GPS Signals Jammed during Tank Trials

By Grau, Lester W. | Military Review, March/April 2001 | Go to article overview

GPS Signals Jammed during Tank Trials


Grau, Lester W., Military Review


Based on 6 August 2000 reports in The Sunday Times of London, Agence France-Presse and the 25 September 2000 Elevtheos Tipos, Athens

The highly accurate Global Positioning System (GPS) supports modem ground forces as they move and shoot. Maps and compasses stay in cases as digitized forces quickly use GPS to determine their location and the enemy's. Although map-reading skills atrophy, few worry that GPS may suddenly provide erroneous information or cease working. Still, US Army equipment has already faced attacks on GPS functions-by allies.

In August 2000 the Greek government sponsored a tank competition at Litokhoro to determine the Greek army's next tank-a deal worth $1.4 billion for 250 tanks. Competitors included the British Challenger 2E, the US M1A1 Abrams, the German Leopard 2A5 and the French Leclerc. During the trials, the British and US tanks had navigation problems despite using multiple GPS satellites to determine their positions precisely. After the embarrassing performance, officials discovered that the GPS satellites were being jammed-by a French security agency. Less than a foot high, the jammers transmitted stronger signals than satellites on the same frequency. The jammers were reportedly hidden on the firing range and remotely activated as US and British tanks were tested.

Greek defense officials found the jamming episode rather amusing and discounted the associated technical problems. The threat remains: if an ally can create such havoc during a test, what effect could hostile GPS jamming have during combat?

Interference-Intentional and Otherwise

Jamming is quite simple, both in principle and in practice. Almost anyone who is interested can do it. Jamming only becomes difficult when the user wants to limit the effect to a narrow, specific range of frequencies over a great distance. Such a power and focus is important to high-tech users who depend on their own electronic systems. A less sophisticated user who wishes only to deny an adversary's capability has no reason for concern. Careful planning could allow a low-tech adversary to conduct electronic countermeasures-such as jamming-- and wreak havoc on a high-tech opponent while the low-tech force uses relatively crude electronics such as frequency-modulated radios at short ranges.

The Defense Science Board and the National Research Council confirm that GPS jamming poses a real threat.13 David E. Lewis, Magnavox Electronic Systems Company, quoted the Defense Science Board's November 1993 Tactical Air Warfare study, "Current GPS receivers are vulnerable to jamming in acquisition mode at very long ranges from low-power jammers and will lose track at moderate range for reasonable jammer threats.14 Lewis states that a 100-watt jammer can affect a standard military GPS receiver, such as a precision-guided munition would carry, as far away as 600 miles (or line-of-sight distance, whichever is less) during initial GPS acquisition. Even when the missile has acquired the GPS signal and is using it to track its progress to the target, such tracking could be interrupted within 28 miles of the jammer.

Commercial television, very high-frequency transmitters, aeronautical satellite communications and Mobile Satellite System terminals can also degrade GPS signals, and natural occurrences can cause interference that would pose distinct problems for users, including the military.15 Designers were aware that GPS signal interference was possible but faced a basic problem-placing weight in orbit is costly. To reduce costs and extend operating life, GPS satellites were designed to produce weak signals; only a few milliwatts. The decision makes good engineering sense but, unfortunately, makes GPS signals easy to jam.

Former Assistant Deputy Undersecretary of Defense for Strategic Aeronautical and Theater Nuclear Systems, Dr. Stanley B. Alterman, stated that a 1 watt (cellular phone-size) jammer located 60 kilometers away (line of sight) can prevent a good commercial GPS receiver from acquiring Navigation Satellite Timing and Ranging (NAVSTAR) signals. …

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