Galileo: no approach with out time




Europe’s Galileo is the world’s most exact satellite tv for pc navigation system, offering metre-level accuracy and really exact timing to its 4 billion customers. A necessary ingredient to make sure this stays the case are the atomic clocks aboard every satellite tv for pc, delivering pinpoint timekeeping that’s maintained to some billionths of a second. These clocks are referred to as atomic as a result of their ‘ticks’ come from ultra-rapid, ultra-stable oscillation of atoms between totally different power states. Sustaining this efficiency calls for, in flip, much more correct clocks down on the bottom to maintain the satellites synchronised and guarantee stability of time and positioning for customers.  


ESA’s ESTEC technical centre within the Netherlands is continously monitoring the ‘Galileo System Time’ on the coronary heart of Europe’s satellite tv for pc navigation system – on an impartial foundation from the operational Galileo system itself.  

For this, the institution hosts in its UTC Laboratory an ‘ensemble’ of high-performance atomic clocks which can be saved in thermally stabilised cleanroom situations. This assortment of fridge-sized atomic clocks, along with means to measure and examine them, gives steady, correct timing usually correct to a billionth of a second, virtually ten instances higher than Galileo System Time. 

Galileo satellite tv for pc in orbit

Serving to set international time 

Pierre Waller, overseeing the Lab, explains: “Our UTC Lab is so named as a result of it – together with the Navigation Help Workplace‘s comparable clocks operated at ESOC in Germany – is used to set a standard timescale referred to as UTC(ESA), which in flip is without doubt one of the inputs for the setting of Coordinated Common Time, UTC – the world’s reference timescale – maintained by the Paris-based Bureau Worldwide des Poids et Mesures, BIPM.” 

ESTEC’s ensemble of atomic clocks has been working constantly for greater than a decade now, enduring the COVID-19 pandemic and a change of location – when the clocks have been progressively transported and re-synched from their authentic website to 1 additional down the hall. The extra contribution of the clocks positioned at ESOC since November 2021 has additional boosted UTC(ESA)’s robustness. 

Ensemble of atomic clocks

ESA microwave engineer Cedric Plantard feedback: “To measure something correctly, an appropriate yardstick is important. So UTC(ESA) can be utilized to carry out an impartial examine on Galileo timing efficiency, in addition to assessing the efficiency of candidate atomic clocks for the approaching set on-board of Galileo Second Era satellites, or another checks requiring ultra-stable and correct time reference”.  

“Consider the clocks right here as working collectively on this rigorously maintained atmosphere like an orchestra, contributing to a weighted common time worth. If one clock goes out of vary of the remaining then an alarm is triggered mechanically.” 

Atomic clocks aboard satellite tv for pc

Counting nanoseconds to measure distance 

Why does precise positioning require exact timekeeping? Due to the way in which Galileo works, turning timing into distance. The precept resembles the way in which kids are taught to estimate the space of a thunderstorm: see a lightning flash, then depend the seconds till a crack of thunder reaches them. Besides on this case the timing entails billionths of a second, multiplied by the velocity of sunshine. 

Galileo satellites orbit 23 222 km above Earth, transmitting alerts downward that incorporate a time stamp. A satnav receiver on the bottom picks up 4 or extra Galileo alerts to repair its place on the bottom. By the point the alerts attain it they’ve taken round a twelfth of a second down from orbit – as revealed by the distinction between the sign time stamp and the receiver time. 

How Galileo works – infographic

The receiver multiplies this distinction by the velocity of sunshine – about 30 cm per nanosecond, a billionth of a second – to derive its precise distance from every satellite tv for pc in orbit, then combines these measurements in a triangulation to compute its total place. If the clocks are in error by greater than three nanoseconds then this positioning worth already exceeds a metre in extent. A second’s error would imply the receiver may as effectively be on the Moon.  

Twin ‘passive hydrogen maser’ atomic clocks are the grasp clocks onboard every satellite tv for pc, measuring time to an accuracy of 1 second in three billion years. Two smaller rubidium clocks present an impartial and different time supply, correct to a few seconds in a single billion years. 

Galileo on the bottom – infographic

However in apply these onboard clocks are vulnerable to drift barely over time. So a worldwide community of Galileo floor stations retains a steady tab on the satellites’ alerts, to establish any clock drift in comparison with ‘Galileo System Time’ (in addition to any slight orbital drift). Any errors are then corrected for inside an up to date navigation message, compiled and add to the satellites for rebroadcast inside Galileo navigation alerts each 100 minutes or much less. 

The very best time for time 

The overall availability of Galileo and the opposite international satellite tv for pc navigation techniques, every with their very own system instances (and their very own offsets relative to UTC), is contributing to an enormous increase to worldwide timing accuracy – used as an example for the synchronisation of communications, finance and energy networks. However ultimately, the one positive approach to examine the efficiency of a clock is with one other clock – therefore the necessity for UTC(ESA) and comparable reference timescales. 

Checking Galileo System Time

“UTC(ESA) is an engineering device that can be utilized for every kind of functions,” provides Pierre.

“So as an example we’re presently trying into the efficiency of time switch through optical fibre with the Dutch VSL Nationwide Metrology Institute, as a way of exporting exact time to wherever it’s wanted, together with assessing Galileo efficiency, candidate future {hardware} and providers.” 

Galileo constellation

About Galileo    

Galileo is the world’s most exact satellite tv for pc navigation system, now serving greater than 4 billion customers across the globe. All smartphones offered within the European Single Market are actually assured Galileo-enabled. As well as, Galileo is making a distinction throughout the fields of rail and maritime transportation, agriculture, monetary timing providers and rescue operations.   

Galileo is a flagship programmes of the EU House Programme, managed and funded by the European Union. Since its inception, ESA has been main the design and improvement of the area and floor techniques, in addition to procuring launches. EUSPA (the EU Company for the House Programme) acts because the service supplier of Galileo, overseeing the market and software wants and shutting the loop with customers.  

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