Photo: RNZ / Claire Concannon

Imagine you're on a boat lost at sea, too far from the coast for a VHF radio or mobile phone to work.

Maybe you've got a personal locator beacon, but you forgot to check the battery and it's dead.

Drifting at night, you can see pinpoints of light passing overhead - satellites orbiting the Earth. If only there was a way to signal to them: I'm here.

Now imagine you have a device that can do that. It needs no batteries, can be folded away until required, and can be detected by not just traditional radar, but a kind of radar used by satellites, meaning you can be spotted anywhere there's one passing overhead.

A few months ago, researchers and the New Zealand Defence Force successfully tested the latest prototype of just such a device, called SAR4SaR, in a final sea trial.

"There were tears all around," NZDF's Defence Science and Technology (DST) director Dr David Galligan says.

"I got a picture through of the snap that they got back from the radar satellite and you could very clearly see the bright spots on the water."

The hope is that one day, a SAR4SaR device could be carried on even the smallest of boats - and save lives.

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Dr Tom Dowling stands in the gentle wash of waves at Mission Bay beach in Auckland, trouser legs