The Effect of Space Weather Phenomena on Precise GNSS Applications

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1 FUGRO SATELLITE POSITIONING Doc. Ref.: A TCBRC1 The Effect of Space Weather Phenomena on Precise GNSS Applications December 2014 PUBLIC

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3 Table of contents The Effect of Space Weather Phenomena on Precise GNSS Applications Table of contents... i Executive summary...1 Introduction...1 Solar phenomena...2 Solar wind...2 The Solar Cycle Sunspots...2 Coronal Mass Ejections (CME)...3 Coronal holes...3 Solar flares...4 The Earth s magnetosphere...4 Solar flux...5 The ionosphere...5 Ionospheric disturbances...8 Solar cycle Effects of increased solar activity...8 Possible solutions Improved ionospheric models Multi-frequency data New GNSS signals and systems Data links Ionospheric scintillation Conclusion Fugro Satellite Positioning i Public A TCBRC1

4 Executive summary The performance of Global Navigation Satellite System (GNSS) receivers on Earth can be adversely affected by certain Space Weather phenomena whose occurrence is usually related to the 11-year solar cycle. Solar Cycle 24 is currently (2014) peaking and consequently, the impact on GNSS positioning, especially precise GNSS positioning, is heightened. The solar cycle is measured by counting sunspots, which develop periodically on the surface of the sun and which correlate well with the levels of solar wind (hot plasma [charged particles] emitted at great speed by the Sun). Solar wind disturbs the Earth s ionosphere and affects its magnetic field, with elevated levels causing geomagnetic storms, aurorae and comet tails in addition to reducing GNSS positioning performance. Coronal mass ejections (CMEs) and coronal holes also periodically discharge large quantities of fast solar wind which merge with the ambient solar wind and can cause severe geomagnetic storms and ionospheric disturbances on shorter temporal scales. The orientation of the Earth s own Interplanetary Magnetic Field (IMF) varies unpredictably and this alters the effect of the solar wind. As well as the charged particles of solar wind, the Sun emits electromagnetic waves over a range of frequencies (X-ray, ultraviolet, visible, infrared and radio waves), termed Solar Flux. Though total energy emitted changes little, the radiation sources which cause ionisation in the Earth s atmosphere display large fluctuations. Radio signals from GNSS satellites, passing through the ionosphere are slowed down by the presence of free electrons so the fluctuations in the above phenomena, which are not easy to forecast reliably, have a direct effect on GNSS satellite range measurements. The ionosphere itself exhibits diurnal changes to its Total Electron Count (TEC) which may also delay certain frequencies of electromagnetic energy, especially at dusk. Large gradients in the TEC may result from the relatively unpredictable presence of plasma bubbles, wave-like disturbances, streams etc. in the ionosphere, and are frequently also associated with the existence of smaller structures that display amplitude and phase scintillation effects. When a GNSS signal encounters large gradients in TEC, GNSS satellite range measurement error is difficult to model, and scintillations can even cause a receiver to lose lock on a satellite if the area of scintillation falls between the satellite and receiver. These effects vary significantly, both spatially and temporally, with polar latitudes and equatorial areas being particularly susceptible to ionospheric disturbances, areas where offshore oil exploration is ongoing. Use of improved ionospheric models, multi-frequency GNSS range data, additional signals and as many constellations as possible can all be employed to reduce the general effects of space weather on GNSS range measurement, and use of robust, redundant broadcast infrastructures and overspeed corrections reduce the risk of failed correction message delivery. Scintillations however present one of the greatest challenges to precise offshore GNSS positioning, particularly in areas such as Brazil and South East Asia, and Fugro has focused efforts on developing a monitoring system that is capable of forecasting the likelihood of scintillations for a given location, up to 24 hours ahead. Introduction Several space weather phenomena, when strong enough, may create disturbances in the Earth's magnetosphere and ionosphere that degrade performance and satellite tracking ability for Global Navigation Satellite System (GNSS) positioning receivers; this has most significance for the more precise techniques, e.g. PPP-RTK and Network-RTK. The phenomena include high-speed solar wind streams, Coronal Mass Ejections (CME), large solar flares, coronal holes etc., which are usually (but not always) related to the 11 year sunspot or solar cycle. Cycle 24 started in December 2008 and is presently (2013/2014) at solar maximum, consequently the impact on precise GNSS positioning is heightened. This document gives a brief explanation of space weather phenomena related to the solar cycle, their impact on GNSS positioning and the actions taken by Fugro to mitigate their effects Fugro Satellite Positioning 1/17 Public A TCBRC1

