The Fragility Of The Satellite Signal

AAlmost everything said in public about satellite navigation concerns the finding of places, and almost nothing of consequence depends on it. The signal a receiver locks onto carries a position only incidentally; what it carries first is the time, kept by caesium and rubidium clocks aboard satellites some 20,000 kilometres overhead and broadcast at a power that arrives at the ground fainter than the light of a household bulb seen from the same distance. Power companies use that time to hold transformers in step, exchanges use it to put transactions in order, and mobile networks use it to hand a call from one mast to the next. A dependency of that kind is invisible while it works. It is also, and this is the point most often missed, not a flaw that better engineering of the satellites could remove. The weakness lies in the last few hundred kilometres of the journey, where a signal that has crossed the emptiness of space can be drowned by a transmitter costing less than a pair of shoes.

BElena Diarra, a signals engineer at the Nordvik Institute of Positioning, holds that the risk which deserves attention is the crude one. Between 2016 and 2022 her group monitored the navigation band at thirty-nine airports for a total of 4,100 hours and logged 2,780 separate interference events. Of these, 87 per cent were traced to what the trade calls personal privacy devices, cheap jammers plugged into the cigarette lighter by drivers who would rather their employers did not know where the lorry has stopped. The median event lasted six minutes and degraded reception out to roughly 1.8 kilometres. Diarra's argument is not that these gadgets are formidable in themselves, but that the accounting is wrong: a profession that lavishes its ingenuity on elaborate deception leaves the ordinary fault unattended, and an approach to a runway broken by a lorry on the perimeter road is not made safer by the fact that nobody intended it.

CThe claim yields a prediction, and Diarra has tested it. If the source is a device in a cab, interference should follow the roads rather than the runways, and it should thin out when the roads are empty. Both held. Seventy-one per cent of the events her team logged fell within 500 metres of a motorway, and the hourly rate between two and five in the morning was under a third of the daytime figure. She is nevertheless explicit about the boundary of the finding. The pattern was established at installations still fitted with single-frequency receivers of a design now being replaced, and she states that her conclusions are confined to that equipment. Ninety-eight of the monitoring stations involved were of that older type when the study began, and the handful of multi-frequency units already in service recorded too little to be analysed separately.

DTomás Reyes, a geodesist at the Cape Meredith Observatory, objects that a count of detected events measures the detector. Jamming announces itself, since a receiver starved of signal reports that it has lost lock; a well-made deception does the opposite, delivering a plausible position that no legacy monitor has any reason to question. Between 2019 and 2023 Reyes followed shipping through a single strait and found that 63 per cent of vessels passing on certain days reported themselves, for a quarter of an hour at a time, sitting on an identical circular track several kilometres inland, an artefact, he argues, of a transmitter ashore rather than of any fault at sea. He concedes two things. His sample ran to eleven vessels whose logs he was permitted to read, and the strength of the interfering signal had to be inferred from the behaviour of the receivers, because no instrument was in place to measure it directly.

EThe argument has consequences outside the journals. A directive adopted in 2021 obliges operators of critical infrastructure in several countries to hold a timing source that does not depend on satellites, and the ground-based systems dismantled in the 1990s are being rebuilt at public expense under the name eLoran; insurers have begun to ask shipping companies which of the two they carry. Reyes points out, awkwardly for his own case, that nothing in his work shows the deceptions he documents to have been aimed at the vessels that suffered them, since a transmitter shielding a coastline will mislead every receiver within range indifferently. Diarra replies that intention is precisely what a regulator cannot legislate for, and that the remedy must therefore be designed for accidents rather than for enemies. Both object to the habit of publishing single dramatic incidents in place of counts of what happens in ordinary weeks.

FWhat is emerging is less a victory than a division of labour. Where the receiver is moving quickly and the cost of losing it falls due at once, an aircraft on final approach or a lorry on a toll road, the crude fault dominates, and the answer is patient monitoring on the ground. Where the receiver stands still and what is drawn from it is the time rather than the place, deception is the greater danger, for an error of a microsecond is not visible to anyone watching a screen. The question nobody has answered is whether a rebuilt ground-based network would in practice be held to the standard its advocates promise, since no country has yet run its grid on the backup for a week. What can be said is that a service which is free at the point of use has merely moved its costs somewhere less easy to see.