The UK will no longer have access to the military grade signal of the EU's Galileo system after Brexit - unless it renegotiates access - and it is looking at plans to build an alternative at a cost of £3-5bn.
But the US, Russia, China, Japan and India also have sat nav systems in operation, each with their own high precision signal for security use and a less accurate public signal.
Bleddyn Bowen, an expert in space policy at Leicester University, told Sky News that the entire spectrum of useable radio frequencies has been shared out between the countries, with no spare capacity.
"If Britain wants to build its own system it would have to get all those countries to agree where to take spectrum from. That is a geopolitical headache for Britain to try to resolve if it goes ahead," he said.
In 2003 Galileo was almost scuppered by the US, which feared it threatened the integrity of its GPS system. Ironically Britain sided with the Americans against the newcomer.
Negotiations for access to radio frequencies would be at the International Telecom Union.
"But Britain does not have many negotiators at the ITU because it is not a capacity Britain has had to develop," said Dr Bowen.
"In the past it has been done on a European level, either through the EU or the European Space Agency."
Britain would still have access to Galileo's public signal, so car sat-navs would still work.
But without access to the military signal Britain would have to rely on the encrypted stream from the US GPS system.
If that was jammed by enemy forces - or Britain was denied access by the Americans in a time of crisis - our satellite-guided weapons would be rendered useless. Galileo would have given Britain a back-up.
Sam Gyimah, the science minister, said it was unlikely Britain would be able to do a deal with the EU for continued access to Galileo.
But he was more optimistic that Britain could negotiate a signal frequency for its own satellite navigation system.
"We speak to a number of allies and there is opportunity here for us," he said.
"We can't discuss everything because this is part of our security needs.
"There is a way in which we can deliver something that is consistent with our military needs but with other allies willing to work with us."
Procurement of a British-built replacement for Galileo would be a bonanza for the space industry.
Glasgow-based Clyde Space has proposed a radical alternative using 1,000 nanosatellites the size of a loaf of bread in a very low orbit.
The constellation would give highly precise positioning across a wide region for two hours a day.
Craig Clark, the company's founder, said: "It would be a tenth of the cost of a traditional system. It would save billions potentially."
He said the idea had been rejected by the government.
"There is a danger when you are too innovative that people think there is too much risk," he added.
"But we are here and happy to continue the conversation."