Editor’s Note: See the report from the European Space Agency here.
An official with the European Space Agency has confirmed that the sequence of maneuvers to adjust the orbit of the second of two Galileo satellites launched into a wrong orbit in August 2014 has been completed.
The orbit of the first satellite, known variously as GSAT0201, Galileo FOC-FM1 or Galileo 5 (with COSPAR ID 2014-050A and NORAD ID 40128) was raised during operations carried out in November, and the satellite began transmitting L-band signals on Nov. 29.
Maneuvering of the second satellite (GSAT0202, Galileo FOC-FM2 or Galileo 6, with COSPAR ID 2014-050B and NORAD ID 40129) began around Jan. 15. The procedure took somewhat longer than that for the first satellite as it also involved changing the mean anomaly of the satellite to be about 180° away from that of the first satellite.
The locations of the satellites in the Galileo constellation are shown in the accompanying figure. Satellites in green are transmitting a full complement of L-band signals. Galileo 4 (GSAT0104), one of the in-orbit validation satellites, suffered a power anomaly and only transmits on the E1 frequency. Galileo 5 is transmitting L-band signals but its orbit cannot be properly represented in the Galileo broadcast almanac. Galileo 6 has not started transmitting valid L-band signals yet.
Officially, all Galileo signals are currently declared unavailable during an extended period of testing following ground segment upgrades. However, signals continue to be monitored by stations participating in the International GNSS Service Multi-GNSS Experiment.
Galileo’s Ground Mission Segment in the Fucino Control Centre in Italy oversees Galileo navigation services and satellite payload operations.
News from the European Space Agency
In Roman times the milestone was the central method of navigation, with all distances fixed from a ‘golden milestone’ in the imperial capital. Today, navigation satellites have become the modern equivalent of milestones — but Rome still has a role to play.
Inside the Galileo System Evaluation Equipment facility, based at Thales Alenia Space in Rome.
The Thales Alenia Space plant in the eastern suburbs of Rome is home to the Galileo System Evaluation Equipment facility, which provides a troubleshooting platform for the Galileo ground network and an assessment of the performance of Europe’s under-construction satnav constellation.
Based in the main plant building, it is equipped with a secure data link to the Galileo Control Centre in Fucino, 90 km away, which oversees Galileo navigation services. This link gives it direct access to all the data gathered by the global ground segment, from the sensor station data to the navigation messages uplinked to the satellites, including satellite orbits and onboard clock corrections.
The facility can then apply separate software to these inputs, rather than that used in the Galileo Mission Segment, to provide a “second opinion” on Galileo performance. In addition, a van measures Galileo performance in the field, gathering data across a range of vehicle and rural environments.
The River Tiber flows through the historic centre of Rome, seen in high-resolution detail by France’s Spot-5 satellite.
“The facility is being routinely operated by the Thales Alenia Space team,” explains Enrico Spinelli, overseeing it on the ESA side. “It is being upgraded to automatically process the data received from the Galileo control centres, perform troubleshooting analyses and provide inputs for the monthly Early Service Key Performance Indicators report. These reports are provided in turn to the European Commission’s European Global Navigation Satellite System Agency, as part of Galileo’s Early Services preparatory activities.”
The facility made the Rome area one of the two main centres of activity during Galileo’s In-Orbit Validation phase, along with the ESA’s ESTEC technical centre in Noordwijk, the Netherlands. In-Orbit Validation was the extensive system testing performed on the ground during late 2012 and early 2013 to ensure the embryonic four-satellite system was performing as designed, including Galileo’s historic first position fix of longitude, latitude and altitude on March 12, 2013.
The Galileo System Evaluation Equipment facility hosted at Thales Alenia Space in Rome is equipped with a van measures Galileo performance in the field, gathering data across a range of vehicle and rural environments.
“The facility was developed for that phase, but has performed so well that it was decided to keep it in operation during succeeding phases,” adds Enrico. “Along with its intended use in monthly reporting, its direct access and processing of Galileo Control Centre data will make it a powerful tool for system troubleshooting for both Galileo’s upcoming services. It can give us independent analyses of factors such as the availability and quality of data from Galileo Sensor Stations and the Orbit Determination and Time Synchronisation process which keeps the overall Galileo system in sync.
“It can also allow us to check the accuracy of software models used to compensate for ionospheric delay, the accuracy of almanacs charting satellite orbital positions and to analyze the efficiency of the ground-to-satellite contact plans for the uplink of the navigation message which the satellites rebroadcast, even to verify the navigation message is being broadcast in its correct structure.”
The improved facility should help to ensure the timely and reliable introduction of initial Galileo services, planned in 2016.
The latest Galileo satellite, formally known as FOC FM06, arrived at the ESTEC Test Centre in its protective container on Dec. 18, after traveling from OHB in Bremen, Germany. Photo: European Space Agency
The latest Galileo satellite has arrived at ESTEC, in the Netherlands, and is undergoing a full checkout to prove its readiness for space.
The satellite was carried by lorry from its manufacturer in Germany, cocooned within an environmentally controlled container. It arrived inside ESTEC’s cleanroom environment on Dec. 18. The container was then opened up to begin preparations for testing.
The first six Galileo satellites are already in orbit, launched in pairs in 2011, 2012 and August this year.
The last pair was delivered into the wrong orbit by a faulty upper stage, but the fifth satellite’s orbit has since been changed to allow checking of its navigation payload, which began at the end of November.
The sides and top of the Galileo satellite container were sprayed clean before it was taken inside the bay of the ESTEC Test Centre to keep any contamination from entering the pristine cleanroom. Photo: European Space Agency
Meanwhile, down on the ground, production of further satellites continues steadily, taking the Galileo series into double figures overall.
Following on from the first four In-Orbit Validation satellites, 22 of these Full Operational Capability satellites are being built by OHB in Bremen, Germany, with navigation payloads from SSTL in Guildford, UK.
Numbered Flight Model 6, or FM06 for short, this latest of the newer satellites is now reunited under the test centre’s roof with three others. FM03 and FM04 have completed their acceptance testing, culminating in the weeks-long thermal-vacuum test. Each satellite was subjected to the same vacuum and extreme temperature conditions experienced in orbit, as well as radio-frequency testing of their navigation payloads and antennas inside an anechoic chamber isolated from the external universe. This pair is now in storage in the centre pending the results of their concluding acceptance review.
The other satellite, FM05, recently ended its own thermal-vacuum trial. It is now being reconfigured for radio-frequency testing, planned to take place after the Christmas break. The latest unboxed Galileo satellite will undergo its own thermal–vacuum test in January.
ESTEC is an essential stop on the way to space for Galileo. It is equipped with all the facilities needed to simulate space conditions under a single roof, including an acoustic chamber, earthquake-strength shaker tables, and anechoic and vacuum chambers, along with a range of specialised measuring equipment.
Once ESTEC gives the satellites its stamp of quality then they are in principle ready to be flown to Europe’s Spaceport in Kourou, French Guiana. ESA and the European Commission are currently deciding on the launch schedule for these next Galileos.
The container containing the latest Galileo satellite, FOC FM06, was carefully hoisted off the lorry that carried it from OHB in Bremen, Germany. Its underside was then carefully cleaned before it was taken out of the bay into the cleanroom environment. Photo: European Space Agency
They said it wasn’t possible — well to be frank, I said it wasn’t possible – but one of the two “misplaced” Galileo satellites, plucky Doresa, has delivered an early Christmas present to the European GNSS community by providing a first fix on Tuesday, December 9. The signal was received at the European Space Agency’s (ESA’s) technical centre in Noordwijk, the Netherlands and at the Galileo In-Orbit-Validation (IOV) test station at Redu in Belgium. Doresa teamed with the remaining three functioning Galileo IOV satellites to provide a Galileo positioning data first fix with horizontal accuracy better than two metres.
