Testing Controlled Reception Pattern Antenna (CRPA) Enabled Global Navigation Satellite System (GNSS) Receivers

Verifying products, designs, and algorithms used to mitigate global navigation satellite systems (GNSS) interference, jamming and spoofing

There are currently four Global Navigation Satellite Systems (GNSS) providing Position, Navigation and Timing (PNT) services globally. USA’s GPS, EU’s Galileo, China’s Beidou and Russia’s GLONASS satellite constellations collectively provide essential global services invaluable to everyday life. GNSS use in navigation might be top of mind for many, but its importance in position and timing are also not to be overlooked. Everything from banking to autonomous vehicles and most modern communications infrastructures require precise time coordination for network management, time synchronization, and location-based services. Because GNSS is a critical infrastructure, keeping it operational and secure is a significant challenge. The signals from the GNSS satellites are coded to provide the satellite’s identity, precise timing, and positioning data. In addition, the codes are cross correlated in the receiver, providing the processing gain necessary for robust and reliable PNT services. Because the GNSS system is based mostly on satellites in Medium Earth Orbit (MEO), the RF signals they emit are attenuated to extremely low power levels by the time they reach GNSS receivers on or near the Earth. While GNSS is designed to provide robust PNT services, receivers can still be interfered with both intentionally and unintentionally. Unfortunately, disruptions of PNT services are happening more frequently, with ever-increasing impact on everyday life.

Many GNSS receivers are designed with a single omni-directional antenna, which allows them to have a horizon-to-horizon view of space. This increases PNT accuracy by allowing the receiver to access the maximum number of satellites in view. The potential disadvantage is that unwanted signals are also picked up by the omni-directional antenna. These interfering signals can overpower a GNSS receiver to the point where the RF signals originating from the navigation satellites in space can no longer be recovered. These types of interfering signals are typically called jammers. Jammers are a general categorization and can be almost any RF signal from a sinewave to wideband noise and any modulated signal in-between that interferes with the normal functioning of the GNSS receiver. On the other hand, signals classified as GNSS spoofers are unique because unlike a jammer, they do not need to overpower the receiver to create erroneous results. In fact, spoofers are designed to be acquired by the receiver and processed as “normal” GNSS signals. They can be low power RF signals closely resembling the actual signals from space, which makes them difficult to detect. When spoofers go undetected it can lead to catastrophic impacts beyond simply losing PNT as can happen with a jammer. One method utilized to counter interference is to employ controlled reception pattern antennas (CRPA) and receivers. CRPA enabled receivers utilize multiple antennae, typically four to seven, sometimes more to allow the receiver or antenna to do additional processing on the incoming GNSS signals, steering reception nulls in the direction of the unwanted signals. This extra processing can effectively apply a directed attenuation towards the unwanted jammers and spoofers. The time and phase of arrival of the satellite signals arriving at each antenna along with advanced signal processing helps differentiate between genuine GNSS signals and non-genuine signals.

To evaluate GNSS receivers in the lab, developers, engineers, and technicians utilize a real-time GNSS emulator. A GNSS emulator could be considered a very specialized RF signal generator, but instead of creating RF signals from signal (IQ) data stored in an arbitrary waveform memory, GNSS emulators utilize real-time processing to emulate all the proper modulation, signal coding and perturbations of the signals as they would be received from each (in-view) satellite in the GNSS constellation given a particular GNSS receivers position, orientation and motion. Or more specifically stated, the antenna’s position and orientation on the GNSS receiver. This is an important distinction as we move to testing multi-antenna configurations utilized in CRPA systems. Effectively the same testing approach is utilized (generation of the realistic, real-time signals) when testing CPRA receivers. But in the case of CRPA receivers the GNSS signals are generated as they would be received by each and every individual antenna in their exact location in the CRPA system.

The challenge in creating realistic real-time emulation for CRPA systems is that each antenna port must be presented with the navigation satellite signals, including the appropriate coding and propagation permutations as that specific antenna would receive from the actual GNSS constellation. The same constraints are necessary for interference signals. Because accurate time and phase at each antenna position is essential for the receiver to properly calculate where to steer nulls, even small errors in the phases between the multiple signals being created can lead to inaccurate development, testing and ultimately errors in the operation of the anti-jam, anti-spoofing capabilities of the CRPA system. Therefore, the emulation systems phase accuracy and stability over time is essential to comprehensive testing of CRPA systems.

Testing CRPA-enabled GNSS receivers requires real-time baseband GNSS emulation and the ability to present time-synchronized, phase-coherent signals to the DUT. Keysight has taken a unique approach to this challenge, providing a test solution in a close collaboration with Syntony, a leading PNT solutions company. Through this partnership, Syntony’s powerful, scalable computational engine is coupled with Keysight’s leading multi-channel RF signal generator to create one of the most compact and capable CRPA test systems in the industry. The Syntony Constellator ™ (Keysight Model R4454A) simulates all GNSS constellations, ionosphere, and troposphere models, required attenuation, movement and Doppler shift models with changing jamming and spoofing scenarios. The resultant composite signaling environment is streamed from the R4454A Constellator optically as digital IQ data into the Keysight M9484C VXG-C vector signal generator, which upconverts them to the multiple CRPA antenna feeds at GNSS RF frequencies. Each M9484C supports up to four time-aligned, phase-coherent RF outputs in a single chassis, using direct digital synthesis architecture. Users can synchronize multiple Syntony-Keysight chasses to support any channel-count needed for a DUT, enabling testing of CRPA devices with seven or even more antennae.

This unique combination of Syntony’s GNSS computational engine and Keysight’s industry leading signal generator provides instrument-grade stability and ease of setup. Users can be confident of their testing results due to the traceable specifications and performance associated with the M9484C.