Tag: satnav signals

ESA plans for low-orbiting navigation satellites

New PNT satellites will operate in low Earth orbit (LEO). (Image: ESA)

News from the European Space Agency (ESA)

Satellite navigation is headed closer to users. ESA’s Navigation Directorate is planning an in-orbit demonstration with new navigation satellites that will orbit just a few hundred kilometers in space, supplementing Europe’s 23,222-km-distant Galileo satellites.

Operating added-value signals, these novel low-Earth-orbit (LEO) positioning, navigation and timing (PNT) satellites will investigate a new multi-layer satnav system-of-systems approach to deliver seamless PNT services that are much more accurate, robust and available everywhere.

Global in coverage, free for everyone to use, GNSS such as Europe’s Galileo have already transformed our society, and due to their sheer omnipresence their influence continues to grow. In 2021, the population of satnav receivers reached 6.5 billion around the world, and the sector is projected to maintain a 10% annual growth rate in the years ahead. But in various respects the standard GNSS approach is nearing the limits of optimum performance — to get even better, added ingredients are becoming essential.

“Satellite navigation has enabled a vast range of applications in recent years, but this very success is inspiring still more demanding user needs for the coming decade,” said Lionel Ries, head of ESA’s GNSS Evolutions R&D team, overseeing the agency’s LEO-PNT studies.

“For use cases such as autonomous vehicles, ships or drones, robotics, smart cities or the industrial internet of things for control of factory systems, the positioning requirements are growing from the current meter-scale to centimeter scale or even more precise, based on continuously reliable signals that are available anywhere, anytime — even indoors —while able to overcome interference or jamming.

“Up until now we have relied for positioning on the classical solution of GNSS such as Galileo, located in medium Earth orbit and based on L-band signals. Standard GNSS alone is not going to be able to fulfil all these future user demands. Instead Europe needs to seize the opportunity to investigate the potential of the kind of LEO constellations that are already on the way in the global market to enable new kinds of PNT services.”

Simply by virtue of physics, with less of a distance to cover down to Earth, the signals from these LEO-PNT satellites can be more powerful, able to overcome interference and reach places where today’s satnav signals cannot reach.

Additionally, by adopting novel navigation techniques and a wider range of signal bands the satellites can address particular user needs: for instance at lower orbits the satellites themselves move more rapidly relative to Earth’s surface — think of the International Space Station at 400 km that orbits the Earth every 90 minutes — which offers possible advantage in the time needed to reach very accurate positions. Also some bands could offer greater penetration in difficult environments while other bands could offer higher robustness and precision.

Mega-constellations of hundreds or even thousands of low-orbiting satellites offer a means of acquiring continuous coverage for telecommunications services or Earth observation. (Image: ESA)

The purpose of ESA’s plan to perform an in-orbit demonstration of low Earth orbiting satnav satellites is precisely to consolidate the types of signals, enabling technologies and their potential for future services.

The plan is to build and fly an initial mini-constellation of at least half a dozen satellites to test capabilities and key technologies, as well as demonstrating signals and frequency bands for use by a follow-on operational constellation, in the same way that Europe’s GIOVE test satellites paved the way for Galileo. Success will place European industry in pole positions for follow-on commercial undertakings, as well as planned institutional programs.

“Each individual satellite would be comparatively small, below 70 kg in mass, compared to a 700 kg current Galileo operational satellite,” added Roberto Prieto-Cerdeira, Galileo Second Generation satellite payload manager and LEO-PNT project preparation manager as part of ESA’s FutureNAV program.

“They can be comparatively more streamlined because they can benefit from other means to calculate the accurate time without extremely precise atomic clocks on board — including relayed signals from the Galileo satellites above them. These satellites would also be built on a rapid batch production basis to save time and cost — we are targeting three years at the most from signing the contracts to the first satellites in orbit, the same kind of timescale achieved by GIOVE-A in the early 2000s.”

A vision of the future shows layered satellite navigation stretching from Earth to the Moon. (Image: ESA)

“It is ESA’s ambition to ensure Europe maintains a world-class space industry, and navigation today forms the single largest downstream space sector, worth about €150 billion annually and growing at the rate of 10% per year,” said ESA Director of Navigation Javier Benedicto-Ruiz. “Standing still is not an option; instead we need to explore new technical avenues to spur European competitiveness and commercialization.”

An operational version of the LEO-PNT constellation would represent a whole new layer for PNT delivery, combined with traditional GNSS as well as 5G/6G-based positioning on the ground, and fused with data from sensors in the user terminals.

Interest from industry

ESA has been researching core elements of the LEO-PNT concept since 2016. Now, with numerous low Earth orbit constellations already taking shape around the globe, the time is right to move from basic research to in-orbit demonstration.

Interest from European industry in the LEO-PNT project has been very high, shown by a recent Request for Information where ESA presented details of how companies and institutions might participate and a large number of companies registered and presented possible concepts and contributions.