5 Solar phenomena Solar wind The Effect of Space Weather Phenomena on Precise GNSS Applications Solar wind is hot plasma 1, streaming away in all directions from the Sun s corona 2, with an average velocity of about 400 km/s, see Figure 1. This streaming solar plasma interacts with the Earth s magnetic field and results in a number of phenomena such as geomagnetic storms, aurorae and the plasma tail of comets. The Solar Cycle Sunspots Figure 1 Solar wind (from Sunspots are relatively cool areas that appear as dark blemishes on the surface of the Sun, see Figure 2. They are formed when extremely strong magnetic field lines just below the Sun s surface become twisted and poke through the solar photosphere. A sunspot will appear, progressively attaining a diameter as large as 80,000 km before dissipating, typically after a few days or weeks. Figure 2 Sunspots (from Seen from the Earth sunspots appear to rotate over the surface of the Sun, following the Sun s Carrington Rotation period of about 27 days. Sunspot populations quickly rise and more slowly fall on an irregular cycle about every 11 years; the socalled sunspot cycle or solar cycle. Sunspots are a useful indicator for the level of solar activity since they correlate with the intensity of solar wind. Significant variations of the 11 year period are known over longer spans of time. For example, from 1900 to the 1960s the sunspot count (at solar maximum) trend has been upward; since then, it has diminished somewhat. The sunspot number is basically the sum of the visible dark areas on the surface of the Sun with adjustments for the instrumentation used. Figure 3 shows the sunspot numbers between 1700 and 2008 (the end of solar cycle 23 or the beginning of solar cycle 24). 1 Solar plasma is hot ionised gas, consisting of electrons and protons (95% of positively charged particles) with a mean density of about 5 per cm 3 2 The corona is the gaseous region above the Sun s surface that extends millions of kilometres into space, it comprises plasma so hot it has escaped the Sun s gravitational field 2014 Fugro Satellite Positioning 2/17 Public A TCBRC1

6 Figure 3 Sunspot numbers between 1700 and 2008 (plotted from Coronal Mass Ejections (CME) A CME is the eruption of a magnetised plasma bubble from the Sun s corona, see Figure 4. CMEs have velocities from 400 km/s up to 2000 km/s. Though short-lived, a typical CME can carry more than 1 trillion tons (10 15 kilograms) of hot plasma into interplanetary space. As the material from the CME merges with the solar wind, it can create a shock wave that accelerates particles to high energies and speeds. Fast CMEs contain strong magnetic fields that interact with the Earth s geomagnetic field; in the case of longer duration (3 hours) CMEs, those with a southward-oriented magnetic field can interact with the Earth s geomagnetic field, resulting in severe geomagnetic storms and consequently ionospheric disturbances. Figure 4 Coronal Mass Ejection (from Though it takes about three days for material from a CME to reach Earth, it is difficult to forecast its direction or severity, hence what the effect will be on GNSS positioning. Coronal holes Coronal holes appear as vast dark areas on x-ray and ultraviolet photographs of the Sun. They are low density magnetic voids in the corona from which solar wind continually streams out into interplanetary space. Coronal holes are usually located in the Sun s polar regions and are the origin of fast solar wind, e.g. 800 km/s; solar wind from the equatorial regions, on the other hand, is slower. Long-lived coronal holes may reappear on the Sun s disc as the Sun rotates around its axis. Coronal holes are distinct from sunspot groups, which have a magnetic structure that rises out and then reconnects nearby, thus inhibiting its contribution to the solar wind Fugro Satellite Positioning 3/17 Public A TCBRC1

