GNSS Constellations Monitoring Using a Phased Array Antenna System ^†

Abstract

This study presents a novel approach to enhance GNSS (Global Navigation Satellite System) signal reception using an eight-port beamformer tailored for satellite navigation applications. The beamformer architecture integrates key stages including filtering, amplification, phase shifting, and signal combination to optimize signal reception. For demonstration, a GNSS signal generator simulates a four-element phased array antenna, emitting GPS L1 constellation signals across four outputs connected to the beamformer ports. By manipulating phase settings on each port, the beamformer enables beam steering, showcasing improved signal detection. The results highlight the efficacy of the custom beamformer in enhancing GNSS signal reception and suggest promising implications for satellite signal tracking and navigation. This research contributes valuable insights into advanced antenna design and signal processing strategies for optimizing GNSS performance.

1. Introduction

Global Navigation Satellite Systems (GNSSs) play a crucial role in modern navigation and timing applications worldwide. However, ensuring their reliability and security requires advanced monitoring systems capable of robust signal reception and tracking. This paper introduces a novel approach using a phased array antenna system for GNSS reception, aiming to achieve superior performance compared to traditional antennas.

The main objective of this research is to develop and evaluate a sophisticated GNSS antenna system using phased array technology [1]. Unlike traditional antennas, phased arrays offer higher gain and directional capabilities, allowing precise pointing towards specific locations of interest. Additionally, the beamforming capability of phased arrays can help to reduce interference by creating nulls for interference mitigation [2].

The final proposed antenna system features a modular 64-element dual-frequency (L1/L5) array, providing flexibility in configuration and adaptive beam shaping tailored to multiple GNSS signals. It integrates advanced filtering, amplification, and phase-shifting stages to improve signal integrity and mitigate interference. By dynamically adjusting antenna element phases, the system optimizes beam direction and shape, enhancing reception performance in challenging environments.

The significance of this work lies in its potential to advance GNSS monitoring capabilities, making them more resilient to interference and multipath effects. Ultimately, this research contributes to the ongoing evolution of robust GNSS solutions for critical applications.

This paper proceeds with an overview of phased array principles and the challenges in GNSS signal reception, followed by detailed descriptions of the custom beamforming antenna system design and implementation. Experimental setups and methodology, including Spirent GNSS signal simulations, are discussed, followed by result evaluations and conclusions with future research directions.

2. Materials and Methods

2.1. Beamforming Antennas

A beamforming antenna is an antenna capable of radiating energy only in a chosen direction. This is especially useful in applications where the locations of two devices communicating are known. This is a way to increase the efficiency of the antenna by having a smart usage of the radiofrequency space [3].

It is also a nice way to enhance the gain of the antenna for applications where long range is needed, for example, satellite-to-earth communications. But in that scenario, except for geostationary orbits, satellites are moving and, thus, would require the beam to move to track the satellites.

Today, the most used technique is to use parabolic antennas that are steered using motors. While being very functional, there are other ways of undertaking beamforming [4], including a lens antenna system or, in our case, a phased array antenna system. The advantages are multiple: no mechanical parts, which means less maintenance is required and, thus, more uptime and faster beam steering; a phased array can steer its beam really fast by using its electronics circuits instead of motors. Lower volume for the antenna part: a phased array can be designed to be very low profile. Reconfigurability and multi-beam operation: depending on the design, a phased array antenna can be used to operate multiple beams at the same time.

While other studies have implemented beamforming antenna arrays for a GNSS application [5,6,7], it is still not common to do so with analogue phase shifting on a significant number of elements.

2.2. Phased Array Antennas

A phased array antenna consists of multiple antenna elements that can be arranged in a linear or planar array generally. By controlling the phase and amplitude of the signal at each element, the antenna is able to steer the main lobe (the beam) of the radiation pattern in the desired direction thanks to constructive and destructive interference, as shown in Figure 1.

