Many experts forecast the space weather to be stormy. After years of inactivity, the Sun is waking up, perhaps profoundly affecting Earth’s space environment and the satellites orbiting through it – including the imminent Galileo constellation.
Problems of a stormy Sun
Our parent star makes its presence felt in a variety of ways, from the solar radiation all life relies on to the steady stream of the solar wind, made up of ionised nuclei and electrons. Then there is the sudden eruption of billions of tonnes of highly energetic charged particles during solar storms – two of which occurred last month.
It typically takes a couple of days for these solar streams to reach our vicinity, where their interactions with Earth’s magnetic field can cause spectacular low-latitude auroras, sometimes damaging electrical infrastructure. One 1989 event caused major power outages across Canada.
The uppermost layers of the atmosphere making up the electrically charged ‘ionosphere’ are also perturbed.
“The frequency of solar storms varies with the 11-year solar cycle, reflected by the amount of sunspots visible,” says Stefano Binda, an ESA engineer in the Galileo project.
“Right now we are in the upwards phase, with maximum solar activity predicted between 2012 and 2014.
“This is an issue of concern for all satellites and therefore also for Galileo, as we begin launching this October and are scheduled to begin operations by mid-2014, right in the heart of the ‘solar max’.”
Effects on satellite navigation
Solar storms affect satellite navigation in various ways, starting with the satellites themselves.
Components can be unexpectedly upset and gradually degraded by the particle bombardments. Galileo’s medium orbit at an altitude of around 23 200 km is less protected by the geomagnetic field than other orbit regimes, like low orbits.
This orbit also takes the satellites through the outer Van Allen radiation belt, one of the two toroidal regions where incoming charged particles are funnelled by the magnetosphere – meaning they will actually have more lifetime radiation exposure than their higher-altitude geostationary equivalents.
Satnav users on the ground, along with the terrestrial infrastructure overseeing the constellation and producing navigation signals, will also experience unwanted effects.
“Propagating through an energised ionosphere leads to signal delay,” explains Stefano.
“Ordinary telecommunication systems can just boost through broadcast energy but satellite navigation uses the signal delay to calculate the user’s position.
“Just a billionth of a second’s delay can cause a 30 cm error, and the ionosphere can cause errors in the order of several metres.”
The Galileo satellites are not being launched blind, however. Two ‘Galileo In-Orbit Validation Element’ satellites – GIOVE-A and GIOVE-B – were launched on 28 December 2005 and 27 April 2008, respectively.
Both carry radiation monitors continuously watching over the radiation flux. “This element of the GIOVE mission is becoming more important as solar activity increases,” explains Stefano.
“We didn’t have direct experience of medium-Earth orbit but the design of Galileo has been guided by radiation models that say, crudely, if you are going to operate at a given altitude then you need a certain radiation shielding.
“GIOVE-A carries two different radiation monitors – the UK-designed Merlin and French CEDEX, while GIOVE-B carries a single ESA-designed Standard Radiation Environment Monitor. They maintain a count of the particles hitting them.
“Other ESA missions including Integral, Proba-1, Rosetta, Herschel and Planck, are carrying similar radiation monitors, helping our specialists to build up a broader picture.
“Integral’s is most applicable, because it’s highly elliptical orbit briefly crosses through the Galileo altitude.”
The good news is that GIOVE’s observed radiation matches ESA models, meaning the planned 12-year working lifetime for each satellite remains reasonable.
“It’s been a little harsher than expected,” Stefano adds. “The big question is what happens now, as the upturn in activity is forecast to increase.
“We want to go on validating our model in the worst-case conditions, so the GIOVE satellites will remain important even after the first Galileo satellites are launched later this year.”
The ionospheric effect has been a particular problem for civil GPS users. A basic ionospheric model is used to remove more than half of the uncertainty, but ‘iono-delay’ remains the single largest contributor to errors in GPS positions.
Across Europe, the EGNOS overlay service broadcasts more accurate ionospheric corrections for GPS signals based on data from Ranging and Integrity Monitoring Stations across the continent.
Galileo will overcome ionospheric effects for civil users in a similar way, with worldwide sensor stations tracking satellite signals at three different frequencies.
Comparing the different frequencies allows the delay to be calculated and corrections to be uploaded to the satellites within the navigation message.
Galileo users with small receivers – such as those in car dashboards and mobile phones – will benefit from this accurate ionospheric correction by using the same model and the broadcast values.
Larger user receivers can also exploit Galileo’s multiple frequencies to correct for most of the delay directly – a feature that the latest generation of GPS has introduced recently for civil users, previously reserved only for military users.
