Terror tension to make GPS useless?

26 September 2001

Terror tension to make GPS useless?

By Tom Allen-Stevens

THE CURRENT build-up of military tension over the terror attacks in the USA could have serious consequences for most GPS equipment used worldwide.

The scrambling of positioning signals from US satellites, which was turned off 18 months ago, may be turned back on to help conceal the position of US troops.

This would alter the accuracy of handheld GPS units and standard satellite navigation equipment from 5-10m to around 100m.

“Thatll make them useless for agricultural purposes,” said Simon Parrington from SOYL at a Precision Update Event at Silsoe Research Institute on Wednesday (26 September).

Others at the event had heard the “selective availability” of satellite signals could be switched back on as early as Wednesday next week (3 October).

The decision to scramble the signal from the satellites, which are owned by the US Department of Defence, would come direct from the US president himself.

Many are not convinced this action would be taken. “During the Gulf War the scrambling was turned off,” said RDS Systems Peter Nelson.

“Many of the ground troops had handheld GPS units, which can only be used with unscrambled signals, to help them pinpoint their position.”

Those who pay for differential GPS systems, costing around 2500 to buy plus an annual fee of around 500, would be unaffected by any signal scrambling.

Under such systems, a corrective signal is generated by another satellite or a land-based station, giving you 2-5m accuracy.

The standard GPS, used in handheld units and car navigation systems, for example, takes signals from up to 26 positioning satellites orbiting the Earth.

Since they were launched their signals have been scrambled so that they can be accurately used only the US Department of Defence.

The scrambling was turned off 18 months ago to give free, 5-10m accurate GPS to all, triggering a boom in GPS equipment from as little as 100 for a position logger.

Two additional satellites over the Atlantic and Pacific Oceans give users in the USA an additional signal, currently on test, enabling free, sub-metre accuracy.

A similar system is due to be launched in Europe in the next two to four years.


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