Experimental verification of GPS jam immunity has shown its vulnerability under the simple intentional interference. The jammer is a CW transmitter of a pure sine-wave of a frequency, close to the carrier of satellite signals. As a result, the carrier and jamming signal beat and make impossible reception of data by the correlator.

GPS uses phase manipulated (PM) signals [1], which are thought to be of a high resistant to interference [2]. The PM signal is a sine-wave and in the predetermined time its phase is reversed step-like. The GPS receiver is based upon the correlator, where the multiplication of the already known waveform and the received signal portion is done, followed by integration over the low-pass band. Integration of the 1024 signal portions results in general reception of the single bit [3].

In [4] the jam immunity of the PM receiver is considered. The optimized jamming interference is suggested, obtained by the variation methods. The same article proves, that CW monochromatic signal (pure sinewave with the constant frequency) is practically equal to the optimal jamming signal (1% difference). This fact can be explained by the beating between the carriers of the data signal and jamming interference l.

Let’s suggest, that both signals have the equal amplitudes, then:

Asin(wt)+Asin(w+Dw)=2Acos((Dw/2)t)cos((w+Dw)t/2),           (1)

where A - signal amplitude, w - carrier of the PM signal, Dw - difference between the carriers. As far as the monochromatic CW is the simplest jamming signal [3], the GPS jamming immunity against this signal is under the question. The correlator input band cannot be less than approx. 1/t, where t is the signal portion length. For the increasing of the reception precision the band width must be even greater. If both the satellite signal and the jamming signal will be received at the correlator input, then the beatwave (1) occurs, thus suppressing the speed measurement. The carrier is then amplitude modulated by the amplitude of the beat wave, and the phase in the near maximums of the beatwave is shifted by p/2. If Dw~1/t, then beat wave phase changes in the same way, as of the signal to be received. The signal sum (1) is far different from the satellite PM signal, thus the signal component at the correlator output would be almost suppressed. Then GPS reception in the direct-vision zone of jamming transmitter action must be suppressed. To test this idea the experimental work was done.

PM modulation drive signal

PM modulated signal

Superposition of the PM and the jamming signals.

Experimental method.

The tests were done with the GPS receiver OEM "Sensor", supplied by "ASHTECH" (12 channels, f=1575,42 ÌÃö with C/A code), software ver. 1E11DRP. GPS antenna P/N AT 575-12, supplied by "Aero Antenna Technology Inc." with the cable length 10 m (typical). Antenna height was 15 m over the ground. The test transmitter was Russian generator "G4-78", max. output power 0.0001W with selectable attenuation of 0..-130dB. The generated signal was CW, with 0.2% frequency setting precision. The transmit antenna was half-wave dipole at the height of 1 m. The distance between the jamming transmitter and the receiver was 200 m. The indication of GPS reception status was provided by the supplied routine - "ashmono.exe" on the IBM PC, connected to the receiver.

TEST RESULTS

1. When transmitting at 0.0001W with -14dB attenuation in the band 1576-1579MHz there was no reception of GPS signals. 

2. In the band of 1575-1576 MHz the satellite reception was registered, but unstable.

3. When transmission was switched off, the 5 satellites were received (see fig-s below).

4. The receiving antenna directivity doesn’t seem to influence on the GPS jamming efficiency.

5. The scaling of the experimental data shows, that at 1W output power jamming at the direct vision line will occur in 500 km range.

6. It must be remembered, that only one type of GPS receiver was under test.

The dependence of the signal reception on the jamming frequency

The dependence of the GPS reception on the jamming power on 1576.25

That’s why very weak jamming signal (-60..-70dB) disrupts GPS reception. Jamming transmitter of the 1W output power will set -60dB signal power at 20 km and GPS signal will not be received. Some experts predict 20..40dB gain of the anti-jam countermeasures [5]. Also, Pentagon has plans to increase the satellite output power up to 5000W. Then GPS signal will be lower at least -30dB - still no jamming immunity.

The PM signals are not jamming-immune even in general reception (with integration). 

CONCLUSION.

The strong dependence of the GPS signal reception on the presence of CW sinewave jamming signal exists. Thus, both the civilian and military channels can be jammed without knowledge of GPS codes. 

REFERENCES

1. Yarlikov M. S. Statistical theory of radionavigation // Moscow, 1985

2. Tuzov G. I., Sivov V. V., Prytkov V. I. Jamming immunity of radiosystems with complex signals // Moscow, 1985

3. Fink L. M. Signals, interferences, errors // Moscow, 1984

4. Ovcharenko L. A., Poddubny V. N. Jamming immunity of the phase manipulated signals// Radiotekhnika, 1992, N7-8, p.13-19.

5. JAMMER AND INTERFERENCE LOCATION SYSTEM - DESIGN AND INITIAL TEST RESULTS// Alison Brown, Dale Reynolds: NAVSYS Corporation; Capt. Darren Roberts, Major Steve Serie: Air Force Space Battlelab.