RoDAR the Equaliser

When jamming is detected, the RF signals from two antennas are passed to the null-steering weight calculation unit after down conversion and digitisation. See Figure 3.

Authentic signals are summed up non-constructively while the spoofing and jamming terms are combined constructively.

Then, the second antenna signal undergoes phase rotation and gain compensation and is removed from the first antenna signal. The resultant samples are adjusted and combined, are jammer and spoofer free, then passed on to the tracking and position, velocity and time (PVT) solution module. See Figure 4.

Figure 3. Dual-antenna based null steering. S1 denotes the signal arriving at Antenna 1. Antenna 2 observes the same signal with a phase offset delay




Figure 4. Dual-antenna process to obtain jamming-free signals.




Figure 5. Changing the antenna gain pattern (orange) to attenuate interference.

“Now, why is the spatial domain so important?” Gerein posed.

“When a jammer comes along, in the time domain you could do pulse-blanking and you could take out just the part

of the time when the bad guy was in there jamming. Frequency domain is, ‘Oh, I’ve got somebody interfering, I’m going to dial in my spectrum analyser, create a digital filter and I’m going to take out that frequency.’

“Spatial domain doesn’t care what’s happening in time or frequency. You can take anything that you could do in the time and frequency domains and do it spatially. It’s really the way forward in terms of anti-jam protection.”

“The only way to go against wideband noise is spatial processing: you stop that from coming in. The threats of wideband noise, by the laws of mathematics, can only be resisted with the use of spatial processing.”

To understand how spatial processing is applied for mitigation, see Figure 5. Using two antennas enables performing a weight calculation to determine there’s interference coming in. Using RoDAR, the receiver changes the antenna gain pattern in that direction.

A RoDAR user doesn’t have to worry about characterising the type of GNSS jamming encountered, whether it’s continuous wave, pulse or chirp jamming; someone relying on time or frequency domain signal processing would need to know the type of jamming before being able to mitigate against it. No matter what method the jammer throws at you, with RoDAR you have protection and mitigation to steer the interference right out of your system’s inputs.

Taking continuous action

RoDAR jamming detection and mitigation doesn’t require post-event data analysis to identify whether the signal was spoofed or jammed or not. RoDAR keeps an active lookout while tracking the signal, so users can go on doing their job.

“RoDAR will give you an indication, an alert that jamming is happening, but it just keeps on trucking,” Gerein said. “If it didn’t give me that alert, if that little red light didn’t come on, I wouldn’t even know because I would never lose my position. If you didn’t happen to be looking at the red light, you’d just be happy as can be because you’re still getting your solution.”

It’s almost a no-brainer solution. The user doesn’t have to worry about jamming. The mitigation is happening right there in the receiver.

“Now, a few things on the full disclosure side,” Gerein added. “With any anti-jam solution, RoDAR or GAJT, you name it, there may eventually come a time that you get so close to the jamming source that the interferer is going to get so loud that your solution will start to degrade. It’s like we tell customers, you can put on your noise-cancelling headphones and walk toward a jet airplane, but you’re eventually going to get so close that it’s going to damage your hearing.

“It’s the same thing with any interference. As you get closer to the jammer, your solution is going to start to degrade. But what’s your choice? You can keep your mission going on much longer, or carry out missions in some places where before you wouldn’t be able to at all, because now you have anti-jam protection.

“So, it is important for us to put on that red light and say, ‘Oh by the way, RoDAR’s active.’ Because as you get closer to the jammer, you might be able to say, ‘You know what, it is getting worse. I had better go see what the heck’s going on here. Maybe somebody is jamming me because they’re coming to shoot down my UAV, or make it crash. Let’s get that UAV home now because it’s a valuable asset.”

The importance of speed

“To be able to do the resiliency-building we’re doing, it has to be done at a really high speed,” Gerein said.

“And by high speed I mean megahertz, the centre speed where the OEM7 is doing signal processing. That’s where the magic happens. It’s unique to NovAtel because we built it into our MINOS7 application-specific integrated circuit (ASIC) for the OEM7 years ago. Now we’re bringing that to life.”

“We did that upfront. That’s what enables you to null-steer and do it really, really super-fast, with some big-boy mathematics.

“Now the markets are ready and the technology is ready for people to start saying, ‘Oh, I can do anti-jamming now because I don’t need to have the same [high] level of performance I’d have on a GAJT.’”

The GAJT portfolio is used worldwide because they work very reliably to protect against jamming and interference, no matter what the environment and no matter what GNSS receiver the user has.

GAJT antenna systems, comprised of controlled reception pattern antennas (CRPAs) and sophisticated electronics, use multiple antenna elements to control the amount of signal received from a particular direction. When GAJT senses interference from one direction, it turns down the antenna gain for that direction, similar to turning down the volume. This reduces the amount of interference received so true and accurate GNSS signals can be received from other directions. See Figure 1.

“What GAJT does,” Gerein explained, “is sniff the signal, detect interference and say the jamming is coming from that direction so I’m going to make the antenna less powerful in that direction.”

GAJT antenna systems can be used with any GNSS receiver, that is to say they are GNSS-agnostic. They can be installed with and perform anti-jamming for any GNSS receiver, however its weight at 7.5 kilos (16.5 lb) and measuring 29 x 12 cm (11.5 x 4.75 inches) typically dictates the kind of platform that can take advantage of GAJT. Some GAJT models are also restricted to military use, as they are export controlled.

RoDAR users won’t need to put the added SWaP and cost of additional hardware into their systems; that’s what makes it really powerful. And it is available only on OEM7.

RoDAR offers 30 dB of nominal protection. It is not subject to ITAR or Canada Controlled Goods regulation.

RoDAR recovery from jamming

Even when subjected to and eventually overcome by a very powerful and very close jammer, a RoDAR-protected receiver recovers more quickly from the incident. See Figure 6.

“Figures 6 and 7 show the introduction of two different types of interference, chirp and wide band jammers, and the effect on the positioning capability of a RoDAR enabled receiver (green lines) and a non-RoDAR OEM7 receiver (pink lines),” Kennedy said. In this test, only the L1 band is used, she said, both for jamming and positioning. The jammer power is increased until the RoDAR receiver loses positioning capability, and then decreased until the receivers both recover a position solution. With both types of jammers, RoDAR’s spatial mitigation approach is very effective, hanging onto a position much longer and recovering very quickly as the jammer fades.

In a wideband jamming situation, RoDAR can provide additional 27 dB protection versus a non-protected receiver, again recovering faster. See Figure 7.

Figure 6. RoDAR versus single antenna OEM7 with a chirp jammer. The RoDAR receiver recovers faster than the non-RoDAR solution.

Figure 7. RoDAR versus single-antenna OEM7 with a wideband jammer. The protected receiver recovers faster.

“With the introduction of a wideband jammer, whose power level is shown in the blue line,” commented Kennedy, “we again examine the reported position standard deviation in both receivers. And again, the RoDAR receiver maintains high quality positioning under a much higher interfering power level than the unprotected receiver and then recovers much more quicker, after being jammed out.”

A powerful solution

Once available, RoDAR firmware can be installed on NovAtel’s PwrPak7D dual-antenna receiver card through a firmware upgrade. This enables a powerful receiver-based anti-jam capability when using two antenna elements. The benefit for users is the smallest SWaP anti-jam solution available—and it’s widely available for commercial applications.

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