INVESTIGATION OF THE EFFECTS OF SPECIFIC SOLAR STORMING EVENTS ON GNSS NAVIGATION SYSTEMS

calibration of GNSS antennas, investigation into solar storm effects to ground position, satellite movement theory. Geomatics Faculty Technology, College. Lecturer/student supervisor in advanced land surveying specializing in GNSS, accuracy and precision, and BuildingSMART. Research interests: examining GNSS error sources, calibrating GNSS systems, Building Intelligent Modeling (BIM). Publications: numerous technical reports, research reports, and conference papers. Abstract. Global positioning system (GPS) satellites operate at 1.2 and 1.5 GHz. The GPS signals travel through the atmosphere and are affected by space weather in the same way as other technological systems in space and on the ground. Space weather has been defined as the condition where the sun influences solar wind, the magnetosphere, and the ionosphere and thus can upset the performance and reliability of space borne and ground based technological systems. Adverse conditions in the space environment can cause disruption of satellite operations, communications, and navigation. Solar storms can add small delays to the GPS satellite signals and therefore impact accuracy. The purpose of this article is to investigate and to determine the effects of specific solar events on GNSS navigation systems. In parts of the Nordic countries, GPS is available with a precision of 1 centimetre through an auxiliary system of permanent tracking stations called position accuracy on the centimetre level (CPOS). This paper discusses the possible effects of space weather activity and uses these tracked data to investigate the effect of specific solar storms on single point positioning. Comparisons are made between the effects in northern and southern Norway.


Introduction
GNSS is a satellite system that is used to identify the geographic location of a user's receiver anywhere in the word. Positioning with GNSS is based on computing the user position from satellite position and user range measurements that are biased and contain random errors.
Inside the global positioning system are many different technologies that all combine and work together to find position on land, at sea, and in the air. The global positioning system depends on a number of GPS satellites orbiting the earth. GPS satellites transmit a unique navigational signal centred on two L-band frequencies of the electromagnetic spectrum: L1 at 1575.42 MHz and L2 1227.60 MHz. Each of the satellites continuously transmits on both frequencies the following information: exact orbit information, rough orbits of all other GPS satellites, and general system health. The radio signals travel a long distance before they reach the GPS device. During this stage, it passes through vacuum (in space) and the different layers of the earth's atmosphere.
Since the beginning of global satellite positioning, it has been a challenge to eliminate and correct the error sources that affect positioning accuracy. Scientists have found mathematical solutions to reduce these errors as much as possible. Some of the errors can be totally eliminated, while others can be corrected to a certain degree. Some of the errors, like ionospheric errors, are still being examined and modelled.
The goal of this paper is to examine whether the errors in a satellite's orbital location, due to high solar activity, can affect the accuracy of ground receivers and, if so, how these errors can be controlled.
When a ground receiver determines its position, there are many possible sources of errors: -Ionospheric and tropospheric delays -signal delays due to the signal passing through various layers of the atmosphere; -Orbital errors (ephemeris errors) -errors caused due to satellites transmitting inaccurate orbit parameters; -Signal multipath -these errors can occur when the signal is reflected off objects before reaching the receiver; -Receiver clock errors -the receiver clock is not as accurate as the atomic clocks on the satellites, which can lead to timing errors; -The number of visible satellite -accuracy is better if the receiver observes more satellites; -Geometry of the satellites -relative position of the satellites in the sky affects the accuracy, best if the satellites are spread widely. Researchers have been trying to find out ways to eliminate these errors. Some of the methods used are: -Differencing; -Using more signal frequencies; -Modelling ionospheric errors. The first sources of errors, ionospheric and tropospheric delays and orbit errors, are affected by levels of solar radiation activity. It is generally assumed that satellites' orbit errors are eliminated by differencing, in other words by using two or more receivers simultaneously (Būga 1999).