7 Solar flares The Effect of Space Weather Phenomena on Precise GNSS Applications Solar flares are also an important source of space weather effects. They are usually associated with the strong magnetic fields seen in sunspots and produce a rapid increase in extreme ultraviolet and X-ray emissions. Photons emitted by solar flares also impact low frequency navigation systems e.g. eloran and HF radio communications in the sunlit hemisphere. The Earth s magnetosphere The solar wind carries with it a magnetic field of its own, called the Interplanetary Magnetic Field (IMF). The Earth s magnetosphere, see Figure 5, which is the region of space influenced by the Earth s own magnetic field, shields us from most of the charged particles in the solar wind. IMF orientation is important in defining the shape of the Earth s magnetosphere: Southward IMF conditions enable a very efficient transfer of the energy in the solar wind to the magnetosphere, but if the IMF turns suddenly north, a sharp expansion can occur with associated ionospheric and magnetospheric perturbation. However when the IMF is oriented northwards, the Earth s magnetic field is less affected by even a strong solar wind shock resulting from a CME. Figure 5 The Earth's magnetosphere (from Nevertheless, charged particles and electromagnetic energy from the solar wind do enter the magnetosphere; whence they are captured by the magnetosphere and flow along its geomagnetic field lines towards the North and South poles. When the Earth s electrically neutral atmosphere encounters proton radiation from the solar wind, it lights up over the Polar Regions in a phenomenon known as an aurora (see Figure 5). Aurorae are the only visible manifestations of the collision between solar wind and geomagnetic field. Depending on the severity of the solar wind, the auroral footprints can extend into lower latitudes. Figure 6 An aurora in the high latitudes 2014 Fugro Satellite Positioning 4/17 Public A TCBRC1

8 Solar flux The Effect of Space Weather Phenomena on Precise GNSS Applications In addition to solar plasma resulting from the above-described phenomena, GNSS positioning is also affected by solar flux: electromagnetic waves radiated by the Sun over a wide range of wavelengths, including the X-ray, ultraviolet, visible, infrared and radio waves. The total radiated energy per second across all wavelengths is relatively unchanging 3 : In fact within the spectral range of maximum energy flux (the visible and neighbouring infrared and ultraviolet ranges) the intensity of solar radiation is practically constant, varying by less than 0.3%. Only the minor radiation sources (the radio, extreme ultraviolet and X-ray spectral regions) which cause ionisation in the Earth s atmosphere, display large fluctuations, depending on solar activity. The solar flux on the radio wavelength of 10.7 cm (2,800 MHz) is well correlated with X-ray, EUV, and UV fluxes. This 10.7cm radio flux is measured daily at the Algonquin Radio Observatory, near Ottawa at 17:00 UT and is known as the F10.7, or Covington index (CI). It varies from a minimum near 65 (corresponding to sunspot number zero at solar minimum) to a maximum of about 200 corresponding to a sunspot number of about 150 to 160. Although 10.7 cm radio flux does not play any role in the formation of the Earth s ionosphere, because of a high correlation of F10.7 with X-ray, EUV and UV fluxes (and also because it is relatively easy to measure on the ground) it is one of the most commonly used indicators of solar activity. Note that the F10.7 index also shows a periodic variation which correlates with the Sun s c. 27 day rotation period (see Figure 9) as well. The Kp index is a quasi-logarithmic index, computed on a three-hour basis, which represents the overall level of planetary geomagnetic field disturbance. It is derived from ground-based magnetic field measurements and ranges from 0-9, with each scale step being ten times more disturbed than the previous step at the higher end of the scale. A typical quiet day will have Kp values less than three. Kp values between three and five correspond to a moderately disturbed field, while Kp values of six or greater occur during a major magnetic storm. The Ap index, another parameter describing planetary geomagnetic activity, is a 24-hour average of overall disturbance levels on a linear scale. For quiet days, the Ap index is generally below eight, while values greater than 20 indicate significant disturbances, which are usually encountered in high latitude auroral areas and along the geomagnetic equator. The Ap index ranges are as follows: less than 16: Quiet; 16-29: Active; 30-49: Minor Storm; 50-99: Major Storm; : Severe Storm. The highest value the Ap index has reached recently was 204 (2003) during the so-called Halloween storm at the peak of Solar Cycle 23, see Figure Ap Kp Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Nov Nov Nov Nov Nov Nov Nov Nov Nov Kp Ap The ionosphere Figure 7 Kp and Ap indices around 29 October 2003, when the Halloween storm occurred at the peak of Solar Cycle 23 (based on data from ftp://ftp.ngdc.noaa.gov) As solar wind encounters the upper reaches of the Earth s otherwise neutral atmosphere it ionises it (causes it to release free electrons). The amount of ionisation depends primarily on the Sun and its activity level, as reflected by the 11-year solar cycle. 3 The total energy at the top of the Earth s atmosphere (at a Sun-Earth separation of 1 astronomical unit) is 1370 watts/m 2 and is called the solar constant Fugro Satellite Positioning 5/17 Public A TCBRC1

