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.
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.”
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.
After more than four years of service as a Galileo test-bed satellite, GIOVE-B was retired on July 23. Its navigation transmitters were switched off and, according to an announcement from the European Space Agency, the satellite’s height was subsequently raised in a series of steps to place it in a so-called “graveyard” orbit where there will be no danger of it interfering with the operational Galileo satellites or other spacecraft.
After the first delta-V orbit manoeuvre, NORAD/JSpOC lost the satellite — at least NORAD/JSpOC stopped providing updated two-line orbital element sets for it. Eventually, 24 days later, the agency found it and resumed issuing element sets.
Just before the orbit manoeuvres, GIOVE-B had a mean motion of 1.70959839 orbits per day according to NORAD/JSpOC, which translates to an orbit semi-major axis value of approximately 29,544 kilometres. When NORAD/JSpOC recovered the satellite, its mean motion was 1.65377594 orbits per day with a semi-major axis of 30,205 kilometres, a change of 661 kilometres.
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.
By Matthias Söllner, Christian Kurzhals, Wolfgang Kogler, Stefan Erker, Steffen Thölert , Michael Meurer, Maktar Malik, and Manuela Rapisarda
GIOVE-B has been in orbit for just over one year. How well is it performing? In particular, what can we say about one of GIOVE-B’s pioneering features: its E1 CBOC signal? In this month’s column, we take a detailed look at a particular monitoring and assessment program set up to examine the GIOVE-B signals and discuss some of its initial CBOC results. The successful operation of this program bodes well for its use in future validation campaigns.
INNOVATION INSIGHTS by Richard Langley
THE SECOND GALILEO TEST SATELLITE, GIOVE-B, was launched on April 27, 2008, and began transmitting navigation signals a few days later. It joined its older sibling, GIOVE-A, which was placed in orbit over two years earlier. Standing for Galileo In-Orbit Validation Element, the GIOVE satellites constitute the first in-orbit test phase in the development of the Galileo navigation system.
In addition to securing the frequencies for the system, the satellites are being used to assess key technologies for the full Galileo constellation. The GIOVE test phase will be followed by the In-Orbit Validation (IOV) phase during which four IOV satellites will be launched, two at a time, aboard Soyuz rockets from Europe’s spaceport in French Guiana. Together with a preliminary ground network, the IOV satellites will be used to validate the Galileo system as a whole, using advanced system simulators. The launches are expected to occur by the end of 2010.
But before the IOV phase can begin, a thorough analysis of the performance of the GIOVE satellites must be carried out to minimize any difficulties with the IOV satellites. This includes monitoring and assessing the different signals broadcast by the satellites.
The GIOVE satellites can transmit on all three Galileo frequencies, E5, E6, and E1 (also known as L1) but only on two simultaneously (either E1-E5 or E1-E6). A variety of modulation types can be transmitted on the different frequencies by both satellites to test their use for the different Galileo services to be implemented for the operational constellation. These include alternative binary offset carrier (BOC) and quadrature phase shift keying on E5 and cosine BOC (BOCc) and binary phase shift keying on E6. On E1, the satellites have different capabilities. Although both satellites can transmit BOCc on this frequency, GIOVE-A can additionally transmit a single BOC signal with a subcarrier frequency of 1.023 MHz and a spreading code chipping rate of 1.023 MHz (BOC(1,1) ) whereas GIOVE-B transmits a more versatile multiplexed composite BOC or CBOC, which linearly combines BOC(1,1) and BOC(6,1). The CBOC signal is being transmitted by GIOVE-B to explore its performance, usability, and any possible side effects including its use in receivers designed to track a BOC(1,1) signal.
GIOVE-B has now been in orbit for just over one year. How well is it performing? In particular, what can we say about one of GIOVE-B’s pioneering features: its E1 CBOC signal? In this month’s column, we take a detailed look at a particular monitoring and assessment program set up to examine the GIOVE-B signals and discuss some of its initial CBOC results. The successful operation of this program bodes well for its use in future validation campaigns.
