Artist’s rendering of GIOVE-A in orbit. (Image: ESA)
News from the European Space Agency
Europe’s first prototype satellite for Galileo, GIOVE-A, has been formally decommissioned after 16 years of work in orbit. The GIOVE-A mission in 2005 secured Galileo’s radio frequencies for Europe, demonstrated key hardware, and probed the then-unknown radiation environment of medium-Earth orbit.
“If not for GIOVE-A, the 26 Galileo satellites in orbit today would not exist,” said Paul Verhoef, ESA’s director of navigation. “Its speedy development and launch opened the way for our working constellation to follow.”
ESA had begun designing Galileo at the turn of the century, and radio frequencies had been set aside for the new system by the International Telecommunications Union. But these frequency filings came with a deadline attached: the frequencies had to be used from orbit by mid-2006 or they would lapse.
GIOVE-A was launched by Soyuz from Baikonur cosmodrome in Kazakhstan on Dec. 28, 2005. (Photo: ESA)
GIOVE-A Sped to Orbit
Galileo In-Orbit Validation Element-A, or GIOVE-A, was produced at a breakneck pace to meet this deadline. Developed in the second half of 2003, the satellite was designed, built and tested before the end of 2005, and launched on Dec. 28 of that year.
“At the time there was a lot of uncertainty: Would we make it or not?” recalled Javier Benedicto, head of the Galileo Project Department, ESA. “GIOVE-A transmitted its first Galileo signal-in-space on Jan. 21, 2006, meaning that Europe was formally in the navigation business.”
That March, ESA formally confirmed it had brought the Galileo-related frequency filings into use, three months ahead of the official ITU deadline.
Europe’s first navigation satellite GIOVE-A, short for Galileo In-Orbit Validation Element-A, during flight preparation. (Photo: ESA)
The mission also carried a prototype rubidium atomic clock — proving its functionality for the operational Galileo satellites that would follow — as well as a radiation instrument. Medium Earth orbit, 23,000 km altitude, was terra incognita at this point for European satellites, but it was known to possess enhanced radiation levels from the impinging of the outer band of Earth’s Van Allen radiation belts.
A second Galileo prototype, GIOVE-B, followed in 2008, this time hosting a prototype passive hydrogen maser — the second type of atomic clock that Galileo relies on — along with an enhanced payload able to transmit for the first time the GPS-Galileo common signal.
GIOVE-A Succeeded at New Mission
Once the first Galileo satellites were in orbit and working well, ESA ended use of GIOVE-A in 2012. The satellite was placed in a graveyard orbit 100 km above the operational satellites’ orbits, as was GIOVE-B after its own four-year mission.
Control of GIOVE-A passed to manufacturer Surrey Satellite Technology Ltd (SSTL) in the United Kingdom. GIOVE-A was then employed for various in-orbit experiments, including demonstrating the reception of satellite navigation signals from GPS satellites orbiting below it — based on spillover sidelobe reception from satellites on the other side of Earth.
GIOVE-A was able to make use of signals emitted sideways from GPS antennas, within what is known as “side lobes.” (Image: ESA)
This proof that satnav can be relied on further out into space means that satellites in geostationary orbit are making use of satnav for positioning. As a next step, ESA is planning to extend satnav coverage all the way to the Moon.
The satellite also continued its radiation survey of medium-Earth orbit, acquiring a unique record extending across more than 10 years, analyzed by the Surrey Space Centre with ESA support. Multiple scientific papers have been written on these results, which encompass the “electron desert” of 2008-9 during the lowest levels of solar activity of the space era, followed by one of the largest electron storm events on record in April 2010.
A new model of the outer Van Allen belt electron fluxes, MOBE-DIC, has been produced from this dataset, helping to guide future satellite designs.
“Actually, the satellite itself is still operating well,” said Sarah Lawrence, SSTL. “The reason for ending the mission is software obsolescence in our control center. The decommissioning procedure involved transitioning the satellite to Earth-pointing mode, turning off the reaction wheels and setting the attitude and orbit control system to standby mode, before finally switching off the on-board computer and transmitter.”
“GIOVE-A over-delivered on its original lifetime and mission goals – an inspiring and game-changing mission on so many levels,” said Martin Sweeting, SSTL executive chairman.
SSTL went on to provide navigation payloads for operational Galileo satellites. Today, 26 Galileo satellites orbit the Earth. Galileo has become the world’s most precise satnav system, delivering meter-scale accuracy to more than 2.3 billion users around the globe.
It was not a big wager as wagers go, at least not in monetary value, but the underlying premise of the wager spoke volumes. It all began innocently enough in 2005 when the first test, or proof of concept, Galileo satellite known as GIOVE-A was launched.
In March of that year, a group of PNT experts made a simple wager that there will be:
10 or fewer operational Galileo satellites by 12/31/15
or
11 or more operational Galileo satellites by 12/31/15
Galileo’s GIOVE-A retired in June 2012.
About 20 PNT experts took the bet, evenly divided on both sides, which essentially said that given that the first test (GIOVE) Galileo satellites were launched in 2005 and 2008 respectively, surely there would be at least 10 operational satellites on orbit or about one per year by 2015.
The stakes were modest, but as I said, the import of the faith (or lack of faith) in the European Union and its ability and understanding of the difficulties involved in the Galileo endeavor spoke volumes. As the chief scientist at Air Force Space Command stated at the time, “This is rocket science; this is hard.”
Chutzpah and/or naïveté
But the Europeans refused to believe it was a very hard problem. Indeed, after the second GIOVE launch, GIOVE-B in 2008, the European ministers announced, with incredible chutzpah and/or naïveté, that the Galileo constellation would be fully operational (24 fully operational on-orbit satellites) by 2013.
Of course, nothing of the sort has happened. Following the in-orbit validation (IOV) satellites, the first operational satellite launch did not occur until October 2011, almost six years later.
As of May 2016, there were 12 operational Galileo satellites on orbit along with two in early orbit or checkout stages — a far cry from the predicted 24 operational satellites. This is not a criticism of the Galileo system; rather, a validation of those who took the pessimistic side of the wager and of the chief scientist who clearly stated the obvious: this is indeed, as a popular euphemism states, a DARPA hard problem.
So the Europeans have been going about this PNT business since the initial decision to proceed in 2003 — 13 years. The United States has been producing and launching GPS satellites continuously since the first test launch of a NAVSTAR satellite in 1977 (39 years), with a continuously fully operational system (FOC) since 1995 (21 years), and guess what? It is still a hard problem. No one denies that. Which brings us to GPS III.
GPS III Update
Since the United States — specifically the United States Air Force (USAF) — has been in the space-borne PNT business longer than any other nation, you would think we would have this down by now. But it is still a hard problem with, fortunately, a long string of successes and very few (only two) failures.
To date, the U.S. government has launched a total of 72 GPS satellites. There are 31 active operational GPS SVs (satellite vehicles) on orbit, with seven additional in residual or test status; 32 have been retired into a parking orbit where they will not interfere with the operational constellation. That equates to 1.85 GPS satellites launched per year on average, or one every 6.5 months — an enviable record, failures and all.
Plus, there are GPS IIA satellites still on orbit that have been there for more than 22 years. Not bad for a satellite built to last (contracted service life) for 7.5 years.
Amazingly, the payloads on every GPS satellite to date were built, in part, in partnership with or completely by one company, now known as Harris, nee Exelis, nee ITT. Of course, the complexity of the payloads being built by Harris for the GPS III satellites is a far cry from the payloads built in 1975 for launch in 1977. According to GPS III program manager and VP Mark Stewart and his cohorts at Lockheed Martin (LMCO), the aerospace company building the GPS III satellites, GPS III
“…will deliver three times better accuracy, provide up to eight times improved anti-jamming capabilities and extend spacecraft life to 15 years [ed. contracted life], 25 percent longer than the [ed. latest family of satellites on orbit today]. GPS III’s new L1C civil signal … will make it the first GPS satellite to be interoperable with other international global navigation satellite systems.”
While many of you may look upon that LMCO statement as marketing hype, in fact it is a rather incredible prophesy. To a PNT expert it translates to: almost all GPS users globally will have sub-meter level positional accuracy from a group of signals that will rarely if ever be completely jammed, from an SV with a projected lifetime of 30 years that has more signals and greater signal strength, flexibility and interoperability than ever before. By the numbers GPS is still, far and away, the world’s gold standard.
So exactly where are we in relation to a launch of the first evolutionary GPS III satellite? After all, the last IIF launch, number 12 in the series, built by Boeing, occurred in February, so by the law of averages we should have the first GPS III launch later this month. That is not going to happen, but then what is a few months among friends when iterated over 39 years?
Currently the first GPS III launch date, according to the USAF, is scheduled for May 2017. All indications are the government is on track to meet that date with, interestingly enough, the availability of a suitable launch vehicle being the LIMFAC (limiting factor), not the availability of an GPS III SV to launch.
SV 01 in testing at Lockheed Martin’s Denver facility. (Photo: LMCO)
According to my sources, GPS III SV-01 is fully integrated, has completed all environmental testing and is essentially ready to ship to Cape Canaveral,. It would be available for launch (AFL) sometime before the end of the calendar year if there were a launch vehicle, a ground control system and range availability.
GPS III SV-02 will undergo full integration (“core-mating”) completion sometime this fall and — following successful completion of its environmental tests — should certainly be AFL in 2017.
The complete navigation panel (from Harris) for GPS III SV-03 should arrive in the LMCO Denver facility early next year. Providing the vehicle stays on track through testing, it should be AFL in 2018.
The government has yet to complete the contract award process for GPS III vehicles SV-09 and SV-10 to LMCO, but I am assured the award is imminent.
My sources confirm that Harris is continuing to pump money, expertise and technology into the GPS III payload development process, a manufacturing tour de force, and the company should be back on schedule early next year.
As for OCX, the future GPS Ground Control Segment, that is another tale for another time. For all other GPS III segments, all in all it is a positive message for development and deployment. Which is an admirable feat — after all, it is rocket science!
