Europe’s first generation Galileo constellation is already the world’s most precise satellite navigation system – delivering metre-scale positioning to more than 3.5 billion users worldwide – but Galileo Second Generation will enable still better performance and an expanded range of services. Essential elements of the G2 system are currently being evaluated in ESA laboratories, including key algorithms to synchronise satellite timings and determine orbits as well as test versions of a satnav receiver and emergency beacon.

With their first launches due in the middle of this decade, G2 satellites will be much larger than existing Galileo satellites and represent a major technical step forward. Employing electric propulsion for the first time, and hosting an enhanced navigation antenna, their fully digital payloads are being designed to be easily reconfigured in orbit, enabling them to actively respond to the evolving needs of users with novel signals and services.

Two independent families of satellites, amounting to 12 Galileo Second Generation satellites in total, are being procured from Thales Alenia Space in Italy and Airbus Defence & Space in Germany. Back-compatible with the current constellation, the G2 satellites will incorporate numerous technology upgrades, developed through EU and ESA research and development programmes.

Algorithms at the heart of Galileo Second Generation

At the heart of satellite navigation is the ability of the satellites to determine where they are in space and the precise time down to a few billionths of a second as they transmit their navigation signals – the greater the precision of these factors, the greater the accuracy of the positioning for users, because Galileo receivers take the time between the signals being transmitted and received and turn it into a measurement of distance. Signals from four or more satellites are used to pinpoint the receiver’s location.

The Advanced Orbit Determination and Time Synchronisation (ODTS) Algorithms Test Platform evaluates the advanced software that will perform these calculations for G2. Developed by Thales Alenia Space through an EU Horizon 2020 project coordinated by ESA, the platform is now installed and running in ESA’s Navigation Laboratory, based at the Agency’s technical heart, the ESTEC establishment in the Netherlands where it is helping simulate how the G2 satellites will operate in practice.

“This platform represents a dynamic, highly performing environment for algorithm experimentation in both real time and post-processing modes, using either real or simulated data,” comments Francisco González, the project’s Technical Officer.

“It contains the algorithmic core of Navigation for Earth Orbit Determination and Identification Segment, NEODIS, which is the suite of algorithms developed by Thales Alenia Space for precise orbit determination of the satellite constellation. These algorithms allow the real-time estimation of orbits and clocks, as well as the generation of Galileo navigation messages, with an estimated accuracy in the tens of centimetres.”

Gustavo Lopez-Risueno, Head of ESA’s Galileo G2 System Engineering Unit, adds: “Important evolutions aimed at improving the estimation of clocks and orbits are being incorporated, such as the integration of composite clock algorithms for a stable and robust reference timescale, the dynamic modelling of satellite and station clocks based on their known behaviour and the processing of auxiliary measurements such as laser range measurements – where lasers are reflected off satellites to measure their orbital position, delivering a ranging accuracy down to under a centimetre, significantly better than the half metre or so available from radio ranging – as well as the intersatellite links that G2 will also be equipped for.”

First G2 receiver up and running

Another outcome of ESA-led H2020 research is also up and running in the lab: the first very G2 receiver prototype ‘breadboard’, developed by GMV.

“Its development has been key to supporting the fine-tuning and assessment of some signals design options we are currently considering,” remarks Jose A. Garcia-Molina, leading the G2 Signal-In-Space design at ESA, and Technical Officer of this activity. “Representative mass-market receiver processing architectures and techniques have been considered to assess the final benefits a user would receive.”

Miguel Manteiga Bautista, heading ESA’s G2 Programme adds: “This first G2 receiver breadboard allows us to better understand the performance G2 can achieve in different user conditions, such as the urban environments that many Galileo users are based in today.”

Meanwhile two parallel activities have been started for development of the G2 Test User Receiver, which will be taken outside the lab for all kinds of test activities ahead of the first G2 launches and then for in-orbit testing and validation.

Radio cries for help under study

Nearby in ESTEC’s Telecommunications Lab is the Galileo Second Generation Search and Rescue Test Beacon simulator, which is now operational following Site Acceptance Testing.

Just like their first generation predecessors, the G2 satellites will pick up SOS signals from emergency beacons down on the ground and relays them on to a ground station for forwarding to local emergency services – in the process helping contribute to the saving of more than 2000 lives annually.

This new simulator to model the performance of these SOS beacons was developed over the course of three years by Thales Alenia Space, under ESA leadership through a G2G System Engineering Technical Assistance Activity.

Eric Bouton, ESA’s Galileo Search and Rescue Engineer explains: “Equipped with state-of-the-art signal generation and processing capabilities, coupled with a high power amplifier of 200W, this new simulator offers several enhanced functionalities over first generation simulators, including the transmission of the new second generation beacons developed by the Cospas-SARSAT organisation and the simulation of complex operational scenarios of up to 15 parallel distress beacons.

“Its development is really a crucial step to gaining a better understanding of the in-orbit behaviour of Galileo’s First and Second Generation search and rescue payloads with the new waveforms of second generation beacons and with the growing beacon population and associated alert traffic. It will be used for an initial test campaign already in preparation, and in future to support the commissioning of all new Galileo search and rescue systems.”