How to Calibrate an RTK System After Installation and What to Do When Fix Is Lost

After mounting an RTK receiver, you need to complete the initial initialization: establish the precise position of the base station (or connect to an NTRIP/CORS network), wait for correction data to arrive, and let the system reach Fixed mode, which typically takes 30 seconds to 5 minutes under good sky visibility. If fix is lost, check signal quality (PDOP below 3, at least 5 satellites visible), the integrity of the radio link or internet connection, and the absence of obstructions near the antenna. When the system drops to Float mode, accuracy degrades from 1-2 cm to 0.3-1 m, so critical measurements should be paused until Fixed is restored.

What RTK Calibration Is and Why It Matters

RTK (Real-Time Kinematic) is a satellite positioning technology that delivers 1-2 cm horizontal accuracy and 2-4 cm vertical accuracy. To reach that level of precision, the rover (mobile receiver) must receive differential corrections from a base station placed at a known point.

The calibration process has two components: geodetic tying of the base and phase initialization of the rover. Without proper calibration, even an expensive receiver will report errors in the range of tens of centimetres or even metres.

Step-by-Step RTK Calibration After Installation

Step 1. Setting up the base station

The base is placed on a point whose coordinates are known to better than 3 cm accuracy - a geodetic control monument, a previously determined point via static GPS, or coordinates sourced from a CORS network. The antenna must have an unobstructed sky view from 15 degrees above the horizon and higher.

• Centre the antenna phase centre over the ground mark with no more than 2 mm offset.
• Measure and enter the antenna height (HI) to the nearest 1 mm.
• Enter base coordinates manually, or average them over 4+ hours of observation if the point is new.
• Confirm the base is tracking at least 6 satellites with SNR above 35 dB.

Step 2. Connecting the rover to corrections

The rover connects to the base via a radio link (typically UHF 410-470 MHz, range up to 10-15 km on flat terrain) or via an NTRIP network over GPRS/LTE. When using NTRIP, make sure the network base station is within 30-50 km, since residual atmospheric errors grow significantly beyond that distance.

Step 3. Initialization (searching for fix)

Once corrections are received, the rover resolves integer carrier-phase ambiguities - this is the core initialization process. Initialization time depends on several factors.

Once initialization succeeds, the system enters Fixed mode. Do not take measurements in Float mode - errors can reach 0.5-1 m with no warning.

Fixed and Float Modes: Key Differences

Understanding the difference between these modes is critical for correctly assessing measurement quality in the field.

Most field controllers display modes with different colours or letters: F (Fixed), FL or Q (Float). Some firmware versions (such as u-blox F9P) also show a number from 1 to 5, where 1 means Fixed, 2 means Float, and 5 means single-point code solution with no corrections.

What to Do When Fix Is Lost

Causes of dropping from Fixed to Float

• Obstruction above the rover antenna (tree, roof, embankment).
• Radio link or mobile connection interrupted for more than 5-10 seconds.
• Sudden deterioration of satellite geometry (PDOP above 4-5).
• Strong ionospheric disturbances (Kp index above 4).
• Multipath: signal reflections from metal surfaces or glass facades.

Fixed recovery procedure

1. Check the correction link status on the controller and confirm corrections are flowing.
2. Move 5-10 m away from potential interference sources, then stand still for 60-90 seconds.
3. If the system does not regain Fixed within 2-3 minutes, perform a hardware reset of the rover receiver.
4. Check PDOP on the controller: if it exceeds 3.5, wait for better satellite geometry (10-20 minutes).
5. In difficult environments, use multi-point observation with position averaging.

When re-initialization takes more than 10 minutes

This indicates a systemic problem. Check rover antenna quality and cable connections, correction format settings (RTCM 3.x, CMR+), and elevation mask configuration (10-15 degrees is recommended). Also check whether the rover is operating in an area with active radio-frequency interference.

Practical Tips for Stable Operation

• Mount the rover antenna at the highest available point on the pole or tripod, with no metal objects within 30 cm.
• For urban environments, consider receivers with L5/E5 support - they are significantly more resistant to multipath.
• Monitor PDOP in real time: 1.0-2.0 is optimal, up to 3.0 is acceptable.
• Keep a measurement log that records Fixed or Float status at every point.
• If using NTRIP, configure a backup connection through a second mobile carrier.