By the way, the Galileo wager is open to interpretation. There were certainly more than 10 Galileo platforms on orbit on the last day of December 2015, but only nine of them were operational at the time. Both sides are claiming victory. What a surprise!
A product to save your hearing
The EB15LE with Hearing Defenders with accessories. (Photo: ERI)
Before I close, I want to mention a product I have tested as extensively as I can in a limited environment. I agreed to test this non-GPS product because of all the emails and letters I receive concerning tinnitus and how it negatively affects our warfighters. Several emails make clear the necessity and criticality of a good sight picture or display for GPS guidance, especially where exfiltration is concerned.
When warfighters or law enforcement officers are suffering the ill effects of extremely loud noises, it is often disorienting. Much like the effects of a flash-bang device, a victim can lose his bearings and needs to have a clear visual of how to exit the threat environment.
The best solution would be not to suffer the devastating effects of the loud noises in the first place. This is where a company named Etymotic Research Incorporated (ERI) comes into play. ERI has developed electronic hearing protection for law enforcement officers and military users.
The version I tested was designated the EB15 for law enforcement. It functioned well as electronic hearing protection and amplification where needed. The device is essentially an electronic hearing aid that amplifies natural or quiet sounds up to five times, and a hearing defender that electronically blocks loud, harmful sounds by up to 25 decibels.
While I was not able to test the hearing defenders in actual combat, the testing I did perform demonstrated that the EB15-LE is an impressive product with a plethora of earplugs for various noisy environments that may help save a user’s hearing. Our warfighters and law-enforcement officers deserve the best technology available, especially if it helps them retain their orientation in a dangerous environment and saves their hearing.
Until next time, happy navigating, and remember: GPS is brought to you free of charge courtesy of the USAF.
Ten years ago saw the launch of Europe’s very first navigation satellite. A decade of hard work later, more than a third of the Galileo constellation has followed it into orbit and a ground network sharpening the satnav system’s accuracy encompasses the globe.
Galileo’s GIOVE-A retired in June 2012. (Photo: ESA)
GIOVE-A, short for Galileo In-Orbit Validation Element-A, was launched by Soyuz from Baikonur cosmodrome in Kazakhstan on Dec. 28, 2005.
“Much work had already taken place on the ground, but GIOVE-A marked the first time that Galileo hardware went into orbit,” recalls Didier Faivre, heading ESA’s navigation directorate.
“It had a crucial role: to claim operating frequencies that had been set aside for Galileo by the International Telecommunications Union, to demonstrate essential technologies such as Galileo’s rubidium atomic clock, and to gather data on the radiation-rich environment of medium-altitude orbit, which was a relatively unknown region for Europe.
Didier Faivre (Photo: ESA)
“So GIOVE-A opened the way to all that followed — the follow-up GIOVE-B in 2008, and then the launches of the Galileo satellites proper — a total of six double-satellite launches from 2011 to earlier this month, with 12 satellites placed in orbit so far, on the way to the full 30-satellite constellation.
“Our three launches this year doubled the number of satellites in orbit, and the pace of progress is set to increase further in 2016, when we will see Ariane 5 called on to launch Galileo for the first time, doubling the number of satellites released on each flight.”
Progress in space has been mirrored on Earth, with the establishment of Galileo’s ground segment: two control centers in Fucino, Italy, and Oberpfaffenhofen, Germany, sit at the heart of a worldwide network of ground stations.
The Soyuz launcher carrying GIOVE-A climbs into the sky above Kazakhstan on Dec. 28, 2005. (Photo: ESA)
This ground segment, as one of the most complex infrastructures ever overseen by ESA, has the task of keeping all the satellites healthy and their navigation signals synched — the timing system accurate to a few billionths of a second that provides the submetre precision.
Sensor stations to monitor Galileo signals and return data to the control centers are required all across the vast footprint of the system. Their locations range from the Arctic to Antarctic to lonely mid-ocean islands, all connected back to Europe via satellite.
Initial Galileo services are set to be available during next year. New generations of commercial satnav receivers are already Galileo-ready, having received extensive technical checks from ESA’s Navigation Laboratory.
GIOVE-A in a clean room at ESA’s ESTEC technical centre in the Netherlands, having instrumentation sensors installed in advance of testing in 2005. (Photo: ESA)
And national governments are already preparing to make use of Galileo’s Public Regulated Service — PRS, the most secure and precise class of service, restricted to authorized users. Belgium, France, Italy and the UK have all performed their own PRS acquisitions and positioning testing.
As for GIOVE-A, the satellite that started it all, its ESA mission concluded in 2012, once the first four Galileo satellites followed it to orbit. Remarkably, it is still operational, controlled by builder Surrey Satellite Technology Ltd in the UK.
In the past it has carried out tests of high-altitude satnav detection. Its radiation monitor continues monitoring the medium-altitude orbit environment, helping to forecast the future long-term performance of its Galileo descendants.
Launch poster for the upcoming launch of Adriana and Liene, also known as Galileo 11 and 12.
The two spacecraft for Arianespace’s latest Soyuz launch at the service of Europe’s Galileo satellite-based navigation system — and the company’s record 12th mission overall in 2015 — have been integrated at the French Guiana Spaceport in preparation for their Dec. 17 liftoff.
The launch of Galileo satellites 11 and 12 — dubbed Adriana and Liene — will conclude a year when the number of Galileo satellites in orbit will have doubled.
With this launch, ten years after the launch of GIOVE A, on Dec. 28, 2005, the Galileo constellation will become a reality.
During activity in the Spaceport’s S3B clean-room facility, the spacecraft pair was mated with their dispenser, which will deploy the satellites by firing a pyrotechnic system for separation in opposite directions at the orbital insertion point.
The satellite/dispenser combination is ready for integration on Soyuz launcher’s Fregat upper stage, followed by the payload fairing encapsulation. This will create the “upper composite,” which is to be installed atop Soyuz once the vehicle has been moved to its Spaceport launch site.
Arianespace’s Dec. 17 mission, designated Flight VS13 in the company’s launcher family numbering system, will orbit two Galileo FOC (Full Operational Capability) satellites, further augmenting the European constellation of navigation spacecraft. The satellites are built by prime contractor OHB System in Bremen, Germany, with their onboard payloads supplied by UK-based Surrey Satellite Technology Ltd (SSTL), which is 99 percent owned by Airbus Defence & Space.
Galileo 11 and 12 are prepared for launch in the clean room. (Video capture: ESA)
The Galileo system is designed to provide high-quality positioning, navigation and timing services under civilian control. Its Full Operational Capability phase is managed and funded by the European Commission, with the European Space Agency (ESA) delegated as the design and procurement agent on the Commission’s behalf.
Flight VS13 will close out a busy year — complementing the 11 previous missions in 2015, which were composed of two other launches with the medium-lift Soyuz, three using the light-lift Vega and six with the heavy-lift Ariane 5.
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.”
The European Space Agency’s (ESA’s) retired GIOVE-A navigation mission has become the first civilian satellite to perform GPS position fixes from high orbit. Its results demonstrate that current satnav signals could guide missions much further away in space, up to geostationary orbit or even as far as the Moon.
GIOVE-A has been able to fix its position, velocity and time from GPS signals, despite orbiting more than 1000 km above the downward-pointing U.S. satellites.
“Satellite navigation has become almost as indispensable for most low-orbiting satellites as it is for car drivers and other terrestrial users,” said ESA’s Steeve Kowaltschek. “Satellites equipped with satnav receivers can continuously monitor their orbit in space, enabling largely autonomous operations with limited ground intervention. GIOVE-A’s three months of data show that future geostationary satellites could operate in the same way, bringing real competitive advantage to the multi-billion-euro telecommunications satellite market.”
Launched in 2005 to claim radio frequencies and test hardware for Europe’s Galileo satnav constellation, the Galileo In-Orbit Validation Element-A, or GIOVE-A, mission far outlasted its original two-year design life. It was formally decommissioned by ESA in the middle of last year, once the first Galileo satellites completed their orbital commissioning. Having been moved into a graveyard orbit about 100 km above Galileo’s orbital altitude of 23 222 km, control was passed to its prime contractor Surrey Satellite Technology Ltd. of Guildford, UK.
ESA had originally worked with SSTL to customize one of the company’s existing satnav receivers for testing on GIOVE-A, an activity supported through ESA’s Advanced Research in Telecommunications Systems (ARTES) program. In the event, the satnav receiver was activated for only 90 minutes during the very beginning of the satellite’s seven-year operational life, with GIOVE-A’s main tasks given priority. Once the formal mission was over, ESA and SSTL took the opportunity to switch the receiver on again.
“We have been really encouraged by the initial results from our receiver,” said Martin Unwin at SSTL. “Our patience has finally been rewarded, and we would like to make the best of this unique opportunity.”
SSTL is able to upload new software to the receiver in orbit, and has been able to apply sophisticated software algorithms to help detect faint satnav signals. Further work is planned to refine operation through the use of an accurate onboard clock and orbit-estimating algorithms.
GIOVE-A has been able to make use of signals emitted sideways from GPS antennas, within what is known as side lobes.
GPS Side Lobes. GPS satellites — like those of Galileo, Russia’s Glonass or their Japanese, Chinese and Indian counterparts — aim their antennas directly at Earth. Any satellite orbiting above the GPS constellation can only hope to detect signals from over Earth’s far side, but the majority are blocked by the planet. For a position fix, a satnav receiver requires a minimum of four satellites to be visible, but this is most of the time not possible if based solely on front-facing signals.
Instead, GIOVE-A makes use of signals emitted sideways from GPS antennas, within what is known as side lobes. Just like a flashlight, radio antennas shine energy to the side as well as directly forward.
“These side lobes are not typically well measured because this is energy that doesn’t reach users on Earth,” explained Kowaltschek. “Antenna designers seek to minimize them, but the laws of physics mean they will always be present in some form. Measuring these GPS side lobes has shown them to be stronger than anticipated, and the combination of side lobes and signals spilling over from the other side of Earth mean that a position fix can be maintained throughout GIOVE-A’s orbit.”