Since then fixes have also been performed using Galileo’s Public Regulated Service (PRS), the civilian encrypted highest-precision signal and one of the constellation’s unique selling points.
The satellite had transmitted its first navigation signal in space on November 29, following its attainment of a safer, more stable, and more circular orbit with the perigee some 3,500 kilometres higher than its original placement.
Doresa’s salvage has been a slow and steady journey since it was placed, with sister satellite Melina, into a fairly useless orbit in August following a launch anomaly. The original orbit, with a 26,000-kilometer apogee and a 13,800-kilometer perigee, prevented their use for navigation services because they were too low during part of their orbit to sense the horizon and correctly determine their own position. They were also getting a daily dose of radiation from the Van Allen belts.
Elevation
The elevation of the satellite started in late October and involved 11 firings of Doresa’s on-board thrusters. The craft now has only 15 kilos left from its original 65 kilo fuel payload but, given the fact that normally Galileo satellites are not required to make regular orbital manoeuvres, ESA engineers estimate this should be enough for a good 12 years of operation in the new orbit.
The next stage will be to repeat this manoeuvre with the second Full Operational Capability (FOC) satellite, Melina, according to a plan to get that into a similar orbit by the New Year. Pending tests of their positioning, navigation, and timing payloads, the two spacecraft are then likely to be able to contribute to the future Galileo navigation constellation. This was confirmed by Didier Faivre, ESA’s director for navigation, during the agency’s ministerial council meeting on December 2 in Luxembourg.
This end result is the best possible scenario given where the satellites were left after launch and is a considerable triumph for ESA’s mission control teams and flight engineers. Doresa is now able to use its Earth sensor continuously and keep its antennae orientated towards the Earth. Despite more than a month’s exposure to the Van Allen radiation, testing so far has shown no ill effects.
“The very good geometry of the satellites in the sky relative to the receivers helped us to achieve this result, plus the signal strength of the fifth satellite,” explained Gustavo Lopez Risueno, coordinating the receiver team at the Navigation Laboratory in ESA’s ESTEC technical centre.
The satellite signals should be usable immediately, in combination with additional navigation message information provided through ground networks, with mass market receivers. In fact the ESTEC Navigation Laboratory, working in conjunction with the European Commission and the European GNSS Agency (GSA), have already performed position fixes with both Galileo and GPS satellites using only navigation-assistance information.
With some adjustments to the Galileo network’s ground infrastructure, it looks like Doresa and Melina will be able to carry out most of the roles they were originally designed to do. They are the first of 22 Galileo FOC satellites to be built by OHB and launched by ESA over the next few years.
Toasted antennae
More good news. The problem with Galileo’s fourth IOV satellite, named Sif, that took it out of action at the end of May seems to have been characterised and — again — indicates that the satellite is not a complete loss to the constellation. While Sif’s E5 and E6 frequency bands are definitively blown, the satellite’s E1 Open Service band should be capable of broadcast.
The problem appears to have been a defective antennae. The four IOV satellites utilise one antennae design, while the FOC satellites have a different design. Fortunately there is no sign of a similar issue with the three other IOV craft, but they have been operating on reduced power as a precaution while the root cause of Sif’s failure is determined. ESA is currently fail-testing an example of the culprit antennae in the laboratory to see if the failure mode can be characterised.
“One of the possible root causes links the problem with the power emitted by the antenna. When we know more we’ll decide what to do with the other three. Since this event occurred in May and June, no more issues have arisen,” Faivre said.
Agreement
This is all a remarkable turnaround and good news for the wider European GNSS community and those stakeholders who have invested in the Galileo programme and its burgeoning application industry. Let’s hope the good fortune continues through 2015.
The administrative side of things is certainly moving on with the signing in October of an agreement which delegates a range of exploitation tasks for Galileo from the European Commission to the GSA, providing a framework and budget for the development of services and operations through to 2021.The signing of the agreement is an initial step towards the full Galileo Exploitation Phase. Current planning calls for this exploitation phase to be progressively rolled out from 2015, with full operability scheduled for 2020.
“With Galileo, we aim to provide a tangible service to European citizens, and this Delegation Agreement ensures we have the tools and funding necessary to achieve this,” said GSA Executive Director Carlo des Dorides. The agreement was signed by Daniel Calleja Crespo of the European Commission and des Dorides. The document specifically sets the actions to be implemented, the amount of funding provided, and the conditions for the overall management.
Innovation
In the same month, the First Satellite Masters Conference took place in Berlin on October 23 and 24. The conference encompassed the 2014 edition of the European Satellite Navigation Competition (ESNC). The event was a great showcase for the innovation, skill, and passion of the entrepreneurs, usually young, who are building the satellite application market in Europe.
For example, the winner of the GSA special prize at ESNC 2014 is developing Galileo modules for the Google Ara modular smartphone concept, a potential game-changer for positioning in the mobile-phone market. Ara uses interchangeable modules to deliver a smartphone that can be whatever a user wants it to be, complete with first- and third-party components including sensors, cameras, radio antennas, and more. Consumers will be able to order them as of January 2015.
Google developers believe an Ara smartphone will last multiple years, much longer than current hardware, since it won’t be obsolete nearly as quickly. Further, Ara could open the smartphone market to billions of new users across the globe.
I spoke with Giovanni Vecchione of Deimos Space, who received the € 40 000 GSA/ESNC prize during the awards ceremony at Deutsche Telekom’s magnificent headquarters in the German capital.
“With a traditional chip structure, all of a smartphone’s functions are currently combined into a single component, which makes it difficult to add or change a function,” explained Giovanni. “With a modular structure, you have the option to simply switch out a component, meaning a smartphone’s capabilities can be easily enhanced.”
Vecchione’s innovation is to use another of Galileo’s unique selling points: the E5 broadband signal. While mass market smartphones will use the E1 signal, the availability of high-end phones offering enhanced accuracy through the use of the E5 signal will appeal to many users. A second module will implement an external antenna interface. Together these developments could deliver an ARA phone offering high precision (centimetre-level accuracy) positioning and multipath-resistant solutions.
Wishing you all a very peaceful and prosperous New Year and hoping Santa has your coordinates accurately entered in his sleigh satnav!
Arianespace and the European Space Agency (ESA), acting on behalf of the European Commission, have signed a contract for three launch services with Ariane 5 ES to step up deployment of Galileo satellites.
With this new launch contract and thanks to the performance of Ariane 5 ES, a total of 12 Galileo FOC (Full Operational Capability) satellites will be launched using three dedicated Ariane 5 ES launch vehicles, each carrying four satellites. The Ariane 5 ES launches will take place from 2015 onwards.
Arianespace will be responsible for ensuring all of the 22 FOC satellites manufactured by the German group OHB System alongside the British company Surrey Satellite Technology Ltd. are taken into circular orbit at an altitude of 23,522 km using a combination of five Soyuz launch vehicles (two satellites per launch) and three Ariane 5 ES launch vehicles (four satellites per launch). The 22 operational satellites will join the four IOV satellites launched successfully by Arianespace from the Guiana Space Center in 2011 and 2012.
Arianespace and its subsidiary Starsem were responsible for launching in 2005 and 2008 from the Baikonur Cosmodrome the initial satellites in the Galileo constellation, GIOVE-A and GIOVE-B, which were able to secure the frequencies allocated to the constellation.
The contract for Arianespace’s three Ariane 5 launches to orbit a total of 12 Galileo FOC satellites was signed at the Guiana Space Center by Chairman and CEO Stéphane Israël (seated, at left) and Didier Faivre, ESA director of the Galileo Program and Navigation-related Activities. Joining them were ESA Director General Jean-Jacques Dordain and Daniel Calleja Crespo, director general for Enterprise and Industry, European Commission.