The radiation pattern of a linear phased array is determined by the array factor, $\mathrm{A}\mathrm{F}\left(\mathsf{\theta }\right)$ [9,10], which describes the combined effect of phase and amplitude variations across the antenna elements. For a Uniform Linear Array (ULA) with N elements spaced at $d$ meters, the array factor is given by

$$AF\left(\mathsf{\theta }\right)={\displaystyle \sum _{n=1}^N}{w}_n{e}^{j\left(n-1\right)(\mathrm{k}d\cos \left(\mathsf{\theta }\right)+\mathsf{\beta })}$$

(1)

where the following is true:

• ${\mathrm{w}}_{\mathrm{n}}$ is the complex weighting coefficient for the ${\mathrm{n}}^{\mathrm{th}}$ element;
• $\mathsf{\beta }={\displaystyle \frac{2\mathsf{\pi }}{\mathsf{\lambda }}}$ is the phase constant $\mathsf{\lambda }$ is the wavelength;
• $\mathsf{\theta }$ is the angle relative to the array boresight.

In the firmware, the required phase shift for each element to produce the desired pointing angle for a linear array [11] can be described as follows:

$$\mathsf{\Delta }\mathsf{\phi }=\left({\displaystyle \frac{2\mathsf{\pi }}{\mathsf{\lambda }}}\right)d\sin \left(\mathsf{\theta }\right)$$

(2)

where the following is true:

• $\mathsf{\Delta }\mathsf{\phi }$ is the phase shift between two successive elements;
• d is the distance between the radiating elements;
• $\theta$ is the beam steering angle, relative to the array boresight;
• $\lambda$ is the wavelength at the operating frequency.

A planar array is a 2-dimensional array where the antennas (radiating elements) are distributed on a grid shape. The array factor of such an array is then described as

$$\mathrm{A}\mathrm{F}\left(\mathsf{\theta },\mathsf{\varphi }\right)=\sum _{\mathrm{m}=1}^{\mathrm{M}}\sum _{\mathrm{n}=1}^{\mathrm{N}}{\mathrm{w}}_{\mathrm{m}\mathrm{n}}{\mathrm{e}}^{\mathrm{j}\left(\left(\mathrm{m}-1\right)(\mathrm{k}\mathrm{d}\sin \left(\mathsf{\theta }\right)\cos \left(\mathsf{\theta }\right)+{\mathsf{\beta }}_{\mathrm{x}})+(\left(\mathrm{n}-1\right)(\mathrm{k}\mathrm{d}\sin \left(\mathsf{\theta }\right)\sin \left(\mathsf{\varphi }\right)+{\mathsf{\beta }}_{\mathrm{y}}\left)\right)\right)}$$

(3)

where the following is true:

• M is the number of rows in the array;
• N is the number of columns in the array;
• $w_{mn}$ is the complex weighting coefficient for the ${(m,n)}^{th}$ element;
• ${\beta }_x{=\beta }_y={\displaystyle \frac{2\pi }{\lambda }}$ is the phase constant ($\lambda$ is the wavelength);
• d is the spacing between adjacent elements in both the x and y directions;
• $\theta$ is the elevation angle relative to the array boresight;
• $\varphi$ is the azimuth angle relative to the array boresight.

In this expression, ${\mathrm{e}}^{\mathrm{j}\left(\left(\mathrm{m}-1\right){\mathsf{\beta }}_{\mathrm{x}}\mathrm{d}\sin \left(\mathsf{\theta }\right)+\left(\mathrm{n}-1\right){\mathsf{\beta }}_{\mathrm{y}}\mathrm{d}\sin \left(\mathsf{\varphi }\right)\right)}$ represents the phase shift applied to the ${(m,n)}^{th}$ element of the array based on the desired angle of arrival $\left(\theta ,\varphi \right)$ of the signal.

The array factor for a 2D planar array allows for steering the main beam in both the azimuth and elevation planes by appropriately adjusting the phase shifts across the rows and columns of the array. This capability enables directional control of the antenna radiation pattern to track and receive signals from specific angles in space.

