Understanding GNSS Antennas

Global Navigation Satellite System (GNSS) antennas are designed to receive signals from satellite constellations like GPS (United States), GLONASS (Russia), BeiDou (China), and Galileo (European Union). They are designed to capture these weak signals from satellites that are used for Position, Navigation and Timing (PNT) applications.

Once the satellite signals are captured by a GNSS antenna, they are filtered to remove unwanted noise and then amplified to reach a signal level that can be processed by a receiver. The receiver uses the timing data from these signals to calculate the distance to each satellite. By combining data from at least four satellites, the receiver can determine the user's precise position using a method called trilateration.

Frequency bands used by GNSS Antennas

GNSS antennas operate on specific frequency bands that correspond to the signals transmitted by different satellite navigation systems, such as GPS (U.S.), GLONASS (Russia), BeiDou (China), and Galileo (EU). Each satellite constellation also has multiple bands. Each band allows GNSS antennas to receive a specific set of critical timing and positional data from satellites, supporting various applications from basic navigation to high-precision surveying.

The key GNSS frequency bands include: 

• L1 Band (~1575.42 MHz): Primarily used by GPS, Galileo, and BeiDou, the L1 band is the main frequency for civilian applications, providing broad compatibility with most GNSS receivers. 
• L2 Band (~1227.6 MHz): This band is utilized mainly for military applications in GPS but is also available for certain civilian uses to improve signal robustness, especially when combined with L1 in dual-frequency systems. 
• L5 Band (~1176.45 MHz): Often referred to as the “Safety of Life” band, L5 is designed for applications that require high reliability, such as aviation, by offering enhanced resistance to interference. 
• E6 Band (1260-1300 MHz) and B3 Band: Used by Galileo (E6) and BeiDou (B3), these bands provide additional signals for advanced services, enabling multi-frequency accuracy and enhanced integrity for specialized applications. 
• In addition to this GPS has the L6 Band, Galileo has the E1, E6 and E5 Bands, GLONASS has the G1, G2 and G3 bands and BieDou has the B1, B2 and B3 Bands.

Using multiple GNSS bands, especially with dual or tri-band receivers, allows for improved positioning accuracy and reliability, reduced susceptibility to signal errors, and increased resilience against interference, making GNSS antennas versatile across diverse and demanding environments. This is especially useful in environments where one satellite system’s signal may be weak or obstructed.

Types of GNSS Antennas 

There are two primary types of antennas: Active and Passive.

Active GNSS Antennas

Active GNSS antennas are equipped with an integrated low-noise amplifier (LNA) that amplifies weak satellite signals before they reach the receiver. This amplification is crucial in environments where satellite signals may be weak or obstructed, such as urban canyons, inside vehicles, or during high-precision applications like surveying. By enhancing signal quality, active antennas improve reliability and accuracy, making them suitable for applications that require robust performance in challenging conditions. These antennas can also compensate for signal loss due to long cable runs, ensuring that the receiver receives a strong and clear signal. As a result, active GNSS antennas are commonly used in fixed installations, high-accuracy applications, and scenarios where environmental factors might interfere with satellite signals.

Passive GNSS Antennas

In contrast, passive GNSS antennas do not incorporate any amplification; instead, they directly transmit satellite signals to the receiver. This design relies on the inherent processing capabilities of the receiver to handle the incoming signals. Passive antennas are best utilized in environments with strong satellite reception, such as open fields or rural areas with minimal interference. Their simplicity and lack of electronic components contribute to a lightweight and compact form factor, making them ideal for handheld devices and applications where low power consumption is a priority. While passive GNSS antennas can sometimes be paired with external low-noise amplifiers to boost the signal if needed, they generally perform optimally in conditions where the satellite signals are strong and stable, allowing for efficient operation without the added complexity of active components. 