The satellite has also acquired detailed profiles of the signal side-lobe characteristics of the various GPS design blocks.
Geostationary satellites reside in set orbital slots, some 80-km across, up in the 36,000-km-altitude belt. Chemical thruster firings are needed every fortnight or so to correct for drift, checked against radio ranging from the ground.
Harnessing satnav would be a way of automating station-keeping functions. It also meshes with the current move to all-electric comsat designs, such as ESA’s Electra. Electric propulsion would do the job of conventional chemical thrusters, delivering more compact satellites capable of flying on cheaper launch vehicles while offering longer mission lifetimes. But electric propulsion provides lower thrust and therefore requires almost permanent ground ranging. Continuous position fixes via satnav could perform this task onboard, maintaining the orbit position with better accuracy.
SmallGEO. (Image: ESA)
In addition, constant orbit determination and close-to-perfect time knowledge also improves pointing accuracy on comsats that use startrackers as their main attitude sensor.
All-electric comsats using satnav could gradually steer themselves up to geostationary orbit following launch, further slashing the required launcher size, onboard fuel and ground support.
“We envisage a future satnav receiver that can track not only GPS, but also Galileo and Glonass signals at high altitudes, meaning a near-continuous availability of accurate position and time for geostationary and other satellites,” says Martin.
As a next step, a receiver will be flown on ESA’s SmallGEO telecom mission, due for launch in 2014. Building on the positive results of the GIOVE-A experiments, SmallGEO will be the first civilian mission to use satnav in geostationary orbit.
CYGNSS, Others Deliver Now and in Future for Global Weather Forecast
Editor’s Note: This article reproduces the acceptance speeches given by the winners of GPS World’s 2012 Leadership Awards, at the Leadership Dinner in Nashville in September. The Leadership Dinner was sponsored by Lockheed Martin and Deimos Space.
Martin Unwin, Surrey Satellite Technology Limited; Principal GNSS Engineer, winner in the Satellites category. He is a key member of the team that built the GIOVE-A satellite (recently retired) and is now working on the Galileo FOC satellites. He is also recognized for his work on space-borne receivers.
Headshot: Martin Unwin, Surrey Satellite Technology, winner in the Satellites category.
I feel privileged and honored to receive this award from GPS World, and I am truly sorry now that I chose this year not to attend the ION-GNSS conference to receive it!
With respect to the achievements in GIOVE-A and Galileo, I cannot claim this award on behalf of myself, but I will claim it on behalf of the people in Surrey Satellite Technology Limited (SSTL) who made the projects possible, and to those in the team here who have been working tirelessly to make the payloads and satellites happen. We are of course partnered with others in Europe that have been laboring equally hard, so it has been a true team effort.
With respect to the spaceborne GPS and GNSS activities, my achievements have only been possible thanks to the top-class staff we have in the receivers team, and thanks are also due to the support we have had from the rest of SSTL.
In the 20 years I have been in the company, Surrey Satellite Technology Ltd has grown from a small university-based department to a major player in the international space scene, and I am immensely proud to have been part of this story.
A Few Words for the Future
Whilst it cannot quite match the early heady days of GPS, I still think nevertheless we are entering an exciting time in the GNSS world. We have two operational systems, and within a few years, we will be seeing two more reaching operational capability. Dual- and even triple-frequency civil signals will soon become operationally available, and some very wide bandwidth signals will be sent down, in particular, by Galileo. There is bound to be a steep learning curve in understanding how to exploit these new signals, with a few crevasses to be negotiated during the climb. But these new signals are bound to lead to an expanded vista of increased accuracy and robustness, and undoubtedly some unexpected destinations.
Taking perhaps the highest perspective, spaceborne remote sensing is a good example that has surprising relevance to the rest of us still on the ground. In this case, GNSS satellites are used as radar sources, and all that is required on a low-Earth orbiting (LEO) satellite to change the world is a GNSS receiver. GPS radio-occultation measurements from low-Earth orbit are now already the third most important data source for our global weather forecasts, thanks to the like of the COSMIC and MetOp satellites.
Furthermore, a new constellation of satellites called CYGNSS has recently announced by NASA that will be using ocean-reflected GPS signals to probe inside hurricanes and typhoons, and for the first time will enable the sensing of the wide-scale ocean roughness, leading to improved global wind and wave knowledge. By adding to this spaceborne receiver the ability to accommodate signals from GLONASS, Galileo, and Compass, plus any other available GNSS-type signals, the number of measurements is instantly quadrupled, and a new capability in sensing the atmosphere, waves, and even ice and land is likely to be seen. Meteorologists already view GPS as an emerging utility for weather and climate sensing, but I think this new role for GNSS will be reinforced and expanded into yet another area where GNSS incontrovertibly, if indirectly, makes such a significant difference to our daily lives.
As with many other applications where GNSS has become important or even critical to our modern world, this is, at the same time, both a blessing and a matter for some caution.
An experimental GPS receiver, built by Surrey Satellite Technology Limited (SSTL), has successfully achieved a GPS position fix at 23,300 kilometers altitude – the first position fix above the GPS constellation on a civilian satellite. The SGR-GEO receiver is collecting data that could help SSTL to develop a receiver to navigate spacecraft in geostationary orbit (GEO) or even in deep space.
GPS is routinely used on Low Earth Orbit (LEO) satellites to provide the orbital position and offer a source of time to the satellite. Spacecraft in orbits higher than the 20,000 km of the GPS constellation, however, can only receive a few of the signals that “spill over” from the far side of the Earth, meaning that the signals are much weaker and a position fix cannot always be secured.
With the support of the European Space Agency (ESA) and the ARTES 4 program, SSTL included the SGR-GEO receiver on the GIOVE-A satellite to prove that a receiver could achieve a position fix from a higher orbit. The SGR-GEO is adapted from SSTL’s SGR range of receivers and incorporates a high-gain antenna and a precise oven-controlled clock. It will demonstrate special algorithms to allow reception of weak signals and an orbit estimator intended to allow a near continuous position fix throughout orbit.
“The results from the SGR-GEO receiver are really encouraging,” said Martin Unwin, principal GNSS engineer at SSTL. “We’re getting higher signal strengths than anticipated and also acquiring side lobes from the GPS transmit antennas, which improves the availability of the usable signals for navigation. With the success of the SGR-GEO receiver, GPS, in combination with Galileo and GLONASS, could soon be helping navigate spacecraft much further away from Earth.”
The experimental GPS receiver onboard GIOVE-A has been inactive for six years while the satellite has been used for its primary purpose of transmitting prototype Galileo signals. GIOVE-A’s retirement in June 2012 has allowed the commissioning of the experiment and is now providing valuable data to SSTL and ESA in support of the future use of spaceborne GNSS receivers at GEO altitudes. Engineers at SSTL will continue operations, testing out, tuning and improving the receiver software onboard GIOVE-A to achieve the best possible performance.
By Richard B. Langley, Simon Banville, and Peter Steigenberger.
For a brief period, and for a few hours on certain days, signals from the first four orbiting Galileo satellites could be received by state-of-the-art multi-frequency, multi-constellation GNSS receivers. Although not intended for actual positioning tests, the satellites did provide a first opportunity to assess the prototype Galileo signals in the positioning domain. The results obtained bode well for the future operational Galileo constellation.
The launch and successful operation of the two Galileo In-Orbit Validation Element (GIOVE) satellites — GIOVE-A and GIOVE-B — followed by the two Galileo In-Orbit Validation (IOV) satellites — ProtoFlight Model (PFM) and Flight Model 2 (FM2) — were important steps in the development of Europe’s Galileo satellite navigation system.
The GIOVE test-bed satellites were orbited to secure the use of the frequencies allocated by the International Telecommunication Union for the Galileo system; to verify the most critical technologies of the operational Galileo system, such as the on-board atomic clocks and the navigation signal generator; to characterize the novel features of the Galileo signal design, including the verification of user receivers and their resistance to interference and multipath; and to characterize the radiation environment of the medium Earth orbits planned for the operational Galileo constellation. The IOV satellites, of which there will be four with two more to be launched this fall, are prototype operational satellites designed to validate the Galileo concept in both space and on Earth. Once all four IOV satellites are in orbit, it should be feasible to carry out positioning exercises using just Galileo satellite signals. It was not intended for the GIOVE plus two initial IOV satellites to be used for positioning demonstrations. However, it turns out that (before GIOVE-A and GIOVE-B were recently decommissioned) for a few hours on certain days, signals from all four satellites could be received simultaneously by state-of-the-art multi-frequency, multi-constellation GNSS receivers.
Dual-frequency measurements from the GIOVE satellites and triple-frequency measurements from the IOV satellites have been archived by a number of continuously operating receivers including those in the COoperative Network for GIOVE Observation (CONGO) and those contributing to the International GNSS Service’s Multi-GNSS Experiment (M-GEX) observing campaign. Before joining the M-GEX campaign as the receiver at station UNB3 at the University of New Brunswick (UNB) in Fredericton, Canada, a Trimble Navigation NetR9 receiver fed by a Zephyr Geodetic II antenna was continuously tested at UNB for a couple of months and its 30-second-interval measurements were locally archived. These measurements included (in the terminology used by the Receiver Independent Exchange (RINEX) version 3 format): C1X, L1X, and S1X (pseudorange, carrier-phase, and carrier-to-noise-density-ratio measurements for combined data-plus-pilot tracking of the Open Service signal on the E1/L1 carrier frequency (1575.42 MHz)); C5X, L5X, and S5X (the corresponding in-phase and quadrature (I+Q) measurements on the E5a carrier frequency (1176.45 MHz)); C7X, L7X, and S7X (the corresponding I+Q measurements on the E5b carrier frequency (1207.140 MHz)); and C8X, L8X, and S8X (the corresponding I+Q measurements on the effective E5a+E5b carrier frequency (1191.795MHz)).
The first two of four Galileo IOV satellites were launched on October 21, 2011. Credit: ESA.