Description of research data
The sun goes through an 11-year cycle from stormy to quiet and back again. Solar maximum appears when sunspots are most numerous and solar minimum occurs when the number of sunspots is lowest. Even though sunspots are darker and therefore cooler areas on the sun, the sun is generally hotter during solar maximum than at solar minimum. Regarding solar cycles, there are 250 years of observations of which only the last 150 years are considered truly reliable.
Sunspots are dark areas on the surface of the sun. They appear dark because their temperature is lower compared to the overall surface temperature of the sun. Sunspots are magnetic regions on the sun that are thousands of times stronger than the earth's magnetic field. They typically last for several days, but large sunspots can remain for several weeks. The magnetic field is stronger on the darker area of the sunspot (umbra) and is weaker on the lighter area of the sunspots (penumbra). Sunspots are active areas that are strong emitters of ultraviolet light and X-rays. Sunspots are often sites for the emission of solar flares.
Predicting the activity level of a solar cycle is very important. An active sun can cause geomagnetic storms which may disrupt communications and power systems on earth. Additionally, mission planning for space requires prediction of the sun's activity years in advance. Nowadays there is a prediction panel for solar cycle. This panel includes members of NOAA, NASA, ISES and other US national representatives together with some representatives from European countries. The objective of this panel is to forecast how an upcoming solar cycle will develop, based on the records from previous observations and cycles (Kunches 2007).
There are different techniques used for predicting the amplitude of a solar cycle. Prediction of the maximum is more reliable 2 to 3 years after the solar minimum. Predictions of maximum before that time are not so reliable.
Although the onset and the peak of the cycle can be predicted to the nearest year or two, the occurrence of sunspots and especially solar flares cannot be reliably predicted because the number of these events is not constant from cycle to cycle. The current cycle is on Cycle 24, which began in January 2008 and at the time of writing has not yet reached its peak. The generally reported opinion in scientific circles is that this will be a medium high cycle, the same as or somewhat higher than Cycle 23 (Kunches 2007;Jakowski et al. 2002).
The largest cycle in the history of storm records is Cycle 19, which began in April 1954 and ended in October 1964. This cycle peaked in 1957 with a yearly sunspot number of 190, just at the time that the first space physics satellites were being launched (Odenwald et al. 2008).
Solar flares are violet explosions in the sun's atmosphere with energy equivalent to tens of millions of hydrogen bombs. They can last from minutes to a couple of hours and they cause solar storms, which are categorised as: -geomagnetic storms, -solar radiation storms, -radio blackouts. Geomagnetic storms are a disturbance in the earth's geomagnetic field caused by solar wind pressure that usually reaches earth's magnetic field 24 to 34 hours after the event.
The solar wind energizes electrons and ions in the magnetosphere and they enter the earth's upper atmosphere near the polar regions. By interacting with the molecules and atoms of the upper atmosphere, the electrons and ions start to glow in different colours known as auroras. Geomagnetic storms can also have a significant impact on GNSS satellite systems by pushing the satellites away from their predicted orbits and in this way affect the ground positioning accuracy (Baker 2005).
Solar radiation storms are caused by elevated levels of solar radiation that occur when the number of energetic particles (protons) increases. These storms have a negative impact on the flight industry and also space flights. Radiation exposure of astronauts and also of passengers flying on polar routes is extremely high.
Radio blackouts are a disturbance of the ionosphere caused by X-ray emissions from the sun. Radio blackouts can influence communications by degrading communications facilities for hours or even days.
The Space Environment Centre in Boulder, Colorado, USA, grades solar storms from minor to extreme.
With the use of data obtained from the Space Environment Centre and the NOAA archive, a priority list of major solar storms was extracted. From these lists, a major event in October 2003 was selected for study compared with an apparently completely storm-free period in October 2010. On 2003.10.30 there were geomagnetic storms of level G1 to G5, and a solar radiation storm S3. On 2010.10.30 there was no solar activity.
In parts of the Nordic countries -Norway, Denmark, Finland and Sweden -GPS is available with a precision of one centimetre through an auxiliary system called position accuracy on the centimetre level (CPOS). This is a Norwegian service for both GPS and GLONASS users who need to determine position to the centimetre level without a base station of their own. To support this service, a number of permanent tracking stations have been established. Two of these stations were chosen for this research. The CPOS station called NYA1 is located in Svalbard (Spitzbergen) north of northern Norway and the second station, KRSS, is located in southern Norway (Fig. 1). The coordinates of these stations are computed in two independent ways: -Precise point positioning (solution in ITRF2005, transformed to EUREF89); -GNSS baselines from the four closest EUREF89 benchmarks.

Observations
The experimental work in this paper is based on the theoretical methods of satellite orbit and ground receiver position determinations. The investigation procedure is made up of a sequence of several steps: -Compute the broadcast satellite orbit positions; -Compute precise satellite orbit positions; -Compute ground position at KRSS and NYA1 using both the precise and broadcast satellite positions; -Compare broadcast and precise receiver positions. Determination of the start and end time for selected days of high solar activity was chosen from 22.30 on the day when the solar burst started, and up to 23.30. An interval of 5 minutes was discussed.
Broadcast ephemerides were obtained from the KRSS and NYA1 CPOS stations. Precise orbits, with a 15-minute sample interval were obtained from the IGS website managed by the Jet Propulsion Laboratory of the California Institute of Technology. The IGS website provides GNSS data and products to the scientific community. The IGS website maintains precise orbit records from 1992 which are stored in the SP3 format. The accuracy of precise final orbit data is generally less than 0.05 meters.
Days with solar activity and days without solar activity are analysed to be able to compare the differences between the coordinates based on broadcast ephemerides and the known high precision coordinates of the CPOS stations. The differences are presented in Tables 2 and 3.
The results show that the differences are significant. In general, the differences at KRSS and NYA1 show extreme position error values, especially in the vertical dimension (Figs 2 and 3).
The graphs for storm-free 2010 (Figs 4 and 5) and table 3, however, generally show that the differences for positions are quite small. On the other hand, there is a sudden increase in the height difference between 23.05 and 23.10 at NYA1.   1. This paper concentrates on solar disturbance on GPS satellite orbits. The idea was to investigate differences between broadcast ephemerides compared with two chosen CPOS stations. 2. In general, looking at north and east positioning of 2003 and 2010, it is noticeable that these differences are mostly within 100-metre vector distance of the true position. This is within GNSS navigation accuracy specifications. 3. Nevertheless, the north and east curves for the storms of 2003 are noticeably further apart and less smooth. This indicates disturbed system stability under storm conditions. 4. For 2010 the height vector of KRSS is smoothly changing. At NYA1 the same height vector appears unstable, particularly with a change of some 100 metres over 5 minutes. This can be explained by the fact that NYA1 is so far north that it detects satellites over the pole that are invisible from KRSS. 5. Further, the satellites over the pole will inevitably be low and therefore particularly vulnerable to atmospheric effects. 6. The same can be seen under stormy conditions, but much emphasised. 7. Meanwhile, both KRSS and NYA1 during 2003 exhibit a significant discontinuity around 2300. The discontinuity is visible in all three axes but most significant in height. This is unsurprising because information taken from the Space Weather Center indicated that this was a time of the most severe storm activity. 8. In other words, solar storm activity clearly affects the stability of GNSS positioning, with particularly emphasis on Polar latitudes. 9. Therefore, it is argue that, where positioning stability is important (especially in height), it is most relevant to maintain a high awareness of the levels of solar activity. This can be done by closely monitoring how position coordinates change over time. The rule seems to be that unstable coordinates indicate the possibility of solar activity disturbance.