Satellite-navigation systems such as GPS are at risk of jamming

THE PHRASE “critical infrastructure” conjures up solidly earthbound images: road and rail networks, water and sewage pipes, electricity grids, the internet, and so on. Such stuff is so wound into the warp and weft of life that it is simultaneously both essential and taken for granted. One piece of infrastructure which has become critical over recent decades, though, is anything but earthbound. This is the various constellations of satellites, the most familiar of which is probably America’s Global Positioning System (GPS), that orbit about 20,000km above Earth, broadcasting to the world precisely where they are and exactly what time it is.

The original purpose of the GPS and its European (Galileo), Russian (GLONASS) and Chinese (BeiDou) counterparts was to enable suitably programmed receivers on or near the ground to calculate their whereabouts to within a few centimetres, by comparing signals from several satellites. In this role they have become ubiquitous, running everything from the navigation systems of planes, ships and automobiles, both military and civilian, to guiding the application of water and fertiliser in precision agriculture. But global-navigation satellite systems (GNSS), to give their collective name, now do much more than that. By acting as clocks that broadcast the time accurate to within a few dozen nanoseconds, they are crucial to jobs ranging from co-ordinating electricity grids and mobile-phone networks to time-stamping financial transactions and regulating the flow of information in and out of data centres.

Location, location, location

GNSS networks do, though, have a weak spot. The satellites’ transmitters broadcast with the wattage of a refrigerator lightbulb. Their signals are so vanishingly faint that they arrive “beneath the noise floor” of ambient electromagnetic radiation, as engineers like to put it. This makes them vulnerable to interference, both accidental and deliberate. The more uses which GNSS constellations are put to, the more this matters. So those engineers are looking at ways to harden and back up the whole idea.

Jamming sometimes happens accidentally. In January, for instance, it emerged that GPS failures which had been plaguing aircraft near Wilmington International Airport, in North Carolina, were caused by wireless equipment at an unnamed nearby utility. GNSS networks are also vulnerable to “natural” jamming by the arrival from the sun of coronal-mass ejections of electrically charged particles. Most often, though, jamming is deliberate.

Local problems can be caused by personal privacy jammers (PPJs). These are devices—widely available for sale even though generally illegal to use—which scramble GPS signals to stop vehicles being tracked by nosy employers or suspicious spouses. Thieves also find them useful. They are, for example, involved in 85% of vehicle thefts in Mexico.

Further up the commercially available scale are wide-area jammers. These devices, which are about the size of suitcases, do have legitimate quasi-civilian uses, such as protecting potential targets, public or private, from attack by GNSS-guided drones or missiles. But misused, whether deliberately or accidentally, they can disrupt GNSS across an area the size of a city. In this context it is notable that the northern Black Sea, where many Russian bigwigs, supposedly including the country’s president, Vladimir Putin, have country estates, is a hot-spot for GNSS outages that affect shipping in the area.

At the high end of GNSS disruptors are military systems, which can muddle signals for hundreds of kilometres around. Collateral effects from these are a growing problem. A paper published in March by Eurocontrol, an air-traffic-control body based in Brussels, noted a “massive rise” in GNSS interference reported by airline pilots. In 2019 the number of recorded incidents reached 3,564—nearly 22 times more than had been noted two years previously. Most hotspots were near war zones. But long-range jammers are also used deliberately for low-level “asymmetric” warfare. South Korea’s capital, Seoul, for instance, often experiences GNSS outages for which the only plausible explanation is jamming from North Korea, the border with which is only about 40km away.

All this jamming, both actual and conceivable, together with the more subtle problem of spoofing, in which bogus GNSS signals are generated to confuse navigation systems, has led to a search for alternative, more robust means of geolocation and time-stamping. In America that search has been reinforced by the National Defence Authorisation Act, which became law on January 1st. This obliges the country’s armed forces to generate “resilient and survivable” positioning and timing capabilities by 2023.