In the firmware, the equation used to calculate the phase shift needed in a planar array is then

$$\mathsf{\Delta }\mathsf{\phi }=\left({\displaystyle \frac{2\mathsf{\pi }}{\mathsf{\lambda }}}\right)\left(x_m\sin \left(\mathsf{\theta }\right)\cos \left(\mathsf{\varphi }\right)+y_m\sin \left(\mathsf{\theta }\right)\sin \left(\mathsf{\varphi }\right)\right)$$

(4)

where the following is true:

• $\mathsf{\Delta }\phi$ is the phase shift for the specified element;
• $x_m$ and $y_m$ are the respective positions of the antenna element in x and y;
• $\theta$ and $\varphi$ are the desired elevation and azimuth angles of the beam direction in radian;
• $\lambda$ is the wavelength at the operating frequency.

2.3. Design of the Antenna System

The antenna array was composed of multiple antenna elements. For this project, we chose to use ceramic patch antennas which are dual-band (L1 and L5) and Right-Hand Circular Polarization (RHCP). These antennas were off-the-shelf components and were placed on a custom PCB which accommodated 8 antenna elements (2 columns, 4 rows). The outputs of each antenna were connected to a channel of the beamformer.

The pitch between 2 consecutive antenna was 0.1 m, which is a compromise between the half wavelength of the L1 band (1575 MHz) and used to limit the coupling between the elements.

To validate the circuit, first, a single antenna was measured in an anechoic chamber, then measured again when mounted on the 8-element PCB. Measurement results for the single antenna showed gains of 3.5 dB and 2.2 dB at 1575 MHz and 1176 MHz, respectively, creating a clean RHCP radiation pattern with axial ratios greater than 20 dB for both frequencies.

The beamformer was made of several stages including filtration, amplification, phase-shifting, and combination. All the components’ operating frequencies were L1 (1575 MHz) and L5 (1176 MHz). This is shown in Figure 2 in a single beam of a 4-element configuration.

Each channel was independent until the combiner, which combined 4 or 8 channels into 1 output depending on the hardware configuration that was chosen.

The filtering and amplification stages used standard, off-the-shelf components, which were GNSS SAW filters and GNSS LNAs. The phase shifter used was the PE44820 from Peregrin Semiconductor. The frequency range of this IC could be extended to work with signals down to 1.1 GHz, which were measured and validated. Then, the combiners were 2-stage (4:1) and 3-stage (8:1) Wilkinson dividers [12,13] designed on a standard FR-4 PCB substrate with CST Studio.

In our case, the Wilkison divider was used as a combiner since this component could be used in both configurations.

All the stages were made of multiple PCBs that were stacked to create the complete system, with each of them being 8 independent channels. The different stages were connected using SMA connectors to make the system completely modular.

2.4. Software Implementation

To control the beamformer, a custom controller was designed to provide power and control to the phase shifters with a standard microcontroller. The coding platform used was the Arduino framework with a command line interface to send commands to the beamformer.

Multiple functions were coded using the equations seen previously in Section 2.2 to implement linear and planar beamforming. The phase of each element was calculated using input variables such as the frequency of interest, the inter-element spacing and the wanted elevation and azimuth angles. A Python API was also developed to interact with the firmware in an easy to program way.

3. Results

3.1. GNSS Signal Generator

For our experiments, we decided to use Spirent GSS9000, which is a multi-port GNSS signal generator [14] that can be seen in Figure 3. Spirent GSS9000 is a high-performance GNSS simulator designed for testing and validating GNSS receivers and systems under controlled laboratory conditions. This device can be configured to simulate a GNSS constellation and output the signal on a defined antenna setup.

In our case, we configured Spirent to simulate a constellation of GPS satellites on four antenna elements. In the configuration, we lowered the absolute power level of all the signals to simulate a scenario close to reality where the GNSS signal is weak when arriving on the antennas. The four antenna elements were distributed in a square fashion, centered at the origin. The space between two consecutive elements was 0.1 m to replicate our antenna setup.

