Trimble ProPoint Engine: Next-Generation Centimeter Positioning and Orientation

Original publication translation: https://geospatial.trimble.com/en/resources/blog/trimble-propoint-engine-next-generation-centimeter-positioning-and-orientation

Trimble® ProPoint® GNSS technology, integrated into a variety of Trimble GNSS receivers, delivers exceptional performance under challenging GNSS observation conditions. This system incorporates innovative measurement management that benefits from the latest advancements in the fusion of GNSS sensors and inertial measurement units. We invite you to delve into the following white paper to learn more about this industry-leading technology.

 

Introduction

In 1992, Trimble introduced the world's first commercial GPS (Global Positioning System) receiver capable of real-time kinematic (RTK) positioning, achieving centimeter-level accuracy. Although this first-generation system required the user to position the rover in a known location for initialization, it marked a revolutionary milestone in the geospatial context, enabling the execution of topographic surveys, as-built surveys, and layout in real time.

Subsequent applications, such as machine control in construction and agriculture, spurred the development of positioning engines that have successfully offered rapid initializations even while moving (on-the-fly), high update rates, and low-latency positions. This white paper provides an overview of Trimble ProPoint GNSS technology, the fifth generation of RTK/RTX precision positioning engines, designed to provide accurate position and orientation by fusing GNSS signals, global correction services, and measurement data from various sensors.

 

System Architecture

⇲ Support for the latest constellations and frequencies

The space segment of the Global Navigation Satellite System (GNSS) is constantly being modernized with new generations of satellites. While these new signals improve the performance of user equipment, they present a challenge for receiver manufacturers as they must adapt to changes in the transmission structure of these signals. Trimble receivers equipped with Trimble Maxwell™ 7 and ProPoint technology are prepared to leverage all current GNSS signals transmitted on all frequencies. These include:

• GPS: L1 C/A, L1C, L2E, L2C, L5
• GLONASS: L1 C/A, L1P, L2P, L2 C/A, L3 CDMA
• Galileo: E1, E5A, E5B, E5AltBOC, E6
• BeiDou: B1C, B1i, B2i, B2A, B2B, B3i
• QZSS: L1 C/A, L1S, L2C, L5, LEX
• IRNSS: S1 C/A, L5
• SBAS: L1 C/A, L5
• MSS: Trimble RTX, OmniSTAR

ProPoint technology is capable of using all available signal inputs, provided that an RTK base receiver is transmitting corrections for the same signals. This has resulted in a more flexible and precise solution in challenging GNSS environments. Additionally, ProPoint technology allows for flexible signal management, which helps mitigate the effects of signal degradation and provides constellation-agnostic operation. For example, when individual frequencies and constellations are interfered with or blocked, the receiver continues to provide positioning using the available measurements.

 

⇲ Integration of IMU data with RTK and RTX technologies

The ProPoint positioning engine has been designed from the ground up with inertial navigation and sensor fusion in mind. Raw GNSS measurements, along with inertial data from the gyroscope and accelerometer, are combined into a single position and orientation solution. The inertial measurement unit (IMU) in most products is an internal design built on the same board as the GNSS receiver. This tight integration provides a robust solution that maximizes the benefits of GNSS and INS.

 

 

⇲ Extensive signal filtering and precise GNSS error modeling

Thanks to advancements in the GNSS signal spectrum and low-power mobile computing technology, ProPoint was designed with an approach that improves data signal filtering. By combining all measurements into a single filter and simultaneously estimating integer carrier ambiguities with an expanded set of filtering states, an optimal solution is achieved. Although more computationally intensive, this approach offers a more flexible use of all available GNSS signals.

The result of this new approach to signal filtering is that it enables the use of any or all available signals in the RTK position solution, as the variety of traditional signal combination-based methods have become redundant. While dual-frequency data is necessary to mitigate atmospheric effects on GNSS signals in RTK positioning, improved filtering technology means the processor can use any or even all signals, including individual signals in complex tracking environments, generating an optimal solution.

 

⇲ Dynamic models adapted to the application

Trimble ProPoint allows users to select a dynamic model that best suits their application. Based on data collected from various applications, models have been created that are used as input for the positioning engine.

 

⇲ Robust estimation techniques for outlier detection

ProPoint identifies any measurement within the received input data that does not match a stochastic model. For each measurement that does not match its stochastic model, the engine will either reject the measurement, adjust the stochastic model assigned to the measurement, or correct the measurement. This method generally allows for providing an accurate position estimate even in the presence of measurements containing one or more outliers.

 

⇲ Positioning Modes

ProPoint supports a variety of positioning modes, both GNSS individually and the GNSS/INS combination. These include the following:

• Precise positioning: GNSS-SBAS, GNSS-DGNSS, GNSS-RTK, GNSS-RTX
• Precise positioning with orientation: GNSS/INS-SBAS, GNSS/INS-DGNSS, GNSS/INS-RTK, GNSS/INS-RTX

Depending on the availability of differential corrections, the engine will seamlessly switch between positioning modes to provide uninterrupted positioning.

 

⇲ Integrated RTK xFill

During periods of radio signal loss, i.e., when the reference station correction message is not available to compute the RTK baseline, or when there is no access to corrections provided by an RTK network like VRS (Virtual Reference Station), Trimble xFill® correction service provides the technology that allows positioning to continue for brief periods of time with centimeter accuracy. This not only eliminates interruptions in positioning but also allows for a brief foray into an area exempt from the reference station's radio signal, but still visible to GNSS constellations. To achieve centimeter GNSS positioning in these conditions, Trimble xFill provides a specialized correction stream transmitted via satellite in the L-band, which is generated using Trimble Real-time eXtended (Trimble RTX®) technology.

