Six Key Parameters to Consider When Comparing GNSS Antennas

Between GPS, Galileo, GLONASS and BeiDou, there’s no shortage of global navigation satellite systems (GNSS) to choose from when designing devices and applications that require ultra-accurate positioning and timing information. Each constellation has its unique attributes, such as the frequencies it uses and the selection and capabilities of signals that are available for all civilian users and ones reserved for authorized users.

When it comes to GNSS antennas, several key considerations apply to all constellations. Their signals are relatively weak by the time they arrive from the satellites in medium Earth orbit. Signals can be further attenuated by conditions on the ground, such as when the receiver is in urban concrete canyons or in places with dense tree cover.

Even the most sensitive, selective GNSS receiver is only as good as the antenna it’s paired with. That’s why systems designers should carefully consider the type and design of each antenna model when deciding which one to use. Here are six top performance-related antenna parameters to keep in mind for the most common GNSS use cases. (Ultra-high-precision geodesy or similar applications have a separate set of unique considerations.)

Efficiency and Gain

Although efficiency and gain are separate attributes, they’re typically discussed together because they’re intrinsically related:

Gain = Efficiency * Directivity

Directivity is what it sounds like: how much the antenna “focuses” in any direction. Gain and directivity are both variable across an antenna pattern. Datasheets or specifications will typically reference peak and/or average gains, with peak gain being by far the most common. This is the highest gain across the pattern.

Efficiency refers to power conversion: how well the antenna transfers conducted power at its terminals to radiated power (or vice-versa). Higher efficiency is desirable in nearly every situation but typically also a key challenge, especially with compact antennas.

Efficiency is usually described in percentage or dB (with 0 dB being 100%, -10 dB being 10% etc.). Gain is usually referencing Peak Gain and given in dBi, or dB referenced to a theoretical isotropic radiator.

Polarization

This describes the shape of the far-field radiated electromagnetic waves. Polarization can be anything between a straight line (called linear) through a circle (circular polarization), with all real waves being somewhere in between. Circularly polarized waves can rotate in either sense (right- or left-hand), and linear polarization can be rotated in any direction. GNSS signals are Right-Hand Circularly Polarized (RHCP).

Axial Ratio

This parameter describes the purity of polarization. An imperfect circularly polarized wave is elliptical, with a major and minor axis. The ratio of these axes is the axial ratio. It is nearly always shown in dB (in industry literature), with a perfect circle having a ratio of 1:1 or 0 dB. A very common target is 3 dB (or 2:1 in linear terms).

Group Delay Variation

GNSS receivers use signals from multiple satellites: at least four, typically five, and even more, depending on the application and its requirements. These signals arrive at the receiver’s antenna at different times from different directions, a phenomenon known as group delay variation (GDV). These differences affect the codes available to the receiver, which is why GDV is sometimes referred to as code phase variation (CPV). Expressed in time units, this is ideally under 20 ns within each band.

Phase Center Offset (PCO)

The easiest way to understand this parameter is in terms of transmitting antennas, where phase center offset is the physical/geometric location from where far-field waves appear to originate. Because antennas are reciprocal, the phase center is the same when receiving signals. The phase center can be offset from the geometric center of the antenna in any direction and can change with frequency. This parameter is described in terms of length.

Phase Center Variation (PCV) describes how the PCO varies across the antenna’s radiation pattern (or across the sky). This is also described in terms of length. This number should be small but more precise applications will require better (lower) values.

There is more to phase center offset than that, but this article focuses on this aspect because the goal is to provide a general overview of the top considerations when choosing an antenna for different GNSS use cases. For a deeper dive into GNSS, visit this page. 

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