9 The ionosphere ranges from km above the surface of the Earth, however during the evening hours the lower bound of the ionosphere rises to 200 km above the surface because as the magnetosphere turns away from the Sun fewer solar particles interact with the atmosphere. Radio signals from satellites pass through the ionosphere and experience a propagation delay or a travel time that is greater than would occur in a vacuum, due to the presence of free electrons. It is well-known that the ionosphere is a dispersive medium and the type of interaction between a radio wave (from KHz to GHz) and the ionosphere depends on the frequency of the wave. Most of the electrons are concentrated in the F-layer, at a height of about 300 km, see Figure 8. The ionospheric delay is a function of the Total Electron Content (TEC) of the ionosphere along the signal path and the frequency of the radio signal: lower frequencies having longer delays. One TEC Unit (TECU) is electrons/m 2. At night, as the lower extreme of the ionosphere rises, low frequency signals will bounce off this lower limit, causing the skywave or skip effect well known to users of terrestrial systems such as Loran-C. For radio operators skip enables them to transmit and receive over much longer distances (depending on the frequency of operation) while for the navigator attempting to position his vessel skip can cause incorrect range measurements inducing errors in his position. Figure 8 The Earth s atmosphere (from As an example, for the GPS L1 frequency ( MHz) the ionospheric delays corresponding to one TECU is equal to 16.2 cm. The total slant range delay caused by the ionosphere is in the range of 1 50 m over the course of a day. Generally, during relatively stable ionospheric periods the TEC and its gradient is predictable over a period of several days. For GPS, a US Air Force Master Control Station predicts the ionosphere, based on a simple empirical model which the model parameters estimated by fitting to ionospheric measurements, and uploads the parameters to the satellites. A user s GPS receiver then receives the parameters via broadcast GPS navigation message to compute the ionospheric delay and uses it as part of its position computation. This predicted model of the ionosphere is called the Klobuchar model after its developer. In Figure 9, the global ionospheric daily variation and Kp and F10.7 indices over 45 days are shown. In this figure, the pentagrams and bars show respectively location and level of the daily maximum TEC in the ionosphere. The periodic variation of the F10.7 index due to the Sun s rotation (period of ~27 days) is clearly visible. Also, the periodic variation in the level of ionospheric maximum TEC can be seen Fugro Satellite Positioning 6/17 Public A TCBRC1