Once the contract had been signed, Stéphane Israël, chairman and CEO of Arianespace, made the following statement: “With its Ariane 5 ES heavy-lift launch-vehicle, Arianespace is able to provide the most appropriate solution for stepping up the deployment of the entire Galileo constellation. Ariane has once again demonstrated its excellence as it lends its expertise to Europe’s ambitions in space. With the three Ariane, Vega and Soyuz launch-vehicles operated from the Guiana Space Center, European spaceport, Arianespace is giving Europe guaranteed access to space and suitable solutions to meet its wide-ranging needs. I would like to extend my heartfelt thanks to the European Commission and European Space Agency (ESA) for their continued trust. Being the launch operator of the Galileo program is an immense source of pride for Arianespace, its employees and its partners.”
Arianespace has decided to postpone the launch of Soyuz flight VS09 carrying Europe’s fifth and sixth Galileo satellites. This is due to unfavorable weather conditions over the Guiana Space Centre.
Another launch date will be decided depending on the evolution of the weather conditions in Kourou.
Europe’s latest Galileo satellites have been sealed within their launch fairing, atop the Fregat upper stage that will carry them into their final orbit on August 21, ushering in the system deployment phase and paving the way for the start of initial services. Galileo SATs 5-6 are scheduled to lift off at 12:31 GMT from Europe’s Spaceport in French Guiana on top of a Soyuz rocket.
The two Galileo satellites had been attached together on the dispenser that secures them during flight, and then delivers them into orbit. Then August 14 saw the follow-on installation of the stack — the two satellites plus dispenser — onto the Fregat stage. The following day was the last time the two Galileo satellites were seen by human eyes, as the two halves of the protective launch fairing were sealed around the satellites and their upper stage.
Meanwhile, on August 18, the satellites’ three-stage Soyuz launcher was moved by rail onto its launch pad then lifted to the vertical position. The launcher’s mobile gantry was then moved into position around the upright launcher. This allows the next step of the launch campaign to take place, the hoisting up and attachment of the entire upper composite — the launch fairing containing the Galileo satellites, their dispenser and the Fregat fourth stage. At three hours, 47 minutes and 57 seconds after liftoff, the satellites will then be deployed from their Fregat by the dispenser’s pyrotechnic separation system, once their final 23,500 km altitude is reached.
These new satellites will join four Galileo satellites already in orbit, launched in October 2011 and October 2012 respectively. This first quartet were in-orbit validation satellites, serving to demonstrate the Galileo system would function as planned. Now that work has been done, the Full Operational Capability (FOC) satellites being launched on Thursday are significant as the first of the rest of the Galileo constellation.
The payloads generating navigation signals to Earth have been manufactured by Surrey Satellite Technology Ltd in the UK, while the satellites carrying them have been built by OHB in Germany.
A steady stream of launches is planned for the next few years, with two Galileo satellites flown per Soyuz launch and four Galileo satellites flown per launch of an Ariane 5 variant currently in preparation.
The definition, development and in-orbit validation phases of the Galileo program were carried out by ESA and co-funded by ESA and the EU. The Full Operational Capability phase is managed and fully funded by the European Commission. The Commission and ESA have signed a delegation agreement by which ESA acts as design and procurement agent on behalf of the Commission.
The Galileo FOC satellite named “Milena” is mated on its Soyuz dispenser unit, joining the already-installed “Doresa” satellite.
The fifth and sixth Galileo satellites met on their dispenser on August 11 in the S5 payload preparation facility of the Guiana Space Centre.
The completed dispenser unit is ready to be transferred from the S5 payload preparation facility for its integration atop Soyuz’ Fregat upper stage.
The fifth and sixth satellites meet for the first time, attached to the dispenser that will carry them into medium-Earth orbit (August 11).
Fueling of the two satellites (August 7-8) allows them to fine-tune their orbits and maintain their altitude over the course of their 12-year lifetimes.
Galileo satellites fastened to upper stage.
Galileo satellites lowered onto upper stage.
Galileo mission logos have been applied to the payload fairing, which encapsulates the two-satellite payload and their dispenser system.
The local (Kourou) poster of the launch.
Flight Operations Director Hervé Côme at ESOC.
Artist’s rendering of an OHB-designed Galileo satellite. OHB in Germany and SSTL in the UK are building the next 14 Galileo satellites.
Photo: European Space Agency
The medium-lift workhorse has been raised into a vertical orientation as the mobile gantry is moved into position.
Arianespace has decided to postpone the launch of Soyuz flight VS09 carrying Europe’s fifth and sixth Galileo satellites, because of unfavorable weather conditions over the Guiana Space Centre.
Another launch date will be decided depending on the evolution of the weather conditions in Kourou.
The next satellites in Europe’s Galileo satellite navigation system will be launched on August 21, ushering in the system deployment phase and paving the way for the start of initial services, according to the European Space Agency (ESA).
Galileo SATs 5-6 are scheduled to lift off at 12:31 GMT (14:31 CEST, 09:31 local time) August 21 from Europe’s Spaceport in French Guiana on top of a Soyuz rocket. They are expected to become operational, after initial in-orbit testing, in autumn.
The two satellites will join the four Galileo in-orbit validation satellites already in space. Launched in pairs in October 2011 and October 2012, these four satellites — the minimum required to obtain a position fix — served to demonstrate and validate the space and ground segments of the system.
Galileo SATs 7-8 are scheduled to follow end of year 2014. Then the constellation will be gradually deployed with six to eight satellites launched per year using a series of Soyuz and Ariane launches from Kourou, along with remaining elements of the ground network.
Satellite “Midwives”
Galileo’s post-launch team at ESA has finalized its preparations for taking control of the twin satellites. Following launch, the most crucial point in the flight comes when the two satellites separate from their upper stage — and the Launch and Early Operations, or LEOP, phase begins, run from ESA’s Space Operations Centre, ESOC, in Darmstadt, Germany.
If the moment of separation is the point when satellites are born, then the LEOP team can be thought of as midwives.
Any tumbling from the satellites being pushed away pyrotechnically must be corrected, and their positions stabilized in space. Next, they have to deploy their solar wings, to ensure a steady flow of power.
Flight Operations Director Hervé Côme at ESOC.
Then comes time to switch on and check out all the satellite systems one by one, to ensure everything has endured the launch in working order.
If all goes well, LEOP should take about a week before control of the satellites can be handed over to the Galileo Control Centre in Oberpfaffenhofen, overseeing the satellites, and ESA’s Redu centre in Belgium, for detailed payload testing.
Galileo’s LEOP team has been in training for months, explained Hervé Côme, flight director for Galileo at ESOC, with preparations stretching back two and a half years. “A simulation campaign has been running since March and the system and its operators have performed flawlessly,” Côme said. “To date, 20 simulations, in both nominal and contingency cases, have been conducted.”
Testing Teams and Technology
The satellites themselves participated in multiple end-to-end system compatibility tests to ensure that they are fully compatible with the various elements of the Galileo ground segment, extending to far-flung ground stations variously belonging both to ESA and to France’s CNES space agency, the Agency’s partner for LEOP.
A joint team from ESA and CNES oversaw LEOP for the first four Galileo satellites, similarly launched in pairs in 2011 and 2012. That work was carried out from CNES’s LEOP and Network Operations Control Centre in Toulouse, France.
This time, ESOC is hosting the LEOP team, with mission control and flight dynamics systems inherited from the first four in-orbit validation satellites adapted for these new Full Operational Capability (FOC) Galileo models.