Although the IOV satellites are in synchronized orbits in the same plane with mean orbital periods of 1.70475 orbits per day, those orbits are not coordinated with those of the GIOVE-A and GIOVE-B satellites, which had mean orbital periods of 1.69434 and 1.70960 orbits per day, respectively. (The orbit of GIOVE-B was recently raised, following decommissioning.) This means that all four satellites will not generally be in view at a ground station at the same time. However, at a given location on certain days, four-satellite visibility did occur for periods up to a few hours. We identified several such days but were hampered in our efforts to obtain positioning solutions due to the testing programs of the satellites.
Our first constraint concerned GIOVE-A. The European Space Agency carried out tests with this satellite for more than six years and decided to decommission the satellite for its purposes on June 30, 2012, and switched off the navigation signals. This narrowed our window of possible four-satellite-visibility days. Secondly, the clocks on the IOV satellites were allowed to drift so that their offsets with respect to GPS System Time could be very large with offset values of tens to hundreds of milliseconds. Some GNSS receivers cannot make usable measurements when presented with such large clock offsets. This behavior further limited our windows of opportunity for four-satellite Galileo positioning. Nevertheless, we found that on May 17, 2012, the receiver at UNB successfully tracked the four satellites with a period of common visibility of two and a half hours. See Figure 1 for the time series of the occurrences of actual measurements made by the receiver. Common visibility extended from 03:04:30 to 05:34:30 GPS Time with the receiver tracking the satellites without any elevation-angle cutoff imposed.
In the remainder of this article, we describe the procedures used to obtain precise positions from the measurements, including the technique used to determine precise orbit and clock data for the Galileo satellites, and the results we obtained.
Figure 1. Visibility of Galileo satellites from UNB on May 17, 2012.
Generating the Orbits and Clocks
GIOVE and IOV satellite orbit and clock parameters are determined at Technische Universität München with a modified version of the Bernese GPS Software in a two-step procedure based on GPS and Galileo observations of 23 CONGO stations. After a common preprocessing step (detection of outliers and cycle slips), GPS and Galileo observations are treated separately. Station coordinates, tropospheric delay parameters and receiver clocks are obtained from GPS observations only. GPS satellite orbits and clocks as well as Earth rotation parameters from the Center for Orbit Determination in Europe (CODE) are kept fixed in this step. In the second step, the ionosphere-free linear combination of E1 and E5a observations is used to estimate the Galileo-related parameters, namely the satellite orbits and clocks. The station coordinates and the troposphere and receiver clock parameters are fixed to the GPS-derived results of the first step. To account for systematic differences between the GPS and Galileo code signals as well as biases between the different receivers, differential code biases (DCBs) are estimated for all stations but one. Separate biases are set up for the GIOVE and IOV satellites. To strengthen the stability of the orbital arc, five daily solutions are combined into a multi-day solution and consistent Galileo clock parameters are recomputed. Only the middle day of the5-day solution is used for the positioning discussed in this article. Based on internal consistency tests and satellite laser ranging residuals, the accuracy of these orbits is assumed to be on the one-to-two-decimeter level.
The Positioning Technique
A preliminary assessment of the quality of Galileo-only positioning could be achieved using the four satellites simultaneously in view at UNB. The second author’s GNSS positioning software was used to process the UNB data. Applying a 7.5-degree elevation cutoff angle to remove low-elevation-angle measurements resulted in an observation session of 1 hour and 48 minutes. The east or longitude dilution of precision (DOP) component starts out at 0.829 at the beginning of this session, gradually dropping to 0.688, and then rising to 1.285 at the end of the session; while the north or latitude DOP component starts out at 2.683 at the beginning of the session, rising to 4.233 at the end (see Figure 2).
Figure 2. North (N), east (E), vertical (V), and geometrical (G) dilution of precision (DOP) values.
Even though the receiver was tracking signals on E1, E5a, E5b, and E5a+b, carrier-phase and code observations on E1 and E5a were selected to match the satellite-clock datum. Ionosphere-free combinations were formed to eliminate first-order ionospheric effects, while the tropospheric delay was modeled using local measurements of temperature, pressure, and relative humidity provided by UNB’s meteorological station. No residual delay was estimated. Phase center offsets (PCO) and variations (PCV) for the Trimble Zephyr Geodetic II antenna were obtained through anechoic chamber calibrations (see Further Reading). The same satellite PCO as the ones used in the generation of the satellite orbits and clocks were applied, and no satellite PCV were considered. Other error sources required for precise positioning were also modeled such as solid Earth tides, ocean tide loading, and phase wind-up.
Since separate biases were set up in the estimation of the GIOVE and IOV satellite clock estimation, the same approach should be used on the user side. Unfortunately, solving for this additional parameter in the navigation filter is not possible when tracking only four satellites. To overcome this limitation, a GIOVE/IOV offset was estimated using 24 hours of combined GPS-plus-Galileo observations in static mode (one position solution for the whole observation period), and was introduced as an additional correction in the Galileo-only solution. The estimated coordinates from this combined solution were also used as a reference in computing the errors in latitude, longitude, and height presented next.
Results and Discussion
Three solutions were computed to demonstrate the quality of Galileo-only navigation. In the first scenario (see Figure 3), ionosphere-free code observations solely contributed to the epoch-by-epoch estimation of receiver position and clock offset. The estimated coordinates are largely contaminated by code noise, which is amplified by a factor of approximately three when forming the ionosphere-free combination. In the absence of redundancy, any errors in the observations (such as noise) propagate directly into the estimated quantities and, in this case, affected particularly the latitude component. An analysis of the noise and multipath characteristics of each signal revealed the presence of time-varying effects in the C5X observations. Further investigations are required to properly identify the cause of those effects. As a result, the root-mean-square (RMS) error of the latitude, longitude and height components were 3.084, 0.658 and 1.617 meters, respectively (see Table 1).
Figure 3. Code-based solution. Differences in latitude, longitude, and height with respect to reference coordinates.Table 1. Summary of the RMS errors for the three solutions computed.
As a second step, both code and carrier-phase observations were combined into a single adjustment (see Figure 4), yielding what is often referred to as precise point positioning (PPP). To accommodate the initial carrier-phase ambiguities, additional parameters were estimated in the filter. While adding carrier phases clearly reduces the noise in the solution, the estimated coordinates do not converge to cm-level accuracies, as typically expected in PPP. Despite weak geometry and range errors, the main reason for poor convergence is again the presence of biases in code observations. Without redundancy, carrier-phase observations only act as a filter for code observations, without reducing the contribution of biases. The RMS errors are 0.422 meters in latitude, 0.150 meters in longitude, and 0.389 meters in height.
Figure 4. Combined code and carrier-phase solution. Differences in latitude, longitude, and height with respect to reference coordinates.
To get an independent assessment of carrier-phase observations, a phase-only solution was computed (see Figure 5). For this test, a different methodology was adopted in which we simulated starting the positioning at a known precise location. At the first epoch, the receiver coordinates were constrained to the estimated values from the 24-hour GPS-plus-Galileo positioning solution, and the receiver clock offset was fixed to an arbitrary value (in this case zero). This initial epoch thus allowed estimation of the carrier-phase ambiguities, which remained constant for the rest of the session. For subsequent epochs, the receiver position and clock offset were estimated on an epoch-by-epoch basis. Even though the errors in the initial ambiguity estimates propagated into the following epochs, the estimated coordinates remained at the centimeter level throughout the nearly two-hour common observing period.
Figure 5. Phase-only solution, starting at a known location. Differences in latitude, longitude, and height with respect to reference coordinates.
Conclusion
We have obtained what we believe to be the first positioning results using observations from the four Galileo satellites launched to date. The results are very respectable given that the observing geometry was far from ideal and there was no redundancy for epoch-by-epoch solutions. Furthermore, the satellites were not operating at a performance level to be expected for the fully operational future constellation. Both GIOVE satellites have been retired and we must now wait for the second set of IOV satellites to be orbited before we can continue our investigations in Galileo-only positioning with live signals.
Acknowledgments
We thank the operators and station managers of the CONGO network for supplying the data used to model the orbits and clocks of the Galileo satellites.
Richard B. Langley is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick (UNB) in Fredericton, Canada.
Simon Banville is a Ph.D. candidate in the Department of Geodesy and Geomatics Engineering at UNB. He is also working for Natural Resources Canada on real-time precise point positioning.
Peter Steigenberger is a staff member in the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München in Munich, Germany.
FURTHER READING
“Anechoic Chamber Calibrations of Phase Center Variations for New and Existing GNSS Signals and Potential Impacts in IGS Processing” by M. Becker, P. Zeimetz, and E. Schönemann, presented at the IGS Workshop, Newcastle upon Tyne, England, June 28–July 2, 2010. Available online: http://www.igs.org/event/newcastle2010/ (scroll to “0205” and click on “PDF.”)
“Precise Orbit Determination of GIOVE-B Based on the CONGO Network” by P. Steigenberger, U. Hugentobler, O. Montenbruck, and A. Hauschild in Journal of Geodesy, Vol. 85, No. 6, 2011, pp. 357–365, doi: 10.1007/s00190-011-0443-5.
Artist’s impression of GIOVE-A in orbit. (ESA, P. Carril)
With the initial satellites of the Galileo constellation working well in orbit, it has been decided to end the mission of ESA’s pioneering GIOVE-A navigation satellite, reports the European Space Agency.
Launched on December 28, 2005, this first experimental satellite performed the vital task of securing the radio frequencies provisionally set aside for Galileo by the International Telecommunications Union.
It also flight-tested Galileo atomic clocks and other equipment in space for the very first time and investigated the radiation environment of medium-altitude orbits, never used before by a European mission.
ESA formally ended GIOVE-A’s mission at the end of June, although it will go on being operated for now by prime contractor Surrey Satellite Technology Ltd of Guildford, UK, to gather radiation data and performance results from a GPS receiver.
“GIOVE-A had a design life of only 27 months, so to continue operating for 78 months is impressive,” said Valter Alpe, managing GIOVE activities for ESA.
“In August 2009, the satellite was moved into a graveyard orbit around 100 km above its normal 23,222 km to make way for the Galileo validation satellites.
