Expert Opinions: Challenges faced by multi-constellation GNSS receiver designers

February 8, 2018  - By

Javad Ashjaee
President and CEO,
Javad GNSS

Q: What is the biggest challenge facing designers of multi-constellation GNSS receivers today?

A: The biggest challenge now is spoofing.

Some years ago the issue was jamming —the hot issue of LightSquared — that would hurt GNSS. To solve that problem we created the J-Shield and showed that J-Shield technology could protect against LightSquared and similar signals. We manufactured dozens of units that were successfully tested by several independent laboratories.

Now GNSS faces the spoofing issue. Reports of Black Sea spoofing and other examples show the urgency of paying attention to this problem. When a spoofer is successful, both position and time are spoofed.

A Nov. 3 CNN video report on this subject gives an example of how little people know about spoofing and about the work that has been done on this subject. The report claims that GNSS technology companies have not done much or spent money on this subject. Obviously the reporter doesn’t know what we have done, as I will report here.

I’ll review the spoofing methods and how we counter them.

Source: Javad GNSS

Source: Javad GNSS

Spoofers use three methods: One simple way is to broadcast GNSS-like signals that provide the wrong ranging information which, when used, creates wrong position and time solutions. Most probably this is the method that Prof. Todd Humphreys used to spoof the GNSS receiver on the $80 million yacht [“GNSS Lies, GNSS Truth,” November 2014 GPS World.] This method fools the GNSS receiver into ignoring the correlation peak of the real satellite signal and using the correlation peak of the spoofer signal. To deal with this type of spoofer we take advantage of the 864 tracking channels and over 130,000 fast acquisition channels of our TRIUMPH chip. We assign more than one channel to each satellite signal and we track all their peaks: The real peak and the spoofer’s peaks. Then in Step 1, below, we exclude all signals with more than one correlation peak.

Method Two is broadcasting spoofed signals for satellites that are below the horizon in the spoofed area or for satellites that do not exist. In this case only one correlation peak exists. Our equipment and OEM boards can download valid and certified almanac data from our website to know the status of satellites and their visibility ahead of their mission. Almanac data can be used for several weeks.

Method Three is to cover the signal of a visible satellite with noise and on top of the noise add the spoofer signal with more power. We recognize such spoofers by their unreasonable signal power and the background noise.
In the first counter-spoofing step we ignore these signals:

  1. Those with more than one peak;
  2. Those that according to our almanac should not be visible;
  3. Those with unreasonably high or inconsistent signal-to-noise ratio (SNR);
  4. Systems whose satellites all have similar SNR.
  5. Satellites that do not generate complete multi-frequency signals (spoofers usually generate only C/A code).

After removing all questionable signals, we use the remaining signals to compute our approximate position. We need at least 4 signals from the many available signals of GPS L1, L2P, L2C, L5, GLONASS L1, L2, L3, and the many signals of BeiDou, QZSS and IRNSS.

In the second step we validate all questionable signals against the approximate position that we have calculated and keep only those that pass our validation. We then re-compute the more precise position using all good signals. We consistently throw away the spoofer correlation peak and use the real satellite signal.

If all signals of all satellites are spoofed, then we warn the user to ignore the GNSS signals and use some other sensors (like compass and gyro) to get out of the spoofed area. A spoofer that can spoof all signals of all satellites will be very expensive to build and deploy.

In a very difficult situation, the user can enter their approximate position to quickly understand if spoofers exist, and then identify them.

All the counter-spoofing methods that I have discussed here are the subject of patents for which we have applied.

Since currently most of spoofers spoof the L1 C/A code, we can simply initially ignore the C/A signals to compute the initial approximate position and use it to identify the spoofed signals.

It is vital that in areas that spoofing danger exists, users employ OEM boards that provide more satellite systems and more signals, rather than using a simple GPS C/A code, for example.

Finally I would like to challenge Prof. Todd Humphreys [professor and director, Radionavigation Laboratory, University of Texas-Austin] to spoof any of our receivers that have this anti-spoofing option. We offer this as an option on all of our OEM boards.

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