The first measurements of the navigation signals transmitted by the second Galileo test satellite, GIOVE-B, were recorded during the night of May 7, 2008, following the successful launch from Baikonur a little over a week earlier on April 28. During the In-Orbit Test (IOT) phase of the mission, which lasted about three months, a program of intensive measurements was carried out by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt or DLR) and the Astrium subsidiary of the European Aeronautic Defence and Space Company (EADS) using the 30-meter high-gain antenna at Weilheim/Lichtenau, near Munich, Germany. These and follow-on activities were performed under a bilateral agreement between the European Space Agency (ESA) and DLR’s Institute of Communication and Navigation on the exploitation of the GIOVE satellites.
The first measurements indicated that the navigation payload suffered no damage or degradation with regard to power and signal-in-space (SIS) quality during the launch of the satellite. After the successful completion of the IOT phase, the satellite configuration was maintained for long intervals to allow stability and long-term quality assessments within the ESA GIOVE mission activities and to stabilize ground segment operation. DLR and Astrium continued their measurements and analysis on GIOVE-B signals, focusing on modulated power and on modulation and correlation quality. The observation intervals included an eclipse season, when GIOVE-B spends a fraction of its orbit in the Earth’s shadow. Any impact of the corresponding temperature variations on signal transmission characteristics and key navigation parameters was of special interest.
This article focuses on modulated spectral power analysis of GIOVE-B signals for the detection of frequency and elevation-angle-dependent variations in the transmitted signal spectrum and power. Also, a detailed in-phase/quadrature-phase (I/Q) sample analysis is presented, focusing on modulation correctness and correlation distortions. These assessments are based on sample files of a few seconds of navigation signals as received by a calibrated high-gain antenna and recorded with the BayNavTech Signal Experimentation Facility (BaySEF). We have evaluated, in particular, correlation loss and S-curve bias since these parameters are very sensitive to onboard signal distortions although their reliable evaluation is quite challenging. In this article, we concentrate on the analyses of the E1 composite binary offset carrier (CBOC) modulation.
We discuss the GIOVE-B measurement and evaluation parameter results from the initial IOT phase and from later phases, including those from an eclipse period. This comparison of measurement results — spread over the first year of operations — demonstrates the excellent stability of signal power, modulation, and correlation quality of the GIOVE-B signals.
GIOVE-B E1 Signal
GIOVE-B can transmit navigation signals either simultaneously in the Galileo E5 and E1 bands or in the E6 and E1 bands. At E1, with a center frequency of 1575.42 MHz, GIOVE-B transmits three signal components called E1-A, E1-B, and E1-C. The E1-A signal has a BOCcos(15,2.5) modulation, with 2.5575 MHz code chip-rate and a binary cosine-type subcarrier modulation of 15.345 MHz. The B- and C-components have CBOC(1,6,1,10/1) modulation. Within this type of multiplexed BOC implementation, the code chips are provided at a constant rate of 1.023 MHz, modulated with a composite quaternary subcarrier with rates of 1.023 and 6.138 MHz. The latter part, called the BOC(6,1) subcarrier, has a relative power of 1/11 and is added to the BOC(1,1) binary subcarrier in CBOC-B and subtracted for CBOC-C. From the beginning of May 2008 until July 2009, GIOVE-B transmitted signals 96.8% of the time. Considering the experimental nature of the satellite, this represents a very successful operation.
Signal Quality and Relevance
In GNSS operations, signal quality assessment generally refers to the behavior of the navigation signals as transmitted from individual satellites. In this article, we assess two major aspects: the transmitted signal power and the frequency-transfer distortions of the satellite relative to the ideal signal definitions.
Why should we consider these aspects in particular? For transmitted signal power, the answer is obvious. In addition to proof of compliance with regulatory declarations, verification and monitoring of signal power is relevant for the system provider to commit to certain navigation service performance.
