A friend of yours is worried that his wife might find out where he has been parking on Wednesday afternoons when he should have been at work. Given that he has been careful to clear his satnav...

A friend of yours is worried that his wife might find out where he has been parking on Wednesday afternoons when he should have been at work. Given that he has been careful to clear his satnav history, explain to him why GPS alone cannot be used to track his movements.

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