Introduction
Standard GNSS gives you meter-level accuracy. DGNSS gives you decimeter-level. But for centimeter accuracy, needed for surveying, construction, and precision agriculture, we need RTK: Real-Time Kinematic positioning.
The Breakthrough: Using Carrier Phase
GNSS signals have two components:
- Code: The coarse pattern (meter-level precision)
- Carrier: The radio wave itself (centimeter-level wavelength)
| Signal | Wavelength | Precision Potential |
|---|---|---|
| L1 Code | 293 m (chip length) | ~3 m |
| L1 Carrier | 19 cm | ~2 mm |
RTK measures the phase of the carrier wave. But there's a catch: the carrier wave is just a sine wave. All cycles look identical. Which cycle are you on?
The Ambiguity Problem
Your receiver can measure carrier phase to millimeters, but it doesn't know which cycle it's measuring. This is the integer ambiguity problem.
Think of it like measuring distance with a tape measure in the dark. You can read the markings precisely, but you don't know where the zero end is.
How RTK Solves This
- Base station and rover both measure carrier phase to the same satellites
- They communicate via radio or internet
- Software solves for the integer ambiguities (how many whole cycles)
- Once ambiguities are fixed, centimeter accuracy is achieved
RTK Components
Base Station
- High-quality receiver
- Survey-grade antenna on known position
- Radio transmitter or internet connection
Rover
- Mobile receiver with similar quality
- Radio receiver or cellular modem
- Data collector/controller
Data Link
- UHF radio (typical, 2–10 km range)
- Cellular/NTRIP (unlimited range with coverage)
- Spread spectrum (license-free bands)
RTK Limitations
- Range: Typically 10–30 km from base
- Initialization: Takes 30 seconds to several minutes to fix ambiguities
- Loss of fix: If signal is lost, must re-initialize
- Multipath: Still a problem, choose sites carefully
- Cost: Professional RTK systems cost $5,000–$20,000
RTK Performance
| Condition | Time to Fix | Accuracy |
|---|---|---|
| Open sky, short baseline | 10–30 sec | 1–2 cm |
| Light trees, short baseline | 30–60 sec | 2–3 cm |
| Open sky, long baseline (20 km) | 1–2 min | 3–5 cm |
| Heavy trees, long baseline | May not fix | Float (20–50 cm) |
Network RTK
Instead of a single base station, a network of permanent stations (CORS) provides corrections over a wide area:
- Accuracy: 2–4 cm
- Range: Entire network coverage area
- No base station setup needed
- Requires cellular data for corrections
Vital Points
- RTK uses carrier phase measurements for centimeter accuracy
- Integer ambiguity must be solved, the "fix"
- Base station + rover + data link are required
- Range limited to 10–30 km from base
- Network RTK provides coverage over larger areas
- Essential for surveying, construction, precision agriculture