GPS - God artikkel fra BBC

 

By David Hambling5th October 2020

Satellite navigation keeps our world running in ways many people barely realise, but it is also increasingly vulnerable. What could we use instead?

When satellite navigation was jammed at Israel’s Ben Gurion airport last year, only the skill of the air traffic controllers prevented serious accidents. The jamming was apparently accidental, originating with  Russian forces fighting in Syria, but it highlighted just how dangerous interruptions to the global positioning system – better known as GPS – can be.

“There is a growing recognition of the need to protect, toughen, and augment GPS,” says Todd Humphreys, a communications engineer at the University of Texas, Austin. GPS now underpins a surprising amount of our everyday lives. In its simplest form it tells us where on Earth at any time a GPS receiver is. We have them in our mobile phones and cars. They enable boats to navigate their way through difficult channels and reefs, like a modern-day lighthouse. Emergency services now rely upon GPS to locate those in distress.

Less obviously, ports would cease to operate, as their cranes need GPS to find the right container to move, and they play a crucial role in logistics operations, allowing car manufacturers and supermarkets to take advantage of just-in-time delivery systems. Without it, our supermarket shelves would be emptier and prices would be higher.

The construction industry uses GPS when surveying and fishermen use it to comply with strict regulations, But GPS is not only about identifying locations, it is also about time.

The constellation of 30 satellites held in orbit around the Earth all use multiple, extremely precise atomic clocks to synchronise their signals. They allow users to determine the time to within 100 billionths of a second. Mobile phone networks all use GPS time to synchronise their base stations, while financial and banking institutions rely upon it to ensure trades and transfers occur correctly.

 

Global positioning systems are an important tool for researchers when studying animal movements and behaviour (Credit: Alamy)

We really would be lost without satellite navigation. But is there anything out there that could replace it? And how might we cope without this ubiquitous system?

A loss of satellite navigation for five days would cost the UK alone more than £5.1bn ($6.5bn) , according to an assessment by the London School of Economics for the British Government. A failure of the GPS system would also cost the US economy an estimated $1bn (£760m) a day, and up to $1.5bn (£1.1bn) a day if it occurred during planting season for farmers in April and May.

But GPS outages are surprisingly common – the military regularly jams it in certain areas while testing equipment or during military exercises. The US Government also regularly performs tests and exercises that lead to disruption of the satellite signal, but also some technical problems lead to worldwide issues.

There are, of course, other global navigation satellite systems available – the Russian Glonass, Europe’s Galileo and China’s BeiDou all work on a similar basis to GPS. But increasingly, interference or deliberate jamming can also lead to interruptions in the signals from satellite positioning systems.

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“The military are coming up against jamming quite frequently now,” says Charley Curry, fellow of the Royal Institute of Navigation and founder of Chronos Technology, which works in this field.

The military has especially good reason to be worried. Satellite navigation was originally developed by the Pentagon, and now guides everything from strategic drones and warships down to individual smart bombs and foot soldiers. And it is under threat.

A massive solar storm, one like the Carrington Event of 1859, could bring down the entire GPS satellite network

Criminals also use GPS jammers, easily bought online, to foil the systems used to track stolen cars, not caring who else is affected in the surrounding area. And there are bigger dangers.

“There is also the remote threat that the whole GPS constellation could be rendered inoperable in the initial salvo of a war targeting the US economy by attacking critical infrastructure,” says Humphreys.

Natural forces could be similarly disastrous. A massive solar storm, one like the Carrington Event of 1859, could bring down the entire GPS satellite network as surely as a military strike.

But if GPS and its international cousins were to suddenly disappear – what alternatives could we turn to in an attempt to keep all our world moving?

One possible backup for GPS is a new version of Long Range Navigation (Loran), which was developed during World War Two to guide allied ships while they were crossing the Atlantic. Instead of satellites, however, it consisted of ground-based transmitters with 200-metre (660-feet) tall aerial masts broadcasting radio navigation signals.

 

While good old fashioned maps can help us find our way, many aspects of our modern lives would cease to function without GPS (Credit: Alamy)

At first Loran was only accurate to within a few miles, but by the 1970s it could give a location within a few hundred metres. The UK and other countries decommissioned their Loran transmitters in the 2000s when GPS made them redundant, but a modern, enhanced version, known as eLoran could be as accurate as GPS. It uses more advanced transmitters and receivers than the original version, along with a technique known as differential correction – where the signal is monitored by reference stations and corrected – to improve its accuracy. 

This enhanced version is reportedly capable of pinpointing locations to an accuracy of less than 10m (32 feet). Unlike GPS, it is also able to penetrate buildings and tunnels – primarily because it uses a lower frequency and higher power than satellite signals. The powerful eLoran signals are much harder to jam and there are no vulnerable satellites. But someone would have to fund it.