“The first two of these were launched on 21 October 2011 and are performing well, so while GIOVE-A has served ESA well it no longer has a job to do.”
Built to a tight deadline by SSTL, GIOVE-A carries a rubidium atomic clock accurate to three seconds in a million years.
On 27 April 2008 it was joined by GIOVE-B, built by an Astrium-led consortium, which carries an even more accurate passive hydrogen maser clock — the first to be flown in space for navigation, accurate to one second in three million years — as well as a second rubidium clock. Operational Galileo satellites carry two pairs of both kinds of clock, for redundancy. They are very different missions in other ways too. The GIOVEs were modified from existing satellite platforms: a prototype geostationary minisatellite for GIOVE-A, and a commercial French Proteus platform typically used for Earth observation for GIOVE-B.
Galileo satellites are based on an entirely new platform and improved payload, specifically engineered for extremely high reliability, only intended to go into safe mode for a few days over their planned 12 years of operation thanks to a robust design based on reconfigurable redundancy.
Even when entering ‘intermediate safe mode’ they can continue to supply navigation signals, although without the usual service guarantee. GIOVE-B, with an orbital lifetime of 50 months and counting, will be used in payload fine calibration tests this summer with the two Galileo satellites.
Then, in September, it will be manuvered into a graveyard orbit 300 km higher. At this point, GIOVE-B’s own mission will end.
“Early October will see the launch of the next two Galileo satellites by Soyuz rocket from Europe’s Spaceport in French Guiana,” added Valter.
“This will be an important step forward because four satellites are the minimum to perform navigation measurements, so Galileo system testing can proceed.” A follow-up batch of full operational capability Galileo satellites is being built by Germany’s OHB and SSTL, with initial Galileo services forecast to be available by 2014.
This month there is good news — great news, actually — where GPS and PNT (Position, Navigation and Timing) systems are concerned. On October 22, a Russian Soyuz rocket placed in orbit the first two validation satellites, built by EADS Astrium Germany, in the Galileo PNT constellation after making its maiden launch from Kourou. Don’t confuse these recent satellites with the earlier experimental satellites, GIOVE-A launched in 2005 followed by GIOVE-B launched in 2008. These initial satellites served to preserve the Galileo ITU frequency filings and test the first-ever space borne Hydrogen Maser atomic clock, which by all accounts is proving to be extremely accurate.
The Soyuz launch of two Galileo IOV satellites.
While it is interesting the Europeans decided on a Russian vehicle for the first Galileo dual launch, the U.S. recently pinned its hopes on a European Ariane Five (pictured at right) to launch a commercially hosted U.S. government payload known, appropriately enough, as the “Commercially Hosted Infrared Payload” or CHIRP sensor, which was specifically developed by the U.S. government as a test payload to test both the payload sensor capability and the commercially hosted options for sensor payloads in GEO. The CHIRP sensor features a fixed telescope that can view one quarter of the Earth from geosynchronous orbit. So it appears that hosted payloads and international launch cooperation efforts are growing and are apparently working successfully.
The two newest Galileo satellites deployed four hours after the Soyuz rocket lifted off from Kourou, in French Guiana.
The Soyuz launched the first two of four validation Galileo satellites designed to validate the Galileo concept by testing both space and ground operations. Two additional validation satellites are scheduled to follow in the summer of 2012. Once the In-Orbit Validation (IOV) phase is completed, an additional 12 satellites will be launched to reach an Initial Operational Capability (IOC) of 16 satellites sometime in 2014, and that date looks extremely doubtful.
According to our own Richard Langley, “During initial operations, the [Galileo] satellites will be controlled by a joint ESA and CNES French space agency team in Toulouse, France. Once that week-long phase ends, the satellites will be handed over to the Oberpfaffenhofen Galileo Control Centre near Munich, [Germany], operated by the DLR German Aerospace Center, which will be responsible for routine operations. Operating the satellite payloads to provide navigation services will be the task of the Fucino Control Centre, near Rome, operated by Telespazio.”
Now, does that sound like a confusing and expensive ground support system? Everybody and every country insist on their piece of the pie, regardless of efficiency and continuity of operations. Who knows this might work; only time will tell.
The approximately $7.5 billion Galileo constellation will eventually, hopefully, comprise a retinue of 27 operational satellites with three on orbit spares by 2020.
The PNT business is obviously good for the Russian launch business. Russia successfully launched a GLONASS-K1 test satellite back in February, followed by three GLONASS-M satellites this month into a constellation that finally, after 29 years, accounts for 23 operational and three hopefully soon-to-be operational satellites. The first operational GLONASS-K1 is not scheduled to be launched until sometime early in 2012. GLONASS satellites have historically proven to be fragile affairs with extremely short lifespans; it remains to see how long this number and capability will be maintained. Hopefully the new K1 and M generation GLONASS satellites have resolved many of the longevity issues. Only time will tell when and if the Russian GLONASS will ever regain Full Operational Capability (FOC), which requires 24 simultaneously operating satellites. The Russians were briefly FOC in December 1995, but unfortunately only for a few months. The word “simultaneous” is important as Russian scientisst frequently state they have 25 or 27 GLONASS satellites in orbit, but unfortunately only 22 or 23 of them are operating. But it is possible, miracles still happen, that by the time you read this GLONASS may actually legitimately have achieved FOC once again.
Now on the Boeing IIF side of the house, more good news as it was announced this week that the second IIF satellite (IIF-2), which has been operational with an elevated signal strength for several months, now has its signals back within the specified signal strength and is good to go. GPS IIF-3 was originally scheduled for launch this coming summer, but the latest launch schedules show the launch in September 2012, about 11 months from now. With 30+ operational GPS satellites on orbit plus residuals, hopefully this will be soon enough.
Apple & GLONASS
Always betting on the come, we now know that the late genius Steve Jobs directed his enterprising engineers to include GLONASS PNT software in the latest iPhone 4S; the latest version iPhone that sold 1.3 million units in one day. This effectively gives the iPhone 55 potential satellites to choose from for PNT information as well as the Wi-Fi, cellular tower, and SkyHook Wireless PNT information. With the addition of the GLONASS PNT resources, the iPhone may now well be the most versatile and capable general-purpose PNT platform that exists today. Is that a sad commentary for other GPS and mobile phone providers, a marketing challenge, or merely a positive sign of the technologically advanced times in which we live? It may in fact simply be a true reflection of the capabilities of the most recognized and profitable corporation in the world today. Apple is doing many things right, and one of them is listening to the consumer and giving them more than they expect. Consequently, customers are loyal and Apple Inc. surpassed Microsoft in market capitalization in 2010, and in 2011 became the most valuable consumer-facing brand in the world. Apple is a company Fortune magazine has named the most admired company in the United States for the last three years running. Apple iPhones and numerous PNT applications are certainly in use by thousands of our warfighters in and out of theater. Interesting, to say the least, plus food for thought and a topic for a future column.
The Bad
The bad news not surprisingly comes via the U.S. government and no, it is not about LightSquared, because that situation continues to be worse than merely bad. No, the bad news comes in the form of a recently released but curiously out-of-date publication concerning GPS by the Congressional Budget Office (CBO). In late October 2011, the CBO released a publication entitled The Global Positioning System for Military Users: Current Modernization Plans and Alternatives.
I was unfortunate enough to receive both a soft and hard copy; and to make matters worse I don’t own a parakeet. The good news is we do have several fireplaces in our home and winter is rapidly approaching. Truthfully, the report is that bad and out of date, but at least it is boring and long. Fortunately hardly anyone is likely to actually endure the pain and suffering required to read through the entire document. However if you are a masochist and/or suffering from acute insomnia I highly recommend this CBO report as a possible cure. Some of you might justifiably complain I have no business giving medical advice because I am not a medical subject matter expert (SME) and I wholeheartedly agree, just as I agree that the CBO is definitely not a GPS SME and should stay with what they do know. Whatever that is.
I can assure you when and if the military needs advice concerning future GPS operations and options the last place they will or should turn is to the CBO. For example, the preface of the document clearly states, “In keeping with CBO’s mandate to provide objective, impartial analysis, this study makes no recommendations.” Contrary to what you may think this is actually good news, since now we don’t have to waste valuable time dealing with flawed recommendations; garbage in, garbage out. Now if only the analysis were impartial or objective, which it is decidedly not. I would even settle for accurate, which it is definitely not. The information in this document is in some cases, as in M-Code satellites, erroneous and confusing; it is out-of-date where the GPS III nomenclature and options are concerned, especially the spot-beam; and it is always misleading concerning objectivity that presents facts not in evidence. There is so much erroneous and misleading information in this report that I sincerely hope no one else reads it, especially our military users.
Against my better judgment I am including a link to the CBO document for those of you who practice self-flagellation. I truly regret the number of tree lifespans cut short to produce this confusing, misleading, out-of-date, and totally unnecessary document. Sometime I will tell you how I really feel.
The Really Ugly
The “really ugly,” as you have probably surmised by now, refers to LightSquared and the clueless FCC. Can you believe we have been dealing with this fiasco for more than 12 months? You are probably tired of it all, I know I am, but I see that as a true danger signal. The situation is very clear technically, the LightSquared signals, both from the terrestrial transmitters and receivers, will significantly impair and jam GPS signals to the detriment of all GPS users. Of course the political and business ineptness continues apace so who knows how long we will be dealing with this issue, but we cannot afford to let down our guard. Although this is exactly what LightSquared, the FCC, and the current administration, in an upcoming Presidential election year, obviously hope will happen. They hope we will all just get tired of dealing or even hearing about this LightSquared mess and then they win by default. We all have more important matters demanding our attention, right? Of course we cannot and are not going to allow that to happen. We will continue to use LightSquared as a verb when necessary and keep the real facts front and center, right here in GPS World, until all aspects are resolved. You can count on it.