The defining factor of a GNSS Antenna is its frequency range of operation. However, there are certain types of antennas that are often used for GNSS applications due to specific properties. We talk about five of them below:

1. Patch Antennas

Patch antennas are flat, rectangular or circular, compact antennas. They are directional antennas that typically support RHCP (Right-Hand Circular Polarization) to match GNSS satellite signal polarization, enhancing reception. However, they often suffer from multipath interference without proper mounting. Patch antennas rely on a ground plane to reflect signals, making them suitable for stationary and portable devices. They are ideal for fixed installations like vehicle navigation, drones, and stationary sensors.

2. Helical Antennas

Helical antennas use a helical coil that allows for omnidirectional reception, even without a large ground plane, which makes them very versatile. They provide high gain with circular polarization and help reject multipath signals. They perform well in challenging environments but may be larger than patch antennas. Helical Antennas handheld devices, portable GNSS equipment, and low-profile applications like wearables.

3. Choke-Ring Antennas

Choke-ring antennas feature rings around the central radiating element, which helps mitigate multipath interference by suppressing ground reflections. These antennas offer high multipath rejection, stable phase center, and robust performance, particularly for professional, high-accuracy applications. They are often used in high-precision applications, including geodetic surveys, meteorology, and static GNSS stations.

4. Chip & Loop Antennas

Chip antennas are miniaturized low-profile antennas that are designed to fit in a constrained form factor. Chip antennas have low gain and are more susceptible to interference, making them suitable mainly for devices where size is a major constraint. These antennas are primarily used in small, lightweight devices like wearables and IoT sensors. They are best suited for applications where size and cost are prioritized over high-precision signal accuracy.

5. Multi-Element and Cross-Dipole Arrays

Multi-element arrays and cross-dipoles offer enhanced directional capabilities and stronger signal rejection for unwanted polarizations. They offer high gain, excellent multipath rejection, and support for multiple GNSS bands. These antennas are used in advanced applications requiring multi-frequency or high-precision GNSS, such as in drones or automotive applications with Real-Time Kinematic (RTK) capabilities.

Each of these antennas provides unique advantages, making them suitable for a wide range of GNSS applications, from handheld devices to highly accurate surveying instruments.

Key Properties of GNSS Antennas and Their Impact on Performance 

Several critical properties of GNSS antennas significantly influence their functionality and overall performance.

Frequency Coverage: This refers to the range of frequencies that an antenna can effectively receive. GNSS antennas must cover multiple frequency bands, including L1, L2, and L5, to ensure compatibility with various GNSS signals, providing flexibility in diverse applications.

Gain Pattern: The gain pattern of an antenna describes how it receives energy in different directions. A well-designed gain pattern ensures optimal signal reception from the satellites, maximizing the antenna's effectiveness in various environments.

Circular Polarization: GNSS antennas must have circular polarization. Antennas using circular polarization can receive signals from satellites regardless of their orientation. This feature helps improve signal integrity and reduces the effects of multipath interference, which is crucial in urban and dense environments.

Multipath Suppression: This property involves techniques to minimize the impact of reflected signals, which can distort the received signal. Effective multipath suppression is vital for enhancing accuracy and reliability, especially in complex environments.

Phase Center: The phase center is the point within the antenna where incoming signals are considered to be perfectly aligned. Variations in the phase center can lead to errors in positioning, making its stability critical for accurate GNSS performance.

Impact on Receiver Sensitivity: The design and properties of the antenna directly affect the sensitivity of the GNSS receiver. Antennas with high gain and low noise characteristics enhance receiver sensitivity, allowing for better performance in challenging conditions.

Interference Handling: The ability of an antenna to manage and mitigate interference from other electronic devices is essential for maintaining signal quality. Robust interference handling capabilities ensure that GNSS systems can operate reliably in diverse environments.

Understanding these properties is essential for selecting the right GNSS antenna for specific applications, ensuring optimal performance and accuracy. GNSS plays a pivotal role in modern positioning and navigation systems, influencing a wide range of applications from precision agriculture to autonomous vehicles. The Antenna is a defining part of this technology.