A faulty TV digital box sparked a rescue mission from RAF Kinloss by sending out a signal identical to those transmitted by vessels in distress.
A coastguard helicopter was asked to make checks in the area
The Kinloss site in Moray, which co-ordinates rescue operations across the UK, detected an "SOS" call from the Portsmouth area on 5 January.
A coastguard helicopter spent two hours searching the harbour area before the signal was traced to dry land.
An RAF spokesman said the signal had been a "complete freak".
Telecoms regulator Ofcom was asked to look into the signal and confirmed the source.
RAF spokesman Michael Mulford said the Aeronautical Rescue Co-ordination Centre at the airbase had picked up the beacon from one of five orbiting satellites.
He said it was transmitting on the major emergency frequency.
"We traced it to Portsmouth Harbour, checked and found out there were no vessels in the area or missing planes."
The rescue centre then contacted Ofcom which was able to establish it was coming from a household.
Mr Mulford added: "This is very very unusual, it's a complete freak and the odds of a digibox sending out such a signal must be astronomical.
"The guy who owns it really should do the lottery because the chances of sending out a signal from a digibox and sending out precisely and exactly on a major emergency channel are far more than 14 million to one."
Ofcom has since removed the £50 Freeview box for tests.
An Ofcom spokesman said: "This is a real one-off as digital boxes only receive signals.
"They shouldn't be sending out signals, let alone maydays.
"The householder was happy to hand it over to our engineers who are trying to get to the bottom of the defect."