Reporter: Brian Davage married his childhood sweetheart, Jill, 54 years ago.
They've lived in the same road in Romford all their married lives.
But since Jill was diagnosed with Alzheimer's disease 10 years ago,
she's been wandering off, and getting lost.
Brian: I went into my doctors', which is right in Romford Market, St. Edward's Way,
And I was in there, and she walked out,
and she was found, 2 1/2 hours later, in Primark's.
Reporter: They're now part of a pilot scheme run by Havering Council.
Jill wears a satellite tracking device which fits around her wrist
and gives off a GPS signal which is monitored by a control center.
If Jill is detected away from home,
the center calls Brian to make sure she's not on her own.
And he can call them, if Jill goes missing.
Brian: The mind's at rest, when she goes out with it on,
that is the main thing. Mind at rest.
Reporter: Over a third of Havering's residents are over 50.
That's the largest elderly population in London.
And as well as keeping people safe, the trackers can save councils money.
And keeping people out of hospital is not only good, if you like,
from a kind of financial point of view - it's also good for the person.
So allowing them to stay in their own home,
it's got to be a better way of caring for people in their old age.
Reporter: There are currently 800,000 people with dementia in the UK,
with numbers set to rise to over one million by the year 2021.
If this scheme's a success, it could be extended
to help more vulnerable people stay independent for longer.
Angela Walker, BBC News.