Trimble IonoGuard™: protecting RTK GNSS from ionospheric disturbances
Trimble IonoGuard Overview
Solar activity peaks every 11 years with the next maximum predicted in 2025. This has a direct impact on delays and stability of GNSS signals and can have a negative impact on precision positioning. This paper identifies the challenge to GNSS users and manufacturers and how Trimble IonoGuardTM technology is mitigating the effects in ProPoint® GNSS technology enabled receivers.
Trimble IonoGuard Introduction
High precision GNSS users operating around equatorial and high latitude regions are familiar with position degradation from ionospheric disturbances. The last solar cycle, which peaked in 2014, was a relatively mild cycle compared to the recent historical record. Understanding that the follow on cycles may not be as moderate or geographically constrained, Trimble embarked on a data collection and development exercise early in the current cycle to ensure hardware and software was ready to maximize customers’ productivity. The result is Trimble IonoGuard technology.
We will take a look at how the ionosphere affects GNSS positioning and how Trimble IonoGuard is being used in the field today to optimize accuracy, availability and integrity in critical applications.
What are ionospheric disturbances and how do they affect GNSS?
Solar Cycle
Sunspots are temporary areas of the sun, caused by active magnetic flux reducing convection. The more sunspots, the more areas with magnetic activity. These areas can eject particles which add to the solar wind and may be carried to Earth. As more particles hit earth, the layer of atmosphere known as the ionosphere becomes more charged, and the GNSS signal delay resulting from the ionosphere increases. The Sun’s magnetic field flips once every 11 years and sunspot activity is correlated with this 11-year cycle. It is difficult to predict the magnitude of the solar cycle. Early models predicted that the maximum in 2025 would be similar to the previous. See the NOAA model below. However, new models and measurements indicate a cycle more similar to the cycle which peaked around 2002.
Ionosphere
The ionosphere is an ionized layer of the upper atmosphere that has a large number of electrically charged atoms and molecules, which cause a delay in the GNSS signals passing through it. The ionosphere varies over time, with significant differences between night and day, when the solar energy source is present. The impact on radio waves is dependent on frequency. The delay is inversely proportional to the square of the frequency, as a result L1 (at a higher frequency) has less delay than L2. A common metric describing the ionosphere is TEC or Total Electron Content. This is the total number of electrons integrated between two points, e.g., from the receiver to the satellite in a straight line. The units are electrons per meter squared; with the frequency of a signal, this can be converted to an equivalent signal delay.
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 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 thus experience a much higher delay. In the absence of a geomagnetic storm, the ionosphere is correlated with solar activity and hence the peak delay is in the early afternoon, with a lower delay overnight.
Equatorial Effects
During the evening hours around the geomagnetic equator, plasma rises in the ionosphere. This can lead to instability within the ionosphere and result in scintillation. This is an effect where the GNSS signals are impacted by varying electron densities in the ionosphere, which can result in very rapid phase and amplitude change, leading to poor tracking, complete loss of lock, and/or carrier phase cycle slips. When the instability occurs, it may be limited to certain regions or bubbles of the ionosphere and thus only a subset of satellites may be affected. In South America, where many of our agriculture and mining customers operate, scintillation occurs an hour or two after sunset and will typically last for 4-5 hours. It also follows an annual cycle with most disturbance between September and March and severity dependent on the 11 year solar cycle.
Polar Effects
Sunspots can eject material from the sun that travels a few 100 km/s to a few 1,000 km/s. This phenomenon is called Coronal Mass Ejection. If the material is ejected with an Earth-bound trajectory, it will typically take a few days to reach Earth. Due to the Earth’s magnetic field, it tends to travel to the poles, where it can significantly impact the ionosphere at either pole. In addition to impacting GNSS performance, this can sometimes be observed as the Northern (or Southern) lights, a phenomenon referred to as aurora borealis (or aurora australis). During more intense storms, the Northern lights can extend to the continental US, and the impact on the ionosphere can affect GNSS signals at lower latitudes.
While a loss of lock and cycle slips can occur in the polar region, data typically shows less severe amplitude scintillation compared to the equatorial regions, with limited cycle slips and less disruption.
Global Effects
Although most noticeable disturbances occur around the geomagnetic equator and northern latitudes, we have also observed an increase in the ionospheric delay measurement globally as we approach the solar cycle maximum. While dual and triple frequency techniques are leveraged to mitigate these effects by using an ionospheric free combination, this also increases measurement and position noise. With the potential for large solar storms to cause disruptions in mid-latitude operations, ionospheric protection has become a critical global requirement for GNSS receivers.