Using Microwaves and Laser Ranging for Precise Orbit Determination
By Erik Schönemann, Tim A. Springer, Michiel Otten, and Matthias Becker
Though Galileo’s GIOVE-A is a test satellite not necessarily ready for scientific use, orbit analyses with a reduced accuracy can help to identify weaknesses and suggest improvements. This month, the authors share work being carried out to precisely determine the orbit of GIOVE-A using SLR and microwave observations. This preliminary investigation will benefit the procedures to be implemented for the future Galileo constellation.
INNOVATION INSIGHTS by Richard Langley
WE USE THEM FOR LISTENING TO MUSIC, for routine surgeries, for making a point in a presentation, and even for hanging pictures straight. Of course, I’m talking about lasers. Invented in 1960, the laser (an acronym for light amplification by the stimulated emission of radiation) has become ubiquitous in modern society. Every CD and DVD player has one. Many printers use them. But lasers are also used in a wide range of industrial and scientific applications including determining the orbits of satellites through satellite laser ranging (SLR).
In the SLR technique, pulses of laser light from a ground reference station are directed at satellites equipped with an array of corner-cube retroreflectors, which direct the pulses back towards a collocated receiving telescope. By accurately measuring the two-way travel times of the pulses and knowing the location of the station and other operating parameters, the positions of the satellites can be determined. A network of SLR reference stations around the globe is used to monitor the orbits of satellites over time and their variations have been used by scientists to improve our knowledge of the Earth’s gravity field; to study the long term dynamics of the solid Earth, oceans, and atmosphere; and even to verify predictions of the General Theory of Relativity.
The first SLR measurements were obtained from the Beacon Explorer-B satellite, which was launched in October 1964. Since then, dozens of satellites equipped with corner-cube retroreflectors have been launched including a number of radio-navigation satellites. Every GLONASS satellite is equipped with retroreflectors and two GPS satellites have been equipped—SVN35/PRN05 and SVN36/PRN06. The COMPASS-M1 satellite in medium Earth orbit carries retroreflectors, as do both GIOVE-A and –B, the Galileo test satellites.
Precise orbit determination of radio-navigation satellites using SLR has the advantage of being unaffected by any onboard satellite electronics and associated signal biases. Radiometric observations of a satellite’s microwave signals, on the other hand, are influenced by the satellite’s clock, for example, and its effect must be estimated to obtain precise (and accurate) satellite orbits for navigation and positioning. Therefore, a comparison of SLR- and microwave-derived orbits can be very useful for studying the performance of the data measurement and orbit-determination processes of both techniques.
In this month’s column, we take a look at some work being carried out to precisely determine the orbit of the GIOVE-A test satellite using SLR and microwave observations. This preliminary investigation will benefit the procedures to be implemented for the future Galileo constellation.
“Innovation” is a regular column that features discussions about recent 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 at the University of New Brunswick, who welcomes your comments and topic i deas. To contact him, see the “Contributing Editors” section on page 6.
The navigation office of the European Space Operations Centre (ESOC) is engaged in various activities using observations of the Galileo test satellite, GIOVE-A (Galileo In-Orbit Validation Element-A), recorded at the Galileo Experimental Sensor Stations (GESS). The work includes the assessment of the quality and performance of GIOVE satellite observables and the testing and improvement of orbit-determination software. These activities support the long-term goal of advancing the scientific applications of the future Galileo constellation.
Since the launch of GIOVE-A on December 28, 2005, various tests have been carried out to analyze the quality of the new code (pseudorange) and carrier-phase observables derived from tracking the satellite’s microwave signals. All of these tests demonstrate the advantages of the new signal structure compared to that of legacy GPS signals. In general, the reduction of the noise by factor of 4-5 as well as a reduction of the code multipath by approximately a factor of 1.2 (GPS C1C versus GIOVE-A C1B/C1C) could be seen.
As the comparison of observations is done indirectly (GPS and GIOVE-A have different orbits) and the databases used for most analyses published up to now is sparse, a deeper analysis of the signal quality parameters seems appropriate, especially as data quality has a direct impact on the precision of orbit determination. Our analyses, presented in the first half of this article, are based on a broad base of data from most of the stations in the GESS network. Because of the difficulty in accessing the phase multipath directly, we first evaluated the signal strength and the code multipath, which gave the first hint of the multipath behavior. In order to compare GPS and GIOVE-A data directly, only data received from the same elevation angles and azimuths were used. Subsequently, we present an analysis of the phase residuals derived by precise point positioning.
The second part of this article focuses on the precise orbit determination or POD of the GIOVE-A spacecraft. The Navigation Package for Earth Observation Satellites (NAPEOS) software used at the ESOC Navigation Support Office allows microwave (radiometric) and satellite laser ranging (SLR) observations to be used either separately or together. The two methods are different due to different tracking networks and the different sensitivity of the observables to atmospheric effects and in their noise levels. We will present the orbit results focusing on internal orbit consistency checks and SLR validation of the microwave-based orbits.
Data Analysis
We first describe the procedures used for analyzing the microwave data followed by those used for the SLR data.
Microwave Analysis. For the GIOVE-A signal analysis and precise orbit determination we used the RINEX data from all of the GESS stations available from the GIOVE archiving facility (see TABLE 1). All stations are equipped with GPS/Galileo antennas, built by Space Engineering S.p.A. and Galileo Experimental Test Receivers (GETRs), built by Septentrio. The data, containing tracking data of all GPS satellites and the GIOVE-A satellite, is given in the RINEX 3.00 data format with a sampling interval of 1 second. To save on storage space for the long-term analyses, such as orbit determination, the RINEX data is decimated to 30-second samples and Hatanaka-compressed, using a test version of the Hatanaka software for the RINEX 3.00 format.
The signal analyses shown here were carried out using GNU Octave, an open-source program for performing numerical computations similar to Matlab, and different scripts developed by the Institut für Physikalische Geodäsie at the Technische Universität Darmstadt. These analyses cover a selection of the designated Galileo signals recorded by the GESS within the time span from December 16 to 27, 2006. Within this time period, the current GPS signals, as well as the GIOVE-A signals E1 and E5, shown in TABLE 2, were recorded. The table also shows the signal components as well as the RINEX observation-type identifiers, which we use in this article.
The stations used for the analyses show a quite similar level of performance in general. There are stations with different behaviors for single signals, as for example GIEN with a stronger code multipath behavior on C1B and C1A, but no station with a considerably different performance level could be identified. The averaging over the data from all sites reduces the station-dependent effects such as multipath and the atmosphere to a large extent, and gives a good indication of the mean signal performance.
The analyzed phase residuals were taken from the processing carried out for the second part of this article. Hence, they include observation data over an extended period of 149 days and were limited to the GIOVE-A C1C/L1C and C7Q/L7Q signals.
This extended data period is from December 12, 2006 (day of year 346), until May 26, 2007 (day of year 146). During this interval, there is a period where no GIOVE-A data was available due to maintenance of the spacecraft. This gap occurred from February 12 to 28, 2007. So in total we have analyzed 149 days of microwave data. Because there are some differences between the results before and after this gap in February, many of the statistics are given for the first and second part separately. The first part covers December 12, 2006, until February 11, 2007; the second part covers March 1, 2007, until May 26, 2007.
We performed the precise orbit determination using the NAPEOS software, a general-purpose software package for orbit determination, prediction, and control, supporting all phases of an Earth-observation mission in terms of mission preparation and operations.
For the GIOVE-A analysis, the three main NAPEOS programs we used are GnssObs, Bahn, and Multiarc. GnssObs reads, cleans, and decimates the RINEX data and converts the data into the NAPEOS internal tracking-data format. The NAPEOS tracking-data format contains the ionosphere-free linear combination, for both code and phase, of the RINEX observations. For GPS, the ionosphere-free linear combination is based on the combination of C1P and C2P code and L1P and L2P phase measurements. GIOVE-A offers several different observables allowing for many different ionosphere-free observations. For most of the work presented in this article, we have used the ionosphere-free linear combination of the C1C and C7Q and L1C and L7Q observations for code and phase respectively.
The next module, Bahn, performs the parameter estimation. In this step, we use the ionosphere-free code and phase observations at a sampling interval of 5 minutes, and we have applied an elevation angle cut-off of 5 degrees. The data is processed in batches of 24 hours, thus resulting in 1-day-arc solutions. The estimated parameters in these daily solutions are the GIOVE-A state vector (position and velocity), five dynamical orbit parameters from the extended Center for Orbit Determination in Europe (CODE) orbit model, a GIOVE-A clock offset for each epoch, all receiver clock offsets for each epoch, one GPS-GIOVE-A “intersystem bias” parameter per day for each station except for a selected reference station, and the carrier-phase ambiguities (integers not resolved). The station coordinates are estimated but tightly constrained (1 millimeter) to their a priori value. We obtained the a priori station coordinates by combining the full set of daily solutions.
Despite the fact that the 13 GESS stations provide very good global coverage, it is expected that 24-hour solutions will not give the most precise GIOVE-A orbit estimates. To generate longer arc solutions, we have used the Multiarc program. This is a tool that has recently been added to the NAPEOS software package. It allows for a rigorous combination of normal equations, also referred to as normal equation stacking, which are generated by Bahn. During the normal equation combination, the satellite orbit parameters may also be rigorously combined, thus effectively leading to multi-day orbital arcs. For the work presented in this article, we have used Multiarc to generate solutions with arc lengths of 1, 2, 3, 4, and 5 days. We also used Multiarc to compute accurate a priori station coordinates by stacking all available 1-day normal equations.
Satellite Laser Ranging
Besides the 13 GESS stations, GIOVE-A is also tracked by more than 17 different SLR stations around the world. For most periods of the mission, the tracking has been consistent enough to allow for GIOVE-A POD using only the SLR data. As the SLR data is completely independent of the microwave data, the resulting orbit solutions will be to a large extent independent as well and thus can be used to give an indication of the achieved precision of the different microwave solutions.