The LEOP procedures and timeline have been fully validated, and system configurations frozen. From here on in, ESOC’s Mission Control Team — following a short summer break — will concentrate on further fine-tuning their organization and procedures in advance of next month’s launch.
The Galileo FOC satellite named “Milena” is mated on its Soyuz dispenser unit, joining the already-installed “Doresa” satellite.The completed dispenser unit is ready to be transferred from the S5 payload preparation facility at the Spaceport in French Guiana for its integration atop Soyuz’ Fregat upper stage.The local (Kourou) poster of the launch.
The troubled Galileo E20 satellite restarted E1 signal transmission Wednesday evening, August 6.
Galileo E20, also known as GSAT0104, the fourth in-orbit validation (IOV) satellite, has been set “unavailable until further notice” according to the European GNSS Service Centre because of a sudden, unexpected loss of power on May 27.
Based on a selected set of IGS MGEX stations and all CONGO stations, the first signals were tracked at AREG, AUT0, LLAG, and UNB3 at 23:13:00. No E5 signals and no navigation messages are currently transmitted. However, some JAVAD GNSS receivers report from time to time false E5a locks with zero or extremely small C/N0.
In an update to our July 2 story (recapped below), correspondent Peter de Selding wrote in Space News on July 3 that the trouble aboard the fourth in-orbit (IOV) Galileo satellite arose from a sudden, unexpected loss of power. The power outage flashed on May 27, shutting down the satellite’s E1 signal. The signal “re-established itself almost immediately. But as soon as it was back in service, the two other channels’ power dropped and did not recover. The full satellite then was shut down by ground teams,” reported de Selding.
European Space Agency (ESA) officials stated on July 3 that they would power-on the satellite again sometime this week (July 7–11) to continue investigating the problem. That investigation has been ongoing since the shutdown but has not identified a cause; officials state they have established that it is not related to the onboard atomic clocks.
The four IOV satellites currently aloft differ in both technology and manufacturer from the next phase of Galileo satellites to be launched. Two of these newer generation are at the Guyana spaceport awaiting a possible late August lift date.
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July 2 GPS World story:
Galileo GSAT0104, the fourth in-orbit validation (IOV) satellite, has been set “unavailable until further notice” according to the European GNSS Service Centre. International observers (not associated with the European Space Agency, ESA) including those of the International GNSS Service tracking the satellite have not detected a signal from GSAT0104 since May 27. A constellation update appeared June 26 at www.gsc-europa.eu/system-status/Constellation-Information, and is reproduced here.
Speculation by unofficial sources is mounting that something is wrong with the satellite, in particular with its passive hydrogen maser, used for timing the signal for synchronous transmission with other Galileo satellites. The hydrogen maser has “a known problem” according to one source. This is why the web site shows GSAT0104, also known as FM04 and E20, as currently using a rubidium atomic frequency standard.
No statement has been made by the ESA.
According to reports, the root cause of the outage is under investigation. Some unofficial sources have gone so far as to speculate that GSAT0104’s useful transmission life may be over.
The setting of unavailability may be due to in-orbit validation testing, as the website implies may be the case, but no further official statement has appeared. On May 27, an active user notifications (NAGU) appeared at www.gsc-europa.eu/system-status/user-notifications regarding GSAT0104 stating ” Unavailable from 2014-05-27 until further notice.” On June 26, another NAGU appeared for “All” satellites and stating “potential performance degradation.” A footnote states “The Galileo system is undergoing its in-orbit validation campaign. During this campaign of tests, users may experience periods of signal degradation.”
According to the ESA website, “The Galileo satellites carry two types of clocks: rubidium atomic frequency standards and passive hydrogen masers. The stability of the rubidium clock is so good that it would lose only three seconds in one million years, while the passive hydrogen maser is even more stable and it would lose only one second in three million years. However this kind of stability is really needed, since an error of only a few nanoseconds (billionths of a second) on the Galileo measurements would produce a positioning error of metres which would not be acceptable.”
Tim Reynolds is director of Inta Communication Ltd. and a long-term Brussels observer writing on many aspects of European government policy and implementation for a range of clients and publications. He is the contributing editor for GPS World’s new quarterly e-newsletter, EAGER: the European GNSS and Earth Observation Report. Subscribe free at env-gpsworld-integration.kinsta.cloud/subscribe.
The message to the recent European Space Solutions conference in Prague was simple enough: EGNOS is here, so let’s use it; Galileo is almost here, so let’s promote it.
Neither task is straightforward.
Take the European Geostationary Navigation Overlay Service (EGNOS), the European piece of a near-global network of terminals on geostationary satellites linked to networks of ground stations to verify GPS signal accuracy, primarily for aviation but with further applications as well. Other pieces of this global network are the Wide Area Augmentation System (WAAS) in the United States, the System for Differential Corrections and Monitoring (SDCM) in Russia, GPS-aided GEO-augmented Navigation (GAGAN) in India, and Multi-functional Satellite Augmentation System (MSAS) in Japan.
EGNOS is operational. It works. Once airports publish the required specificafions for localizer performance with vertical guidance (LPVs), aircraft with EGNOS terminals ultimately will be able to use EGNOS for flight terminations up to as low as 200 feet above the runway. Gone is the need for runway infrastructure, and welcome to the long-promised world of satellite-based augmentation systems. “It offers cheap solutions for precision approach,” said Fabio Gamba, chief executive of the European Business Aviation Association.
In the United States, where business aviation is a bigger market than in Europe, some 3,400 LPVs have been published for 1,670 airports. In Europe, the equivalent figure is 108 LPVs at 77 airports.
Why the sluggish response? Gamba cited a long list of issues, including some that appeared more political than technical. Part of the reason, some said, was that the EGNOS backers, including the company under contract to manage the system — European Satellite Services Provider (ESSP) of Toulouse, France — have not done enough to get the word out.
After all, these observers said, EGNOS suffered multiple delays, and its bigger younger brother, Galileo, has had bad press for years as its business model, ownership, regulatory backing, and schedule took turns in making eyes roll in Europe.
But that’s yesterday’s issue. Thierry Racaud, chief executive of ESSP, said EGNOS posted greater than 99 percent availability in May for its safety-of-life service, which is currently available on none of the other regional GPS augmentation systems except WAAS.
Racaud promised that the 108 LPVs signed so far would grow to 180 by the end of this year, and that 200-foot level approaches would be certified by late 2015. He said he hoped all 28 member nations of the European Union would have concluded their EGNOS regulatory approvals by 2017 or 2018.
“What we need now is more users,” Racaud said.
If EGNOS is not well known on its home turf, imagine its status in Africa, where European companies are trying to sell its adoption. Abdel Nasser Saint’Anna, director of the EGNOS-Africa Joint Program Office, said Africa should be Exhibit A for an EGNOS success pitch. Of the 2,500 runways in Africa, he said, only 177 were equipped with instrument landing systems (ILS), the system EGNOS and Galileo ultimately would like to replace.
Galileo, with Four, in Fourth
Galileo, too, appears headed for a successful adoption in many areas around the world even if, once operational, it likely will be the fourth global GNSS system in place, after GPS, Russia’s GLONASS and China’s BeiDou — not counting the large regional Indian and Japanese systems now being developed.
For those with scorecards, recall that four Galileo satellites, designed to validate the system’s performance, are in orbit. Carlos des Dorides, director of the European GNSS Agency (GSA) in Prague, said tests in May proved Galileo’s interoperability with GPS.
More importantly, des Dorides said the tests demonstrated how much better it is for consumers when their terminals access GPS and Galileo together. That should be obvious. Less obvious: Results were much better than with terminals tracking both GPS and GLONASS, he said.
The more satellites, the better? Yes, at least up to a point. Whether terminal manufacturers will see fit to incorporate all four global GNSS constellations, plus one or two of the regionals, in their hardware remains to be seen.
