RTK GPS for Pixhawk/ArduPilot Systems: Elevating Precision in UAV Navigation

Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS) is revolutionizing the way unmanned aerial vehicles (UAVs) achieve positioning accuracy. With a focus on accurate positioning for professional surveying and research applications, RTK GPS systems, particularly when integrated with Pixhawk and ArduPilot platforms, provide significant enhancements to conventional GPS data. Through a combination of groundwork and high-frequency corrections transmitted from a base station, RTK GPS can achieve horizontal accuracies of 1–3 cm, an improvement that is crucial for applications needing precision.

What is RTK GPS?

RTK GPS is a GNSS correction technique that employs a base station and a rover. The base station is located at a known position and broadcasts Real-Time Correction Message (RTCM3) signals to the rover. This signal allows the rover to adjust its measurements with significant precision, dramatically reducing positional errors that commonly occur with standalone GPS systems.

The RTK system operates by determining the integer ambiguity of the carrier phase measurements collected, which allows for the high-precision positioning to be achieved. This method stands in stark contrast to standalone GPS systems, which are subject to significantly higher positioning errors due to atmospheric conditions, satellite geometry, and other perturbations.

Accuracy Levels of RTK GPS

When discussing the capabilities of RTK GPS, it is essential to distinguish between the various levels of accuracy provided by different positioning systems:

How RTK GPS Works

The operational framework of RTK GPS centers around its fundamental components: the base station and the rover. The base station, positioned at a known survey point, collects GNSS signals and calculates its precise location based on known coordinates. It then generates corrections (RTCM3 format) to compensate for errors inherent in the GNSS signal. These corrections are continually broadcasted to the rover.

The rover, usually mounted on a UAV, receives these correction signals and applies them to its GNSS data. The critical aspect here is the resolution of integer carrier phase ambiguities, a calculation that allows the rover to achieve fixed positioning as opposed to float positioning. Floating positioning provides less accuracy than the fixed solution but can still enhance the usability of standard GNSS measurements.

Time to First Fix (TTFF)

Another important aspect of RTK GPS systems is the Time to First Fix (TTFF). This is the time required for the system to acquire enough GNSS satellites and corrections to provide an accurate position fix. Typical TTFF characteristics include:

• Float RTK: approximately 30 seconds
• Fixed RTK: typically between 1 to 5 minutes

Overview of Here3 and Here4 RTK GPS Units

Here3 (CubePilot)

The Here3 unit is a powerful RTK GPS module developed by CubePilot, featuring the u-blox F9P chipset. This unit supports dual-frequency bands (L1/L2), enhancing its robustness in various satellite environments, namely GPS, GLONASS, Galileo, and BeiDou.

• Accuracy: 2.5 cm CEP RTK fixed
• Connection: DroneCAN interface
• Compass: ICM-20948

Here4 (CubePilot)

The Here4 is a next-generation RTK GPS unit that builds on the success of Here3 but introduces several key improvements.

• Chipset: u-blox F9P, supporting L1/L2/L5 tri-frequency
• Faster Convergence: Achieves fixed positions more rapidly
• Connection: Continues with DroneCAN interface

Here3 vs Here4 Specification Comparison

Base Station Setup

Setting up a base station is crucial for effective RTK GPS operation. The base station consists of a second Here unit that connects to a laptop via USB. The corrections are then fed to a ground control software like Mission Planner or through Networked Transport of RTCM via Internet Protocol (NTRIP).

1. Connect the Here unit: Set up the Here unit (Harsh environment protection recommended) to the laptop.
2. Software Configuration: Run your correction service (e.g., using Mission Planner or another NTRIP-compatible service).
3. Broadcast Corrections: Ensure your base station is broadcasting RTCM3 corrections.

ArduPilot RTK GPS Parameters

For configuring ArduPilot to work with RTK GPS units, it’s essential to set appropriate parameters related to the GPS type and blending options:

• GPS_TYPE: Set to 9 for UAVCAN/DroneCAN systems.
• GPS_TYPE2: Set to 9 for the secondary unit.
• GPS1_TYPE: Optional setting to define how to blend the two GPS units.

GPS Blending

The blending of signals from two GPS units facilitates accuracy improvement by averaging the measurements when both units have a good signal. This feature is controlled through the GPS_BLEND_MASK parameter, ensuring that the system effectively uses the available signals for enhanced reliability.

Survey-Grade Precision for Professional Applications

The integration of RTK GPS with the Pixhawk platform supports survey-grade precision crucial for professional surveying applications. With RTK fixed solutions, the Pixhawk can achieve horizontal positioning accuracies of 1–3 cm, allowing it to surpass conventional GPS capabilities significantly.

This level of precision opens up a host of applications ranging from mapping and topography studies to precision agriculture and infrastructure monitoring. Moreover, with a robust RTK system, UAV operators can reduce the number of ground control points needed in a survey, thus saving time and resources.