Forward to FutureNAV

LEO-PNT is supported through the ESA Directorate of Navigation’s FutureNAV programme, which also includes the GENESIS satellite to measure the shape of Earth more accurately than ever before while also boosting the positioning performance of satnav satellites. The FutureNAV programme, which includes both GENESIS and the LEO-PNT initiative, is up for decision at ESA’s next Ministerial Conference, taking place in Paris on Nov. 22-23. Read the fact sheet here.

Until now, all navigation satellites have flown in medium-Earth orbit – up at 23,222 km in the case of Galileo, which delivers meter-level accuracy. At such altitudes the satellites move slowly across the sky, helping ensure global availability of satellite navigation signals, albeit at relatively low power.

ESA’s LEO PNT constellation would move to a “multilayer system of systems” approach, with medium-Earth orbit signals supplemented by those from LEO satellites at altitudes of less than 2,000 km — along with additional inputs from terrestrial PNT systems and user-based sensors, made up of approximately a dozen satellites, helping European companies move forward at a time when worldwide commercial interest is high in LEO constellations of all kinds, especially for telecommunications and PNT.

The satellites themselves can be stripped down compared to current navigation satellites, because they would essentially be relaying satnav signals from MEO. This is a key point because there will need to be many more satellites to ensure global coverage — because the lower the orbit the faster each individual satellite will pass across the sky. This fact also opens the way to a more agile “New Space” approach to satellite construction for European firms, with smaller payloads and simplified operations from the ground.

Their signals will be much stronger (potentially able to penetrate indoors), and transmitted on novel frequencies, which – along with the new geometries made possible by LEO satellites – should enhance overall service resilience. LEO PNT will also deliver faster position fixes and enable rapid two-way authentication checks. And overall signal availability will be boosted enormously, especially in high-latitude and polar regions.

October 31, 2022
Tests begin of Galileo’s OSNMA signal authentication service

In a first for any satellite navigation system, Galileo has achieved the first position fix based on navigation signals carrying authenticated data, according to the European Space Agency.

Galileo’s Open Service Navigation Message Authentication (OSNMA) is intended as a way to combat malicious spoofing of satnav signals.

OSNMA receivers successfully calculated an OSNMA-protected position fix after Galileo satellites started transmitting authentication data at 15:28 UTC on Nov. 18, 2020. The first tests used eight Galileo satellites for around two hours on Nov. 18. Tests have continued ever since, for intermittent periods, and will continue over the next months ahead of a public observation phase.

The first authenticated signal position, velocity and timing fixes were made using a total of eight Galileo satellites for around two hours on Nov. 18, 2020. The tests represent a first proof-of-concept for an eventual operational service offering positioning with authenticated data to users. (Image: ESA)

Pioneering a long-awaited service

The Galileo OSNMA authentication mechanism allows GNSS receivers to verify Galileo information, making sure that received data are indeed from Galileo and not modified in any way.

“Ensuring the validity of positions elaborated by GNSS is one of the main challenges before addressing an entirely new set of applications demanding dependability and resilience,” said Matthias Petschke, director of space at the European Commission, DG DEFIS. “Galileo is now set on course to deliver on this highly anticipated feature and has many more novel features in store for the coming years.”

Testing is taking place at ESA’s Navigation Laboratory at ESTEC in the Netherlands, the same site where the first Galileo positioning fix took place in 2013.(Photo: ESA)

Increased robustness

OSNMA test signals are being broadcast by the Galileo constellation using the spare bits from the current navigation message, therefore not impacting the legacy OS receivers implementing the current OS Signal-In-Space Interface Control Document (OS SIS ICD).

“Galileo’s Open Service Navigation Message Authentication is one of its key differentiators,” said Rodrigo da Costa, executive director of the European GNSS Agency. “The additional robustness that it will provide to the Galileo signal will be critical for many applications, particularly those where security and trustworthiness are a priority, making the OSNMA a key component in any resilient PNT solution.”

OSNMA works on a comparable basis to everyday encryption, where sending a digitally signed document involves both sender and recipient using compatible cryptographic keys (private and public) to validate the document’s source of origin.

“Up until now, as a navigation satellite disseminates navigation and timing data, there is no way of confirming these data are indeed coming from their apparent originator,” explained Paul Verhoef, director of navigation at the European Space Agency. “As a result, the data could be falsified, a phenomenon known as spoofing, where corrupt false signals mislead receivers about their position, misleading their users in turn, with potentially serious consequences.”

An ESA Navigation Directorate team at the ESTEC technical centre in the Netherlands worked with their European GNSS Agency (GSA) counterparts at the twin Galileo Control Centres in Italy and Germany and the Galileo Service Centre (GSC) in Spain to develop and test the OSNMA.

Next steps

Upon successful completion of the internal testing phase, a public observation phase will begin, in which the OSNMA signal will be publicly accessible. In preparation for this phase, the OSNMA user Signal-In-Space Interface Control Document (OSNMA SIS ICD), receiver implementation guidelines, and the necessary cryptographic materials will be published. This will allow receiver manufacturers and application developers to test and prepare their products.

During the public observation phase, feedback will be gathered from users, leading to the consolidation of the service.

