Medium-Tech Attackers
Consider the demented al-Qaeda terrorists who struck the World Trade Center and the Pentagon. Their training includes low- and medium-tech strategies such as concealing nitroglycerine as perfume or aftershave lotion. Surreptitiously carried aboard an aircraft, it's detonated using nothing more than a battery and a concealed blasting cap.
Speaking of concealable devices, there are those who contend that a Global Positioning Satellite (GPS) jamming system, built in a home workshop, can disable the reception of GPS signals. Designs for such jammers are now kicking around the Web.
All it takes is a small transmitter modulated to emit Gaussian noise centered at GPS's main 1.57542 MHz frequency. Carefully aimed, the jammer might be able to wreak havoc with consumer GPS spread-spectrum receivers, such as those handhelds that were pressed into service in Desert Storm during the Gulf War. A user might be able to jam military GPS receivers, too.
Distance Is the Weakness
GPS receivers rely on signals from four or more visible satellites in a constellation of orbits at distances of more than 13,000 miles. "The distance and power of the constellation signal is the chief weakness of GPS, that is, jamming or spoofing of the low-power signal," says Murray Rosen, Technical Director of New Jersey-based Electro-Radiation Inc. (ERI).
In a paper entitled Protection Solution to GPS Vulnerability, Rosen points out that the key to precise GPS navigation is processing very weak GPS signals. "A commercial GPS receiver is vulnerable to a simple 1 W jammer at up to 135 km," says Rosen.
"A typical tracking military receiver using inertial aiding (with about 62 dB of jam immunity) can lose code track within 54 km of a 1 kW CW jammer, and within 17 km of a 100 W noise jammer."
Homebrew GPS Denial
Consider the GPS jammer that 22-year-old Romain Lievin built while a student at France's ESISAR-INPG Grenoble engineering school. Lievin's GPS jammer may eventually be available commercially. Here's a photo of one of his prototype circuit boards.
One "hobby" GPS-denial system that I found on the Web uses a Motorola MC145151 PLL-based frequency synthesizer, a Micronetics M3500-1324S VCO module, and a Fujitsu MB506 divide-by-256 pre-scaler.
The VCO feeds the pre-scaler, generating a 1.575 GHz signal that's divided down and routed to one port on the PLL's comparator. The reference is an oscillator running an El Cheapo 10 MHz crystal. Ultimately a signal is derived that's in the audio range, where it's easy to deal with. No strip-lines in that domain, eh?
Augmenting the PLL is a hash generator based on Zener diode noise. A 12-cent 2N2222 transistor amplifies the noise. This develops broadband crud out to 100 MHz.
In this home-workshop GPS jammer, the hash is filtered and then boosted by a ubiquitous National Semi LM386 audio amplifier IC, and then mixed with the VCO's error-tune signal. The result? RF looking like random noise centered on 1.57542 GHz.
Boost That Signal
Next comes a bit of RF amplification. The Web article shows a Sirenza Microdevices SGA-6289 module as an RF pre-driver, feeding a Toko 4DFA-1575B-12 ceramic bandpass filter (Digi-Key p/n TKS2609CT-ND). Once filtered, the signal is applied to a Watkins-Johnson AH102 power amp.
For an antenna, the author suggests an off-the-shelf GPS receiving antenna, or a Ramsey Electronics Model DA25 microwave discone. The DA25 is advised for omnidirectional use, but the article emphasizes that a log periodic array can boost the signal, and help shield the user's own GPS receiver from being jammed or detected!
Would you prefer building your homebrew jammer with surface-mount technology? According to the same Web primer, dielectric GPS patch antenna elements can be used. Recommended is Toko's DAK Series (Digi-Key p/n TK5150-ND). "The small antenna-element size is perfect for hidden or portable operations," says the article's author.
Russian Capitalism At Work
Is this improbable? Maybe so, but my ears perked up when I caught a blip on the evening TV news about portable GPS jammers that are being sold to the Iraqis by the Russians. Presumably these units come from the Moscow-based Aviaconversiya Ltd. electronics company.
