Galileo: European GPS (update)

(To Tiziano Ciocchetti)

Last December the European GPS satellite system became operational (Global Positioning System) Galileo.

Il global positioning system arises from the need for a flexible system that can also be used in areas without navigational aids that also offer wide applications in the military sphere. During the Cold War, these demands drove the two superpowers to launch parallel programs that would then be transferred to the GPS and GLONASS systems in the 1990s.

The American GPS, developed and controlled by the Department of Defense, became operational in the 1993 with an 24 satellite apparatus placed in circular orbits at 20.000 km in height.

The Soviet GLONASS also envisaged a conformation similar to the American one, with 24 satellites on three orbital planes at 19.000 km, however financial problems consequent to the collapse of the Soviet Union led to a drastic resizing of the program, with only half of the satellites and without civil applications.

The American system has proved to be efficient but has some limitations, the most important of which are those relating to accuracy and reliability. In fact the precision level can be extremely variable as it is conditioned by the place and time and, due to the same configuration of the orbital planes, the system leaves vast areas of the earth uncovered at higher latitudes. As for reliability, the quality of the signal is not guaranteed, nor are there indications to alert the user in the event of failures or malfunctions. Furthermore, since it is an essentially military system, in times of crisis the manager can block the signal to civil users.

To overcome the technical limits and existing political-military constraints, the European Union, through the European Commission, has taken the decision, in close collaboration with the European Space Agency, to develop its own GPS system, called Galileo, characterized by higher levels of precision, reliability and safety.

The European system offers not only greater precision but also more constant, due to the particular structure of its composition and ground network.

A significant improvement, compared to the American GPS, is the control of the quality of the SIS signal (Signal in Space), also called information to test this quality by transmitting the results to the two main control centers that will alert a satellite to problems after a few seconds; a non-secondary safety measure, as it ensures very high levels of safety in the case of applications in air, sea and land transport.

Among the main features is the interoperability of Galileo with GPS and GLONASS so that the user can read his position provided by the three systems using the same receiver. This way you can have 60 satellites instead of 30. Thanks to Galileo the operator is able to see up to 20 satellites, considerably increasing the accuracy as well as the availability and continuity of the service.

The system Galileo is based on 30 satellites (active 27 and reserve 3) arranged in three MEO orbits (Medium Earth Orbit) 56 ° inclined circular on the Equator at a height of 23.616 km, with a period of about 14 hours, in order to give an optimal coverage of the land up to 75 ° North latitudes, corresponding to North Cape and beyond.

It was decided to use a single height in order to have uniformity of performance in terms of both availability and accuracy, minimizing the consequences in the event of failures. With such an arrangement there is a probability of 90% being always in sight of at least four satellites, but in many areas this number will grow to six-eight, allowing for position accuracy up to a few centimeters.

The satellites send the SIS signals to a network of 29 GSS (Galileo Sensor Station) distributed over territories under European control. The GSS collect all the various data, including the distance of the satellite, after which they send them to the OSPF (Orbit and Syncronization Processing Facility) which is the center that processes the orbital data.

It is important to know the orbit of the satellites as this is one of the parameters that affect the performance of the system and it is on the basis of this position that the user then determines his own. However, there is also the problem of the time that the two segments, spatial and terrestrial, must remain synchronized. The PTF (Precise Timing Facility) which has the main task of defining the system reference time; there is a Galileo System Time which, through the network, has passed to the system and in particular to the aforementioned OSPF which, in addition to performing the orbital determination, also defines the algorithms for clock synchronization. This system allows messages to be sent on any anomalous behavior that leads to deviations from the position above the permitted tolerances.

This information is sent to another facility which is the IPF (Integrity Processing Facility) which processes integrity information. The latter is one of the key elements that differentiate Galileo by GPS. Integrity is the ability of the system to self-diagnose, basically to identify a fault and then alert in real time - the latest specifications require within 6 seconds - any user.

Then the GSS collect the observable data coming from the constellation; send them to the OSPF - which with a single process performs the determination of the orbital data and the time synchronization - and to the IPF that determines the integrity.

At this point the data processed by these two facilities go to the MCF (Mission Control Facility) whose task is the management of the entire mission in the role of online monitoring and control center (with the MSF - Mission Support Facility) and mission planning. The MCF, based on these data, through the MGF (Message Generation Facility) creates the base of the navigation message intended for the user through the ULS (Up-Link Station) that are 10, appropriately distributed in the territory, which send the message to the satellite in C band.

The satellite receives the message to which essentially the on-board processor adds time, transforms it into radio frequency and radiates it for the benefit of the user, closing the loop.

The operation of Galileo has a number of relevant interests also in the military sector. The guarantee of reliable signals will facilitate the use of automatic landing systems, both on aircraft and on UAVs, or the automatic navigation for naval units (entry-exit from ports, positioning for mooring, navigation in restricted waters, guide of mine-hunting vehicles), as well as the development of robotized logistics, with vehicles that follow the combat units through predetermined routes.

Problems for Galileo?

Ten atomic clocks on board the Galileo GPS system have reported malfunctions.

According to Jan Woerner, head of the European Space Agency, the problem has affected atomic clocks of different types on satellite platforms of two separate manufacturers.

Satellites use atomic clocks to compensate for the delay of a few microseconds per day that exists between the Earth and their orbital altitude and thus provide coordinates as accurate as possible. Galileo's satellites have four atomic clocks on board - of two different types - but they only need one to correctly detect the position.

In particular, two PHM passive hydrogen maser clocks are mounted on the Galileo satellites (Passive Hydrogen Maser), the main, and two of rubidium, reserve.

The PHMs are produced by the Italian Leonardo (through the old SelexES) in collaboration with the Swiss company SpectraTime, belonging to the group Orolia. According to Leonardo, Galileo's PHM has a precision higher than 1 nanosecond daily and is the most precise of those operating in earth's orbit. Spectratime, together with Airbus Deutschland, also deals with the supply of rubidium watches.

Of the ten atomic clocks that have experienced problems, seven are of the PHM type and three are of rubidium. A PHM clock then resumed operation, thus taking the instruments out of service to nine.

According to an initial investigation by ESA, the root of the problem should not be sought in the clock itself, but in the peripheral circuitry or in some other secondary component which, under certain conditions, tends to cause the failure. "We were not able to replicate the failures during our ground tests, but all the units involved had passed the qualification and validation texts"Javier Benedicto also explained, head of the ESA department. Basically, according to the agency, there could be a particular circumstance, including environmental, that brings out the problem.

Given the redundancy of the system, with as many as three tools back-up, at the moment the functionality of Galileo is not compromised and, as Benedicto explained.

(photo: European Space Agency)