10 Figure 9 Illustration of the ionospheric global variation and Kp and F10.7 indices over 45 days Figure 10 shows solar radiation (F10.7) correlates with vertical ionospheric delay for GPS L1. Figure 10 Two-hourly global maps of vertical ionosphere delay on L1-GPS on 8 Feb Fugro Satellite Positioning 7/17 Public A TCBRC1

11 Ionospheric disturbances The Effect of Space Weather Phenomena on Precise GNSS Applications Since ionospheric delay follows local noon as well as correlating well with the Sun s 27 day Carrington rotation period and the 11-year Solar Cycle, these background components can be predicted with some degree of confidence. However, when a GNSS signal encounters large gradients in TEC, the error in the range measurement is difficult to model and therefore cannot be transmitted to users receivers in the usual way. These gradients can be caused by plasma bubbles, wave-like disturbances, streams, or even sharp boundaries between enhanced or depleted regions in TEC. Frequently associated with these large gradient regions are smaller structures within the ionosphere that can be associated with scintillation effects. Scintillation can cause both amplitude and phase fluctuations that can individually or together cause a GNSS receiver to lose lock. Phase scintillation is typically seen during auroral periods in high latitudes but can occur at other times depending on the level of ionospheric disruption. Both amplitude and phase scintillations occur mostly in equatorial regions during the night hours. Large gradients in TEC are primarily associated with equatorial and polar regions, and are more frequently observed during conditions of high geomagnetic activity. However, large gradients in TEC can also appear in mid-latitudes during moderate to severe geomagnetic disturbances and can significantly impact GNSS users. During the course of the year, the equinoxes (September October, March April) are statistically more likely to see geomagnetic disturbances and ionospheric structuring than other months. This is due to transit of the Sun between the earth s northern and southern hemispheres. During these seasons it is not uncommon to experience periods of degraded GNSS performance in the late afternoon and especially at dusk and after sundown. These periods typically occur at the same time each day and last for several days or a few weeks. Solar cycle 24 In 2008/2009 we were at a solar minimum, which marked the beginning of solar cycle 24. The Solar Cycle 24 Prediction Panel expects this solar cycle to be below average in intensity with a maximum sunspot number of 90; this maximum started in mid-may 2013 and will last until mid-2014 (see Figure 11). Figure 11 Solar cycle 24 prediction (based on data from and Effects of increased solar activity As the solar cycle progresses, the average daily sunspot number rises, creating conditions favourable to solar flares and CMEs. These phenomena add energetic particles and solar materials to the solar wind. Disruption of the ionosphere occurs due to the additional energy striking the magnetosphere and entering the upper atmosphere. The changes are rapid and significant and the ionospheric model used in GNSS receivers to compute their position from single frequency data no longer matches reality, resulting in biased position estimates Fugro Satellite Positioning 8/17 Public A TCBRC1

12 Other solar cycle effects cause damage to satellite electronics and increase drag on spacecraft, altering their orbits. This energetic wind also affects the Earth s magnetic field and can cause significant DC ground currents, potentially disrupting local power grids. During the solar maximum of Solar Cycle 22 (1989), the Hydro-Quebec electric power system failed, resulting in six million people in the US and Canada being without power for nine hours. With increased solar activity, the polar regions and a band extending to about 15 North and South of the geomagnetic equator are most susceptible to ionospheric disturbances, see Figure 12. In equatorial regions the effect is on the TEC gradient, which will change quickly. These regions include Brazil, Central Africa and parts of Southeast Asia where offshore oil exploration is ongoing and where significant GNSS position errors have been reported. There are a number of different types of ionospheric events that occur in these regions which are grouped into a broad category referred to as equatorial anomalies Canada, Alaska and other high-latitude regions are affected as well, due to auroral activity Ionospheric disturbances have the smallest impact on standard DGNSS positioning performance in midlatitude areas like the Gulf of Mexico or the North Sea, although these areas may be affected during moderate to large geomagnetic storms. Typically, a greater number of users are affected by ionospheric disturbances as Kp and Ap indices increase. In areas where ionospheric prediction models (like the Klobuchar model for GPS) are inaccurate, see Figure 14 and Figure 15. The actual TEC values need to be measured to allow users to correct their position with a more realistic estimate of the ionospheric delay. Figure 12 The geomagnetic equator and auroral regions 2014 Fugro Satellite Positioning 9/17 Public A TCBRC1