Concerning frequency-transfer distortions, the answer may be less obvious. The relevance is obtained via the impact of distortions on a receiver’s correlation function. First, the available maximum correlation power for a receiver binary replica may be affected. This is due to changes in the power sharing of the signal components within the complete signal and due to reduced matching of distorted transmitted signals with the ideal receiver replica. Second, the shape of the correlation function may suffer from asymmetric distortions, leading to receiver-dependent biases in the discriminator lock point. The most relevant receiver parameters are the input bandwidth and the discriminator type, especially the discriminator spacing (early-late spacing). With respect to this, user receivers and receivers in the control system ground segment (used to derive the satellite orbit and clock parameters) may differ. This leads not only to timing but also to positioning errors, if the distortions are different for different satellites.
These are the main reasons why we need to analyze and control the corresponding distortions for the Galileo system.
For an accurate assessment of transmitted signal power and frequency distortions, we had to obtain measurements with a highly directive antenna. This is essential in order to lift the instantaneous signal far above the noise floor and to avoid environmental distortions from interference as well as multipath.
But why don’t we consider also other aspects of signal quality such as the correctness of the navigation message or the stability of hardware delays and clocks? Because measurements with the highly directive antenna are not so appropriate for an assessment of these parameters. Instead, continuous monitoring over weeks or longer is preferred, based on measurements from a network of distributed navigation receivers. Such monitoring likely will be discussed in other publications.
Figure 1: Illustration of S-curve bias parameter.
Evaluation Parameters
For the assessment of the satellite’s radiated power in the individual navigation bands, absolute calibrated spectral measurements are evaluated for spectral flux densities integrated over frequency. These parameters provide a first insight into spectral asymmetries and variations over time.
Other parameters are used to evaluate the distortions in the instantaneous transfer characteristics of the satellite. They are derived from wide-band recorded baseband-signal samples of up to a few seconds duration.
Initially, we consider the I/Q probability density of the signal after Doppler frequency shift removal, well known from communication system analysis as scatter plots. Secondly, as introduced in the previous section, we want to quantify the impact of transfer distortions on the navigation performance obtained for ideal navigation receivers. The corresponding acquisition and tracking performance is based on the correlation function.
We define the normalized correlation function with respect to ideal receiver properties in order to separate the satellite transmit distortions of interest from receiver distortions according to the following equation:
with
the preprocessed baseband signal, SBB-PreProc, with down-converted nominal center frequency, full Doppler removal, and brick-wall-filtered to a bandwidth of interest;
the reference-signal, SRef, providing the ideal binary (or, for CBOC, quaternary) baseband receiver replica signal;
the integration period, Tp, often corresponding to the primary code period of the reference signal under consideration.
From this normalized correlation function, we can derive the primary relevant navigation parameters, which are, as previously mentioned:
correlation loss
early-late spacing dependency of the code-loop-discriminator lock point and S-curve bias.
Figure 2: GIOVE-B E1 CBOC measured and ideal normalized power spectral density.
Correlation Loss. For a given (distorted) signal, the correlation loss (CL) quantifies the loss in correlator output power relative to an ideal signal. This can be formulated by
where
It should be noted that the ideal baseband signal here is the multiplexed signal including all signal components, also band limited with a brick-wall filter to the bandwidth of interest.
S-Curve Bias. The navigation receiver obtains the (noiseless) code delay by following the zero crossing of the code discriminator. The output as a function of delay resembles the letter “S” or its reflection and is called the S-curve. For asymmetric distortions in the correlation function, it turns out that different code-tracking loops may have different lock points, as illustrated in FIGURE 1 for an arbitrary example.
To quantify this effect with a reasonable compromise between parameter complexity and practical value, a non-coherent (early minus late) power discriminator is considered over a wide range of early-late spacings, . This refers to the code discriminator of
with its lock point, , defined by
.
Then, the spreading of the lock point is the S-curve bias, SCB, given by
,
considering all δ in the range [0, δmax], with
Those interested in additional candidate signal-quality parameters, more detailed descriptions, or a theoretical analysis of the impact of satellite distortions on navigation performance parameters should consult the paper “GNSS Offline Signal Quality Assessment” listed in Further Reading.
