Introduction
Your GNSS antenna is the first point of contact for signals from space. But antennas aren't perfect, they introduce their own errors. Calibration is how we measure and correct these errors.
The Antenna Problem
An ideal antenna would have a perfect phase centre at its exact physical centre with the same response in all directions. A real antenna has a phase centre that varies with satellite elevation, azimuth, signal frequency, and temperature. These variations cause errors, sometimes centimetres, sometimes more.
Phase Centre Variations (PCV)
- Phase Centre Offset (PCO): Mean position of phase centre relative to the antenna reference point, can be millimetres to centimetres, same for all satellites
- Phase Centre Variation (PCV): How the phase centre moves with elevation and azimuth, a pattern unique to each antenna model, can be several millimetres
Why Calibration Matters
| Application | Why Calibration Is Critical |
|---|---|
| High-precision surveying | 1 mm error matters at control point level |
| Long baselines | Antenna errors don't cancel as they do short-range |
| Mixed antenna types | Different models have different biases |
| Scientific research | Need true, traceable positions |
| Network RTK | Reference stations must be consistent |
Calibration Methods
- Absolute field calibration: A robot rotates the antenna while tracking satellites, measuring phase at all elevations and azimuths. Produces a full PCV map. Accuracy: ~1 mm. Standard: IGS, ISO 17123-8.
- Relative field calibration: Compare unknown antenna to a calibrated reference. Simpler, less equipment. Accuracy: 2–3 mm. Common for survey gear.
- Chamber calibration: Anechoic chamber with signal generator; controlled environment; can test all frequencies. Expensive and rare.
- Robotless absolute: Use many days of data for statistical determination. Less accurate but cheaper.
Calibration Formats
- ANTEX (ANTenna EXchange format): Standard IGS format containing PCO and PCV for all frequencies. Used by all major processing software.
- NGS Antenna Calibration: NOAA's National Geodetic Survey publishes calibrations for many antennas, free online database.
- Manufacturer formats: Trimble .dat, Leica .gs15, etc., often built into proprietary software.
Using Calibration Data
In processing software, select the correct antenna model from the database and the software applies corrections automatically. Important: use the correct antenna model, use the correct radome (if applicable), and know what reference point you're measuring to (ARP, bottom of antenna, etc.).
Common Antenna Types and Calibration Needs
| Antenna Type | Typical PCV | Calibration Recommended? |
|---|---|---|
| Choke ring | 1–3 mm | Yes, for highest accuracy |
| Survey-grade | 2–5 mm | Yes, for precise work |
| Geodetic | 1–2 mm | Yes, always |
| GIS-grade | 5–10 mm | Sometimes |
| Drone patch | 5–15 mm | Rarely |
| Smartphone | Unknown | Not applicable |
Practical Tips
- Surveyors: Use calibrated antennas for control work; keep calibration records; don't mix antenna types on long baselines; know your antenna reference point
- GIS users: Standard antennas fine for metre-level work; consistency is more important than calibration
- Researchers: Always use calibrated antennas with IGS-approved calibrations; document antenna/radome in metadata
Vital Points
- Antennas have variable phase centres, not a single fixed point
- Calibration measures these variations so they can be corrected
- PCV patterns are unique to each antenna model
- Millimetre-level accuracy requires calibration
- ANTEX format is the standard for sharing calibrations
- Always document antenna type and calibration used