The orbit determination strategy used for the SLR solutions is very similar to the one used for the microwave orbits with the main difference being the increased arc-length of 7 days. The same satellite parameters are estimated as with the microwave solutions: the GIOVE-A state vector and five dynamical orbit parameters from the extended CODE orbit model. No further parameters need to be estimated and all corrections applied to the SLR data are according to the International Earth Rotation and Reference Systems Service 2003 standards and, for station coordinates, we used those from the rescaled International Terrestrial Reference Frame 2005 solution. As the noise level of the SLR data is very low, the measurements can also be directly used to give an indication of the precision of the radial position components of the different microwave solutions by computing the SLR residuals without using them in the estimation process itself.
Combined Microwave and SLR Analysis. In this step, the SLR data was added to the microwave data in the 24-hour solutions. For the data weighting, we used 100 millimeters for SLR and 1000 millimeters and 10 millimeters for GIOVE-A and GPS code and phase observables respectively. The only change in the analysis strategy in this case was that we now processed the SLR data in 24-hour solutions and not in 7-day batches. All the processing options remained as described in the two previous sections. The resulting 1-day solutions, or rather the associated normal equations, were used in Multiarc to generate combined solutions of different arc lengths.
Microwave Data Quality
We now take a detailed look at the quality of the microwave data in terms of signal-to-noise ratio (SNR), code-tracking noise and multipath, carrier-phase-tracking noise, and carrier-phase residuals.
Signal-to-Noise Ratio. The SNR (or equivalently carrier-to-noise-density ratio, C/N0) is strongly dependent on the satellite transmitter, the signal path through the atmosphere, and the receiver configuration (ground station, antenna, receiver, cable, etc.). Hence the SNR cannot be seen as an absolute value. The SNR is specific to the position, the equipment, and the time. Furthermore, the determination of the SNR values depends on the receiver and the firmware used. As a result, SNR values from different receivers cannot be readily compared. Nevertheless, using only one type of receiver, assuming similar effects on all the different signals at the same epoch, and taking averages over a long time span, we expect the relationships among the signals to be constant. Based on this assumption, we can use the SNR values given in the GESS RINEX files without adjustment.
To compare the GPS with the GIOVE-A SNR values, we ordered the corresponding SNR values of all stations on all days by satellite position into a grid with widths of 5 degrees in azimuth and 5 degrees in elevation angle. For the evaluation, we took the grid cells occupied by both GPS and GIOVE-A values and computed the median over all the cells of equal elevation angle. The median per elevation-angle bin for each signal is shown in FIGURE 1.
FIGURE 1. Signal-to-noise ratio, GPS versus GIOVE-A
As can be seen from the figure, the signal strength of the GIOVE-A C8Q observable ranks best, followed by the GPS C1C, GIOVE-A C7Q, C5I/C5Q, C1A, and C1B/C1C. The weakest signal is found for the GPS C1P/C2P observable, with a maximum signal strength of 40 (receiver-dependent unit, approximately dB-Hz) at the zenith. Comparing the GPS open signals versus GIOVE-A, GPS C1C is considerably stronger than the GIOVE C1B/C1C. According to the GPS and Galileo interface control documents, GIOVE-A C1B/C1A should show up with a stronger signal strength than GPS C1C. The power levels guaranteed on the Earth’s surface are -160 dBW for GPS and -158 dBW for the future Galileo satellite signals except for the BOC(10,5) and BOC(n,m) modeled signals, for which a power level of even -155dBW is guaranteed. But looking at the SNR values shown in Figure 1, we see that the GIOVE-A C1B/C1C is worse by approximately 4 dB than the GPS C1C. But keeping in mind that GIOVE-A is an experimental satellite, an increase of the signal power for the future operational Galileo satellites should improve the signal performance above that shown in this article.
Code-Tracking Noise. For signals containing data and pilot components, as in the case of those from GIOVE-A, the code-tracking noise can easily be computed as the difference between the data and the pilot signal. The advantage of this computation scheme is that both signals are influenced by identical error sources (atmospheric errors, multipath errors, receiver errors, etc.). Based on the assumption of equal uncertainties in the two components, we divided the resulting noise values by the square root of two to specify the noise level of each part according to the laws of error propagation. TABLE 3 shows the code-tracking noise for the two analyzed GIOVE-A codes sorted by elevation angle. The median code-tracking noise is 0.62 meters for C1B/C1C and 0.35 meters for C5I/C5Q, for observations below an elevation angle of 5 degrees. For the C1B and C1C code measurements, the noise median stays below 0.2 meters for an elevation angle above 25 degrees, whereas the median for the C5I and C5Q code measurements for elevation angles above 35 degrees even comes down below 0.1 meters. The results discussed above are consistent with the code-tracking noise values published previously.
Code Multipath. We computed the relative code multipath effects as code minus phase differences assuming the amplitude of phase multipath to be insignificant compared to the amplitude of the code multipath. Ionospheric effects were taken into account by using the phase measurements on two frequencies in the usual way:
In this equation, CMPx is the estimate of the multipath error on the code, Px and Lx are the code and phase measurements of the same frequency, while Ly is the phase measurement used to correct the frequency-dependent ionospheric effect. The constant, , describes the relationship of the ionospheric behavior for the two frequencies.
In order to compare the code multipath level of GPS versus GIOVE-A, we sorted the multipath values using a grid covering the sky with widths of 5 degrees for both elevation angle and azimuth as before. FIGURE 2 shows the median standard deviation of the code multipath values, derived in each grid cell per day and station, versus the elevation angle. No significant difference between GPS C1C and GIOVE-A C1B and C1C, the open code signals on G1/E1, could be found. The code multipath behavior of the GPS precise codes are comparable with the GIOVE-A C5I, C5Q, and C7Q, whereas the C8Q shows the least code multipath effects closely followed by the GIOVE-A C1A, the public regulated service signal.
FIGURE 2. Code multipath, GPS versus GIOVE-A
Carrier-Phase-Tracking Noise Analyses. In the same manner as that carried out with the code, we computed the GIOVE-A carrier-phase-tracking noise as the difference of the two components (pilot minus data). To accommodate the effect of error propagation, the resulting errors were divided by the square root of two. The resulting phase-tracking noise values were sorted by elevation angle and can be found in TABLE 4.
In conformity with the theory that the phase-tracking noise is independent of the modulation scheme, both signals (L1B/L1C and L5I/L5Q) show the same results in units of cycles. Looking at the results in units of distance, GIOVE-A L1B/L1C shows up with a mean phase noise of 0.7 millimeters and L5I/L5Q with 0.9 millimeters. These values confirm those of previous studies.
Carrier-Phase Residuals. Phase residuals contain the phase tracking noise, multipath, as well as all unmodeled remaining errors such as antenna calibration inaccuracy and tropospheric effects. The magnitude of the residuals can be seen as an indicator for the observation and model accuracy as well as for measurement quality.
The following analyses are based on the ionosphere-free linear combination (GPS L1C/L2P, GIOVE-A L1C/L7Q), computed with NAPEOS. The analyses include data of the 13 GESS over a period of 149 days.
To compare the GPS and GIOVE-A residuals, we sorted them into a grid with a width of one degree in both satellite azimuth and elevation angle. Only data in overlapping grid locations were compared to make sure the data was affected in a similar way by multipath or other disturbances.
To properly interpret the results, we should mention that for GIOVE-A, 0.06 percent of the ambiguities (2501) were not fixed correctly whereas for GPS all ambiguities were fixed correctly. Looking at the GIOVE-A observations that were correctly fixed, we find a significantly larger number of rejected observations. The number of rejected observations is less by one third for GPS (6 percent) as for the GIOVE-A (9 percent) data.
Due to the small number of GIOVE-A observations for elevation angles above 86 degrees, the outlier-cleaned mean as well as the standard deviation at this elevation-angle range are not meaningful. For all elevation angles, GIOVE-A residuals show a lower standard deviation than GPS, indicating a superior performance of GIOVE-A signals.
Phase and Code Validation in Processing. Looking at the quality of the code and phase measurements on the different signals, it is conspicuous that GIOVE-A C1A/L1A and C8Q/L8Q rank best, whereas for the current processing of GIOVE-A data, usually the C1C and C7Q signals are used. This leads to the question of which is the best signal combination for GIOVE-A. Hence, we processed 10 days of GIOVE-A data, using different signal combinations. Presently the processing of the C8Q/L8Q signals is not yet implemented in NAPEOS. However, we were able to process the GIOVE-A C1A/L1A – C7Q/L7Q combination. The root-mean-square (RMS) of the code results were reduced by a factor of approximately 1.4 using L1A/C1A compared to L1C/C1C, whereas the RMS of the phase observations showed only a minor improvement. Furthermore, there is a higher number of rejected observations with L1A/C1A. Further analyses have to be carried out to evaluate the potential benefits of the different signal combinations.
Orbit Quality
In this section, we assess the quality of our precise orbit determination solutions. We have three sets of different orbit solutions. Set 1 is made up of the 7-day solutions based solely on SLR observations. Set 2 consists of the solutions based on the microwave observations using 1- to 5-day arcs. Set 3 consists of the solutions based on a joint analysis of the microwave and SLR observations also using 1- to 5-day arcs.
First, we assess the orbit quality by looking at the internal consistency of the solutions. For the two sets using microwave observations, the internal orbit consistency is done using an orbit fit. This will not tell us much about the absolute quality of the solutions but it will indicate the optimal arc length and whether adding the SLR observations to the microwave data improves the orbit estimates.
Secondly, we validate the orbits by determining the SLR residuals. Of course, the solutions that used SLR observations should perform better than the microwave-only solutions. However, the validation of the microwave orbits against the SLR observations will give us a good impression of the absolute accuracy of our orbits.
As a third test, we compare the best orbit (best arc length) of each of the three sets (set 1 only has one arc length) against each other. This should give us another indication of the quality of the orbits.