13 Figure 13 shows the typical daily maximum 3D position error over the world due to the ionosphere when the ionospheric effect is not accounted for in single-frequency GPS data. When the Klobuchar model is used, the ionospheric effect is reduced by only 50% (see Figure 14). Figure 13 Typical daily maximum 3D positioning error due to ionosphere effects Figure 14 Typical maximum 3D positioning error for a single-frequency code-only GPS receiver with ionospheric effects accounted for using the Klobuchar model 2014 Fugro Satellite Positioning 10/17 Public A TCBRC1

14 Figure 15 shows position errors for a 1700 km baseline between Belem and Recife in Brazil, computed using standard (L1 only) differential GPS corrections and the broadcast ionospheric model parameters. This data was collected during the Halloween storm of October The horizontal position error (95%) is 12.8 m. Figure 15 Horizontal position errors for a baseline of 1700 km in Brazil. Position was computed using GPS L1 corrections and the Klobuchar ionospheric model Figure 16 shows the effect of a severe ionospheric storm on the number of tracked satellites for a receiver in the north-western United States. For comparison, the number of tracked satellites for the corresponding period by the same receiver is also shown when the ionosphere was quiet again (30 days later, the period was shifted by 120 minutes) :00: :15: :30:00.0 Number of satellites 20:45: :00: :15: :30: :45: :00: :15: :30: :45: :00: :15: :30: :45:00.0 Number of satellites (storm) Number of satellites (no storm) Figure 16 Effect of a severe ionospheric storm on the number of tracked satellites 2014 Fugro Satellite Positioning 11/17 Public A TCBRC1

15 Possible solutions Improved ionospheric models The Effect of Space Weather Phenomena on Precise GNSS Applications Employing improved ionospheric models (e.g. than the GPS Klobuchar model) can enable better compensation for ionospheric delays in GNSS range measurements. For example, NeQuick is an empirical model that has been proposed as a real-time ionospheric correction model for single-frequency positioning using the future European GNSS, Galileo. NeQuick can account for approximately 70% of the ionospheric delay. Also, certain regional and global numerical ionospheric models can be dynamically updated with actual data, allowing improved predictions. A drawback of this method is that the models may not accurately represent actual ionospheric delays, especially in the case of severe disturbances; and performance may vary in different areas for a variety of reasons. Also, this approach requires that additional information must be transmitted to the user along with the corrections, which is feasible only if enough bandwidth is available. Multi-frequency data An alternative to modelling is using dual-frequency GNSS data. Since the ionosphere is a dispersive medium, the ionospheric delays depend upon the frequency of the signals and can, to first order, be removed by forming a linear combination of observations at two frequencies, for example, GPS L1 and L2 ( MHz). If triple-frequency data is available, second order effects can be accounted for as well. However, the noise in the resulting ionosphere-free observation will increase. Alternatively, the first order delay can be computed from dual-frequency data. In this case the ionospheric information from the reference site needs to be transmitted over the DGNSS data link along with the standard single frequency range errors. At the mobile, it is then possible to reconstruct an ionosphere-free range error. Figure 17 shows horizontal position errors for the same baseline as Figure 15, but this time the results were obtained using dual frequency data. It is clear from this figure that the errors have been significantly reduced (1.2 m (95%)) and that using dual frequency data is very effective. New GNSS signals and systems Using dual-frequency data provides an effective means to deal with ionospheric errors. However, it may be possible that the ionospheric disturbances (e.g. plasma bubbles) are such that the receiver loses lock on a number of satellites and the number of remaining satellites is too small to estimate a precise position. For GPS, this could in particular occur for the L2 signal, which originally was never intended for civil use, although most receiver manufacturers have found ways around this and are able to provide L2 observations, albeit with a lower signal to noise ratio. However, as part of the GPS modernisation, a new and stronger civil L2 signal (L2C) has been defined. This signal is currently (2014) available on the 13 operational satellites launched since It is expected that by GPS satellites will be transmitting on L2C. To benefit from these new signals, upgraded or new receivers are required Fugro Satellite Positioning 12/17 Public A TCBRC1

