GNSS Interference and Jamming

GNSS Anti-jam Technology

[Keywords]

jam resistant, integrated GPS/IMU navigation/guidance system munition: bombs, artillery rounds, JDAM

jamming environment

form factor: small enough to fit into a 9 in³ NATO standard fuze-well

munition accuracy with anti-jam GPS:

[Introduction]

- GPS signal level: -130dBm C/A code, -133dBm P(Y) code

- GPS receiver jamming resistance, J/S: 22dB for C/A code, 34dB for P(Y) code

- Anti-jamming methods

1) CRPA(controlled radiation pattern array): (N-1) nulls can be formed with N-element array. Beam-steering gain 10log₁₀N (dB)

[Anti-Jamming Antenna Technologoy]

References

[01] West, "Munitions/artillery shell GPS multi-edge slot anti-jamming array," US Pat. 8,138,982 B1, Mar. 20, 2012.

1. Dual stacked patched controlled by PIN diodes, http://waas.stanford.edu/~wwu/frankb/NDPAJGA.pdf : 15-20dB suppression possible. Jammer = helical antenna with 12dBic gain, 2W power, C/A code pulsed at 12.5% duty cycle.

2. Stacked edge slot array in L1 and L2 bands onboard the projectile: US Pat. 8,138,982

Deep null in the forward axis, circular symmetric pattern around the projectile axis.

[Effect of GPS Jamming]

Carrier phase wrap

Carrier phase roll up

Roll ripple due to antenna asymmetry

[Anti-Jamming GPS Receiver]

Out-of-band interference rejection:

1) SAW filter after the antenna and before the LNA, 2) Lowpass antialiasing filter in the RF chip after the mixer to remove signals that would create aliases during the digitization process in the A/D converter, 3) Highpass filter after the ADC to remove DC components that might come from the data converstion and also removes the flicker noise inherent in CMOS circuitry.

In-band interfernce rejection:

- Use a fine-resolution ADC: 5 bit (30dB dynamic range)

- Use of software controlled hardware digital filter banks: detects an interfering signal and blanks them out.

Jamming threshold:

- 3-dB signal de-sensitization: 3-dB signal degradation at the receiver output

Commercial GPS receiver's jamming threshold:

- Typical GPS receiver: -110dBm

- GPS receiver with adaptive filtering: -85dBm (u-blox 5, u-blox 6: adaptive digital filtering)