GPS Interference at Le Mans: What Motorsport Teams – and Device Developers Everywhere – Need to Know

I’ve had the privilege of working closely alongside Aston Martin Racing during this year’s FIA World Endurance Championship. My time in the paddock at Silverstone and Le Mans – and in the Prodrive lab – has yielded some important findings, both for the team, and for GPS engineers in general.

AMR car during race

At Spirent, we started working with Aston Martin Racing (AMR) during the 2015 season. Like many of the teams, AMR had experienced intermittent issues with the FIA’s official GPS telemetry device – a key source of information about each car’s speed and track position, as well as a record used for stewards’ adjudications during and after the race.

Despite extensive experiments with antenna positioning, the team could not trace the source of the problem – so I joined them at pre-season testing and races at Silverstone at the legendary 24 Hours of Le Mans, to confirm whether track conditions or the race car itself might be interfering with the signal.

I also visited the team headquarters in Banbury, Oxfordshire, to characterise the performance of the GPS logging device itself.

Our findings had some important implications – not just for World Endurance Championship participants, but for any engineer developing a GPS-enabled device.

Lesson 1: understanding receiver performance is essential

The GPS logging unit is essential as a source of race telemetry for analysis and a record of vehicles position and speed on the track. But there is only one unit sanctioned by the FIA and therefore no need for comparison, and as a result the team had never evaluated its capabilities.

We therefore used a Spirent GSS6700 simulator and SimGen software to put the unit through its paces. We generated a number static, acceleration and deceleration scenarios, collected the device’s output via CAN bus, and compared them with the known, “truth” data.

As a result, we gathered important information on the unit’s accuracy and resolution – which is essential in using the information for racing decisions, and also understanding the margin for error when observing an imposed speed limit, or braking hard before pit lane entry.

Lesson 2: simple fixes can have unintended consequences

Spirent’s GSS6425 record-and-playback unit was easily portable enough to sit in the car during testing, and record the signal environment at speed, enabling us to recreate complete test laps in absolute fidelity in the lab. Among other benefits, this enabled us to rule out the possibility of the car itself being a source of GPS interference.

To check for interference at the track, we also took a Spirent GSS100D Detector to the test event, and later to races at both Silverstone and Le Mans, where we monitored the RF spectrum throughout the race meeting.

At both races, we found general, narrow-band interference, which grew markedly when team radios were being used most intensively. This was most likely caused by a small number of faulty handsets, but would not usually pose a problem to GPS reception.

But during the Friday at Le Mans – while there were no cars running – we also found interference that appeared to be from a GPS repeater signal. This may have been used to test cars’ systems in the garage, with no view of the sky, but could potentially have interfered with other cars’ systems – causing them to report a stationary position (the position of the repeater), even while they were actually moving.

Lesson 3: testing jamming resistance is for everybody

But perhaps the most unexpected finding came during Le Mans, just after 3 o’clock in the morning. The GSS100D detected the signature interference pattern of a purpose-built GPS sweep jammer, of the kind that is commercially available online and powered by a vehicle’s power socket.

The event lasted for only 12 seconds, and was almost certainly unintentional. Inexpensive sweep jammers are most frequently used by delivery drivers, to disrupt employers’ fleet tracking systems so it seems likely that a courier was making an urgent delivery to one of the teams.

It’s a small incident; the implications, however, are significant. As motorsport – like so many other industries – begins to rely more heavily on satellite positioning data, intentional and accidental jamming has the potential to fundamentally disrupt key outcomes, whether that be race position or a consumer device’s ability to tell the time and location.

And with fleet tracking becoming more common, jammers becoming cheaper, and the effective range often far greater than advertised (leading to events like this one), the issue is only going to become more common – quite aside from the potential for deliberate foul play.

Resistance to jamming is rapidly becoming a key performance characteristic for any GPS-enabled device – and it should be considered as such during development and testing.

Lab-based simulation of realistic interference scenarios should be used to demonstrate how well a design works in the face of interference: whether it can keep a signal, how it responds during a jamming event, and how quickly it can produce an accurate time and/or position afterwards.

And that’s true whether you’re testing a hand-built racing car, or a SatNav for a family saloon.

If you’d like more information on our work with Aston Martin Racing this season, you’ll find a case study here.

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