Conclusion

In conclusion, RTK GPS represents a significant advancement in UAV navigation, especially within Pixhawk and ArduPilot systems. Leveraging the capabilities of advanced modules like Here3 and Here4, operators can achieve unprecedented accuracy in their aerial surveys and research applications. Understanding the setup, parameters, and functioning of RTK GPS will enable users to enhance their UAV operations, driving the future of surveying and mapping with precision engineering.

Frequently Asked Questions

What is the primary advantage of RTK GPS over conventional GPS?

The primary advantage of RTK GPS is its ability to achieve centimeter-level accuracy through real-time corrections, making it ideal for professional surveying and precision applications.

How does a base station work in the RTK GPS setup?

The base station, placed at a known location, broadcasts corrections to the rover, helping to minimize errors caused by atmospheric disturbances and satellite signal variations.

What is the expected accuracy of Float RTK compared to Fixed RTK?

Float RTK can achieve accuracy levels around 0.3–1 m, whereas Fixed RTK typically provides accuracies of 1–3 cm.

How quickly can an RTK GPS system provide a position fix?

Float solutions can be achieved in around 30 seconds, while Fixed solutions typically take between 1 to 5 minutes.

Can I use multiple RTK GPS units with the Pixhawk?

Yes, you can use multiple RTK GPS units simultaneously with the Pixhawk by appropriately configuring the GPS parameters to enable GPS blending for improved accuracy.

For further exploration on the nuances of RTK technology and implementation, please visit our comprehensive resources at MTS UAV.

Sources & References

• ArduPilot: GPS/Compass Setup
• ArduPilot: Here3 Overview
• CubePilot Herelink Documentation
• u-blox ZED-F9P Technical Overview
• ArduPilot GPS Blending

RTK GPS Parameter Configuration Reference

For Pixhawk and ArduPilot RTK installations using DroneCAN GPS hardware such as Here3 or Here4, configure each GNSS receiver consistently and verify that the vehicle receives correction data from the RTK base before flight. Here3 is a DroneCAN-connected L1/L2 receiver with 2.5 cm CEP accuracy when RTK Fixed is achieved. Here4 supports L1/L2/L5, multi-constellation operation, and DroneCAN connectivity.

When using DroneCAN GPS, also confirm the CAN interface is enabled. ArduPilot uses CAN_P1_DRIVER = 1 to enable CAN and CAN_D1_PROTOCOL = 1 for DroneCAN. DroneCAN devices use unique node IDs and are connected on a twisted-pair CAN bus with 120-ohm termination resistors at each end.

Achieving RTK Fix: Troubleshooting Common Issues

RTK operation depends on a base-and-rover workflow. The BASE unit broadcasts correction data, and the ROVER is installed on the vehicle. In ArduPilot, RTK status typically progresses from Float to Fixed. Float indicates sub-meter positioning, while Fixed indicates centimeter-level positioning. Here3 is specified at 2.5 cm CEP when RTK Fixed is achieved.

RTK Fix can take longer when the receiver has poor sky view, limited satellite geometry, or is operated indoors. RTK receivers need clean GNSS reception and reliable correction data; blocked sky view, multipath, or interrupted correction injection can prevent the rover from reaching Fixed status.

HDOP should be evaluated against the mission’s configured acceptance threshold. If HDOP is above the project’s threshold, the GNSS solution geometry is not suitable for precision RTK work, even if correction messages are present.

• Confirm RTK correction source: Verify that RTCM3 corrections are being injected from Mission Planner or from an NTRIP correction source to the vehicle link used by ArduPilot.
• Check BASE and ROVER roles: The BASE unit must provide corrections, and the ROVER must be the GPS installed on the vehicle.
• Verify DroneCAN setup: For Here3 or Here4 on DroneCAN, confirm GPS_TYPE = 9, and for a second receiver confirm GPS_TYPE2 = 9 or the equivalent GPS instance parameter.
• Enable CAN correctly: Confirm CAN_P1_DRIVER = 1 and CAN_D1_PROTOCOL = 1 for DroneCAN operation.
• Inspect CAN wiring: DroneCAN requires twisted-pair wiring and 120-ohm termination resistors at each end of the CAN bus.
• Check node IDs: Each DroneCAN device must have a unique node ID so the flight controller can communicate reliably with the GPS and other CAN peripherals.
• Allow outdoor convergence time: Move the vehicle and base to an open-sky location and allow time for Float to transition to Fixed.
• Watch for correction dropouts: RTK injection failure can be caused by a broken telemetry path, incorrect correction routing, or loss of the RTCM3/NTRIP correction stream.
• Reduce test-flight speed: Use a conservative value such as WPNAV_SPEED = 500 while validating RTK behavior in Auto missions.
• Use dual GPS carefully: If two GNSS receivers are installed, configure both GPS instances and use GPS_BLEND_MASK only after confirming both receivers report stable GPS status.