Up on the Rooftop

Up on the Rooftop

“I met Peter Soar at some conference or other and we got to talking,” Curry recalls. Soar is NovAtel’s Business Development Manager, Military & Defense, based in the UK. “And then we met again, more than once, on similar occasions.”
And, it would seem, they found common ground. “I knew Chronos Technology were specialists in Position, Navigation and Timing [PNT],” Soar says, “with an emphasis on timing, mainly based on GNSS and eLoran. As such they were also experts in mitigation against GNSS interference and timing.”

Now aware of Chronos’ activities on London’s rooftops, Soar decided to join the show.

“Chronos had been in contact with NovAtel’s UK Dealer, Forsberg Services Limited,” Soar says. “Understanding their desire to investigate further the interference they were seeing, we agreed to provide a GAJT to Chronos, on loan, via Forsberg.”

How It Works

NovAtel’s website says, “GAJT, pronounced ‘gadget,’ is the first single-unit GPS anti-jam antenna appropriate for use with military land vehicles, networks, and timing infrastructure.”

A high-powered gadget indeed, specifically intended for military use, but its particular characteristics make it useful to almost anyone with a jamming problem. GAJT is an externally mounted unit, requiring only power and a single RF cable. No additional electronics are necessary.

The GAJT product line is a result of close cooperation between Defense Research and Development Canada (DRDC) and NovAtel, combining DRDC’s Electronic Warfare expertise with NovAtel’s GNSS signal processing and product development heritage.

“Throughout the development of GAJT,” Soar says, “we have sought the optimal mixture of performance, size, and cost. The latest development, our ‘Space Frequency Adaptive Processing’ (SFAP) algorithm, gives a step-change in performance inside the same form factors. Also, by enabling analysis actions like ‘freezing the weights’ of the system to allow for detailed evaluation, NovAtel has enabled user organisations to perform their own tests to verify our claims.”

GAJT’s proprietary technology is a null-forming system that protects the GPS receiver, simultaneously on L1 and L2 frequencies. It passes through the good GPS signals and ignores the jammers by steering nulls in the antenna gain pattern towards those jammers. This means that GAJT only passes good GPS signals to the receiver which is once again clear to operate normally.”

“The first GAJTs used Space Time Adaptive Processing (STAP), in common with other anti-jam systems on the market,” Soar says, “but our GAJTs now use the improved SFAP algorithm: digital processing includes a Fast Fourier Transform (FFT) process to separate the incoming signal into a set number of equal frequency bins, followed by parallel space/time calculations in the form of a Least Mean Square (LMS) power minimization algorithm on each of those frequency bins, and finally an inverse FFT to reconstruct the frequency bands.”

The specific benefits of SFAP are:

• Narrow-band jammers are selectively nulled. SFAP nulls by both frequency and direction of arrival. Narrow-band interference is selectively nulled without attenuating other signals arriving from the same direction.

• Better performance versus wide-band jammers. Compared to STAP-only anti-jam systems, the SFAP algorithm is less sensitive to input filter phase variations and can accommodate edge-of-band group delay variations, allowing for improved performance in the presence of wide-band interferers.

• Step-change in performance: Coverage Improvement Factor (CIF). These changes together show a marked improvement in CIF, the method used to assess and compare the performance of Controlled Radiation Pattern Antennas (CRPAs) of various geometries and numbers of elements. GAJT can now excise jammer signals that are very close geometrically to the wanted GPS signal, giving unmatched anti-jam performance, Soar says.

The SFAP algorithm can also operate in modes at different sensitivities.

“The choice of mode allows for the trade-off between sensitivity in excising the jammer and fixed latency through the system,” Soar explains. “This is particularly helpful when selecting a GAJT configuration for use with helicopters, as the block mode can be adjusted to take into account the frequency of the helicopter’s rotors.”
Without that adjustment, he says, chopped reflections of jammer signals can disrupt the nulling.
Crucially, for Chronos, by selecting a mode with appropriately low latency, protection of timing subsystems can also be facilitated.