Dit bericht is al redelijk oud, 10-03-2003 http://forum.myoms.net/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic
The Pentagon has told it will not implement a global GPS blackout for civilian users if war starts in Iraq, as seems increasingly likely.
"We would not create a global problem for transport out of spite for Saddam," says a spokesman at the US Department of Defense. However, he admitted that the US military does have the capability to jam civilian GPS signals regionally, and did not rule this option out.
A blockade of non-military GPS access in the "theatre of war" could significantly hinder the capabilities of Iraqi forces. But millions of civilian users depend on the US-controlled network of 27 global positioning satellites, in activities including shipping, transportation and power transmission.
Any sudden GPS blackout would trigger chaos, say experts. It might also help break the deadlock in negotiations over Europe's proposed rival to GPS, Galileo. This 30-satellite system would be run for entirely civilian purposes. If agreement is reached it could be operational by 2008 at a cost of $2.8 billion.
Former US president Bill Clinton made military-grade GPS available to the public on 1 May 2000. Previously, civilian GPS signals from the US satellites were "downgraded", reducing their accuracy fivefold from the military precision of 20 metres to 100 metres. But Clinton also stated that "should an occasion arise in which it is in our interests to block GPS on a regional basis, we will have the ability to do so".
The Pentagon spokesman said any decision to block GPS would need to be taken at the very highest level, between President George Bush, Donald Rumsfeld, the defense secretary and Tommy Franks, commander of the US forces. "To reverse Clinton's executive order would be a very serious decision," he said.
But satellite experts contacted by New Scientist say a regional blackout is relatively easy to achieve, and that the US has the technical capability to do it.
"We know that various jamming tests have occurred with that technology," says Richard Langley, a GPS expert at the University of New Brunswick in Canada. "And it's mature enough that they could deploy it in a theatre of war." He suspects the technology has already been used in Afghanistan to deny GPS to Taliban forces.
Langley says the civilian GPS signal relies on a so-called " coarse/acquisition code" (c/a), which enables a receiver to determine the distance to the satellite. But the US military relies on the "precise code" (p).
The p code is transmitted over a much wider bandwidth than the c/a code explains Langley: "So you can jam the narrower c/a code without jamming the wider signal."
Jammers can be deployed on mountaintops or tall antennas, but it is probably most economical to place them aboard aircraft. Langley thinks the US might also use "spoofing", in which fake signals fool the GPS receiver into thinking it is somewhere else.
The Pentagon spokesman said that even regional jamming would be difficult because US troops and strap-on guidance systems for smart bombs still rely heavily on the civilian GPS system. "So it could bring complications for our own operations," he said.
But Langley says that, while this was true in the last Gulf War of 1991, he doubts it is now. In 1991, US forces did not have enough military GPS receivers, meaning they actually had to turn off the signal degradation to allow their personnel to use civilian receivers.
Langley also points out that the Iraqis may have jammers of their own: "You can buy them for a few roubles in Russia." The military argues that the signals transmitted by jammers can be swiftly tracked and destroyed.