Testbed vehicle by ESA’s Navigation Lab. (Photo: ESA)

February 11, 2021
ESA provides new maps of Earth from reflected satnav

News from the European Space Agency

Your phone or satnav receiver routinely picks up signals from navigation satellites in order to tell you precisely where you are. But have you ever thought what happens to those satnav signals afterwards? A foresighted ESA inventor had the idea of using them as a tool for observing the Earth.

More than 120 satellite navigation satellites are in orbit, making up multiple constellations including Europe’s Galileo system, sending down a continuous rain of satnav signals for the benefit of users worldwide. Just like visible light, these microwave signals go on to reflect off Earth’s land and sea surfaces.

The traditional attitude to these reflected signals is to see them as something of a nuisance — known as multipath, they can confuse satnav receivers and reduce their overall accuracy.

ESA microwave engineer Manuel Martín-Neira, inventor of the PARIS reflectometry concept. (Photo: ESA)

But back in 1993 — at the same time as the US GPS satnav system reached its full constellation of 24 satellites — a young ESA microwave engineer called Manuel Martín-Neira came up with the idea of treating these satnav reflections as a scientific resource instead.

“My head of division asked me to come up with a budget-friendly way of increasing the overall sampling rate to build up a fuller picture of mesoscale phenomena, and that led me to start looking into making use of additional signals of opportunity, chiefly satnav signals.

“The initial reaction was mixed, because the forecast accuracy was not as precise as the ERS-1 altimeter could deliver — but on the plus side there would be a lot of these signals to make use of, and the performance has improved a lot since those early days.”

PARIS, detecting reflected satnav. (Photo: ESA)

Inspiration from reflection

The basic idea of what Manuel christened the Passive Reflectometry and Interferometry System, or PARIS, comes down to a two-sided antenna. As the topmost side picks up a satnav signal from the satellites in orbit, the other side picks up the version of the signal bounced back from Earth.

By comparing this initial, overhead signal with its reflected equivalent using a process called interferometry — measuring tiny differences in signal phases – the extra travel time of this reflected beam can be determined, down to an accuracy of less than five centimetres, determining sea height and sea ice thickness.

Additional amplitude waveform processing can deliver further data on wind and wave measurements over the ocean, and soil moisture and biomass over land.

Satellite reflectometry has since grown into a thriving field. This summer, Manuel attended the latest international workshop on the method he first devised 26 years ago.

Reflectometry reaches space

“It’s been fantastic to have experimental evidence, and that’s really been made possible by the growing availability of smaller satellites,” explains Manuel.

“Because satellite reflectometry is a passive form of remote sensing, it makes for an attractive potential payload because it doesn’t need a lot of power to operate. Then one of the results is meteorology data that private companies intend to make money with by delivering to public agencies.”

Surrey Satellite Technology Ltd.’s UK-DMC satellite was the first orbital mission with a reflectometry payload. (Photo: ESA)

In 2003, the UK-DMC satellite was the first mission to fly a reflectometry payload, followed in recent years by, for example, the UK’s TechDemoSat-1, NASA’s CyGNSS constellation to monitor hurricanes and the Spire global constellation of commercial nanosatellites.

“These satellites have really given the reflectometry community a wealth of signals, demonstrating what reflections look like over different surfaces including sea ice, forests, and even inland water bodies such as the Amazon River and its tributaries.

“In parts of the ocean near continental masses and within atolls we are seeing reflected signals from very calm waters which resembled a mirror, giving us very high precision down to 1 cm level. Such measurements could potentially complement current altimetry missions, by for instance measuring sea level rise.”

Example of a CYGNSS Microsatellite Observatory. (Image: Southwest Research Institute)

ESA activities taking flight

ESA meanwhile is active on reflectometry in various ways, having developed and tested a steerable airborne antenna called the Software PARIS Interferometric Receiver or SPIR, capable of steering separate antenna beams to build up a rapid surface picture, and differentiating between different signal sources, such as GPS from Galileo.

Manuel adds: “ESA’s GNSS Science Support Centre, based at the Agency’s European Space Astronomy Centre near Madrid, has been taking a keen interest in these activities.”

Missions are also in development, including a dedicated CubeSat with RUAG-Austria and the University of Graz called PRETTY (for Passive REflecTomeTry and dosimetry, which would also carry a radiation detector), and a small satellite pair called FSSCat from Spain’s Universitat Politècnica de Catalunya, backed through the Copernicus Masters competition, seen as a prototype for a future reflectometry constellation.

ESA’s Directorate of Telecommunications and Integrated Applications is also working with the Spire company to fly enhanced reflectometry instruments, starting at the end of this year.

One of Spire’s Satellite Manufacturing Technicians (Tomasz Chanusiak) tests the Radio Frequency capabilities of a LEMUR2 nanosatellite in Spire’s cleanroom in Glasgow, Scotland. (Photo: ESA)

When it comes to the thriving state of today’s reflectometry community, Manuel recalls the patenting of his idea as a turning point: ‘Having had this idea, which was not particularly well received, the proposal by ESA’s Patents Group to patent it made all the difference. It gave me a feeling of confidence, that somebody else at least saw the potential of this idea — and the rest is history.”

September 23, 2019