Introduction
"Your GNSS system is only as good as the signals it receives - and that starts with the antenna."
Engineers often focus their attention on receivers, software, and correction services, but the antenna is the true starting point of every GNSS measurement. A substandard antenna, or one poorly placed, will introduce errors that no amount of downstream processing can fully correct. Understanding antenna types, their performance characteristics, and their proper deployment is fundamental to precision GNSS system design.
How GNSS Antennas Work
All GNSS antennas must receive Right-Hand Circularly Polarized (RHCP) signals in the L-band frequency range (approximately 1.1 to 1.6 GHz). They convert the arriving electromagnetic wave into an electrical signal that is fed through a cable to the receiver's RF front-end. Most precision antennas include an integrated Low-Noise Amplifier (LNA) near the antenna element to boost the signal before losses accumulate in the cable.
A key performance parameter for any GNSS antenna is phase center stability. The phase center is the effective electrical point from which the antenna receives signals. For centimetre-level positioning, this point must remain stable and well-characterised across all satellite elevations and azimuths. Variations in phase center position with signal direction introduce systematic errors into measurements that can be millimetre to centimetre in magnitude.
Antenna Types
Patch Antennas
Patch (microstrip) antennas are the most common type in consumer and low-cost GNSS applications. They consist of a flat metallic element - typically square or circular - mounted on a dielectric substrate above a ground plane. They are inexpensive to manufacture, have a low profile, and integrate easily into devices. Single-feed patch antennas support L1-only reception; dual-feed designs improve circular polarization quality and support multi-frequency reception.
Patch antennas offer moderate multipath rejection through their directional gain pattern - signals arriving from below the horizon are attenuated. However, they remain susceptible to low-angle multipath, and their phase center stability is generally inferior to geodetic-grade designs.
Helical Antennas
Helical antennas wind conductive wire in a spiral, producing RHCP radiation without requiring a separate ground plane. They provide wider coverage angles, making them well suited to dynamic platforms such as UAVs and autonomous vehicles where the platform may tilt significantly. Their multipath resistance is inherently better than basic patch designs because they do not rely on a ground plane to block low-angle signals.
Choke Ring Antennas
Choke ring antennas are the gold standard for high-precision, reference-grade applications. They surround the central antenna element (typically a patch or dipole) with a series of concentric conductive rings - the choke rings - that act as resonant cavities. These rings are tuned to the GNSS frequencies and create a highly resistive surface for signals arriving from near or below the horizon, effectively blocking multipath from ground reflections.
Two variants exist. A 2D choke ring has rings arranged on a flat horizontal plane, providing excellent suppression of signals from below the horizon. A 3D choke ring (sometimes called a conical choke ring) extends the ring geometry upward, giving even better rejection of low-elevation multipath - the standard for the most demanding scientific and geodetic monitoring applications.
| Feature | Patch | Helical | Choke Ring |
|---|---|---|---|
| Profile / Size | Low, compact | Tall, cylindrical | Large, heavy |
| Ground Plane Required | Yes | No | Built-in |
| Multipath Rejection | Moderate | Moderate | Excellent |
| Phase Center Stability | Fair | Good | Excellent |
| Cost | Low | Medium | High |
| Typical Use | Consumer, mobile | UAV, dynamic platforms | Reference station, surveying |
The Importance of Ground Planes
For patch antennas without built-in choke rings, the ground plane is the primary defence against low-angle multipath. A larger ground plane provides better attenuation of signals arriving from near and below the horizon. For field deployments on tripods or monuments, a metal disc of at least 10 cm radius - and ideally 30 cm or more - significantly improves measurement quality. Ground planes should be kept clean, flat, and horizontal.
Antenna Placement: Critical Decisions in the Field
Even the finest geodetic antenna performs poorly if placed incorrectly. The following placement principles are non-negotiable in precision work.
- Clear sky view: The antenna must have an unobstructed view of the sky above the elevation mask angle (typically 10–15 degrees). Buildings, trees, vehicles, and terrain features all cause signal blockage and multipath.
- Height above reflective surfaces: Antennas placed close to the ground, vehicle roofs, or metal structures will receive stronger multipath. Elevation reduces this effect.
- Separation from interference sources: Keep antennas away from radio transmitters, cellular base station equipment, and high-voltage lines that can desensitise the front-end or introduce RF interference.
- Antenna height measurement: In survey work, the height from the ground mark to the antenna reference point must be measured precisely. An error of even 5 mm in the measured height directly propagates into the vertical coordinate.
Vital Points
- Antenna quality strongly affects measurement accuracy - phase center stability and multipath rejection are the critical performance metrics.
- Bad antenna placement introduces multipath errors that corrupt pseudorange and carrier phase measurements and cannot be fully removed in post-processing.
- A high-quality choke ring antenna can deliver better results than a lesser antenna paired with a more expensive receiver.
- Always consult the antenna's Phase Center Variation (PCV) calibration data when performing geodetic-grade work.