But the pent-up demand for Galileo does now seem better than it was as little as a year ago, despite the fact that some Asian nations attending the conference said they need Galileo to demonstrate its vitality sooner rather than later. Some officials said signal-quality issues with Beidou, and the recent GLONASS outage, will more than make up for Galileo’s delays as long as deployment progress is visible.
The fact remains that by 2020 there will be more than 100 GNSS satellites in medium-Earth orbit, in addition to the augmentation terminals on geostationary satellites.
A graphic presented by SpaceTec Partners’ Rainer Horn, whose company has been charged with preparing the Asian market for Galileo, showed just how dense the Asian skies will be with GNSS assets at the end of the decade. India, China, Japan, Taiwan, and South Korea are SpaceTec’s current Asian targets.
The message from these markets: Launch Galileo now. Drum up support. Occupy the media space.
Did the European Commission get the message? Time will tell. The next opportunity to wave the Galileo flag comes in late August, when the first two of 22 full-operational-capability satelllites will be launched from Europe’s spaceport in South America. Two more are scheduled to follow late this year.
Eight satellites in orbit by Christmas will not make an operational service, whatever the brochures say. But does that matter? Galileo now has secure funding, through 2020, for most — not all — of what it needs to launch a full constellation. Absent a new issue, by 2017 few will remember the delays.
Paul Weissenberg of the European Commission, who has seen the Galileo wars up close, reminded the European Space Solutions audience in Prague that one future Galileo customer sits outside the commission’s offices, waiting for approval to use Galileo’s PRS encrypted service. The U.S. Defense Department’s desire for Galileo does not have an expiration date. Just launch it.
Performance of the Galileo Single-Frequency Ionospheric Correction During In-Orbit Validation
By Roberto Prieto-Cerdeira, Raül Orús-Pérez, Edward Breeuwer, Rafael Lucas-Rodriguez, and Marco Falcone
OFF TO A GOOD START. That’s how we might characterize the European Galileo satellite navigation system. The official beginning of the Galileo program occurred on May 26, 2003, when the European Union and the European Space Agency officially agreed on the first stage of the program (although some work on system concepts took place earlier). The first two prototype or development satellites, Galileo In-Orbit Validation Element-A (GIOVE-A) and GIOVE-B, were launched on December 28, 2005, and April 26, 2008, respectively. The satellites successfully validated key technologies for the full Galileo constellation and secured the system’s frequency allocations.
The first two In-Orbit Validation (IOV) satellites were launched by a single rocket on October 21, 2011, and the third and fourth IOV satellites were similarly launched on October 12, 2012. The two GIOVE satellites and first two IOV satellites provided an opportunity to use Galileo-only receiver measurements and after-the-fact precise satellite orbit and clock data to compute the position of a receiver’s antenna. Joined by two colleagues, I was pleased to report our successful attempt using dual-frequency carrier-phase and pseudorange data collected on May 17, 2012, in an article in the September 2012 issue of this magazine. The two GIOVE satellites were subsequently retired.
The four IOV satellites began transmitting navigation messages with valid ephemerides in March, 2013, and this paved the way for the first real-time single-frequency pseudorange Galileo position fix using the broadcast messages on the morning of March 12 at the Navigation Laboratory of the European Space Research and Technology Centre in Noordwijk, the Netherlands. The position fix included compensation for the effect of the ionosphere on the Galileo signals.
The signals from GNSS satellites travel through the ionosphere on their way to receivers on or near the Earth’s surface. The free electrons populating this region of the atmosphere affect the propagation of the signals, changing their speed and direction of travel. This results in a delay in the arrival of the modulated components of the signals (from which pseudorange measurements are made) and an advance in the phases of the signals’ carrier waves (affecting carrier-phase measurements). The ionosphere is a dispersive medium for radio signals, so by making measurements simultaneously on two frequencies transmitted by a satellite, most of the effect of the ionosphere can be removed. However, single-frequency devices such as most vehicle navigation and handheld receivers don’t have the luxury of dual-frequency correction. These devices must rely on a single-frequency correction model. The coefficients for such a model are included in the navigation messages transmitted by all GPS satellites. Known as the Ionospheric Correction Algorithm or Klobuchar Algorithm, it removes at least 50 percent of the ionosphere’s effect.
The Galileo satellites also include the parameters of an ionospheric algorithm, called NeQuick G, in their navigation messages. In this month’s column, the Galileo system design team describes the novel European way for modeling the ionosphere for single-frequency users and compares its performance to the current GPS approach.
“Innovation” is a regular feature that discusses advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering, University of New Brunswick. He welcomes comments and topic ideas. Write to him at lang @ unb.ca.
Radiowave propagation of GNSS signals is affected by the Earth’s atmosphere and the characteristics of the local environment surrounding the receiver. GNSS systems are based on the broadcasting of radiowave ranging signals in the microwave domain (mainly in the so-called L-band, although some new systems like the Indian Regional Navigation Satellite System are also expected to broadcast in the S-band). These electromagnetic signals may suffer from a number of impairments as they propagate from a satellite to a receiver. In considering these effects, we can divide the Earth’s atmosphere into two parts: the electrically neutral atmosphere (primarily the lowest part, the troposphere), whose main effect is a group delay on the navigation signal due to water vapor and the gas components of the dry air, which, for microwave frequencies, is non-dispersive (independent of frequency); and the ionosphere, the ionized part of the atmosphere. The local environment may affect the navigation signal in various ways, too, such as signal fading or complete signal blockage by vegetation or obstacles such as buildings, and multipath, where the signal is broadened in the time and frequency domains due to reflections and diffraction by surrounding objects. In this article, we will discuss the effect of the ionosphere on GNSS signals and how it is being dealt with by the Galileo satellite navigation system.
The ionosphere owes its existence to solar radiation, primarily extreme ultraviolet light. The radiation ionizes the atoms and molecules in the upper atmosphere at heights of less than a hundred kilometers to a few kilometers above the Earth’s surface, producing a sea of ions and free electrons (collectively known as a plasma). This region is responsible for a number of dispersive (frequency-dependent) effects on navigation signals. Chief among these is a persistent delay of the pseudorandom noise (PRN) ranging codes (and the advance of the phase of the underlying carrier waves), thereby introducing positioning and timing errors if not compensated for. Signals are also susceptible to scintillations — rapid variations of amplitude and/or phase of the signals due to diffraction and refraction caused by plasma irregularities. Furthermore, the ionosphere can bend the signal path, resulting in a slightly longer path than the straight path, and rotate the polarization of the signal.
The ionospheric refractive index (the ratio of the speed of propagation of electromagnetic waves in a vacuum to the speed of their propagation in a medium) is related to the number of free electrons along the propagation path. For this purpose, the total electron content (TEC) is defined as the electron density in a cross-section of 1 square meter, integrated along a slant (or vertical) path between two points (such as a satellite and a receiver). It is often expressed in TEC units (TECU) where 1 TECU = 1016 electrons per meter squared = 0.1624 meters of delay at the GPS L1 frequency. According to the electron density, the mechanisms responsible for such ionization, and the dynamics, the ionosphere is usually sub-classified in layers of different characteristics: D, E, F1, and F2, with the latter largely responsible for the ionospheric effects on GNSS.
All of the propagation effects due to the ionosphere depend on a number of factors such as time of the day, location, season, and solar activity. There is also an interaction between solar activity, the ionosphere, and the Earth’s magnetic field, which, at times, can result in a significant disturbance of the ionosphere, as happens during geomagnetic storms. On a long timescale, solar activity follows a periodic, approximately 11-year, cycle. And spatially, the behavior of the ionosphere can be broadly classified into four main regions: the equatorial anomaly regions, located at around ±15-20º on either side of the magnetic equator, usually presenting the largest TEC values; mid-latitude regions, where the daytime TEC values are usually less than half the values found in the equatorial anomaly regions; and the auroral and polar regions, which present moderate TEC values but with larger variability than at mid-latitudes due to the characteristics of the geomagnetic field.