Figure 3: GIOVE-B E1 CBOC received power flux on May 11, 2008
Weilheim Measurement Setup
As previously mentioned, for accurate measurements of the various signal-quality parameters, the signal needs to be lifted above the noise and multipath, and any interference needs to be suppressed considerably. Moreover, as the signal quality from the satellite needs to be assessed separately, any measurement-system transfer distortions need to be calibrated out as much as possible.
That’s why DLR installed a measurement and calibration system at their 30-meter high-gain dish antenna at Weilheim for GIOVE signal performance measurements. This measurement setup was completed for high capacity signal recording with one rack of the BaySEF equipment in cooperation with EADS Astrium.
The 30-meter antenna, as shown in the opening graphic, is the main core of this verification facility. This antenna is based on a shaped Cassegrain system with elevation over azimuth mount, with higher than 50-dB gain and a beam width around 0.5° at L-band. The absolute position accuracy of this antenna is 0.001° in each direction. The signals are directed from the parabolic main reflector to the measurement cabin via a hyperbolic sub-reflector, a waveguide, and a second flat sub-reflector. One big benefit of this construction is the direct access to the installed feed in the cabin and the possibility to place the complete measurement equipment next to the feed, avoiding long connection cables.
The signals are recorded with BaySEF and for individual frequency bands (selected by switchable band pass filters) and also with a vector signal analyzer (VSA) of at least 80 MHz bandwidth. Moreover, a signal with up to 300 MHz bandwidth can be recorded with a digital oscilloscope, if the VSA is used for down conversion. Interfaces to other measurement equipment, such as navigation receivers, have already been prepared for future extension. The whole setup is referenced to a highly stable cesium frequency standard. We essentially use the BaySEF equipment as a high-capacity multiband bit grabber in this setup.
Figure 4: GIOVE-B E1 CBOC received power frequency asymmetry on May 11, 2008
The main BaySEF applications are the verification and monitoring of GNSS SIS and support for the design of applications based on parametric software-receiver evaluations. The key features most relevant for the signal-quality assessment discussion of this article are:
Simultaneous processing (down conversion, etc.) and synchronized recording of up to four frequency bands;
3-dB RF bandwidth of more than 100 MHz at E5 and more than 50 MHz in other bands (E5a, E5b, L2, E6, E1);
Maximum recording capacity of 120 megabytes per second per frequency band;
Flexible decimation and quantization;
Recording capacity of 0.6 terabytes per frequency band, corresponding to more than 80 minutes at the maximum recording rate;
Remote control of data acquisition from, for example, the EADS Astrium premises in Ottobrunn.
Figure 5: GIOVE-B E1 CBOC received power flux on July 3, 2009Figure 6: GIOVE-B E1 CBOC received power frequency asymmetry on July 3, 2009.Figure 7: Scatter plot of GIOVE-B L1-CBOC (black crosses indicate ideal constellation).
Measurement Calibration
Accurate system calibration is the key to reliable signal quality measurements. To achieve a combined absolute measurement uncertainty significantly less than 1.0 dB (required for EIRP assessments), it is essential to calibrate precisely every used part of the system. In addition to all RF components of the receiving system, this also includes the high-gain antenna itself.
For the characterization of the high-gain antenna, two values are assessed: antenna pointing accuracy and gain. A pointing offset of about 0.04° was measured exactly with a known L-band pilot signal from the Artemis satellite and corrected in the antenna control. For gain characterization over the complete L-band frequency range of interest, the radio “star” Cassiopeia A is used. Cas A (actually a supernova remnant) is one of the strongest wide-band radio emitters in the northern hemisphere. With the help of the well-known flux density of Cas A, the gain-to-noise-temperature ratio (G/T) can be measured. After precise determination of the system noise temperature, T, the antenna gain, G, itself can be found.
For online calibration of absolute gain drifts in the measurement system, a frequency-and-power-stabilized signal generator is used in combination with two power meters.
