ESA : New Galileos join Europe’s satnav constellation .-Nuevos Galileos se unen a la constelación satnav de Europa

http://www.esa.int/Our_Activities/Navigation/Galileo/Launching_Galileo/New_Galileos_join_Europe_s_satnav_constellation

Galileo satellites

8 December 2016

Galileo satellites 13 and 14 have begun transmitting navigation signals as fully operational members of Europe’s satnav constellation.

The two were launched together from Europe’s Spaceport in French Guiana on 24 May.

Their flight into space, and subsequent manoeuvres to reach their final orbital altitude, was only the start of their quest to join the operational constellation.

Next, their navigation and search and rescue payloads were methodically switched on, checked out and their performance assessed in relation to the rest of the worldwide Galileo system.

 

Galileo liftoff

This lengthy test phase saw the satellites being run from the second Galileo Control Centre in Oberpfaffenhofen, Germany, while their payloads’ output was assessed from ESA’s Redu centre in Belgium, equipped for the tests with specialised antennas for receiving and uplinking signals.

The test campaign measured the accuracy and stability of the satellites’ atomic clocks – essential for the timing precision to within a billionth of a second as the basis of satellite navigation – as well as assessing the quality of the navigation signals.

Oberpfaffenhofen and Redu were linked for the entire campaign, allowing the team to compare Galileo signals with satellite telemetry in near-real time.

 

Galileo's 20-m L-band antenna

 

These two satellites were visible in the sky above Redu for a limited time each day, ranging from three to nine hours, so tests were scheduled accordingly.

Now that in-orbit testing is completed, the satellites are transmitting working navigation signals and are ready to relay any Cospas-Sarsat distress calls to emergency services.

The next four satellites, launched together on 17 November, are beginning the same in-orbit testing activity, with the aim of joining the network next spring.

Galileos 13 & 14 under launcher fairing

 

About Galileo

Galileo is Europe’s civil global satellite navigation system. It will allow users worldwide to know their exact position in time and space with great precision and reliability. Once complete, the system will consist of 24 operational satellites and the ground infrastructure for positioning, navigation and timing services.

The Galileo programme is funded and owned by the EU. The European Commission has the overall responsibility for the programme, managing and overseeing the implementation of all programme activities.

Galileo’s deployment, the design and development of the new generation of systems and the technical development of infrastructure are entrusted to ESA. The definition, development and in-orbit validation phases were carried out by ESA, and co-funded by ESA and the European Commission.

The European Global Navigation Satellite System Agency (GSA) is ensuring the uptake and security of Galileo. Galileo operations and provision of services will be entrusted to the GSA from 2017.

ESA

Guillermo Gonzalo Sánchez Achutegui

ayabaca@gmail.com

ayabaca@hotmail.com

ayabaca@yahoo.com

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ESA : Galileo: it’s all about the time

                                                                                                                                             

Galileo timing

3 July 2014

Europe’s Galileo satellite navigation system – the combination of satellites in space and stations on the ground – forms a planetary scale timepiece. Current testing shows it keeping very good time indeed – around 10 billionths of a second on average. 

To put that in perspective, light waves cover a distance equivalent to more than seven times around planet Earth in one second. 

So why does Galileo need to deliver such precise timing, down to this billionth-of-a-second scale?

Because light – and all other electromagnetic waves, including radio – travels at a fixed speed, just under 30 cm per nanosecond ( a billionth of a second), the time it takes for Galileo signals to reach a user receiver on the ground can be used to yield distance measurements. All the receiver has to do is multiply the travel time by the speed of light.

How satnav works

A minimum of four satellites need to be visible: one each to fix latitude, longitude and altitude, with another to ensure synchronised timings.

So such precise timing becomes essential for accurate positioning. Galileo’s initial four-satellite ‘In Orbit Validation’ testing demonstrates a horizontal accuracy of up to 3 m, 95% of the time, set to improve as more satellites join the system later this year.

In addition, the Galileo System Time (GST) made available through satellite signals is set to become an important utility in its own right, essential to synchronise worldwide banking, power and data networks and a boon for scientific teams.

Ground-based atomic clocks

The challenge has been to maintain GST in a stable manner. Determining GST would, of course, be impossible without the latest high-performance atomic clocks, which use an atom’s switching between tightly-defined energy states for their ‘ticks’.

