J-Shield filters out interference

December 10, 2019 - By Tracy Cozzens

The Triumph-LS receiver. (Photo: JAVAD GNSS)

J-Shield is a robust filter on Javad GNSS antennas that blocks out-of-band interference (Figure 1). In particular, J-Shield blocks signals that are near the GNSS bands, including the proposed Ligado Networks (formerly LightSquared) broadband signals, explained Javad Ashjaee, founder and CEO of Javad GNSS.

FIGURE 1. Protection characteristics: The J-Shield filters have a sharp 10-dB/KHz skirt, which provides up to 100-dB of protection. (Image: JAVAD GNSS)

The anti-jam digital filters protect against in-band interference such as the harmonics of nearby TV and radio stations, or against illegitimate in-band transmissions. The anti-jam filters can be combined in pairs for complex signal processing and can simultaneously suppress several interference signals.

“The filters make the near band spectrums available for other uses,” Ashjaee said. “They protect GNSS bands now and in the future.”

In-Band Noise Measurement. The receiver measures the level of interference as a percentage of noise above the normal condition. Figure 2 shows the condition in a clean environment, where eight GPS satellites were visible, according to the almanac. In all, eight C/A, six P1, six P2, six L2C and two L5 GPS signals were tracked. The noise level was 2% on C/A and L5 and 0% on P1, P2, and L2C.

FIGURE 2. Clean environment. (Image: JAVAD GNSS)

Figure 3 shows 290% noise in the GPS C/A signal and 121% noise in Galileo E1. Only one of the eight GPS C/A code and none of five Galileo E1 signals could be tracked because of the high level of interference.

FIGURE 3. High interference levels. (Image: JAVAD GNSS)

Spectrum Analyzer

Filters in the GNSS antenna provide one way to protect GNSS signals from interference. Another is the receiver chip itself. For instance, the Javad GNSS Triumph chip includes an integrated spectrum analyzer — a more efficient solution than using a commercial spectrum analyzer to continuously monitor and evaluate the environment, Ashjaee explained.

The spectrum analyzer monitors the spectrum inside the chip. It has an effective bandwidth of 1 KHz, and can be programmed to automatically record the spectrum (and other information) periodically or according to pre-set conditions. Each spectrum shows the power and shape of any interfering signals and jammers.

Figure 4 shows the shape of the GPS L1 band spectrum when the band is jammed, as indicated by the huge peak in the center where the C/A code is. The number on the bottom left is the height of the peak. The height of the spectrum is 21.1 dB; compared to a calm spectrum of 11.2 dB, this spectrum indicates a jamming impact of about 10 dB.

FIGURE 4. The L1 band is jammed, as shown by the peak. (Image: JAVAD GNSS)

Automatic Gain Control. In addition to monitoring the spectrum, the Triumph chip also keeps a record of automatic gain control (AGC) — another indicator of unwanted external signals. The AGC monitors the environment and adjusts the gain to keep the voltage at a certain level. The change in AGC is an indicator of interference.

Spoofers

“Spoofers are quite different from jammers,” Ashjaee said. “They don’t disturb the environment and the spectrum shape. They broadcast a GNSS-like signal to fool the GNSS receivers to calculate wrong positions. We detect spoofers by digital signal processing.”

With 864 channels and about 130,000 fast-acquisition channels in the Triumph 2 chip, it has the resources to assign more than one channel to each satellite to find all of the signals transmitted with the same GNSS PRN code — including spoofed signals.
“If we detect more than one reasonable and consistent correlation peak for any PRN code, we know that we are being spoofed and can identify the spoofer signals,” Ashjaee said. The chip isolates and ignores the wrong peak.

“Usually more than 100 signals are available at any given time. We need only four good signals to compute position,” Ashjaee said. “We reject infected signals, and then among all the available GPS, GLONASS, Galileo, BeiDou, IRNSS and QZSS signals, we use the healthy ones. It is extremely unlikely that we can be spoofed without our knowledge. We can immediately recognize spoofing and take corrective actions. In the rare case that all signals are affected, we inform the user and guide them to use a compass and altimeter to get out of the jammed area.”

Figure 5 is a screenshot from the company’s Triumph-LS survey receiver, showing the details of each signal tracked. The first six lines in this screenshot show the spoofed signals that were detected as soon as they appeared (number “1” in the C1 column). Percentages show the amount of interference above the normal level.

In the last column, T indicates the signal was tracked by the main channels, Q by the fast-acquisition channels, and U indicates the signal was used in position calculations.

Figure 5. Signal Details: The Triumph-LS receiver provides users with a wealth of information on each signal received, including spoofed signals.

Indicators for Healthy Signals

In addition to the spectrum shape and AGC, these other indicators show the health of GNSS signals:

Number of signals tracked.
Divergence of SNR from its expected value.
Level of additional power and its RMS.
Divergence of AGC from its normal value and its RMS.
Extra noise.
Number of signals spoofed.

As an aid to users, the company’s Triumph-LS receiver can display the status of all GNSS signals received. Figure 6 shows this compact view, with normalized values of the above indicators (0 means good and 9 means poor).

Figure 6. Signal Status. Information on all GNSS signals received as shown by the Triumph-LS. (Image: JAVAD GNSS)

Users of the Triumph-LS can click on any of the signal buttons to see the actual and normalized values of the indicators for that signal. Action buttons provide quick access to View Satellites, View Spoofing, View Spectrum and Take Spectrum. Jamming and spoofing protection is an option on all Javad GNSS products and OEM boards.