.

Live interference tests also run the risk of spurious jamming signals compromising nearby operations that rely on GNSS. One military unit discovered this the hard way in 2015, when their jamming exercise blocked navigation signals used by local farmers, pilots and city workers. Spurious signals also present a confidentiality risk. Can you guarantee a test conducted in public won’t expose the types of jamming you are testing to competitors or hackers?

Again, significant planning is required to work around the risk of spurious interference signals. But this can introduce its own risk. For instance, in June 2016 the US navy planned a six-day jamming exercise in California that was cancelled at the last minute. No reasons were given for the cancellation, but it’s likely that this abrupt change of plan resulted in months of wasted of planning – as a well as cost and disruption.

The importance of interference testing

With so many obstacles standing between system builders and effective live interference testing, finding a way to simulate real world GNSS interference scenarios offers a range of benefits. 
From our extensive experience in GNSS testing, we have seen examples of what both full and partial interference can do to GNSS-reliant equipment. From total signal blocks, to unexpected receiver behaviour, there are many ways interference can affect GNSS functionality.

Recent reports of GPS disruption affecting maritime users in the Black Sea also illustrates the importance of understanding system and receiver behaviour when subject to unwanted radio frequency interference. So, it’s important for any manufacturer to understand how their equipment will react under interference.

The only way to prepare for these complex interference threats is to thoroughly test against realistic scenarios.

Making the untestable, testable

When live-sky testing is impractical, or even impossible, how can you test against realistic scenarios?

To test interference without resorting to live-sky, you can either:

  1. Use a GNSS simulator with an embedded, software interference simulator.  Spirent simulators have had the capability to embed interference transmitters into their simulation scenarios for 10 years. This kind of simulation is possible with no additional hardware being required for the interference generation.
  2. Use an external hardware interference simulator alongside a simulator or RPS. Sometimes there is a need to inject powerful external signals into simulations – Spirent’s architecture allows for this to be done very easily

Both options or a combination are viable, and will suit different equipment and testing environments. There are trade-offs between the use of embedded jammers or external generators which you’ll need to evaluate when deciding on the test scenarios you want to create.

Simulating complex interference environments in the lab offers a range of clear benefits over live range tests, and makes situations that would be virtually impossible to test on a live range feasible through a simulated scenario.

Beyond avoiding the cost and complexity of organising a live-sky testing event, simulation allows for the creation of repeatable GNSS environments. When testing needs to be carried out over a period of days, weeks, or even longer, the unpredictable nature of live-sky makes it difficult to test against repeatable conditions. But with a simulator, you can program and test against the same constellation environments – and get more consistent readings as a result.

Interference simulation also allows you to test against previously untestable scenarios. If you have to conduct over the air testing on a test range, it may not be practical to create the interference scenarios that you really want to test against. But with simulation you can reliably generate and test complex scenarios that include natural atmospheric effects, dynamic and static sources of interference, spoofing and multipath and even segment errors.

How Spirent can help

Our deep experience of GNSS testing, along with our constant development of new test cases and scenarios means that we can help answer any questions you might have about GNSS testing. From how real-world scenarios that contain atmospheric (solar) effects can impact testing, to how you can mitigate the effects of radio frequency interference, spoofing, and even user segment faults, Spirent are here to help.

Spirent has helped organisations in a wide range of industries simulate interference, offering both embedded and external interference generators for over a decade.

Talk to the Spirent team for more information.

Or, if you’d like to stay up-to-date with the latest interference threats and simulation solutions – or discuss your challenges with likeminded engineers – you might also like to join the GNSS Vulnerabilities LinkedIn Group.

 
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