If we ignore some smaller, higher-order terms, the ionospheric group delay (the delay of the “group” of waves making up the PRN ranging code modulations) may be expressed in meters as 40.3 sTEC / f2, where sTEC is slant TEC in electrons per meter squared, calculated along the straight propagation path between receiver and satellite, and f is the carrier frequency in hertz. This effect introduces ranging errors of several meters if not corrected. The higher order terms usually account for differences at the millimeter level (rising to centimeter level during extreme ionospheric disturbances) and may be safely neglected for code ranging. The effect on the carrier phase has the same magnitude as the code delay, but of opposite sign, meaning that the carrier phase is advanced while propagating through the ionosphere. Since the group delay is dispersive, its effect can be mitigated using linear combinations of signals at two separate frequencies.
For single-frequency receivers, GNSSes often rely on correction models driven by broadcast data. For example, with GPS, the Ionospheric Correction Algorithm (ICA, also known as the Klobuchar algorithm) uses eight broadcast coefficients to describe the ionosphere, which is represented as a two-dimensional thin-shell model (the vTEC is assumed to be concentrated in a two-dimensional shell at a given height, relying on an analytical mapping or obliquity function to convert between vTEC and sTEC depending on the elevation angle of the received signal). This model is very efficient in terms of computational complexity, and it usually removes more than 50 percent of the ionospheric error, particularly at mid-latitudes.
Galileo and NeQuick G
Galileo provides dual-frequency services able to mitigate the effects of the ionosphere, but also services to single-frequency users. For a Galileo single-frequency receiver, an algorithm has been developed based on an adaptation of the NeQuick electron density model.
With the launch of the Galileo In-Orbit Validation (IOV) satellites and the initial navigation message broadcast, for the first time the end-to-end performance of the single-frequency correction algorithm for Galileo could be analyzed. The objective of the IOV phase was to launch the first four operational Galileo satellites and to deploy the first version of a completely new ground segment. During this phase, the European Space Agency (ESA) needed to validate — in the operational environment — all space, ground, and user components and their interfaces, prior to full system deployment, including the single-frequency correction algorithm performance starting from April 2013. Results were obtained for the period up to March 2014, coinciding with the maximum of solar cycle 24 and including three equinoxes with increased solar activity. In this article, we present performance results showing that the algorithm is capable of correcting more than 70 percent of the ionospheric group delay error under nominal ionospheric conditions, using only the reduced Galileo infrastructure during IOV (four satellites and a partial set of the Galileo sensor or monitoring stations).
The Algorithm. The Galileo single-frequency correction algorithm is based on an adaptation of the three-dimensional NeQuick electron density model, driven by an effective ionization level calculated with three broadcast ionospheric coefficients.
The original NeQuick model is a three-dimensional and time-dependent ionospheric electron density model based on an empirical climatological representation of the ionosphere, which predicts monthly mean electron density from analytical profiles, depending on solar-activity-related input values: sunspot number or solar flux, month, geographic latitude and longitude, height and UT. It allows us to calculate the TEC through numerical integration of electron density along a path between a beginning and an end point crossing the ionosphere. As an example, a global vTEC map obtained with NeQuick is illustrated in FIGURE 1. The first version of this model (NeQuick1) was incorporated into a previous version of the International Telecommunication Union (ITU) recommendation ITU-R P.532 for TEC estimation in radiowave propagation predictions. Researchers have continued development of the model with updated formulations, and version NeQuick2 is the one currently recommended by the ITU.
FIGURE 1. Global vTEC map obtained with the NeQuick electron density model for a sunspot number of 150 at 13h UT in the month of April (grid resolution 2.5 degrees × 2.5 degrees).
The NeQuick model has been adapted for Galileo single-frequency ionospheric corrections (for convenience, the Galileo version is known as NeQuick G) in order to derive real-time predictions based a single input parameter, Az, which is determined using three coefficients broadcast in the navigation message. The three coefficients are used in a second-degree polynomial as a function of the modified dip latitude (MODIP) of the receiver, to determine Az, which replaces the solar flux input parameter of the parent NeQuick model, with the following equation:
(1)
where ai0-2 are the three broadcast coefficients. MODIP is expressed in degrees. A grid table of MODIP values versus geographical location is provided together with the algorithm. A map showing five different MODIP regions is presented in FIGURE 2, each region usually presenting different behavior.
FIGURE 2. MODIP regions. Contours are modified dip latitudes.
The performance of the Galileo single-frequency ionospheric algorithm, designed to reach a correction capability of at least 70 percent of the ionospheric code delay, had been assessed in the past using GPS data only and using GPS plus Galileo In-Orbit Validation Element satellite data for an offline estimation of the broadcast parameters.
Since the first successful autonomous real-time Galileo-based position fix on March 12, 2013, the Galileo navigation messages have been broadcast by the four IOV spacecraft to the external user community, including the ionospheric broadcast parameters determined with IOV-only observations.
Experiment Period and Performance Indicators
To analyze the performance of the single-frequency ionospheric correction, a number of performance indicators were used:
The root-mean-square (RMS) error of the ionospheric model in meters of L1 code delay, for one station and one day.
The relative correction capability, expressed as an RMS percentage, defined as:
(2)
where STECref is the reference STEC and STECNeQuickG is the STEC obtained with the Galileo correction model. The factor 66 is used to avoid the fact that small absolute errors, which are relatively large due to small reference values, inflate the correction capability; it is linked to a target correction of 70 percent with a minimum absolute threshold of 20 TECU (30 percent of 66 TECU is about 20 TECU).
Performance verification has been assessed for the period from April 2013 to March 2014, which includes the secondary peak of the current solar maximum. The Galileo broadcast data used for this test are the Az coefficients broadcast by the four Galileo IOV satellites. It is important to remember that during the period of this assessment, the IOV infrastructure was reduced with respect to the target full operational capability, including the generation of the ionospheric parameters: four IOV satellites (no other GNSS satellites were used in the estimation) and a reduced number of monitoring stations.
Since the ionospheric correction performance assessment can be done independently of the Galileo signals and analysis of performance is preferred over independent data and locations, reference STEC estimated using dual-frequency observables from GPS at stations from the International GNSS Service (IGS), distributed around the world, were selected for the correction capability performance assessment. This resulted in observations of six to nine satellites for any epoch and with more than 120 stations per day, which assured good global coverage for the test. Performance has been computed individually for each set of broadcast parameters. For this aspect of ionospheric correction assessment, the differences between GPS and full constellation Galileo geometries are considered to be negligible.
As a reference for comparative purposes, for some cases the results have been compared to those obtained with the GPS ICA correction model using the broadcast parameters from GPS satellites.
The reference ionosphere STEC values were computed using dual-frequency carrier-phase GPS observables from IGS stations at a sampling rate of 300 seconds, and using IGS final global ionospheric maps (GIMs) to level the geometry-free combination of carrier phases. In this context, the IGS GIMs are employed to align the geometry-free or ionospheric combination, LI, to compute the ambiguity term (BI) for each satellite-to-receiver arc: (3)
where LI represents the linear combination between signals at frequencies f1 and f2; is the ionospheric delay in meters of LI; and BI is composed of several terms: station and satellite phase inter-frequency biases ( and respectively), LI phase ambiguity (λ1N1j – λ2N2j), phase wind-up, multipath, and noise. And i corresponds to the station and j to the satellite.