The relative frequency transfer distortions of the receiving system are calibrated with two techniques. A network analyzer periodically provides precise measurements of gain and phase of the RF path from the antenna feed to the measurement devices.
To include also down-converting measurement devices such as the BaySEF, the injection of wide-band calibration signals is used, and simultaneously measured with a commercial digitizer (VSA).
The desired in-band measurement system transfer characteristic (relative to the VSA-characteristic, which is assumed to be ideal) is then extracted by means of de-convolving the calibration signal sampled by the device to be calibrated with the VSA-sampled reference signal.
Corresponding transfer characteristics obtained for the RF path (from network analysis) and for the BaySEF (from wide-band calibration signals) were combined to derive the equalization filter applied in post-processing the recorded samples.
Power Measurement Results
For analysis of the transmitted signal power of the GIOVE-B E1 CBOC and its variation, we have recorded a large number of spectral measurements over single satellite passes. A typical example of a single power spectral density measurement — here normalized to unit power — is shown in FIGURE 2, overlaid on the ideal spectral envelope. After absolute power calibration, we integrate the spectral power flux density over a reference bandwidth of 40.92 MHz and over the individual main lobes of the BOC(1,1) and BOCc(15,2.5) components, as illustrated in Figure 2. This procedure is used to detect the variation of transmitted signal power and possible signal asymmetries over time.
Parameter results are shown first for an early satellite pass during the IOT campaign on May 11, 2008 in FIGURE 3. Main variations in this figure are as typically expected from the cut of the measurement pass through the satellite antenna pattern. Also, the overall main-lobe power of the BOC(1,1) and the BOCc(15,2.5) components are similar, but with a strong asymmetry between upper and lower main-lobe power of the BOCc(15,2.5) component.
A closer look at the time-dependency of this asymmetry is given in FIGURE 4, showing a stable low power difference of about 0.2 dB between the BOC(1,1) main lobes and a mean power difference of 0.8 dB between the BOCc(15,2.5) main lobe with ±0.2 dB variations over time.
The second record was captured more than a year later, with similar results as shown in FIGURE 5, which indicate a stable transmission power. Only the mean main-lobe asymmetry of the BOCc(15,2.5) signal, as shown in FIGURE 6, is slightly smaller and with a different shape in its time-dependency. The measurement passes surely provided different cuts through the satellite antenna pattern. Therefore, we assume that these variations mainly indicate some antenna-pattern frequency dependency, as will be emphasized also in the following signal quality measurement results. For precise characterization, measurements and evaluations for many more satellite passes would be required.
Figure 8: GIOVE-B E1 correlation function shape of CBOC componentsFigure 9: GIOVE-B E1 correlation function shape of BOCc(15,2.5) componentFigure 10: GIOVE-B E1 example of correlation-loss results for COBC-B
Signal Quality Results
In this section, we will show some GIOVE-B E1 signal quality results as obtained from BaySEF measurement data collected at Weilheim. The results presented are from two passes with the satellite in E1 CBOC-transmission mode:
May 9, 2008, a few days after the signals were switched on
September 29, 2008, a pass when the satellite was in eclipse for one hour.
The modulation type of the E1 signal can be seen from the scatter plot, shown in Figure 7, which is derived from about 50 millisecond-signal-samples after Doppler removal. The overlay, with the eight phase points of the ideal constant envelope signal, allows clear identification of the interplexed CBOC signal. Due to bandwidth limitation, distortions, and noise jitter, the individual phase states have been enlarged, and transition traces become visible. Also, the symmetry of phase states becomes slightly deformed. How much this affects measurable navigation performance is not directly obvious from such a plot.
A better indicator of navigation performance is the E1 interplex CBOC correlation function shapes as shown in FIGURE 8 (CBOC-B and -C) and FIGURE 9 (BOCc(15,2.5)), respectively. As for all the following evaluations, the E1 signals were brick-wall band-limited to 40.92 MHz (40 × 1.023 MHz), which was the performance bandwidth of interest, and up-sampled to a rate of 575 MHz. The differences in shapes are not due to distortions but due to different signs of the BOC(6,1) subcarrier in the B and C channels. Note the imaginary part due to signal distortions has been amplified 10 times to stress its presence.