16 Figure 17 Horizontal position errors for the same 1700 km baseline as in Figure 15. Position was computed using dual frequency GPS observations Shown in Figure 18 are the signal to noise ratios of the traditional L2 and new L2C signals for the same GPS satellite, observed by a receiver which was initially tracking L2 for one hour, followed by one hour of L2C tracking. Also shown are the L1 signal to noise ratios. The signal to noise ratio of L2C and L1 are virtually the same. Fugro s G2 positioning service already makes full use of the American GPS and the Russian GLONASS, which are both operational. The European Galileo and Chinese BeiDou are currently under development. The impact of using two or more systems instead of just GPS is significant. Figure 19 shows the total number of GPS, GLONASS, Galileo and BeiDou satellites in Recife, Brazil, in April At that time, the number of satellites compared to GPS only almost doubled on average. Fugro is working on extending its G2 service to also support Galileo and BeiDou Fugro Satellite Positioning 13/17 Public A TCBRC1

17 Figure 18 L1 and L2 (left) and L1 and L2C signal strengths for the same satellite and receiver Data links Figure 19 Number of visible GPS, GLONASS, Galileo and BeiDou satellites for Recife in April 2014 GNSS correction data is usually broadcast over satellite or terrestrial radio links. These links can be affected by ionospheric disturbances as well. To reduce the risk of failed delivery, Fugro uses multiple and redundant data links and satellites. These satellites are in different orbit positions, to reduce the risk of failure in case of ionospheric scintillations. Fugro also sends corrections at twice the necessary rate. This allows for missed messages on the data link without impairing performance. In addition to employing redundant satellites, Fugro also has redundant Network Control Centres (NCCs) each generating every Fugro beam. Primary and backup beams are not uplinked by the same NCC, unless one NCC fails. Each NCC also has a physical backup location. Furthermore, in the case of an uplink failure, Fugro has alternate uplink locations for all of the Fugro beams, reachable by either NCC. Figure 20 illustrates how Fugro ensures the independence and robustness of their GNSS positioning services Fugro Satellite Positioning 14/17 Public A TCBRC1

18 Figure 20 Example of the independence and robustness of Fugro s high-precision GNSS positioning services Figure 21 shows the coverage area of Fugro s satellite beams, used to provide GNSS correction data to users worldwide. This figure shows that there is significant separation between the satellite positions and at the same time sufficient overlap between coverage areas so that in the event that one link fails, another can take over. Figure 21 Coverage areas of Fugro s satellite beams, used to provide GNSS correction data to users all over the world Ionospheric scintillation Scintillation of GNSS signals is a consequence of the existence of small-scale spatial irregularities (plasma bubbles) in ionospheric electron density. GNSS signals are diffracted and refracted when passing through the ionospheric irregularities and this leads to rapid fluctuations in signal intensity (Amplitude Scintillation) and phase jittering (Phase Scintillation). Ionospheric scintillation is important because it can severely degrade GNSS receiver performance by causing e.g. signal power loss (even loss of lock) adversely affecting signal tracking and increasing measurement noise level. Plasma bubbles are created due to geomagnetic disturbances, mostly at low and high geomagnetic latitudes and more frequently during periods of increased solar activity. Plasma bubbles in the equatorial region occur in the ionosphere s F-region between sunset and midnight with activity occasionally continuing until dawn Fugro Satellite Positioning 15/17 Public A TCBRC1