Trimble xFill technology is capable of producing RTK positions with accuracy levels similar to traditional differential RTK positioning, given that it mitigates the same error sources. Satellite clock, orbit, and measurement bias errors that are canceled through differentiation in standard RTK processing are modeled and transmitted as part of the Trimble RTX correction stream.

These effects become known quantities and can consequently be considered when processing mobile receiver measurements. Atmospheric errors are handled by algorithms developed specifically for the Trimble RTX system, reducing residual effects to an acceptable level for high-precision GNSS positioning applications. As a result, Trimble RTX data processing generally provides residual error modeling in satellite observations that is comparable to what is achieved in RTK mode.

 

⇲ Optimization of GNSS Processing on Modern Platforms

As the number of GNSS signals increases, so does the computational processing power required to process all observations. To address this challenge, Trimble has launched receivers with enhanced processors and the recent Trimble Maxwell 7 Application Specific Integrated Circuit (ASIC) GNSS. This enables positions to be delivered to control systems with minimal latency while simultaneously reducing overall power consumption.

 

 

BENEFITS OF THE TRIMBLE PROPOINT POSITIONING ENGINE

Among the advantages of including the ProPoint positioning engine in various applications, we find key factors such as accuracy, availability, and integrity.

Many positioning systems can provide some of these features, but Trimble ProPoint delivers the position and orientation necessary for your project to be truly successful. In direct comparison tests with the previous generation RTK/RTX engine in challenging GNSS environments such as areas near trees and buildings, the Trimble ProPoint engine performed at least 30% better across a variety of factors, including time to reach centimeter-level accuracy, positioning accuracy, and measurement reliability.

 

⇲ Accuracy

Open-sky guidance and control applications have been leveraging centimeter-level RTK accuracy for over 20 years. Unfortunately, in the real world, most autonomous applications operate in challenging environments where satellite line-of-sight can be impacted. Trees, buildings, bridges, and other obstructions block and reflect incoming signals. ProPoint, characterized by flexible signal management and a unique optimized filtering approach, offers centimeter-level accuracy results under tree canopy, whereas in the past, decimeter-level accuracies were achieved at best. For applications where most of the sky is obstructed, the integration of IMU measurements into the positioning engine offers the best solution. Trimble ProPoint tightly integrates available GNSS and IMU measurements, offering centimeter accuracy and orientation during these events.

 

 

The latency of computed results is also an important factor in autonomous vehicle control. A position can be highly accurate, but if it is delayed, it has little value for system performance and safety. Trimble ProPoint produces positions and orientations with latency less than 20 milliseconds at an update rate of up to 100 Hz.

 

⇲ Availability

Trimble has always aimed to deliver centimeter accuracy for everyone, everywhere. Enabling our customers to operate in more challenging environments expands the variety of potential applications and offers a better return on investment for the end-user of the technology.

ProPoint in GNSS or GNSS/INS modes delivers centimeter accuracy in dense vegetation and urban environments with tall buildings, where classic positioning engines would have considerable difficulties. Both on-road and off-road vehicles, operating near buildings or under bridges, continue to achieve close to 100% availability in positioning and orientation.

Users in RTK mode benefit from integrated xFill technology by allowing continuous operation when the correction source is lost. This increases availability beyond what RTK users can achieve who are not leveraging Precise Point Positioning (PPP) technology to compensate for the loss of local base corrections.

 

⇲ Integrity

Providing high-quality accuracy estimates is critical for centimeter-level control of autonomous machines. Applications rely on these horizontal and vertical indicators to know when to trust position and orientation information. An incorrect estimate can result in damage to the final product, equipment, or, in the worst case, personnel.

Considerable testing of ProPoint has been conducted in a variety of environments worldwide, resulting in estimates that reflect the quality in determining accuracy, benefiting from improved noise models and the ability to adapt these models to any environment.

 

PROVEN PERFORMANCE

When evaluating ProPoint's capabilities, it was essential for the Trimble team to conduct field tests that closely approximate the types of terrain challenges customers identify daily. To this end, Trimble carried out an extensive testing campaign, employing different types of tests worldwide, each comprising different environments characterized by common GNSS obstacles such as trees and buildings. Examples of RTK GNSS and GNSS/INS tests conducted to compare the previous generation positioning engine with the new Trimble ProPoint positioning engine are presented below, with similar results obtained in RTX modes.

 

⇲ GNSS Performance

The Trimble ProPoint positioning engine provides superior GNSS performance in challenging environments. Observing RTK data from a 19 km baseline with the rover located in a suburban environment (5 m from a 2-story building with nearby trees), an improvement is seen in both the magnitude of position errors and the estimation of position errors. In each set of graphs, the upper axis indicates the horizontal position error while the lower axis indicates the vertical position error.

The blue lines indicate the position error from the known survey point. ProPoint produces a more precise and accurate solution than the previous generation RTK engine.

The red and orange lines indicate the 1-sigma, 2-sigma, and 3-sigma error estimates issued by the receiver in real-time. It can be seen that these have dramatically improved with ProPoint, better estimating the actual position error under these unfavorable conditions. Error estimates are also less erratic. This will help users better evaluate, in real time, whether the position solution meets their accuracy requirements.

 

Trimble ProPoint Positioning Engine

 

CONCLUSION

The Trimble ProPoint engine is an investment for the future, prepared to merge current and future GNSS measurements with inertial and sensor data to deliver premium performance in the most challenging environments. If accuracy, availability, and integrity are key requirements for your application, then ProPoint is the ideal choice.