Over the past 18 months, there has been an increase in the number of incidents of navigation and positioning problems while using the Navstar global positioning system (GPS) system. Seismic exploration contractors and other users of differential GPS (DGPS) have reported errors of 10-20 meters for extended periods. At other times, the L Band DGPS communications equipment has lost lock with its satellite provider and consequently stopped receiving the correction signal resulting in complete loss of DGPS.
These problems have been regional and seasonal: Appearing primarily in Africa and South America, and South East Asia, occasionally in Europe and rarely in North America, starting mid-year and peaking as the earth approaches the sun, with daily variations. These positioning errors, along with communications and power grid problems, are consequences of increased solar activity. This increased solar activity is expected to peak in mid-2000, at the maximum of the current 11-year solar sunspot cycle.
Even before the 600s, when Galileo started viewing the sun with a telescope, astronomers noted that the sun goes through cycles where greater and lesser numbers of visible black spots would develop on its surface. This sunspot cycle has an average of 11 years, peak to peak. The current cycle is referred to as number 23 and is expected to peak during mid-2000.
Sunspots are relatively cool areas that appear as dark blemishes on the face of the sun. They appear and dissipate, lasting days or weeks, rotating over the surface of the sun with an average 27-day period. The sunspot number is basically the sum of the visible dark areas on the surface of the sun with adjustments for the instrumentation used.
Sunspots are formed when extremely strong magnetic field lines just below the sun's surface are twisted and poke through the solar photosphere. The twisted magnetic fields above the sunspots are sites where solar flares and coronal mass ejections (CME) are observed to occur. These and other types of solar events add energetic particles, solar materials, and gravity waves to the solar wind, which over a period of minutes to days, impacts the earth's magnetosphere, disrupting the ionosphere.
This disrupted ionosphere is what causes the positioning errors seen in seismic navigation. The signal from the GPS satellites passes through the ionosphere and is changed, causing the GPS user equipment to compute an incorrect position. The term "space weather" is used to refer to the general condition of the sun-earth connection and the level of disruption in the ionosphere.
Sun-Earth connection
Not until after the first satellites were launched, some 50 years ago, that scientists realized that the space surrounding the earth is not an unchanging vacuum. The reality is that the sun is constantly bombarding the earth with its solar wind of ultraviolet rays, x-rays, energetic protons, as well as other charged particles, solar materials and magnetic fields. This solar wind can have velocities on the order of 250-1,000 km/second. Unlike Mars or Venus, Earth has a strong magnetic field that works to protect humanity from the harmful effects of this energetic solar wind. The geomagnetic equator is a measure of the orientation of earth's magnetic field relative to the geographic equator. In South America, the geomagnetic equator generally passes south of the geographic equator. In Africa and Asia, it passes to the north.
There is significant interaction between the wind and the earth's magnetic field, which results in a teardrop-shaped cavity around the earth called the magnetosphere. Most of the content of the solar wind is forced around the outside of the magnetosphere. At the same time, the magnetosphere expands and contracts, depending on the density and speed of the solar wind.
The particles that do make it inside of the magnetosphere have a significant effect. The ionosphere is created by the solar x-rays and extreme ultraviolet rays which pass into the magnetosphere, causing photo-ionization of the upper atmosphere, creating free electrons. The measure of the number of free electrons in the ionosphere - electron density - is called the total electron content (TEC).
The ionosphere ranges from 50-1,000 km above the surface of the earth, averaging about 450 km. During the evening hours, the lower bound of the ionosphere rises to 200 km above the surface as the magnetosphere turns away from the sun and fewer solar particles interact with the atmosphere.
The solar wind bends and twists the magnetosphere and ionosphere depending on the strength of the wind. This, in turn, causes the earth's magnetic field to be disturbed. One measure of the earth's magnetic field disturbance is the Planetary K index (Kp index) and is computed using a network of ground based magnetometers. An indication of high geomagnetic storm activity is a Kp index value of five or greater.
Radio propagation
The ionosphere is a non-homogeneous and dispersive medium that affects the propagation of any radio signal passing through it. Radio signals from satellites pass through the ionosphere and experience a propagation delay or a travel time that is different than would occur in a vacuum. This delay is called ionospheric refraction. It is a function of the TEC (or electron density) of the ionosphere along the signal path and the frequency of the radio signal: the lower the frequency the longer the delay.
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 radio operators and those familiar with radio ranging systems such, as Loran, ArgoRegistered and SyledisRegistered. For radio operators the skip enables them to transmit and receive over much longer distances (depending on the frequency of operation) while for the navigator attempting to position a vessel, the skip can cause incorrect range measurements, inducing errors in his position.
If the ionosphere is significantly disturbed, then the scintillation effect can occur. Scintillation consists of random fluctuations in the intensity and phase of a radio signal. If the scintillation is of sufficient magnitude, the radio signal can be scrambled to the point where the receiver will lose lock on the broadcast signal, causing total loss of data. Scintillation is typically seen during auroral periods, but can occur at other times, depending on the level of ionospheric disruption.
