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Reporter: Brian Davage married his childhood sweetheart, Jill, 54 years ago.

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They've lived in the same road in Romford all their married lives.

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But since Jill was diagnosed with Alzheimer's disease 10 years ago,

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she's been wandering off, and getting lost.

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Brian: I went into my doctors', which is right in Romford Market, St. Edward's Way,

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And I was in there, and she walked out,

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and she was found, 2 1/2 hours later, in Primark's.

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Reporter: They're now part of a pilot scheme run by Havering Council.

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Jill wears a satellite tracking device which fits around her wrist

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and gives off a GPS signal which is monitored by a control center.

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If Jill is detected away from home,

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the center calls Brian to make sure she's not on her own.

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And he can call them, if Jill goes missing.

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Brian: The mind's at rest, when she goes out with it on,

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that is the main thing. Mind at rest.

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Reporter: Over a third of Havering's residents are over 50.

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That's the largest elderly population in London.

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And as well as keeping people safe, the trackers can save councils money.

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And keeping people out of hospital is not only good, if you like,

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from a kind of financial point of view - it's also good for the person.

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So allowing them to stay in their own home,

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it's got to be a better way of caring for people in their old age.

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Reporter: There are currently 800,000 people with dementia in the UK,

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with numbers set to rise to over one million by the year 2021.

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If this scheme's a success, it could be extended

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to help more vulnerable people stay independent for longer.

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Angela Walker, BBC News.