One approach to doing so is to upgrade the satellites themselves. America’s air force, for example, has begun launching a generation of new “GPS III” satellites built by Lockheed Martin, a defence giant. The first of these began transmissions in January. GPS III offers somewhat stronger signals than its predecessor. But its main advantage is an encryption system, the details of which remain classified, that makes those signals more resistant to jamming. Both of these features will help military users. They will, however, be of less use to civilians, who will not be able to benefit from the encryption.

Signalling for help

Private enterprise is, however, coming to the rescue of those who are willing to pay for reliable geolocation and time stamping by pressing alternative satellite networks into service. Satelles, a firm in Virginia, is using Iridium, a constellation of 66 satellites orbiting at an altitude of just 800km, to re-broadcast encrypted time data sent from a network of high-precision clocks on the ground, together with data about the satellites’ locations (thus mimicking the functions of a GNSS network), to clients including telecommunication firms, data centres, stock exchanges and banks.

The timekeeping and positioning data offered by Satelles’ system are, respectively, a little, and notably less, precise than GNSS. But because Iridium satellites orbit at a mere 25th of the altitude of GNSS constellations, the signals from them are more than 1,000 times stronger, thus shortening jammers’ effective ranges. Satelles’ clients are concentrated in America. That, says Michael O’Connor, the firm’s boss, is where the realisation, “oh shoot, we need backups”, has taken greatest hold. He says, though, that if Satelles had clients in Seoul, they would continue to receive signals during North Korea’s periodic jamming of the city.

Spoofing can be made harder, too. A Belgian firm called Septentrio is designing anti-spoofing antennae that can distinguish signals which have come from the sky, and thus carry tiny distortions imposed by the ionosphere, from cleaner ones generated nearby on the ground as spoofs. Septentrio’s wizardry relies on hardware in the form of a complex array of conductors and insulators inside the antennae. But software can do the job as well. America’s Department of Homeland Security recently released a set of algorithms intended to help signal engineers develop anti-spoofing programs, and Galileo’s masters are testing, with help from Septentrio, what is intended to become a publicly available anti-spoofing encryption service called Open Service Navigation Message Authentication, or OSNMA.

The securest approach of all, though, is surprisingly old-fashioned. It is to back GNSS up with systems on the ground. And the development of cheap, reliable atomic clocks makes this increasingly possible.

Light the beacons

Orolia, a French firm that makes such clocks for satellites, reports high demand for a line of ground-based versions called miniaturised rubidium oscillators, which went on sale last June. These have half the volume of a pack of cigarettes, so are widely deployable. According to Thierry Delhomme, Orolia’s European general manager, they typically drift less than a microsecond per day. That is not bad for a unit sold for a few thousand dollars (as opposed to the $1.5m cost of the best clocks used in satellites), and would certainly tide a user over a temporary outage. But anything more than a day or two and even one of these new devices would get sufficiently out of synch with reality to cause trouble.

OPNT, a Dutch company, has another idea. This is to deliver the precise time as signal pulses sent through fibre-optic cables, rather than by satellite. That could be done using existing fibre-optic networks, by isolating one strand within a cable and dedicating it to the purpose.

To turn the clock back properly, however, some people are trying to revive the idea of land-based navigation beacons similar to the Loran (long-range navigation) towers used by the American and British navies during the second world war. According to George Shaw of the General Lighthouse Authorities of the UK and Ireland, which runs the British Isles’ coastal-navigation system, several countries are now constructing enhanced “eLoran” networks. These include China, Iran, Russia, Saudi Arabia and South Korea. And, on a smaller scale, private enterprise is interested, too. NextNav, a firm based in Silicon Valley, is building in San Francisco a network of about 100 small beacons that will broadcast timing and position signals around the city. This network’s density, and the fact that it can draw power from the grid rather than relying, as GNSS satellites do, on solar panels, means that the signals are roughly 100,000 times stronger than those from such a satellite—and thus hard to jam or spoof.■

A version of this article was published online on May 5th, 2021