The company has been flaunting its handheld GPS jammers at military hardware and air shows since 1999. Claimed to be able to jam American GPS and Russian GLONASS satellite transmissions, the Aviaconversiya emitters strike me as true spy-vs.-spy products.
By the way, a Web search reveals that Aviaconversiya is on a list of hundreds of U.S. Department of Defense contractors whose contracts exceed $25,000. (Curious, don't you think?)
So just what does Aviaconversiya's little battery-powered $40,000 box do? Well, for starters, it delivers a hefty 8 W of microwave RF into its high-gain Yagi antenna. That gives a user possible GPS denial out to hundreds of miles. If fitted with an omnidirectional antenna instead of the Yagi beam, the Aviaconversiya system is touted as capable of preventing a cruise missile from finding its mark.
Threats Real or Imagined?
Whether homemade or off-the-shelf, are GPS jammers a bona fide threat? Mario Casabona, president of ERI, thinks so. Writing in a paid "advertorial" that appeared in the December, 2002, issue of GPS World magazine, Casabona stated: "With the schematics you get on the Internet, you can build a jammer that's a real concern, and the U.S. military is particularly interested."
Some experts are more sanguine, but still anxious. In an article appearing in the October 3, 2001, issue of Global Positioning and Navigation News (Vol. 11, No. 20), the author says that U.S. aerospace and defense spokesmen are confident that the U.S. arsenal of anti-jamming technology, coupled with the ability to knock out brute-force high-power GPS jammers with missiles, would ultimately prevail in a conflict.
ERI's Rosen seems to agree. "Adding 35 dB of additional CW anti-jam and 25 dB of noise anti-jam reduces GPS vulnerability to 1 km," he says.
However, some experts insist that military and civilian GPS signals can be "easily" jammed. The Global Positioning article's author argues that the cost/benefit calculation needed to fix the problem could turn Pentagon military budget planning "on its ear," with huge investments in the anti-jamming systems needed to combat portable jammers.
"In the struggle outlined by President Bush, America's sharpest engineering minds, with millions of dollars at their disposal, may be locked in long-distance intellectual combat against an Afghani radio technician in a mud hut, holding a soldering iron and surrounded by off-the-shelf parts," suggests the Global Positioning author.
In the long run it's more than likely that well-funded Western military will prevail. Rockwell Collins, for one, has already developed an anti-jamming system using so-called pseudo-satellites, or pseudolites. The pseudolite system uses airborne or ground-based signals to mimic GPS satellite signals.
Existing GPS munitions and military GPS receivers can be upgraded with pseudolite software to lock on to these new signals. GPS satellite signals act as pseudolite positioning references, but the pseudolite signals would be local and strong, overpowering brute-force high-power jammers (moreover, tactical scenarios call for jammers emitting GPS denial signals to be detected, and then knocked out).
The Crows Plug It Out
Rockwell Collins, under contract to the Defense Advanced Research Projects Agency, is already building Precision Lightweight GPS Receivers, or PLGRs. Known as "pluggers" by users, the handheld pseudolite receivers are now standard-issue in the U.S. military. Rockwell Collins reports that hundreds of thousands of pluggers have been shipped.
ERI's Casabona says that pluggers offer 30 dB to 40 dB of inherent anti-jamming capability. "That can be increased with special anti-jamming antennas and by interfacing to other specialized receivers," he adds.
Another military contractor, Raytheon Systems Ltd., has designed anti-jam antennas. The company's GPS Antenna System-1, or GAS-1, is an analog configuration. It points antenna nulls in the direction of incoming interference to attenuate jamming signals.
Over a thousand GAS-1s have been deployed, but Raytheon's spy-vs.-spy roadmap includes a digital receiver, too. It will tout nulling and multi-beam steering, and will use DSP to enhance jammer suppression. It will filter out jammer noise, while preserving the current GAS-1 form, fit, and interfaces.
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