GEOCOM presents solution for ionospheric scintillation

Protecting GNSS RTK observations from ionospheric disturbances

 

Solar activity peaks every 11 years, and in 2025 we will experience a new maximum. This phenomenon, evidenced by ionospheric scintillation, negatively affects GNSS RTK positioning, especially in terms of precision and productivity. This article reviews the impact of the solar cycle and ionospheric scintillation on GNSS users and manufacturers. Additionally, it presents GEOCOM's developments and technology to mitigate these effects on satellite positioning.

 

INTRODUCTION

High-precision GNSS positioning in equatorial and high-latitude regions is affected by position and precision degradations due to disturbances in the ionosphere, which are generated by solar cycles. During 2014, we experienced the last solar cycle, which, at its maximum intensity, did not have a significant impact on GNSS RTK positioning. Unlike the 2014 solar cycle, the developing solar cycle is already considerably affecting the productivity and precision of GNSS RTK positioning.

This situation is critical considering that the peak of this cycle is expected in 2025. For this reason, GEOCOM, with the support of Trimble's technology, embarked on a research and development project at the beginning of the current solar cycle to ensure that Trimble's GNSS receivers continue to provide accuracy and reliability under these complex conditions. To contextualize the work developed by GEOCOM, some concepts related to how the ionosphere affects GNSS positioning and how GEOCOM's technology is being used today in the field to optimize the accuracy, availability, and integrity of GNSS RTK positioning will be presented below.

 

 

What are ionospheric disturbances and how do they affect GNSS?

To understand ionospheric disturbances, it is necessary to know some related definitions, among which we find:

Solar Cycle, main characteristics

Sunspots are temporary areas on the sun caused by active magnetic flux that reduces convection. The more sunspots there are, the more areas with magnetic activity are generated. These areas can expel particles that add to the solar wind, potentially reaching Earth. As more particles impact Earth, the layer of the atmosphere known as the ionosphere becomes more charged, resulting in a delay of the GNSS signal. On the other hand, the magnetic field generated by the sun reverses once every 11 years, and sunspot activity is correlated with this cycle, making its prediction difficult.

Ionosphere

The ionosphere is an ionized layer of the upper atmosphere that has a large number of electrically charged atoms and molecules, which causes a delay in GNSS signals passing through it. For its part, the ionosphere varies with time, with significant differences between day and night, when the source of solar energy is present. On the other hand, the impact on radio waves depends on the frequency. In addition, the delay is inversely proportional to the square of the frequency, as a result, L1 (higher frequency) has less delay than L2. A common indicator that describes the ionosphere is TEC (total electron content). This is the total number of electrons integrated between two points, for example, from the receiver to the satellite in a straight line.

The delay through the ionosphere is not fixed and will change based on the time of day, year, and location. The elevation angle between the receiver and the satellite also impacts the magnitude of the delay. A high elevation signal will take the shortest path through the ionospheric layer of the atmosphere, as the path is perpendicular to the ionosphere. A low elevation signal will pass through the ionosphere at an angle and, therefore, will experience a much greater delay.

Equatorial Effects

During the evening hours around the geomagnetic equator, plasma rises in the ionosphere. This can lead to instability within the ionosphere and cause scintillation. This is an effect where GNSS signals are impacted by varying electron densities in the ionosphere, which can result in very rapid phase and amplitude changes, leading to poor tracking, complete loss of tracking, and/or cycle slips. When instability occurs, it can be limited to certain regions or bubbles in the ionosphere and, therefore, only a subset of satellites may be affected. In South America, and particularly in mining, scintillation occurs one to two hours after sunset and usually lasts 4 to 5 hours. It also follows an annual cycle with most disturbances between September and March, where its greatest impact depends on the maximum of the solar cycle.

Global Effects

Although the most significant disturbances occur around the equator and at high polar latitudes, an increase in the measurement of global ionospheric delay has also been observed as we approach the maximum of the solar cycle. Although dual and triple frequency techniques are used to mitigate these effects through an ionosphere-free combination, this also increases measurement and position noise. With the probability of large solar storms, disruptions in mid-latitude operations can be generated, therefore, ionospheric protection has become a requirement for GNSS receivers.

Impact on GNSS RTK operation

As mentioned above, ionospheric disturbance can lead to poor signal tracking and in some cases to a complete loss of GNSS satellite tracking. Disturbances can also be localized, causing difficulties for RTK algorithms when base and rover measurements are affected differently. In our country, mainly mining is the industry that mostly experiences these interruptions, due to the need for centimeter-level positioning in its operations. Therefore, there is a loss of productivity in its operations.

 

What is GEOCOM doing about it?

During the last three solar cycle maxima, GNSS users have experienced problems with GNSS RTK positioning. Each cycle has resulted in focused research and development to improve receiver tracking and processing algorithms. With the number of GNSS users doubling every cycle, significant efforts have been made to ensure that the technology performs optimally to minimize disruptions during future cycles.

