What is baseline length?
The baseline is the straight-line distance between your RTK rover and the reference station providing corrections. In a base-rover setup, that is your own base station. When using an NTRIP correction service, it is the nearest physical reference station in the network.
The shorter the baseline, the more similar the satellite signals seen by both rover and base — which means corrections are more accurate and Fix is faster. As the baseline grows, atmospheric differences (ionosphere and troposphere) between rover and base increase, making it harder for the receiver to resolve carrier phase ambiguities.
0–30 km
Fast Fix, centimetre accuracy
30–60 km
Slower Fix — use VRS
60–100 km
Fix becomes unreliable without VRS
100 km+
Use VRS or another network solution
Practical limits by setup type
| Setup type | Recommended max | Absolute max | Status |
|---|---|---|---|
| Own base station — Single base, radio or NTRIP | 10–15 km | ~30 km | Best accuracy |
| NTRIP network, standard mountpoint — Nearest physical station | 20–30 km | ~50 km | Good in dense networks |
| NTRIP network, VRS mountpoint — Virtual reference station | Any distance in network | Network coverage area | Recommended for >30 km |
| NTRIP network, no VRS — Sparse station coverage | 20 km | ~40 km with degraded accuracy | Use VRS if available |
| PPP (Precise Point Positioning) — No local base needed | Global | Global | Minutes to converge, cm post-fix |
What happens as baseline grows
Longer baselines introduce three problems that affect RTK performance:
1. Ionospheric decorrelation
The ionosphere is a layer of charged particles that delays satellite signals. At short baselines, rover and base see nearly identical ionospheric conditions, so corrections cancel out the delay well. Beyond roughly 20–30 km, conditions diverge enough to degrade corrections, especially during high solar activity.
2. Tropospheric decorrelation
The lower atmosphere also delays signals based on temperature, pressure and humidity. These vary across terrain. At longer baselines, especially with significant altitude differences between rover and base, tropospheric errors become significant.
3. Slower ambiguity resolution
RTK Fix depends on resolving carrier phase ambiguities. At longer baselines, this calculation becomes harder and takes longer, or may not converge at all. The result is Float instead of Fix.
Watch for this sign
If you are consistently stuck on Float and your environment is good (open sky, strong signal), long baseline is often the cause. Check the distance to the nearest reference station in your NTRIP sourcetable.
How VRS solves long baselines
VRS (Virtual Reference Station) is a network feature where the NTRIP server computes a synthetic correction stream as if a real base station existed right next to your rover — typically within 1–2 km. It achieves this by interpolating data from multiple physical stations across the network.
To use VRS, your NTRIP client must send your position (a GGA sentence) to the server. The server uses that position to generate the virtual corrections and streams them back. If GGA is not sent, the VRS cannot generate a local correction and you will receive no data.
VRS setup checklist
Enable GGA transmission in your NTRIP client.
Select a mountpoint labelled VRS, MAC or RTCM3_VRS.
Ensure you have a Single or Float solution first so GGA contains a valid position.
Limits by device type
Different receivers handle long baselines differently depending on their processing engine and the signals they track.
Emlid Reach RS2+ / RS3 / RS4 — ~60 km
Multi-band. Use a VRS mountpoint beyond 30 km.
Emlid Reach RX / RX2 — ~30 km
Pure network rover. VRS is strongly recommended.
Trimble / Leica — ~100 km
Advanced engines. VRS or MAC required beyond 30 km.
DJI RTK drones — ~30 km
Use an MSM5 mountpoint. Use VRS for longer baselines.
u-blox ZED-F9P — ~20 km
Entry-level multi-band receiver. Sensitive to baseline length.
Single-frequency receivers — ~10 km
Short baselines only. No ionospheric correction.
Tips for long baseline situations
Use VRS first
If your NTRIP service offers a VRS or MAC mountpoint, switch to it. This eliminates baseline as a factor entirely and is the single most effective change you can make.
If VRS is not available or you are using your own base station, these steps help:
Move your base closer
For own-base setups, the most direct solution is repositioning the base within 10–15 km of your work area.
Wait for better conditions
During high solar activity (solar maximum), ionospheric delays increase. Working early morning often gives better results.
Use a multi-band receiver
Dual or triple-frequency receivers can model and correct ionospheric delays using the difference between frequencies (L1/L2/L5). Single-frequency receivers cannot.
Increase initialisation time
At longer baselines, ambiguity resolution simply takes longer. Give the receiver 5–10 minutes in a stationary position before starting work.
Check elevation mask
A 15° elevation mask removes low-elevation satellites that carry the highest atmospheric errors at long baselines.
Accuracy degrades with baseline even at Fix
RTK Fix does not guarantee centimetre accuracy at long baselines. At 50+ km without VRS, horizontal errors of 5–10 cm are common even with a Fix solution. For precision work, always verify with known control points.
