Motorola developed the first cellphone system more than 20 years ago, along with the exchanges that connect standard cellphones to the public telephone network. It then went on to develop satellite telephone technology for the Iridium and Teledesic services.

To stress the value of the ideas in its patents, Motorola first explains how the security services eavesdrop on conventional cellphone calls: snoopers set up a monitoring station at the exchange where cellphone calls are connected to the fixed public phone network, and simply tap into the link.

Motorola then points out that this would often not be possible with satellite phone calls. Like cellphone systems, they route some calls through the public phone network via gateways on the ground, and these are easy to intercept.

But other calls are routed directly to the receiving subscriber via a direct space link, making interception much more difficult. If space calls are diverted through a ground station for the benefit of eavesdroppers, the signal is delayed and degraded. This alerts the caller, and makes unobtrusive interception "difficult or impossible", Motorola says.

The solution--which Motorola is patenting, and therefore has to reveal--takes advantage of the way satellite systems use ground control centres to authenticate space calls. When a subscriber dials a number, their satellite phone "handshakes" with the nearest satellite overhead.

The satellite sends a data signal down to the nearest ground station so that it can check the subscriber's details and authorise the call connection. If the call is routed directly between satellites, the speech signal doesn't travel down to Earth. But, Motorola reveals, it can be made to.

To allow eavesdropping, the ground station sends a control signal telling the satellites to send it a replica of the conversation. This signal can then be intercepted by anyone with access to the station. Meanwhile, the space link carries on working normally, so callers cannot tell they are being bugged.

The British human rights pressure group Liberty is already calling for amendments to the Regulation of Investigatory Powers Bill now going through Parliament. If passed, Liberty says the bill will let the police and security services intercept calls if they pose a risk to national security or economic well-being, or involve serious crime.

These criteria are "extremely vague and subjective", warns Liberty. "They could include industrial action and organised protest."

Liberty's director, John Wadham, told New Scientist: "We are very concerned now to learn that a commercial organisation is making it possible to eavesdrop on satellite telephone calls. We want to see it made a criminal offence for any organisation to interfere with any communications without judicial warrant."

This article appeared in the March 11 issue of New Scientist New Scientist. Copyright 1999 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

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