Minimizing the risk of chip-jammer interference for UAVs

Affordable, high-end drones coupled with easy-to-use mission-planning tools, created the perfect environment for drones to flourish.

No longer the preserve of specialists, applications using drones have ventured into survey, inspection and volume analysis.

The impact of drones is little short of revolutionary.

But, in the air, the stakes are higher. When things go wrong, the consequences are invariably much more serious than for a ground-based application. One of the biggest threats to drone safety is GNSS interference.

At the very least, disruptions to satellite signals can degrade position quality. When this occurs it causes fall-backs from high-precision RTK and PPP modes to less-precise modes. In the most extreme cases, interference can result in complete loss of signal tracking and positioning.

 

Self interference

Other components installed on a UAV is often a significant source of interference. The restricted space often means that the GNSS antenna is in close proximity to other electrical and electronic systems.

Figure 1 shows what happened to the GPS L1-band spectrum when a GoPro camera was installed on a quadcopter close to the GNSS antenna without sufficient shielding. The three peaks are exactly 24 MHz apart. This points to their being harmonics of a 24 MHz signal: the typical frequency for a MMC/SD logging interface.

An AsteRx4 receiver, which includes the AIM+ system, was selected for this setup. As well as mitigating the effects of interference, AIM+ includes a spectrum plot to view the RF input from the antenna in both time and frequency domains.

At the installation stage, the ability to view the RF spectrum is an invaluable tool for identifying the source of interference. Plus, it helps with determining the effectiveness of measures such as modifying the setup or adding shielding.

For the quadcopter installation in this example, the loss of RTK was readily diagnosed. The problem was solved by placing the camera in a shielded case. All this while the quadcopter was still in the workshop.

 

External sources of interference

GNSS receivers on-board UAVs can be particularly vulnerable to external sources of interference, be they intentional or not. In the sky, the signals from jammers can propagate over far longer distances than they would on land.

In the case of UAV inspections of wind turbines for example, many countries encourage the construction of windmills next to roads. However, this situation increases the chance of interference from in-car chirp jammers.

Though illegal, chirp devices are cheap and readily available on the internet. For example, an individual using a chirp jammer can drive around undetected by the GPS trackers on the vehicle. Car thieves can disable GPS anti-theft devices on stolen vehicles with chirp jammers.

 

External interference: the effect of a chirp jammer on a UAV flight

Although transmitting with a power of around 10 mW, chirp jammers are powerful enough to knock out GNSS signals in a radius of several hundred meters on land. In the air, unhindered by trees, building or other obstacles, these jamming signals have a far greater reach. Thus, the UAV is much more vulnerable to interference.

Figure 2 shows how a 10mW chirp jammer can knock out RTK positioning over more than 1 km in a high-end receiver.

Even a low-end consumer-grade L1 receiver, being less accurate and thus less sensitive, loses stand-alone positioning over several hundred meters.

With AIM+ activated, the AsteRx4 is able to maintain an RTK fix throughout the simulated flight. It also shows no degradation to its position variance.

 

Solving chip-jammer interference on UAV systems

A comprehensive approach puts interference considerations at the forefront of receiver design and incorporates it into every stage of signal processing. In the case of the AsteRx4 and AsteRx-m2, the antenna signal is immediately digitized after analogue filtering and automatically cleansed of interference using multiple adaptive filtering stages.

As each interfering signal has its own individual footprint, the ability to visualize the RF signal in both time and frequency domains allows drone users to identify sources of self-jamming and adapt their designs accordingly before the drone gets in the air.

When it is in the air, AIM+ is able to mitigate jamming from external sources: a set of configurable notch filters are complemented by an adaptive wide-band filter capable of rejecting more complex types of interference such as that from chirp jammers, frequency-hopping signals from DME/TACAN devices as well as high-powered Inmarsat transmitters.

 

You can shop Septentrio’s line of solutions at Unmanned Systems Source.