ESA - ESA’s Navigation Lab helps set global time

Galileo is based on time

20 February 2013

ESA is helping to set the world’s time. Ultra-accurate atomic clocks of the Agency’s Navigation Laboratory, which will be used to assess performance of the Galileo satnav system, have joined the global effort setting Coordinated Universal Time down to a billionth of a second.

The replacement for Greenwich Mean Time, Coordinated Universal Time (UTC) is part of all our daily lives: it is the timing used for Internet, banking and aviation standards and other international timescales, maintained by the Paris-based Bureau International de Poids et Mesures (BIPM).

Participating measurement institutes and observatories around the globe use collections of atomic clocks to estimate a current value for UTC. These clock data are fed through to the BIPM to be carefully weighted and averaged to derive a combined global value. The complexity of this effort is such that it takes around six weeks to arrive at a definitive final figure.

ESTEC Director Franco Ongaro has signed an agreement with BIPM to mark the international recognition of the ESA timescale and the addition of ESA’s atomic clock data to the UTC calculations.

“This is an independent timing capability that ESA’s Navigation Laboratory – based in ESTEC in the Netherlands – built up to support validation of Galileo timing performances, and before it the experimental Galileo GIOVE satellites,” explained Pierre Waller of ESA’s RF Payload Systems division.

Atomic clocks at ESTEC

“This is an independent timing capability that ESA’s Navigation Laboratory – based in ESTEC in the Netherlands – built up to support validation of Galileo timing performances, and before it the experimental Galileo GIOVE satellites,” explained Pierre Waller of ESA’s RF Payload Systems division.
“But it makes sense to apply it more widely, and this BIPM recognition reflects the quality of our data. Our UTC estimate – formally known as UTC (ESTEC) – is also available for projects within ESA: there are many space applications beyond just navigation, such as precision technical experiments or synchronisation of telecommunications and deep-space ground stations.
“Incidentally, it is important to note that our contribution to UTC does not replace the existing input from the Netherlands’ own national timing metrology institute, Van Swinden Laboratories (VSL) in Delft. Instead we are adding to it, for enhanced global accuracy overall.”

Galileo

Galileo, like all other satellite navigation systems, is based on the highly precise measurement of time. A receiver on the ground pinpoints its position by calculating how long signals from satellites in orbit take to reach it.
Matching the receiver and satellite clocks then multiplying the time taken by the speed of light gives the range between the user and the satellite. This allows the receiver to fix its longitude, latitude and time when in contact with four or more satellites. Atomic clocks on each satellite keep time to a matter of nanoseconds – billionths of a second – synchronised by a worldwide ground network.

Galileo runs on ‘Galileo System Time’ (GST), which will be calculated by two Galileo Precise Timing Facilities equipped with active hydrogen masers and caesium atomic clocks in the system’s twin control centres, Fucino in Italy and Oberpfaffenhofen in Germany.
“UTC is the basis for legal time,” added Galileo’s Alexander Mudrak. “GST is derived independently of UTC but will be kept close to it. Galileo will also disseminate the offset between GST and UTC in its navigation message. Close GST–UTC agreement also ensures Galileo and GPS are interoperable.”
“In addition, Galileo will become an important source of precise time measurement in its own right, in addition to its navigation function, in the same way GPS is now.” 

For now, the current four Galileo satellites broadcast dummy navigation signals with no time data during testing.
That will change later this year, when all four satellites are operational and GST begins to be available to users. From then on ESA’s Navigation Lab will play an important role independently validating Galileo timing performance.

ESTEC

ESTEC’s Navigation Lab is fitted with caesium and also active hydrogen maser atomic clocks – an order of magnitude more accurate than the passive hydrogen masers aboard each Galileo satellite, themselves accurate to one second in three million years.
To ensure their stable running, the clocks are maintained in a carefully controlled environment with conditioned power and a continuous generator back-up.

ESA

Guillermo Gonzalo Sánchez Achutegui

ayabaca@gmail.com

ayabaca@hotmail.com

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ESA - Navigation - Helicopter flight over ‘Galileo valley’ guides future satnav

 Helicopter flight

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 A helicopter flew over the Galileo Test and Development Environment – GATE – in Berchtesgaden, Germany, to gather data on how Europe’s two satellite navigation systems – EGNOS and Galileo – will work together in future. The helicopter flew a variety of manoeuvres, from fast loops to mid-air hovering, to see how satnav signals were received in practice. The promising results from the 24-16 August testing are now being analysed. 