 

Online GNSS Planning Tool

During solar activity maxima, where certain parts of the ionosphere are excited, greater signal delays will occur. Although ionospheric activity can typically occur at key times of the day, such as midday or sunset, that is not always the case. Trimble has established a global ionospheric measurement network, which through the online tool GNSS Planning allows users to plan ahead and avoid working during times when there is a higher probability of disturbance (Figure 1)

 

Data Collection

To devise effective ionospheric mitigation methods in GNSS RTK receivers, GEOCOM and TRIMBLE engineers have collected data in areas where the scintillation problem is critical. This data provided the basis for R&D development. Thus, signal processing techniques and RTK algorithms are refined using reproduced data. Then, the updated firmware is loaded into the receivers on site for real-time evaluation.

 

Technology to mitigate ionospheric effects
Ionospheric mitigation features have been included and improved in Trimble receivers over the last three solar cycles. To ensure readiness for current and future solar maxima, the data collection exercise was used to develop state-of-the-art technology to mitigate ionospheric effects.

 

Signal Processing Mitigation
During ionospheric storms, there can be significant frequency-dependent differences in phase delay. Therefore, it is important that the receiver does not rely on a single frequency to operate. Trimble's GNSS signal processing has been updated to track signals independently. Thus, considering that RTK algorithms depend on carrier phase measurements from all frequencies. During extreme ionospheric events, the receiver may lose carrier tracking for brief periods, often only for a few seconds. Improvements in the phase tracking algorithm with the technology proposed by Trimble have reduced the time required to recover carrier phase tracking and minimize possible disturbances. ProPoint is also key in tracked signals, meaning it can operate with any combination of triple, dual, or single frequency measurements.

 

Trimble ProPoint RTK Mitigation
Ionospheric activity mitigation technology has been tightly integrated into the Trimble ProPoint RTK engine. Optimal performance is achieved when the technology is enabled on both base and rover receivers. With this technology enabled at the base station, ionospheric information for each satellite is transmitted via CMRx or RTCM-MSM protocols to rover receivers, which use this information along with their own ionospheric measurements to optimize calculated positions.

If the mitigation technology is not enabled on the base receiver, then a rover will analyze the standard base messages and determine if ionospheric adjustments are needed. These adjustments are then used to improve positioning performance. This method is not as rigorous as having the more detailed ionospheric information sent from the base, but it can help when using a third-party base receiver.

 

Developments driven by GEOCOM

With the introduction of this technology, an additional traffic light system has been added to the receiver's web interface. A green, yellow, orange, or red icon indicates the level of ionospheric disturbance the RTK base station is experiencing on each satellite. The rover receiver displays this same information using the received base messages (Figure 2).

 

 

Thus, if mitigation messages are not being received from the base, then the circular symbols are replaced by squares and the base's ionospheric activity calculated by the rover is indicated. A history of the base's ionospheric activity is also available on the base's web interface (Figure 3).

 

Figure 3. WEBui spectrum analyzer

 

All ionospheric data is recorded in standard Trimble T04 raw data files, which can help GEOCOM support teams further diagnose issues (Figure 4).

 

Figure 4. Trimble Tools for T04 file analysis

 

Real-World Improvements

Base stations and mobile receivers continuously operating in regions with high levels of ionospheric disturbance have been running with mitigation enabled. During solar storm events, the improvement in positional accuracy is particularly evident. The blue line represents the horizontal positioning error in meters for the firmware operating with the enabled technology. The orange line represents classic RTK positioning and the green line represents the Xfill solution (Figure 5).

Figure 5. Improved horizontal displacement thanks to Ionoguard and the mitigation system developed by GEOCOM R&D

 

Similar results are achieved in terms of vertical accuracy (FIGURE 6). This demonstrates that during high ionospheric activity, mitigation ensures that not only centimeter-level accuracy is continuously available, but also that it is at an acceptable level for mining, construction, agriculture, and geospatial applications.

 

Figure 6. Improved horizontal displacement thanks to Ionoguard and the mitigation system developed by GEOCOM R&D

Conclusions

In equatorial and high-latitude regions, ionospheric disturbances are common, with maximum activity during solar storms and the 11-year cycle maxima. Industries with high operating costs such as mining require 24/7 centimeter-level accuracy.

Unfortunately, high-precision GNSS RTK positioning is most affected by these disturbances, and with increasing solar activity, the problem could become more complex. Using a global network of GNSS stations, Trimble's GNSS planning tool allows users to identify and plan around high ionospheric activity.

GEOCOM, in turn, has taken further measures to ensure the accuracy and availability of its customers' GNSS through the development of mitigation technology. Diagnostic information indicating the actual level of ionosphere on each satellite, along with historical data, is presented in a user-friendly web graphical interface. The mitigation technology, operating worldwide, has already shown significant improvements in positioning performance during periods of high ionospheric activity. Users can be confident that the superior performance of their ProPoint receivers has been further enhanced with the addition of ionospheric mitigation technology.

 

Acknowledgements

GEOCOM R&D for developing the ionospheric scintillation mitigation system.