Then, in order to compute the corresponding BI term for each satellite-receiver continuous arc, the sTEC prediction of the GIM (sTECGIM_map) is computed for each satellite ionospheric pierce point, and then the average is computed as follows: (4)
where the indices i, j, and α correspond to the receiver, satellite, and arc indicator respectively, and the average is performed over the corresponding continuous (no cycle slips) arc (α) of data. is estimated following the mapping function and the procedures to interpolate in space and time recommended by IGS for GIM maps represented in ionosphere-exchange (IONEX) format.
With this estimation, the aligned STEC can be obtained as: (5)
which is the STEC used as an accurate sTEC estimation or “truth” reference value.
Results
The first analysis that we performed was the daily RMS error and correction capability for all stations. Most days have shown very promising performance. To see different levels of performance, results for one “bad” day and one typical “good” day, in the period of experimentation, are presented in FIGURE 3. It is observed that even for the “bad” day, the correction capability is above 70 percent, except for some stations in the equatorial regions. This performance is exceeded significantly for the “good” day, with RMS residual ionospheric errors below 1.5 meters for L1 even at low latitudes.
FIGURE 3a. Performance of the Galileo single-frequency ionospheric correction when using the E11 satellite broadcast, “bad day” RMS error in meters of L1.FIGURE 3b. Performance of the Galileo single-frequency ionospheric correction when using the E11 satellite broadcast, “good day” RMS error in meters of L1.FIGURE 3c. Performance of the Galileo single-frequency ionospheric correction when using the E11 satellite broadcast, “good day” correction capability in percent.FIGURE 3d. Performance of the Galileo single-frequency ionospheric correction when using the E11 satellite broadcast, “good day” correction capability in percent.
The evolution of the RMS residual error both for Galileo NeQuick G and GPS ICA from April 2013 to March 2014 are presented in FIGURE 4. In this figure, ionospheric activity at the equinoxes is clearly observed in the degradation of performance, and the influence of increased solar activity from October 2013 to March 2014 is also evident.
FIGURE 4. Global daily RMS ionospheric residual error in meters of L1 after correction with Galileo NeQuick G (red) and GPS ICA (blue) from April 2013 to March 2014.
The residual error of the Galileo correction model is already at the level of the expected capability for the full constellation. It also shows better performance as compared to the GPS ICA model, especially at equatorial latitudes.
The level of correction capability for each station for the Galileo NeQuick G model and the GPS ICA model are presented in FIGURE 5 for a quiet day in May 2013 and an active day during the spring equinox in 2014.
FIGURE 5a. RMS correction capability (percent, with a lower bound of 20 TECU) of Galileo NeQuick G correction models for day 127, 2013.FIGURE 5b. RMS correction capability (percent, with a lower bound of 20 TECU) of GPS ICA correction models for day 127, 2013.FIGURE 5c. RMS correction capability (percent, with a lower bound of 20 TECU) of Galileo NeQuick G correction models for day 80, 2014.FIGURE 5d. RMS correction capability (percent, with a lower bound of 20 TECU) of GPS ICA (right) correction models for day 80, 2014.
Effect in the Positioning Domain. We have performed two analyses to assess the correction performance in the positioning domain: one using GPS observables and one with Galileo-only observables. In both cases, we used three ionospheric delay mitigation methods: the dual-frequency ionosphere-free combination, the single-frequency GPS ICA correction algorithm, and the single-frequency Galileo NeQuick G correction algorithm.
The performance of the correction algorithm in the positioning domain using GPS observables was performed with data from two stations: Noordwijk in The Netherlands (a mid- to high-latitude station) and Malindi in Kenya (a low-latitude station) for the day of year (doy) 172 of 2013. Results are presented in FIGURES 6 and 7 showing good performance of the NeQuick G correction, in particular at low latitude. The results do not include code smoothing neither for single-frequency nor dual-frequency positioning. In the results, it may be observed that, as expected, the noise level for single-frequency positioning is much lower than that of ionosphere-free, but a higher bias may be present (the residual mean ionospheric error).
FIGURE 6a. Horizontal GPS positioning error on L1 using single-frequency NeQuick G correction (blue), L1 and GPS ICA (red) and dual-frequency ionosphere-free (green) for mid-latitude station in Noordwijk (doy 172, 2013).FIGURE 6b. Vertical GPS positioning error on L1 using single-frequency NeQuick G correction (blue), L1 and GPS ICA (red) and dual-frequency ionosphere-free (green) for mid-latitude station in Noordwijk (doy 172, 2013).FIGURE 7a. Horizontal GPS positioning error on L1 and single-frequency NeQuick G correction (blue), L1 and GPS ICA (red) and dual-frequency ionosphere-free (green) for low-latitude station in Malindi (doy 172, 2013).FIGURE 7b. Vertical GPS positioning error on L1 and single-frequency NeQuick G correction (blue), L1 and GPS ICA (red) and dual-frequency ionosphere-free (green) for low-latitude station in Malindi (doy 172, 2013).
Positioning domain analysis with Galileo-only observations using the four Galileo IOV satellites, and applying the NeQuick G correction, was evaluated for a station in Washington, D.C., for doy 245, 2013, including E1-only, E5a-only, and dual-frequency E1-E5a ionosphere-free observations. (E1 is centered at the GPS L1 frequency, while E5a is centered at the GPS L5 frequency.) These results are presented in FIGURE 8. The single-frequency positioning performance is considered promising considering the limited number of satellites.
FIGURE 8a. Horizontal Galileo IOV positioning error on E1 and single-frequency NeQuick G correction (blue), E5a and single-frequency NeQuick G correction (red) and dual-frequency E1-E5a ionosphere-free (green) for mid-latitude station in Washington (doy 245, 2013).FIGURE 8b. Vertical Galileo IOV positioning error on E1 and single-frequency NeQuick G correction (blue), E5a and single-frequency NeQuick G correction (red) and dual-frequency E1-E5a ionosphere-free (green) for mid-latitude station in Washington (doy 245, 2013).
Conclusions
The performance of the Galileo single-frequency ionospheric correction algorithm, based on NeQuick G, was evaluated using the broadcast navigation messages from the four Galileo IOV satellites, both in correction capability and in the positioning domain for the period April 2013 to March 2014. Despite the reduced infrastructure (broadcast ionospheric parameters estimated using only the IOV satellites at a limited number of monitoring stations), the performance shows promising results, in particular for low-latitude regions where the ionosphere is more problematic and, as expected, it has been confirmed that the correction performance is correlated with solar activity.
Acknowledgments
The NeQuick electron density model was developed by the Abdus Salam International Center of Theoretical Physics in Trieste, Italy, and the University of Graz in Austria. The adaptation of NeQuick for the Galileo single-frequency ionospheric correction algorithm (NeQuick G) was performed by ESA and involved the original developers of NeQuick and other European ionospheric scientists under various ESA projects.
Note to Manufacturers
The publication of the NeQuick G model and the Galileo single-frequency correction algorithm is under preparation for public release by the European Commission.
ROBERTO PRIETO-CERDEIRA is a propagation engineer in the European Space Agency (ESA) at the European Space Research and Technology Centre (ESTEC) in Noordwijk, The Netherlands, responsible for the activities related to radiowave propagation for GNSS and satellite mobile communications.
RAUL ORUS-PEREZ is a propagation engineer at ESTEC, working on activities related to radiowave propagation in the troposphere and ionosphere for GNSS and other ESA projects.
EDWARD BREEUWER is the system integration and verification manager in the Galileo Project Office at ESTEC, responsible for the organization and coordination of all testing activities at the system level. He had overall responsibility for the IOV test campaign.