Because all atoms of a given element and isotope are identical they produce precisely the same high-frequency microwave response when transitioning between states, typically on the order of billions of times per second.

GST is determined using a set of active hydrogen maser atomic clocks at Galileo’s Precise Timing Facility, at Fucino in Italy. The averaged output of these cupboard-sized clocks establish the reference time for the entire Galileo system.

Galileo satellites in space

The satellites themselves house smaller, less accurate rubidium and passive hydrogen maser atomic clocks, the latter being about the size of office fridges. These two designs keep an accuracy of three seconds in one million years and one second in three million years, respectively, though this is not enough by itself to maintain the ultrahigh precision needed.

Corrections are therefore transmitted from the ground on a regular basis, at least once every 100 minutes once the system becomes fully operational, to keep these satellite clocks fully in line with GST.

 

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Galileo time compared to UTC

In addition, while GST itself is determined on a fully independent basis, it is also required to be kept in strict agreement with the international reference time, Universal Time Coordinated or UTC.
This is achieved by continuously comparing GST with the national real time approximations of UTC carried out by  France, Germany, Italy, Spain and Sweden.
The current ‘offset’ between GST and the approximation of UTC (the final measurement of UTC is so precise it is actually calculated weeks afterwards) is determined by Galileo’s Time Validation Facility (the precursor of the Galileo Time Service Provider), which is currently operated by the company GMV in Madrid, Spain.
 

Galileo's UTC compared to 'actual'' UTC

This process means UTC time measurements can also be made available to users to within an designed accuracy of 26 nanoseconds, although in practice Galileo has been able to do better than this – down to about 5–10 nanoseconds.

Finally, GST also needs to be kept in synch with time system underpinning the US GPS system, known as GPS Time.

To enable user receivers to employ both Galileo and GPS in a seamless manner, the current offset between the two reference times needs to be determined and passed to users through the navigation signal, to an accuracy of 5 nanoseconds or less. 

It is officially known as the GPS to Galileo time offset, GGTO. Without it, around 50 nanoseconds’ worth of difference between the two system’s reference times could put users up to 15 m out of place.

Working closely with the US Naval Observatory, which oversees GPS Time, Galileo’s Precise Timing Facility calculates GGTO continuously, the results being distributed through Galileo’s navigation message.

 

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ESA

Guillermo Gonzalo Sánchez Achutegui

ayabaca@gmail.com

ayabaca@hotmail.com

ayabaca@yahoo.com

 

 

 

 

 

 

Galileo works, and works well

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The current quartet of satellites make Galileo time measurements from at least one satellite possible around two thirds of the time to users.

The launch of two more Galileo satellites, set for this August, will lead to worldwide Galileo timing capability for users at almost all times.

 

 

 

 

 

 

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ESA : ESA to certify first Galileo position fixes worldwide

Europe’s Galileo satnav system

12 March 2014

To mark the first anniversary of Galileo’s historic first satnav positioning measurement, ESA plans to award certificates to groups who picked up signals from the four satellites in orbit to perform their own fixes.

In 2011 and 2012 the first four satellites were launched – the minimum number needed for navigation fixes.

On 12 March 2013, Galileo’s space and ground elements came together for the first time to perform the historic first determination of a ground location – the Navigation Laboratory of ESA’s Technical Centre in Noordwijk, the Netherlands.

From this point, generation of navigation messages enabled full testing of the entire Galileo system – not just by ESA and its industry and institutional partners but also by any entity with a customised satnav receiver.

Galileo positioning performance

ESA’s Galileo team heard about position fixes carried out by organisations and companies all over Europe and beyond, including as far away as Vietnam.

A year on, ESA will recognise these Galileo pioneers with commemorative certificates to the first 50 entities who document their achievement of a past or present fix. Details of how to apply are provided here.

The first Galileo services are scheduled to begin later this year, as more satellites are delivered into orbit.

The next launches will occur in the second half of this year, each with two satellites aboard a Soyuz ST-B. They will take place in close succession to build up the constellation.

Many satnav receiver chips are already technically ‘Galileo ready’, requiring only software upgrades from their manufacturer to begin working with Galileo signals along with the US GPS and other international satnav systems.

ESA
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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