Internal Orbit Consistency. To determine the internal orbit consistency of the different solutions we make an orbit fit. For this orbit fit test, we used the middle 24 hours of two consecutive solutions and fit one 48-hour arc through these two parts. The satellite orbit was modeled by estimating the satellite state vector and all nine parameters of the extended CODE orbit model. The RMS of this fit gives us an indication of the internal consistency of the orbit estimates. For longer arcs, the RMS of fit should go down because the solutions are not fully independent of each other. So a lower RMS for the longer arc solutions is expected. On the other hand, this means that if the RMS does not go down with increasing arc length that we have reached the limit of our modeling capabilities. Furthermore, comparing the internal orbit consistencies of equal length solutions will tell us which solution has a better internal consistency. The results of this internal orbit consistency check are given in TABLE 5. The table gives the mean of the 2-day RMS over all processed days. The mean is given separately for the first and second part of the observation interval (see above) and also for the total observation interval.
Table 5 shows several interesting results. First of all, it shows that the results of part 2 of the observation interval are significantly better than the results from part 1. The reason for this is unclear since the statistics from the 1-day solutions, such as the residual RMS and number of observations, did not change significantly after the observation gap. The improvement, however, is very significant. The second observation is that the results including the SLR data are significantly better compared to those using only the microwave data. This is true for all arc lengths! As expected, we see a significant improvement of the internal consistency when going from 1-day arcs to 3-day arcs. The 4-day arcs show only a slight improvement compared to the 3-day arcs. The 5-day arcs do not show a significant improvement. This indicates that with the current observations and modeling techniques, the optimal arc length for precise orbit determination seems to be around 3 to 4 days.
SLR Validation. In this section, we look at the SLR residuals obtained from the different orbit solutions. We generated a clean SLR dataset by using the SLR-only orbit to remove any outliers in the SLR observations. The total number of valid SLR normal points for the entire period is 3520 observations from 17 different SLR stations. (A normal point is an average of a number of individual laser returns.) The number of observations for the first part of the observation period is 796 points from 12 stations and for the second part, there were 2724 normal points from 17 stations. For two of the three solutions, the SLR data has been used in the orbit determination process so the residuals will give a too-optimistic indication of the orbit quality.
As can be seen from TABLE 6, the 3-day solution based on the microwave-only data has the lowest SLR residuals and indicates a radial precision of around 100 millimeters. A similar behavior can be seen in the microwave plus SLR solution with the exception of the 1-day solution (and to a smaller extent also the 2-day solution) where the orbit solution is mainly driven by the SLR data, but the quality as can be seen from the internal consistency of the orbit is poor. Interestingly, there is a large improvement in SLR residuals for the microwave plus SLR solution, although the number of SLR data points is only 2 percent of the total tracking data in the combined solution. The values for the SLR-only solution are included in the table to give an indication of the lowest possible SLR residuals one could expect by combining the microwave and SLR data.
Orbit Comparison. To get an indication of the overall orbit quality, the best solutions were compared against each other for the second period of observation. TABLE 7 gives the RMS differences between the SLR only (SLR), 3-day microwave only (micro), and the 3-day microwave and SLR solution (micro+SLR) for the radial, along-track, and cross-track position components as well as the norm (3D).
As expected, the largest difference is between the SLR-only and microwave-only solutions giving a total (norm) orbit difference of 652 millimeters. As a major part of the SLR tracking from GIOVE-A comes from European stations, the quality of the SLR solutions is directly correlated with the ability of the European stations to track GIOVE-A. Bad weather over Europe can lead to data gaps for more than 24 hours, affecting the orbit quality. It is interesting to see the large impact the SLR data has on the combined solution. As mentioned before, the SLR data is only around 2 percent of the total tracking data but has a significant impact on the orbit solution as can be seen from the difference between the microwave-only and microwave-plus-SLR solution.
Based on the analysis presented above, we conclude that the 3-day solution using both microwave and SLR observations has provided the best orbit estimates.
Conclusion
The analyses of the observation data quality (signal quality) confirmed the good results from prior analyses for code multipath behavior and code noise. GPS C1C and the GIOVE-A C1B/C1C show a comparable multipath behavior, whereas the GPS precise codes C1P/C2P are comparable to the GIOVE-A C5I, C5Q, and C7Q. The least code multipath behavior could be found for GIOVE-A C8Q observable, closely followed by the GIOVE-A C1A. Based on this, the combination C1A/L1A – C8Q/L8Q should show the best noise behavior within the data processing scheme.
The results given in this article demonstrate that the 13-station GESS network allows us to determine the orbit of the GIOVE-A satellite quite accurately (~200 millimeters) using only microwave observations. The SLR validation of the microwave orbits gives an RMS of 100 millimeters (one-way range RMS). This result gives an absolute value for the orbital error. Of course, the SLR observations mainly tell us something about the radial orbit errors; the along- and cross-track errors could be much higher. To obtain accurate GIOVE-A orbit estimates, we need to keep the orbits and clocks of the GPS satellites, tracked simultaneously with the GIOVE-A satellite, fixed using the International GNSS Service (IGS) final orbit and clock products. Furthermore, an arc length of 3 days should be used. The microwave-based orbit estimates may be improved by adding the available SLR observations into the orbit-estimation process. Although there are relatively few SLR observations, they do have a significant positive effect on the orbit estimates, improving the internal consistency from 52 to 41 millimeters. Also, the validation of the orbits using the SLR observations shows a significant improvement. However, this is not an independent validation because the same SLR observations were used in the orbit determination.
The results presented in this article, even though based on observations from the GIOVE-A test satellite, can be considered as a first attempt towards establishing an optimal data processing approach for the future Galileo satellite constellation.
Acknowledgments
This article is based on the paper “GIOVE-A Precise Orbit Determination from Microwave and Satellite Laser Ranging Data – First Perspectives for the Galileo Constellation and Its Scientific Use” presented at the 1st Colloquium on the Scientific and Fundamental Aspects of the Galileo Program, held in Toulouse, France, October 1-7, 2007.
ERIK SCHÖNEMANN studied geodesy at the Technische Universität Darmstadt (TUD), Germany, writing his diploma thesis at the University of New South Wales, Sydney, Australia. Since receiving his diploma from TUD in April 2005, he has been working for the Institute of Physical Geodesy at TUD on GNSS station calibration and validation and analyses of GIOVE-A and GIOVE-B data.
TIM SPRINGER received his Ph.D. in physics from the Astronomical Institute of the University of Berne (AIUB) in 1999. He has been a key person in the development of the Center for Orbit Determination in Europe, one of the IGS analysis centers, located at AIUB. Since 2004, he has been working for the Navigation Support Office (NSO) at the European Space Operations Centre (ESOC) of the European Space Agency (ESA) in Darmstadt. In this group, he has led the development of the new ESOC GNSS software, which is used for most GNSS activities at NSO including GIOVE-A and -B analyses.
MICHIEL OTTEN obtained a degree in aerospace engineering from Delft University of Technology in 2001. He has been working for ESOC’s NSO since 2002. His main role within NSO is the precise orbit determination of low Earth-orbiting satellites equipped for SLR, DORIS, and GPS tracking. He is also responsible for ESA’s International DORIS Service Analysis Centre activities.
MATTHIAS BECKER is a full professor of geodesy and director of the Institute of Physical Geodesy, TUD. He received his diploma and Ph.D. in geodesy from TUD in 1979 and 1984, respectively. He is responsible for research and teaching in the fields of physical geodesy and satellite geodesy.
FURTHER READING
• GIOVE-A
“Meet GIOVE-A: Galileo’s First Test Satellite” by E. Rooney, M. Unwin, A. Bradford, P. Davies, G. Gatti, V. Alpe, G. Mandorlo, and M. Malik in GPS World, Vol. 18, No. 5, May 2007, pp. 36–42.
“Galileo Signal Experimentation” by M. Hollreiser, M. Crisci, J.-M. Sleewaegen, J. Giraud, A. Simsky, D. Mertens, T. Burger, and M. Falcone in GPS World, Vol. 18, No. 5, May 2007, pp. 44-50.
• GIOVE Tracking Network
“GIOVE Mission Sensor Station Receiver Performance Characterization: Preliminary Results” by M. Crisci, M. Hollreiser, M. Falcone, M. Spelat, J. Giraud, and S. La Barbera in Proceedings of Navitec 2006, the 3rd ESA Workshop on Satellite Navigation User Equipment Technologies, Noordwijk, The Netherlands, December 11-13, 2006.
• GIOVE Tracking Performance
“Performance Assessment of Galileo Ranging Signals Transmitted by GSTB-V2 Satellites” by A. Simsky, J.-M. Sleewaegen, M. Hollreiser, and M. Crisci in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 26-29, 2006, pp. 1547–1559.
“Code and Carrier Phase Tracking Performance of a Future Galileo RTK Receiver” by T. Pany, M. Irsigler, B. Eissfeller, and J. Winkel in Proceedings of ENC-GNSS 2002, the European Navigation Conference, Copenhagen, Denmark, May 27-30, 2002.
• Multipath Mitigation in Modernized GNSS
“Comparison of Multipath Mitigation Techniques with Consideration of Future Signal Structures” by M. Irsigler and B. Eissfeller in Proceedings of ION GPS/GNSS 2003, the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, September 9-12, 2003, pp. 2584–2592.
• GIOVE Orbit Determination
“Estimation and Prediction of the GIOVE Clocks” by I. Hidalgo, R. Píriz, A. Mozo, G. Tobias, P. Tavella, I. Sesia, G. Cerretto, P. Waller, F. González, and J. Hahn in Proceedings of the 40th Annual Precise Time and Time Interval (PTTI) Meeting, Reston, Virginia, December 1-4, 2008.
• Satellite Laser Ranging
“GIOVE’s Track: Satellite Laser-Ranging Campaigns” by M. Falcone, D. Navarro-Reyes, J. Hahn, M. Otten, R. Piriz, and M. Pearlman in GPS World, Vol. 17, No. 11, November 2006, pp. 34–37.
“The International Laser Ranging Service: Current Status and Future Developments” by W. Gurtner, R. Noomen, and M.R. Pearlman in Advances in Space Research, Vol. 36, No. 3, 2005, pp. 327–332 (doi:10.1016/j.asr.2004.12.012).
“Laser Ranging to GPS Satellites with Centimeter Accuracy” by J.J. Degnan and E.C. Pavlis in GPS World, Vol. 5, No. 9, September 1994, pp. 62–7.