The positions (x, y) of the antenna elements are then the following:

• A1 (−0.05, 0.05);
• A2 (0.05, 0.05);
• A3 (−0.05, −0.05);
• A4 (0.05, −0.05).

3.2. Measurements

The objective of this measurement campaign is to create a beam and steer it towards simulated orbiting GPS satellites in order to compare it with a classical antenna.

For this purpose, four ports of the Spirent GSS9000 are connected to four ports of the beamformer, which are combined into one output. The low number of ports selected compared to the capability of the beamformer is due to hardware limitation at the time of measurement. The combined output is then connected to Septentrio PolaRx 5, a high-performance professional GNSS receiver.

The outputted data are the Carrier over Noise ratio (C/N0) for each satellite, measured in dB-Hz. In certain configurations, not all of the satellites are received; in that case, the measurement is replaced with null values.

First, a reference measurement is carried out by using only one antenna element (i.e., no beamforming, one channel of Spirent GSS9000 is connected to one channel of the beamformer). Next, a beam is created and steered at different angles.

First, the beam is pointed toward the Zenith $\left(\theta ,\varphi \right)$ = (0, 0). Then, the beam is steered to a 45° elevation and different azimuth values (45°, 135°, 225°, and 315°, corresponding to SW, SE, NE, and NW directions). The results are shown in Figure 4a–i. A sky plot of the measured satellites is also shown in Figure 4j.

The color legend is shared between all the plots. The sky plot in Figure 4j is captured from the Spirent GSS9000 software and shows the position of the satellites in view. Five directions are pointed with the beam: Zenith (Beam 0, 0), South-West (Beam 45, 45), South-East (Beam 45, 135), North-East (Beam 45, 225), and North-West (Beam 45, 315).

4. Discussion

The results of the measurements are shown in Figure 4a–i, and we can compare the results with the sky plot in Figure 4j. Each subfigure contains the signal quality data for all the measurements (i.e., the different beams created, as well as the reference measurements).

First, we can note that all reference levels are in the same range of approximately 32–33 dB-Hz. Also, the reference experiment is receiving the nine satellites correctly.

Then, when we create a beam aimed toward the Zenith (0, 0), we see that the only received satellites are the ones close to the Zenith, i.e., G02, G05, G12, and G30. But their levels are then higher than the reference measurement: between 39 (G12) and 42 dB-Hz (G30). This already demonstrates a performance increase of more than 6 dB.

If we look at the lower elevation beams, we can see that by pointing at the satellites, the signal power become stronger than the reference measurement. For example, satellite G31 is received with a signal superior to 42 dB-Hz when the beam is (45, 135) in Figure 4i. The same is true with satellite G24, which is received at 41 dB-Hz when the beam point is at (45, 45) on Figure 4f.

Satellite G10 is also well received even though the beam is not directly pointed at it (45, 135), with a level a bit higher than 40 dB-Hz in Figure 4d.

To finish, the beam pointing at NE (45, 225) receives satellite G04 with a level greater than 40 dB-Hz again in Figure 4b.

5. Conclusions

This study successfully implemented a custom beamformer antenna system tailored for GNSS signal reception. The experiments focused on showcasing the fundamental concept of beamforming in GNSS applications. Specifically, a Spirent multi-port GNSS signal generator was utilized to simulate a GPS constellation, feeding signals into a planar array comprising four antenna elements.

Within this setup, the simulated antenna elements were connected to the beamformer, which applied varying phase shifts to generate beams at different elevation and azimuth angles. This approach effectively demonstrated the beamforming capability of the antenna system, highlighting its potential for adaptive signal reception and interference mitigation in practical GNSS scenarios.

Moving forward, these initial experiments lay a strong foundation for further investigations into optimizing beamforming strategies and integrating real-world GNSS signals. The next step of this project is to implement the full version of the system with 64 radiating elements. Used in combination with the beamformer and a control unit, the system will be able to track satellites for constellations monitoring purposes.

To conclude, the custom beamformer antenna system presented here represents a promising advancement in GNSS antenna technology, offering enhanced capabilities for precision navigation and signal reliability in challenging operational environments.