RAFAEL LUCAS-RODRIGUEZ is the Galileo Services Engineering Manager for the Galileo project at ESTEC.
MARCO FALCONE is the System Manager in the Galileo Project Office at ESTEC.
FURTHER READING
• Development of NeQuick Ionospheric Model
“A New Version of the NeQuick Ionosphere Electron Density Model” by B. Nava, P. Coïsson, and S.M. Radicella in Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 70, No. 15, December 2008, pp. 1856–1862, doi: 10.1016/j.jastp.2008.01.015.
“A Family of Ionospheric Models for Different Uses” by G. Hochegger, B. Nava, S.M. Radicella, and R. Leitinger in Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science, Vol. 25, No. 4, 2000, pp. 307–310, doi : 10.1016/S1464-1917(00)00022-2.
“An Analytical Model of the Electron Density Profile in the Ionosphere” by G. Di Giovanni and S.M. Radicella in Advances in Space Research, Vol. 10, No. 11, 1990, pp. 27–30, doi: 10.1016/0273-1177(90)90301-F.
• Evaluation of the Galileo Single-Frequency Ionospheric Model
“Assessment of NeQuick Ionospheric Model for Galileo Single-Frequency Users” by A. Angrisano, S. Gaglione, C. Giola, M. Massaro, and U. Robustelli in Acta Geophysica, Vol. 61, No. 6, December 2013, pp. 1457–1476, doi: 10.2478/s11600-013-0116-2.
“GIOVE-A Experimentation Campaign: Ionospheric Related Data Analysis” by R. Orus and R. Prieto-Cerdeira in Proceedings of NAVITEC 2008, the 4th ESA Workshop on Satellite Navigation User Equipment Technologies: GNSS User Technologies in the Sensor Fusion Era, Noordwijk, The Netherlands, December 10–12, 2008.
“Assessment of the Ionospheric Correction Algorithm for GALILEO Single Frequency Receivers” by R. Prieto-Cerdeira, R. Orus, and B. Arbesser-Rastburg in Proceedings of NAVITEC 2006, the 3rd ESA Workshop on Satellite Navigation User Equipment Technologies, Noordwijk, The Netherlands, December 11–13, 2006.
“Advanced Ionospheric Modelling for GNSS Single Frequency Users” by M.A Aragón Ángel and F. Amarillo Fernández in the Proceedings ofPLANS 2006, the Institute of Electrical and Electronics Engineers / Institute of Navigation Position, Location and Navigation Symposium, San Diego, California, April 24–27, 2006, pp. 110–120, doi: 10.1109/PLANS.2006.1650594.
• GPS Ionospheric Model
“Ionospheric Time-delay Algorithm for Single-frequency GPS Users” by J.A. Klobuchar in IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-23, No. 3, May 1987, pp. 325–331, doi: 10.1109/TAES.1987.310829
Belgian frigate Leopold I-F930 in rough water off Norway during Galileo maritime testing. In December 2013 the frigate participated in the first maritime trials outside mainland Europe of the Galileo satellite navigation system.
Results are being processed from the first Galileo maritime trials outside of mainland Europe. The long-range, high-latitude testing spanned the North Sea, following the same historical sailing route that Viking dragon-ships used 1200 years ago.
Ancient manuscripts record Viking navigators relied on “sunstones” to find their way — archaeologists believe these may have been polarizing crystals to pinpoint the Sun even in overcast skies.
By contrast, Belgian frigate Leopold I-F930, participating in the end-of-year trials, carried the most up-to-date equipment possible, with multiple Galileo receivers for both its public Open Service (OS) and secure Public Regulated Service (PRS).
“Galileo is in a transition between its In-Orbit Validation (IOV) phase and follow-on Full Operational Capability phase,” said Miguel Manteiga Bautista, head of ESA’s GNSS Security Office. “This means we are engaging in all kinds of experimental demonstrations of all Galileo services, in particular PRS, which offers the most highly accurate positioning and timing performance, but with access strictly restricted to authorized users.”
The recorded course of Belgian frigate Leopold I-F930 during the first high-latitude trials of Europe’s Galileo satellite navigation system. The frigate sailed first from the Dutch marine base of Den Helder on 4 December 2013 to Stavanger in Norway. From there it progressed north in very rough seas with 10-m high waves, coming close to the Arctic circle on December 17 — a first for Galileo PRS observations — before heading homeward.
The frigate sailed first from the Dutch marine base of Den Helder on December 4, 2013, to Stavanger in Norway. From there it progressed north in very rough seas with 10-meter-high waves, coming close to the Arctic circle on December 17 — a first for Galileo PRS observations — before heading home.
The testing provided tangible in-situ evidence of Galileo signal stability across both its operating frequencies up at high latitudes, equaling low satellite elevations in the local sky.
Following the completion of earlier road, then flight, testing last summer and autumn, the last challenge for Galileo’s IOV phase was to engage in a long-term maritime trial into high latitudes. The testing was performed as part of the PRS Participants to IOV project jointly managed by ESA and the European Commission, in collaboration with the European GNSS Office Agency and several Member States possessing PRS test receiver technology.
The trials were performed by the Royal Military Academy of the Belgian Ministry of Defence, the UK Space Agency in collaboration with Nottingham Scientific Ltd. and ESA, to ensure PRS signals were available whenever the four Galileo satellites in orbit came into view.
Two receivers, seen either side of the main antenna, were carried by Belgian frigate Leopold I-F930 during high-latitude testing of both Galileo’s publicly-available Open Service and secure Public Regulated Service in December 2013.
A dual-test setup was fitted to the frigate at Den Helder. Belgium connected a PRS receiver and an OS receiver, both manufactured in Belgium by Septentrio NV, to a common antenna. The PRS receiver recorded raw PRS measurements on both frequencies while the OS receiver logged data from openly available Galileo, GPS and GLONASS signals at one-second intervals.
Nottingham Scientific installed its Ultra system configured to record radio-frequency samples, allowing the detailed post-processing of Galileo OS and PRS signals.
“As this was a first use of PRS equipment outside EU borders, the security issues were quite challenging,” said Bruno Vermeire, head of the Belgium Competent PRS Authority (Federal Public Service of Foreign Affairs). “Several partners from different countries and industries were involved. At all times the necessary security was assured, though this could not have been possible without the dedicated joint commitment of all partners.”
David Parker, head of the UK Space Agency, commented, “This test is a significant milestone on the road to demonstrating early PRS capability across a range of platforms. It should serve as a model for wider international collaboration between national governments and industry to prove and demonstrate PRS in different applications.”
Belgian frigate Leopold I-F930 at Den Helder dockyard in the Netherlands.
Alain Muls, professor of the Royal Military Academy of Belgium, faced the challenge of coordinating the maritime trial without interfering with the normal operations of the frigate. “Thanks to the cooperation of with the Maritime Component of the Belgium Defence, in particular that of the frigate’s commander and crew, preliminary results look very promising. Reception of Galileo’s OS and PRS navigation services have been practically demonstrated under severe maritime conditions with waves of up to 10 meters in height.”
“This activity is a truly collaborative effort at all levels. The trial involved UK and Belgian governments and industry partners with support from different European bodies as well as officials from the Netherlands and Norway,” said Mark Dumville, Nottingham Scientific general manager. “This team effort has enabled the concept of radio-frequency sampling processing of Galileo PRS signals to be tested in real-world operational environments. We have confirmed that the prototype receiver is now ready to support European governments and associated PRS applications.”
The collaborative nature of this trial was formally recognized as the Leopold I-F930 reached Stavenger. Under the supervision of Belgium’s CPA, Jochen Devadder, the country’s Ambassador to Norway Michel Godfrind provided a Norwegian delegation with details of the testing.
Results from the trial will guide future Galileo developments for years to come.
