GPS Jamming N' Stuff

This page contains resources on GPS Jamming [I am not responsible for any trouble you may get yourself into]

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Project Overview

A low cost device to temporarily disable the reception of the civilian coarse acquisition (C/A) code used for the standard positioning service (SPS) on the Global Positioning System (GPS/NAVSTAR) L1 frequency of 1575.42 MHz. This is accomplished by transmitting a narrowband Gaussian noise signal, with a deviation of +/- 1.023 MHz, on the L1 GPS frequency itself. This technique is a little more complicated than a simple continuous wave (CW) jammer, but tends to be more effective (i.e. harder to filter) against spread spectrum based radio receivers. This device will have no effect on the precise positioning service (PPS) which is transmitted on the GPS L2 frequency of 1227.6 MHz and little effect on the P-code which is also carried on the L1 frequency. There may be a problem if your particular GPS receiver needs to acquire the P(Y)-code through the C/A-code before proper operation. This device will also not work against the new upcoming GPS L5 frequency of 1176.45 MHz or the Russian GLONASS or European Galileo systems. It can be adapted to jam the new civilian C/A-code signal which is going to also be transmitted on the GPS L2 frequency. That said, it will work against the majority of consumer/OEM GPS receivers, provided they are not setup in any advanced anti-jam configuration.

Technical Description

This will be a brief description of each of the major sections which compromise the entire jammer device. Refer to the included schematic diagram as you read along. You should also refer to the component's datasheets for even more detailed information.

Phase Locked Loop

The jammer's main oscillator components consist of a Motorola MC145151 phase-locked loop (PLL) frequency synthesizer chip, a Micronetics M3500-1324S voltage controlled oscillator (VCO) module and a Fujitsu MB506 divide-by-256 prescaler chip. The VCO feeds a portion of its radio frequency (RF) output signal into the prescaler chip, where it is divided by 256. A 1575 MHz signal would be turned into a 6.15234375 MHz signal. This is then fed into one side of the PLL chip. The other side of the PLL is fed with a reference frequency which is derived from a 10 MHz quartz crystal. This crystal reference frequency is divided down 512 times by the PLL to reach 19531.25 Hz. The 6.15234375 MHz prescaler output frequency is also further divided down 315 times by the PLL chip for a final frequency of 19531.25 Hz. This will be the new PLL internal reference frequency. That big bad 1575 MHz microwave signal now looks like a simple audio frequency to the PLL chip and the supporting components. The PLL chip internally compares the phase of the 19531.25 Hz VCO side signal to the phase of the 19531.25 Hz crystal side signal. The PLL chip outputs high or low voltage pulses depending on whether the crystal signal is leading or lagging in phase with the VCO signal. These pulses are then filtered and dampened into a pure DC control signal via a simple passive loop filter. This cleaned up signal is then connected to the VCO's voltage tune control input. When everything is working properly, the VCO's output frequency is locked to whatever frequency you programmed into the PLL chip, 1575 MHz in this case. It will stay on that frequency even through dramatic temperature changes, a problem that a non-PLL VCO would have. If the PLL is not working properly, the red "PLL Unlock" LED will be lit. Due to a quirk with using low cost, easy to obtain components, you'll need to tweak two loading capacitors on the reference crystal. This is unusual, but necessary to move the signal from the default 1575 MHz to the more appropriate 1575.42 MHz (+/- a few hundred Hertz). This is a very important and delicate procedure, and you'll need a frequency counter to accomplish it.

Antenna

A radiating antenna is not shown in the schematic diagram and one will need to be purchased or constructed for proper operation. There are numerous commercial GPS receiving antennas which will work fine for this low power transmitting application. Some of the best pre-made or easily assembled microwave antennas can be purchased directly from Ramsey Electronics. The Ramsey DA25 broadband discone antenna is recommended for omni- directional (transmit in a circle) radiating applications. The LPY2 log periodic Yagi antenna can be used for directional (transmit in a straight line) radiating applications. Using a directional antenna will give you a slight increase in overall transmitted RF power, which increases the jammer's range, and can also be used to shield your own GPS receiver from being jammed (i.e. point it at the enemy). Dielectric GPS patch antenna elements may also be purchased from Digi- Key. Toko DAK series elements, Digi-Key part number TK5150-ND, are perfect for surface mounting directly to the circuit board. They will require a plastic radome to slightly lower their resonant frequency. The small antenna element size is also perfect for hidden or portable operations. Construction Notes

Operation

Once the jammer is operational, you can practice testing it by monitoring the signal on a common consumer GPS receiver or high quality communications receiver. A GPS receiver close to the jammer will not be able to acquire C/A-code lock and any operating GPS in the jammer's radiation pattern will lose C/A-code lock. Higher quality GPS receivers tend to be less susceptible to low power jamming, so you'll need to be in the antenna's near-field radiation pattern (i.e. close) for it to work. Any obstructions near the jammer's own antenna (trees, houses, hills, walls, etc.) will decrease the jamming range. The best placement is where the jammer's antenna is line-of-sight to the antenna of the GPS receiver you're trying to jam. Real world results will vary drastically, but you should be able to obtain a jam radius of a few hundred feet even in heavily obstructed areas with the higher power (AH102) option and a simple antenna. You can even practice counter-jamming methods to protect yourself against hostile or accidental GPS jamming. Try to shield your GPS receiver from the interference source by placing your body, trees, hills, rocks or other obstructions in-between your position and the interference. More advanced methods involve using directional or steerable phased-array antennas on your GPS receiver (pointed skyward) to nullify any ground based interference.

GPS Vulnerability PDF

FBI GPS Tracking Device Found On A Civilian Vehicle

GPS Jammer Schematic:

GPS Jammer Schematic