Credits: ESA - S. Corvaja

A helicopter recently flew over a very special Alpine valley to gather data on how Europe’s two satellite navigation systems – EGNOS and Galileo – will work together in future.

The helicopter flew a variety of manoeuvres, from fast loops to mid-air hovering, to see how satnav signals were received in practice. The promising results are now being analysed.

The airborne testing, which took place in Germany on 24–26 September, was based around prototype signals of the next generation of the European Geostationary Navigation Overlay Service – EGNOS – combined with simulated Galileo signals.

EGNOS, the first pan-European satellite navigation system, works by sharpening the accuracy of US GPS signals. 

Receiving equipment

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Receiving equipment on the helicopter picking up EGNOS and simulated Galileo signals during testing at the Galileo Test and Development Environment – GATE – a giant outdoor laboratory around Berchtesgaden in the Bavarian Alps where prototype Galileo receivers can be used freely without any modifications. Transmitters atop eight neighbouring mountain peaks blanket 65 sq km of territory with satnav signals. Credits: ESA - S. Corvaja

The first four Galileo satellites have been placed in orbit – the minimum needed to provide basic navigational services. It will take many more to provide global coverage.

But there is one place in Europe where full Galileo service coverage is already a reality: the town of Berchtesgaden in the Bavarian Alps has transmitters atop eight neighbouring mountain peaks to blanket 65 sq km of territory with satnav signals.

The result is the Galileo Test and Development Environment – GATE – a giant outdoor laboratory where prototype Galileo receivers can be used freely without any modifications.

 Galileo Test and Development Environment

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 The Galileo Test and Development Environment (GATE) in Berchtesgaden amid the Bavarian Alps is a test range equipped with Galileo-like transmitters placed in high points for testing in advance of this European GNSS system becoming operational. 

Credits: IFEN

Kept busy by European industrial and research teams, GATE is owned by the DLR German Aerospace Center. ESA’s Global Navigation Satellite Systems for Europe (GNSS) Evolution programme uses it to help prepare the design of next-generation systems. The helicopter testing relied on the SPEED platform – Support Platform for EGNOS Evolutions & Demonstrations – enabling a user to receive simultaneous real-time augmentation signals for both GPS and Galileo, in the same way that the intended next-generation EGNOS system will operate.  

EGNOS sharpens GPS accuracy over Europe

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The US GPS global satellite navigation system has an accuracy of 5–10 m. Across our continent that accuracy is greatly sharpened to 1-2 m through the European Geostationary Navigation Overlay Service (EGNOS), an operational precursor to Europe’s coming Galileo global satnav system. EGNOS broadcasts augmented information through a trio of geostationary satellites linked to a network of monitoring ground stations. 

Credits: ESA

Evolving EGNOS

The ESA-designed EGNOS employs a trio of satellites, processing facilities and a network of ground stations to improve the accuracy of GPS satnav signals over European territory.

The service is guaranteed to an extremely high level of reliability set by the International Civil Aviation Organisation: it is allowed just a one in 10 million chance of error.

The satellite-based service provides horizontal and vertical guidance information for aircraft performing safety-critical landing approaches to airports in a similar way to existing Instrument Landing System devices – but with no local ground-based navigation infrastructure needed.

“EGNOS is already certified for European aviation but what we are testing here is how it operates with Galileo,” explained Guenter Hein, head of ESA’s GNSS Evolution programme.

 EGNOS-guided aircraft on approach

One of Eurocontrol's EGNOS pioneers, this Aurigny Airlines Trislander can perform EGNOS-guided approaches using runway procedures published for Southampton Airport in the UK and Alderney Airport in the Channel Islands. This activity took place through the partnership of the UK's National Air Traffic Services (NATS), Aurigny Airlines and Anglo Normandy Engineering, with the support of the UK Civil Aviation Authority and the States of Guernsey. 

Credits: Eurocontrol

“We are seeking to develop the next generation of EGNOS, which should be ready and operational around 2020.

“The ambition for Europe is to have an EGNOS-like system able to manage the data coming from both Galileo and GPS, making the system much more robust.

It will be an important part of a constellation of EGNOS-like satellite augmentation systems covering our entire planet.”

GNSS Evolution is also tasked with designing the next generation of more advanced Galileo satellites, proceeding on the basis that the first generation of satellites will need replacement in the course of the 2020s.

ESA

Guillermo Gonzalo Sánchez Achutegui

ayabaca@gmail.com

ayabaca@hotmail.com

ayabaca@yahoo.com