For the shape of the BOCc(15,2.5) correlation function, a small asymmetry in the real part is visible when compared to the ideal band-limited autocorrelation function. This signal distortion effect might be relevant, leading to a higher chance for false lock in acquisition and tracking. However, current receivers have no problems in tracking these signals.
A direct visual assessment of these shapes does not allow us to draw many conclusions. More quantitative evaluations provide the performance parameters of correlation-loss and lock-point-bias behavior.
Example results of the correlation-power evaluation for the GIOVE-B CBOC-B signal component are shown in FIGURE 10, with the red curve showing the correlation power of succeeding code periods relative to the total signal (which also includes the CBOC-C and BOCc(15,2.5) signal components). A curve of similar shape, shown in blue, is obtained for the ideal reconstructed signal using the specified codes and actual data bits of all components synchronized to the input signal. Obviously, the strong jitter of more than 0.1 dB is due to code cross-correlations. The correlation loss is obtained by taking the difference (see Figure 10b), which has a much lower variation with a maximum value of about 0.02 dB. This variation is still dominated by residual distortion differences of the code-cross-correlation values of the GIOVE-B codes as can be seen when accounting for the fact that equally colored dots in the figure correspond to equal code cross-correlations. Despite the variation, the negative loss is remarkable. This corresponds to a gain in usable signal power relative to the ideal case and is due to a stronger bandwidth limitation. This analysis indicates that the signal power of the wide-band signal component, BOCc(15,2.5), is more strongly reduced than for the narrow-band CBOC component, providing effectively a gain in relative correlation power for CBOC.
Next, we consider the lock-point bias of a noncoherent power discriminator as a function of early-late spacing (over the relevant range) for the CBOC-C signal component in FIGURE 11, evaluated for about 60 succeeding code periods. Again, different colors indicate different code-cross-correlation values. The corresponding S-curve bias is obtained by evaluating the peak-to-peak variation of this lock-point bias and is shown in FIGURE 12. Over this short period of 0.5 seconds, a quite stable value of about 1325 picoseconds is obtained with only 25 picosecond standard deviation mainly due to residual code-cross-correlation effects and residual noise.
Figure 11: GIOVE-B E1 example of lock-point bias dependency on early-late spacing for CBOC-C
It should be noted that all evaluation results include not only the satellite distortions but, in general, measurement distortion contributions. In fact, accurate measurement system calibration is one of the most critical issues for exact signal quality assessment of satellite transfer characteristics.
Mutual evidence of successful calibration is gained from the fact that similar results for correlation loss and S-curve bias of the CBOC-signal components have been obtained from measurements carried out by ESA together with Surrey Satellite Technology and the Science and Technology Facilities Council at the Chilbolton Observatory near Andover, England. However, even for the same measurement periods and perfect calibration, identical results may not be expected, as will be discussed here.
Figure 12: GIOVE-B E1 example of S-curve-bias evaluation result for CBOC-C
In addition to the signal-quality assessment of individual snapshot measurements, variation over observation direction and over time is of major relevance. Therefore, these evaluations have been performed for many snapshots of two single passes almost five months apart. The passes and measurement points mapped to the directions as seen from a satellite-antenna-fixed coordinate system are shown in Figure 13.
In FIGURE 14, the results of correlation loss and S-curve bias are shown for both passes mapped to the satellite antenna off-axis angle. For a monotonic x-axis, a sign was added to this angle here, with negative values for the ascending part of the pass and positive values for the descending part as seen from Weilheim.
Figure 13: GIOVE-B measurement points of BaySEF transformed into satellite antenna coordinates (off-axis angle, antenna-azimuth angle)Figure 14: GIOVE-B E1 correlation-loss variation results over satellite passes.
