Reliable navigation with interference-free GNSS signals

By Markus Irsigler and Sebastian Kehl-Waas

Interference-free GNSS signals are essential for more than just military vehicles and aircraft. Anti-jam systems usually suppress signals from interference sources by means of spatial filtering.

These solutions can likewise be used to protect satellite navigation signals for autonomous driving and flying against interference signals. To allow GNSS receivers to detect interference sources and suppress transmitted interference signals, they must be designed as multichannel systems.

This way the direction of the interference signal can be determined using phase-coherent signal processing of signals from multiple antennas, and the interference can be suppressed. Rohde & Schwarz offers a solution for the verification of interference immunity and interference suppression.

FIGURE 1a. The GNSS antenna in the example on the left has only one element, so its characteristic cannot be modified. A sufficiently strong interference signal can prevent the receiver from processing the GNSS signals, making satellite-based navigation impossible.

FIGURE 1b. In contrast to the individual antenna, the characteristic of the antenna array can be modified by combining and weighting the received signals. The interference signal is suppressed at its angle of arrival, and the GNSS signals can be received. A disadvantage is that GNSS signals from the same direction as the interference signal are also suppressed.

Multi-channel receivers can simultaneously process signals from multiple distributed antennas or from an antenna array. This is useful for determining the direction of incoming signals by means of signal analysis, and for adjusting the antenna pattern so that undesired signals are suppressed. For GNSS-based position determination, this means that signals from global navigation satellite systems (GNSS) can be strengthened and jamming or spoofing signals originating from the ground or the air can be suppressed. Up to now this technology has primarily been used for military applications, but in the future it can also make an important contribution to robust navigation for autonomous driving or flying. Typical interference sources in this regard are harmonics of transmitters in the vicinity, tactical air navigation (TACAN) signals, DME air navigation signals for civil aviation, and LTE signals. Another factor is the growing popularity of so-called personal privacy devices (PPD), which are GNSS jammers that radiate narrowband or broadband signals to disrupt GNSS localization. A new solution from Rohde & Schwarz enables comprehensive testing of the resistance of GNSS receivers to interference signals, if necessary in a realistic hardware-in-the-loop (HIL) environment.

Multi-Channel GNSS Receivers for Interference Suppression

GNSS receivers often use controlled reception pattern antennas (CRPA) to suppress undesired signals. These antennas consist of an antenna array and a signal processing unit. The connected antennas are generally arranged in a strict geometric pattern to achieve full coverage of all possible signal directions. The overall receive characteristic of the antenna array can be altered by suitable weighting of the signals from the individual antennas in the signal processing unit (Fig. 1). This way, interference signals can be specifically blanked out (nulling) or the required GNSS signals can be amplified at their angle of arrival (beamforming). A combination of these two methods is also possible. The antenna arrays typically consist of four to seven elements. The number of interference signals that can be simultaneously suppressed increases with the number of elements.

FIGURE 2a. A four-channel GNSS test system consisting of two R&S SMW200A vector signal generators and an R&S SMA100B analog signal generator for the LO signal (left). The vector network analyzer is used to calibrate the overall system at a user-selectable reference plane in terms of amplitude, phase and propagation time.

FIGURE 2b. A four-channel GNSS test system consisting of two R&S SMW200A vector signal generators and an R&S SMA100B analog signal generator for the LO signal (left). The vector network analyzer is used to calibrate the overall system at a user-selectable reference plane in terms of amplitude, phase and propagation time.

Test System Requirements

Rohde & Schwarz offers a test system for GNSS receivers that use CRPAs. First, it acts as a multichannel GNSS simulator that considers all aspects of a satellite navigation system. It must be able to generate the signals of all standard satellite navigation systems in all GNSS frequency bands, with attention to correct satellite orbits, signal propagation characteristics and realistic modeling of the dynamically changing receive environment. Configuration of the antenna array in terms of geometry and the receive characteristics of the individual antennas also must be included.

Simulating the Interference Signals

Second, the system can simultaneously generate jamming or spoofing signals in order to test the interference suppression functions of the device under test (DUT). A second, identical test system is necessary for freely definable configuration of interference sources with very high transmit power. Here the R&S Pulse Sequencer software assists in the definition of complex interference scenarios. The scenarios cover requirements such as long simulation times, moving interference sources and GNSS receivers, user-defined antenna patterns and antenna scans. In addition, the software calculates the correct amplitude, phase angle and propagation time of the signals as a function of signal frequency, antenna arrangement, and the positions of transmitters and receivers in three-dimensional space for each individual antenna element. Signal generation is handled by the R&S SMW200A high-end vector signal generator.

For the tests, the required GNSS signal as well as the unwanted interference signals must be generated for each antenna input of the GNSS receiver. In order to test a CRPA receiver with four antenna inputs, this means that four signal sources are needed to generate the GNSS signals and an additional four signal sources are needed to generate the interference signals. Fig. 2 shows a pair of test systems that can be used to generate coupled GNSS signals and interference signals for a four-channel CRPA receiver.

Calibration Against the DUT

In order to correctly simulate the directions of the satellite signals and the interference signals, the test systems must be calibrated at the RF interface to the DUT with regard to amplitude, phase and propagation time. This means that the amplitude, phase and propagation time differences between the individual RF paths, resulting for example from cables or RF components, must be compensated. The vector signal generators of each system are phase coherently linked using suitable synchronization. A high-end R&S SMA100B analog signal generator in each system provides the shared LO signal.

Using the R&S RF Ports alignment software, the complete system can be calibrated at any desired reference plane with regard to amplitude, phase and propagation time, so that the properties of the test system do not corrupt the simulated signal differences between the individual antennas. The required measurements are performed with a vector network analyzer.

It is not necessary to calibrate the two test systems relative to each other. For the simulation of realistic scenarios, it is sufficient to run the GNSS and interference source simulations at the same time, since in the real world there is usually no correlation between GNSS satellites and interference sources.

FIGURE 3. Aircraft with a multichannel radar warning system consisting of multiple receive channels, a central processing unit and a display.

Integration in an HIL Environment

The GNSS test system also can be embedded in a hardware-in-the-loop (HIL) environment. In this case a computer streams the motion profile of the GNSS receiver under test, with position, speed, acceleration and vehicle attitude, to the test system at a high data rate. The test system then generates the corresponding satellite navigation signal in real time. This requires very high update rates and low latencies.

Summary

Multichannel GNSS CRPA receivers considerably improve the navigation of ground vehicles and aircraft of all kinds. With the new Rohde & Schwarz test system, realistic multi-channel test signals can be generated for both GNSS simulation and interference simulation. For tests in an HIL environment, motion data also can be streamed to the GNSS test system.