BAE navigation system identifies suspicious signals

A new navigation system could help prevent GPS users from being fooled with fake signals or getting lost when the satellite link goes down.

BAE Systems is developing the technology as a back-up to GPS and other global navigation satellite systems (GNSS) that doesn’t need the internet to locate itself in the way smartphones do and could even help navigate within buildings.

The relatively low cost of the system means individual soldiers or small autonomous vehicles could use it to stop their GPS from being blocked by tall buildings or enemy jamming technology, while firefighters could use it to find their way around smoke-filled buildings.

But it could also help reduce the vulnerability of sectors such as aviation, shipping and telecoms to GPS disruptions. A recent European Commission report found that €800bn (£638bn) worth of economic activity in Europe depended on satellite navigation.

‘Because we’ve become so dependent on GPS, it’s important to have these back-ups to get you through periods of loss,’ said Dr Ramsey Faragher, principal scientist at BAE’s Advanced Technology Centre, who is leading the project.

Mapping method

The Navigation via Signals of Opportunity (NAVSOP) system builds a map of an area using ground-based signals from wireless internet, TV, radio and mobile phone networks but also transmissions from low-Earth-orbit satellites.

Many smartphones use a similar system using nearby mobile signals to locate themselves but this requires access to an internet database of signal landmarks.

NAVSOP is able to quickly build its own database by identifying each different signal and recording its exact position while GPS is still available, making it more suitable for regions that don’t already have the required database or situations where a mobile internet connection isn’t available.

‘Mobile phones weren’t designed for this purpose — you’re kind of jerry-rigging them to do something they weren’t designed to do in the first place,’ said Faragher. ‘We take a much greater variety of measurements.’

Instead of the less accurate method of examining signal strengths, NAVSOP uses the radio signals it collects in a similar way to how GPS works, analysing the repetitive pattern of the signals to work out the time it takes each one to travel from its source and so calculate how far away it is.

‘But you can’t do that straight away because the networks were never designed to operate in that way,’ said Faragher. ‘They don’t announce their location to you like GPS satellites do and they all run with different timing structures, even different transmitters within the same network.’

By learning about these timing structures and calibrating against GPS while it is available, the system can gradually build its map of an area until GPS is no longer needed and so protecting it from signal problems or attacks.

Signal suspicion

GPS signals can be disrupted by space weather such as solar flares but are also vulnerable to man-made attacks such as jamming or spoofing, where a fake signal is used to fool GPS technology into giving a false location reading. This could send a ship off course or fool transport companies monitoring the position of their trucks.

The Royal Academy of Engineering recently drew attention to this problem in a report that highlighted the vulnerability of GPS to spoofing and jamming thanks to the growing availability of suitable equipment.

Not only does NAVSOP create an alternative to GPS, it also offers greater protection from these kind of attacks. ‘Spoofing entails understanding the exact signal structure of a given transmission and then broadcasting a believable copy,’ said Faragher. ‘So for any radio-based system that has unencrypted parts to it, someone could theoretically develop a system to recreate and broadcast those signals.’

However, spoofing all of the signals that NAVSOP uses at the same time would be a much more complicated task, he said. ‘NAVSOP works out where the signals are coming from so if suddenly all these new signals appear that are all very interesting and yet the system works out they’re all coming from the same location that’s highly suspicious.’

The system could help bring greater protection against spoofing to wider usage by the military and in other applications.

‘Very expensive military systems have ways of getting around [signal disruptions] anyway but making radio measurements has always been very cheap and this is just signal processing of radio measurements so this is quite a low-cost way of helping you through these issues,’ said Faragher.

‘Because it’s low cost it’s more applicable for things such as deploying on every soldier or deploying on low-cost autonomous vehicles, which rely very heavily on GPS and can’t just use eyes on a map like a soldier.’

Real-time positioning

BAE is working on two versions of the technology: one for use outdoors that currently relies on laboratory testing equipment and another that is designed for indoor use. It has produced a prototype that can process the signals in real time.

‘We have a real-time demonstrator using medium-wave radio that provides positioning at the 5–30m level, and if you know you are on roads you can snap to them and do even better,’ said Faragher. ‘Our real-time indoor positioning system, tested using VHF FM, 2G and 3G mobile phone signals, gives us 5–10m positioning.’

Prof Jim Norton, a policy adviser who contributed to the Royal Academy’s report, said the difficulty for this kind of system was the need to maintain an up-to-date database as mobile operators regularly reconfigure their networks and wireless internet signals change even more frequently.

‘My concern would be that this would work well in major urban areas, where there are many clearly identified transmitters, but would become increasingly unreliable and inaccurate in more sparsely populated areas, where even mobile phone coverage can be patchy — let alone having a range of wireless signals to triangulate on.’

But Faragher pointed out that NAVSOP will also use signals from low-Earth-orbit satellites and even downlinks to aircraft to help build the map of a region and highlighted that the system would always remain one part of a toolkit in providing location data.