19 The geomagnetic disturbances usually last a few days and as a consequence electron density irregularities are likely to be repeated daily after sunset for a given location in the equatorial region. Plasma bubbles in equatorial regions usually result in both phase and amplitude scintillation and GNSS users may often experience loss of lock. In high latitudes, the ionospheric irregularities are mainly due to charged particle radiation originating from solar wind. The charged particles penetrate the atmosphere and cause ionospheric irregularities in the lower ionosphere (the E-layer, at a height of about 100 km) and can occur during the day or night. At night time these irregularities are visible as aurorae: as the electron density in the E-layer is low, the irregularities usually do not result in amplitude scintillation, so at high latitudes GNSS users mostly experience only phase scintillation. Occurrence of scintillation depends on the signal frequency, local time, season, solar and magnetic activity. It depends also on satellite zenith angle and the angle between the signal path and the Earth s magnetic field. To assist in prediction of this phenomenon, in 2013 Fugro developed a scintillation monitoring system which provides the status of scintillations over the preceding 24 hours for a user location, and forecasts scintillation for the next 24 hours. Phase Scintillation is quantified by the σ φ60 index which is defined as the standard deviation of the GPS L1 signal phase noise over a one minute interval. Figure 22 shows Fugro s scintillation monitoring system results for a user location in 2 nd March 2014 in Brazil at 2230 UTC. In Figure 22, the top panel shows the regional scintillation map, the middle panel depicts L-band scintillation for visible geostationary satellites. And the bottom panel shows observed and predicted 3D position error at the user location due to scintillation (denoted by SDOP Scintillation Dilution Of Precision). Fugro s scintillation forecast service will be provided initially as an interactive website for clients. It will be operational soon after testing and tuning the algorithms is complete. Figure 22 Fugro real-time scintillation forecast (user location of Lat. = 11.2, Long. = 36 ) in Brazil on 2 nd March Fugro Satellite Positioning 16/17 Public A TCBRC1

20 Conclusion The current solar cycle 24 is peaking during During this period Fugro is confident that by using state of the art GNSS technology capable of tracking all available signals and through its independent and redundant data links, it will be able to keep on providing a range of exceptionally robust and trustworthy GNSS correction services to its global user base. Notwithstanding this, unpredictable ionospheric delay errors and ionospheric scintillations continue to plague users periodically in certain geographic locations. Affected users may take certain measures to mitigate the harmful effects of their exposure to these phenomena. Table 1 lists the effects due to unpredictable ionospheric delay errors on Fugro s GNSS services. These errors are seen almost all of the time all over the world. Table 2 summarises the effects of ionospheric scintillations. Scintillations are more sporadic and more likely to occur near the geomagnetic equator, especially in Brazil, West Africa and the Polar regions. Service Exposure Mitigation Notes Starfix/Seastar L1 High Upgrade to dual frequency receivers. Starfix/Seastar HP None Starfix/Seastar XP None Marinestar GPS None Starfix/Seastar XP2 None Starfix/Seastar G2 None Marinestar GNSS None Some single frequency receivers can be upgraded to dual frequency. Table 1 Impact of ionospheric delay errors on Fugro s GPS and multi-constellation (GNSS) services Service Exposure Mitigation Notes Starfix/Seastar L1 Medium Upgrade receivers to track GLONASS Starfix/Seastar HP Medium/low Starfix/Seastar XP Medium/low Upgrade receivers to track GLONASS and use G2. Marinestar GPS Medium/low Upgrade receivers to track GLONASS and use Marinestar GNSS Starfix/Seastar XP2 Low Supports GPS and GLONASS Starfix G2 Low Support GPS and GLONASS; later in Marinestar GNSS Low 2014 also Galileo and BeiDou Table 2 Impact of ionospheric scintillations on Fugro s GPS and multi-constellation (GNSS) services == ENDS 2014 Fugro Satellite Positioning 17/17 Public A TCBRC1