The correlation-loss results for both passes vary about ±0.1 dB around 0.7 dB for BOCc(15,2.5), around -0.5 dB for CBOC-C and around -0.55 dB for CBOC-B. Also, the measured S-curve-bias results are in the same range for both passes, which is about ±100-200 picoseconds around 1200 picoseconds for CBOC-B and CBOC-C and ±2 picoseconds around 23 picoseconds for BOCc(15,2.5). Furthermore, these plots show smooth variations over the antenna off-axis angle, obviously also with some azimuth dependency due to the different shapes for each pass. It is remarkable that the relative shapes of the correlation-loss and S-curve-bias plots are so similar.
Even if measurement system instability may be a contributing factor, most of the variations are thought to be due to the directional dependency of the satellite antenna pattern. See also the similarity of results from both passes at the one end of the high off-axis angles, corresponding to similar azimuth angles. These results indicate that for full characterization of signal quality over the satellite antenna pattern, further well-calibrated measurements over several passes would be required.
During the measurement pass of September 29, 2008, a few measurement points were taken when the satellite was in eclipse. Corresponding results marked by black points in the figures show almost no impact of the eclipse on correlation loss and S-curve bias. Further measurements and evaluations would be required to confirm finally that the larger changes afterwards are not due to the eclipse.
Conclusions
This article described some of the GIOVE-B navigation SIS performance characterizations carried out by DLR and EADS Astrium in collaboration with ESA during the past year. These characterizations were achieved using a very accurate measurement system, calibrated in absolute power, relative amplitude, and phase over several frequency bands. For this purpose, several calibration approaches have been adopted and are still being optimized.
The similarity of results from measurements spaced several months apart indicates excellent long-term stability of the considered characteristics. Also, it has been demonstrated that a satellite-eclipse period seems not to affect modulation and correlation quality in a relevant manner.
As expected and already known from previous work, the satellite provides some frequency-dependent directional variations in its transmitted signal power characteristics, which also affects signal quality.
During the described campaign, the observed variations can be considered as moderate. A continuation of the monitoring activity is under consideration, to further improve the coverage of the characterizations.
During these GIOVE-B measurements and evaluations, all involved teams gained considerable experience in accurate characterization of the navigation signals transmitted from the satellite, and the operation of instruments and signal evaluation was thoroughly verified and cross-validated. The knowledge and experience gained will be very useful for future navigation satellite validation campaigns.
Acknowledgments
We would like to thank DLR’s German Space Operations Center for use of the Weilheim antenna and the colleagues who operate and maintain it. BaySEF is part of the BayNavTech program, which is supported by the Bavarian Government (Ministry for Economic Affairs, Infrastructure, Transport and Technology). The activity reported in this article was performed under a bilateral agreement between ESA and DLR’s Institute of Communication and Navigation on the exploitation of GIOVE satellites and the GIOVE signal-quality-characterization effort has been supported by ESA. This article is based on the paper “One Year in Orbit – GIOVE-B Signal Quality Assessment from Launch to Now” presented at the European Navigation Conference GNSS 2009, held in Naples, Italy, May 3–6, 2009.
Manufacturers
Measurement equipment included a Rohde & Schwarz GmbH & Co. KG (www.rohde-schwarz.com) FSQ26 vector signal analyzer. The EADS Astrium facility did not use commercial receivers to capture the GIOVE-B signals discussed in this article.
MATTHIAS SÖLLNER is senior expert on navigation signal engineering. CHRISTIAN KURZHALS and WOLFGANG KOGLER are navigation signal engineers focusing on payload aspects and performance verification, respectively. All are working at the Astrium GmbH subsidiary of the European Aeronautic Defence and Space Company N.V. (EADS).
STEFAN ERKER and STEFFEN THÖLERT work on GNSS validation and signal analysis at the German Aerospace Centre in Oberpfaffenhof-en with MICHAEL MEURER, who is responsible for performance issues concerning the Galileo system.
MAKTAR MALIK is the payload system manager for GIOVE-B at the European Space Research and Technology Centre in Noordwijk, The Netherlands, where he works with MANUELA RAPISARDA, who is a radio navigation engineer providing system support to the Galileo Project Office.