“eLoran is a great technology that could fill nationwide gaps,” says Humphreys, adding “if there were a commitment to setting it up and maintaining it”.

Other approaches do not require additional infrastructure. Long before radio, sailors navigated with the aid of the sun and stars, using a sextant to measure the angles between them. Celestial navigation continued into the modern age. And surprisingly enough, ballistic missiles like Trident still use astro-navigation during flight. By using fixes from stars it is possible to pinpoint a location on Earth to within a thousand metres or so.

Having large numbers of fast-moving objects to get bearings on means that Skymark can achieve greater accuracy than was possible with slow-moving stars

But US company Draper Laboratory has developed a new generation of celestial navigation known as Skymark which uses a small, automated telescope to track satellites, the International Space Station and other objects orbiting the Earth along with the stars.

Having large numbers of fast-moving objects to get bearings on means that Skymark can achieve greater accuracy than was possible with slow-moving stars. Skymark uses a database of visible satellites – both working satellites and space junk – and has a claimed accuracy of 15m (49ft), making it almost as good as GPS. At times it is capable of greater accuracy, but this depends on how many of these satellites can be seen at once, says Benjamin Lane, group leader of advanced position, navigation and timing instrumentation at Draper.

“The best accuracy for celestial navigation with certainty is within a couple of meters,” he says. “One limitation is the size of the satellite references.”

Another drawback is that it only works with a clear view of the sky. Using infrared light rather than visible light, which can pass more easily through haze and light cloud, helps a little, but in parts of the northern and southern hemisphere where thick cloud and grey-skies are more common, it is likely to be less useful.

 

Tracking fast moving satellites like the International Space Station has helped to improve the accuracy of celestial navigation (Credit: Alamy)

Perhaps a more day-to-day option might be inertial navigation, which uses a set of accelerometers to work out the exact speed and direction that a vehicle is travelling in to calculate its position. Basic versions are already in common use.

“When your car goes into a tunnel and you lose the GPS signal, it’s inertial navigation that keeps your position updated,” says Curry.

The problem with inertial navigation is “drift” – the calculated position gets less accurate over time as errors build up, so the inertial navigator in your car is only useful for short GPS interruptions.

Drift could be overcome with quantum sensors thousands of times more sensitive than existing devices. In the quantum world, atoms and particles start to behave as both matter and waves, and acceleration alters the properties of this behaviour. French company iXBlue is using this technique to build a device to rival GPS precision, and a team from Imperial College London, working with laser specialists M Squared, demonstrated a prototype portable quantum accelerometer in 2018.

The US Department of Transport is now holding a competition to select possible backups for GPS

Such quantum sensors are still confined to laboratories and are years away from a usable end product.

Optical navigation, in which automated systems with cameras use landmarks like buildings and road junctions, may be with us much sooner. An early version, known as Digital Scene Matching, was developed for cruise missiles.

ImageNav, developed by Scientific Systems for the US Air Force, is a modern optical navigation system for aircraft. It has a terrain database of the area being navigated and matches it with input from video cameras to work out its location. ImageNav has been successfully tested on a number of aircraft, but could also find uses in self-driving vehicles.

Swedish company Everdrone also recently carried out the first drone delivery between hospitals without using GPS. Their system uses a combination of optical flow – measuring speed by the rate of which scenery passes below – and landmark identification to find its way from point to point with GPS-like precision. Of course, this method relies on have a complete and accurate image database of the area you are navigating, which is likely to require a lot of memory and frequent updates.

 

Inertial navigation is what takes over when in-car navigation devices lose the GPS signal inside tunnels (Credit: Alamy)

The UK is developing a backup system for the timing synchronisation services that GPS provides in the form of The National Timing Centre program, the first such national service in the world.  When it becomes operational in 2025, it will involve sets of precise atomic clocks at distributed, secure locations across the UK, providing timing signals via cable and radio services. The idea is that if satellite signals go down, there is no single vulnerable centre that could be brought down by an accident, technical glitch or cyberattack.

Ultimately no single system may be able to replace the power of satellite navigation systems such as GPS, and we may end up with a mix-and-match of different solutions for ships, planes and cars. The US Department of Transport is now holding a competition to select possible backups for GPS. There is a real question though over whether any alternative will be in place soon enough.

“There’s now an awareness of the problem, but things are still moving at glacial speed,” says Curry.

We are becoming ever more reliant on accurate navigation. Self-driving cars, delivery drones, and flying taxis are expected to appear on and above our roads over the next decade. All of them will be dependent on GPS.

As Curry notes, one person with a powerful jammer in a could knock out GPS across an area the size of  London from the right place. Unless adequate backup systems are developed, in the future whole cities might grind to a halt at the flick of a switch.