Ionospheric refraction
Variations in the density of the ionosphere (the gradient of TEC) cause the same ranging errors in satellite positioning systems, like GPS, as in traditional radio ranging systems. Also, ionospheric scintillation can cause the GPS receiver to lose data. For GPS, the delay caused by one TEC unit equates to 16.2 cm of delay at L1 (1575.42 MHz) and 26.6 cm at L2 (1227.6 MHz). The total slant range delay can change in the range of 1-50 meters 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, the US Air Force GPS control center predicts the ionosphere, based on sunspot number related indexes, and uploads it into the satellite network. The GPS receiver then downloads that information and uses it as part of its position computation.
This predicted model of the ionosphere is called the Klobuchar model, after its developer, Jack Klobuchar. DGPS service providers have access to this broadcast ionospheric model as well as their own GPS reference network information which allows them to significantly improve upon the computed GPS position. As long as the broadcast GPS model matches the true condition of the ionosphere, all is well.
The solar cycle
As the solar cycle progresses, the average daily sunspot number rises, creating conditions favorable to solar flares and coronal mass ejection's (CME). Disruption of the ionosphere occurs due to the additional energy striking the magnetosphere and entering the upper atmosphere. The changes are rapid and significant. Now, the ionospheric model used in GPS receivers to compute their position no longer matches reality. The computed position is incorrect. When the sunspot number and Kp index are both high then we can expect this kind of ionospheric disruption.
Other solar cycle effects are damaging to satellite electronics, and increased drag on spacecraft, altering their orbits. This energetic wind also effects the earth's magnetic field and can cause significant DC ground currents, potentially disrupting local power grids.
As the solar cycle progresses, the polar regions and a wide band around the geomagnetic equator are the areas most susceptible to ionospheric disturbances. Polar auroral events that are visible in the middle latitudes are possible. In the equatorial region, the effect is on the TEC gradient, which will change quickly. This band includes Brazil, Central Africa, and parts of Southeast Asia, where oil exploration is ongoing and where significant GPS position errors have been reported. In these areas, where the current broadcast ionospheric model is inaccurate, the actual TEC needs to be measured to allow users to correct their position with the true ionospheric delay.
Ionospheric problems
The two effects on GPS due to ionospheric disturbances are range errors and loss of lock. As the TEC changes with the disturbance, the satellite signal delay also changes causing errors in the range measurements to the satellites. Scintillation may so attenuate or change the GPS signal as to cause a GPS receiver to entirely lose lock on GPS satellites.
DGPS works on the theory that the errors seen at the reference site(s) are similar to those at the user location and that any error model used to improve the position computation is accurate. The range errors measured at the reference site are sent to the user location via a data link and applied to the user data to remove inherent system errors.
In the case of the Klobuchar GPS ionospheric model, even if there are errors, as long as the reference site and user see the same ionospheric delays, then the differential GPS processes will normally take care of this. It is when the reference and user site see different ionospheric gradients that a problem arises.
Introducing better models than the Klobuchar model can compensate for the added errors in the GPS range measurements. This might be a more detailed model that is updated with additional ionospheric delay data. It could be a regional or a global model. In DGPS systems this means that additional information has to be transmitted to the user over the data link together with the standard GPS range corrections. The drawback with modeling is that the model may not accurately represent the real world, and may have a different performance in different areas for a variety of reasons.
The alternative is to use the L2 GPS frequency in addition to the L1 frequency. Most GPS receivers in use are single frequency L1 only, as opposed to dual frequency L1/L2 receivers used for precise survey and kinematic applications. Since the delay in the ionosphere is inversely proportional to the frequency of the signal, using the two frequencies allows us to calculate the actual delay in the ionosphere.
In a DGPS service, this method of compensating for the difference in the ionospheric delay at the reference station and at the mobile has proven to be very effective. In this case, the reference site ionospheric information needs to be transmitted over the DGPS data link along with the standard range errors. Both the reference station and the mobile need dual frequency receivers so that each can compute their own ionospheric delays.
The scintillations on the GPS signal are a more severe problem, however, it is much less frequent. Whereas added errors due to an inaccurate model in the GPS system are present for a large part of the day in most equatorial areas (extending up to +/- 30° from the magnetic equator), the severe scintillations are infrequent and confined to smaller regions. Brazil is actually the area with the most severe scintillations.
In the case of scintillation, the dual frequency scheme indicated above may help as more distant reference stations can be used and still maintain the required accuracy if a closer reference station is affected. If the user mobile receiver experiences scintillations to the extent that not enough GPS satellites are tracked for positioning, the problem remains.
The ionosphere plays a large part in determining the level of positioning accuracy for offshore navigation. As we approach the peak of the current solar cycle, we will continue to see increasing problems with single frequency DGPS positioning that relies on the standard ionospheric models. The use of dual frequency GPS to compute the true ionospheric delay is a proven technique that can significantly reduce positioning errors for the offshore oil industry.
