Similarly, a GNSS signal must travel a long and challenging path (21,000 km) through the atmosphere to get to a receiver antenna in order for an operator to determine a perfect distance measurement and an accurate time space scenario. GPS signals are very weak compared to the other frequencies transmitting in a given spectrum—and therefore the signal gets pushed around, altered or blocked along the way.
Signal strength and clarity is the challenge that all GPS receivers face, especially in today’s current environment where everyone is trying to take advantage of the RF spectrum. With older generations of receivers, operators do not have easy or instant visibility into data about signal interferences that can cause cycle slips, lock breaks or lock loss. The only clue that the proper performance specifications are not met would be a loss of satellites or position. The where and why of interference—intentional or not—typically requires expensive and cumbersome spectrum analysis equipment, the logging of a lot of interference raw data, and/or extensive post-processing.
That’s not the case anymore. The next generation OEM7® GNSS receiver card, launched in September 2016, is the first of its kind to incorporate advanced interference mitigation technology. The Interference Toolkit (ITK), one of the most advanced and innovative features available on the NovAtel® OEM7 GNSS receiver platform, is a game changer—particularly with today’s crowded frequency spectrums where potential for interference is high.
The ITK provides any operator on-demand actionable intelligence by measuring the radio frequency spectrum levels, making it much easier and faster to visualize, monitor, quantify and even mitigate interference sources that commonly impact receiver performance.
Within any GNSS receiver, an operator is looking to balance available signals with potential interference by selectively applying filtering protection.
ITK incorporates a spectrum analyzer specific for GNSS frequencies that auto detects in-band and out-of-band interference and allows users to see the RF spectrum levels using NovAtel Connect™ 2.0, a GUI that lets operators communicate and configure receivers via serial port, USB or Ethernet connection.
The analyzer displays a 100 MHz wide frequency spectrum—far more bandwidth than a typical receiver would have or need, which is 25 MHz.
Patrick Marvin Casiano, Applied Technology Group Lead at NovAtel, explains, “The Interference Toolkit allows operators to identify distinct features in the interspectrum displays and then adjust the spectrum for easier viewing. It’s a bigger picture view because of the wider bandwidth. For instance, we can detect if there is a spike in the frequency satellite-based communication systems such as Globalstar or Inmarsat, which might be affecting performance. Other onboard frequency spectrum analyzers don’t offer that; they only offer the bandwidth that the receiver needs.”
The frequency spectrum map produced by the analyzer clearly shows the shape of the spectrum (in the data plot) and the changes due to interference.
Before ITK, Casiano recalls one customer who was in the validation phase of the development of an integrated smart antenna receiver, but he kept getting interference on one of the bands. After considerable time and money trying to determine the interference, the team called in an RF consultant to determine the source of the interference.
“It turns out, the PCs that were used for performance validation were the cause of the performance deficiency. If the development team had the Interference Toolkit, they would have pinpointed the 1.6 GHz signal interference frequency, which would have led them to DDR memory on the laptop that was sitting beside the antenna.”
Mitigation of the interference, in this case, was to simply move the PCs further away from the receiver.
Anyone with a smart phone is familiar with High Dynamic Range (HDR) displays to improve photos. HDR mode, in the GNSS world, is used to amplify the bandwidth details that an operator wants to track and suppresses everything else.
Casiano further explains, “If a transmitting modem is affecting a wide part of the spectrum, the HDR feature will float a selected frequency to a higher level and thereby suppress the unwanted frequencies that are happening in the same spectrum. We’re essentially amplifying the optimal GNSS signal above all else, making sure it has the best chance of making it into the receiver.”
The HDR capability is best used during the integration phase of a system development. The developer would likely incorporate the feature and budget the necessary power in the receiver to give the operator flexibility.
“With HDR, we are basically adding contrast to good and bad signals,” says Casiano. “Bolstering the compromised signals gives the operator a better chance of working through an interference event.”
Developers configure the front end mode for each RF path to the firmware default or specify the HDR mitigation method. The combination of the ITK’s notch filter and HDR mode can also help suppress a continuous wave interferer.
The Interference Toolkit and the HDR features are just two of the capabilities built into the OEM7 receivers. Current development is focused on the holy grail of GNSS interference—automated interference detection, management and mitigation.
Casiano concludes, “Right now, we can detect and manually mitigate interference, this sets the stage towards the automation of interference mitigation without user intervention.”