Entitling its release “From Orbit with Love,” the European Space Agency (ESA) proudly announced today, March 12, 2013, that the first four satellites of the future Galileo Satellite Navigation constellation achieved their first-ever autonomous position fix. The positioning was replicated and confirmed by a team at the NavSAS group of Politecnico di Torino, Italy.
The obtained accuracy lies in the 10-meter range, according to ESA. ESA added that considering the infrastructure is only partly deployed, this fulfills expectations. As with GPS or any satellite navigation system, a minimum of four satellites is required to make a position fix in three dimensions.
The position fix was obtained by ESA’s navigation laboratory in the Netherlands, using the four satellites, launched in October 2011 and 2012, and the Galileo programme’s ground infrastructure, consisting of control centers in Italy and Germany and a global network of ground stations.
“This fundamental step confirms the Galileo system works as planned,” read the official statement.
“Once testing of the latest two satellites was complete, in recent weeks our effort focused on the generation of navigation messages and their dissemination to receivers on the ground,” explained Marco Falcone, ESA’s Galileo system manager.
Measurements of individual Galileo horizontal position fixes performed for the first time using the four Galileo satellites in orbit plus the worldwide ground system between 1000 and 11:00 CET on Tuesday 12 March 2013, showing an overall horizontal accuracy over ESTEC in Noordwijk, the Netherlands, of 6.3 m.
This first position fix of longitude, latitude, and altitude took place at the Navigation Laboratory at ESA’s technical heart ESTEC, in Noordwijk, the Netherlands, early on the morning of March 12, with an accuracy between 10 and 15 meters, which was expected, taking into account the limited infrastructure deployed so far.
“The test of today has a dual significance: historical and technical,” notes Javier Benedicto, ESA’s Galileo project manager. “From the historical perspective, this is the first time ever that Europe has been able to determine a position on the ground using only its own independent navigation system, Galileo. From the technical perspective, generation of the Galileo navigation messages is an essential step for beginning the full validation activities, before starting the full deployment of the system by the end of this year.”
With only four satellites for the time being, the full Galileo constellation is visible at the same time for a maximum two to three hours daily. This frequency will increase as more satellites join the constellation in orbit, along with extra ground stations coming online, for Galileo’s early services to start at the end of 2014.
The European Commission’s program head for Galileo, Paul Flament, granted an interview last week with GPS World, recapping the coming launch activities and expectations for initial and full operational capabilities, the latter with a target constellation of 30 satellites. The interview will appear in the April issue of the magazine, which is specially devoted to Galileo and European navigation initiatives.
With the validation testing activities under way, users might experience breaks in the content of the navigation messages being broadcast, said ESA. In the coming months the messages will be further elaborated to define the offset between Galileo System Time and Coordinated Universal Time (UTC), enabling Galileo to be relied on for precision timing applications, as well as the Galileo to GPS Time Offset, ensuring interoperability with GPS.
Galileo Is Real, and NavSAS Has the Evidence
Almost simultaneously with the ESA announcement, the NavSAS group of Politecnico di Torino and Istituto Superiore Mario Boella in Turin, Italy, also achieved a position fix using the signals of the four In-Orbit Validation Galileo satellites (PFM, FM2, FM3, FM4) that started today to broadcast a valid navigation message. The researchers of the NavSAS team successfully computed the positions by using full software receivers developed by the team.
The positions obtained are depicted in Figure 1, as red squares on the rooftop of the NavSAS Lab in Turin, Italy, where the antenna is positioned (latitude 45°03’54.98767″ N, longitude 7°39’32.28920″ E, height 311.9667 meters). The navigation message was first successfully decoded at 11.28 on March 12.
Figure 1. Position fixes on the rooftop of the NavSAS lab in Turin, Italy.
The configuration of the four Galileo satellites as seen by the NavSAS lab is reported in Figure 2.
Figure 2. Skyplot of the Galileo IOV satellites at the time of the data acquisition for the fix.
The NavSAS team was earlier among the first research teams worldwide able to receive and process the signal of the PFM and FM2 satellites, in December 2011 after the launch of the earliest Galileo IOV satellites, and again at the end of 2012 for the FM3 and FM4.
The milestone in both accounts of Galileo-only positioning is that it is real-time positioning using the Galileo navigation message. Galileo positioning using a post-processing mode had already been demonstrated, and described by Peter Steigenberger, Urs Hugentobler, and Oliver Montenbruck of the Technische Universität München and the German Space Operations Center, in an account in GPS World, February 2012 issue. (scroll down to “First Demonstration of Galileo-Only Positioning”).
The Universal Software Radio Peripheral as RF Front-End
By Ningyan Guo, Staffan Backén, and Dennis Akos
The authors designed a full-constellation GNSS receiver, using a cost-effective, readily available, flexible front-end, wide enough to capture the frequency from 1555 MHz to 1607 MHz, more than 50MHz. This spectrum width takes into account BeiDou E2, Galileo E1, GPS L1, and GLONASS G1. In the course of their development, the authors used an external OCXO oscillator as the reference clock and reconfigured the platform, developing their own custom wide-band firmware.
The development of the Galileo and BeiDou constellations will make many more GNSS satellite measurements be available in the near future. Multiple constellations offer wide-area signal coverage and enhanced signal redundancy. Therefore, a wide-band multi-constellation receiver can typically improve GNSS navigation performance in terms of accuracy, continuity, availability, and reliability. Establishing such a wide-band multi-constellation receiver was the motivation for this research.
A typical GNSS receiver consists of three parts: RF front-end, signal demodulation, and generation of navigation information. The RF front-end mainly focuses on amplifying the input RF signals, down-converting them to an intermediate frequency (IF), and filtering out-of-band signals. Traditional hardware-based receivers commonly use application-specific integrated circuit (ASIC) units to fulfill signal demodulation and transfer the range and carrier phase measurements to the navigation generating part, which is generally implemented in software. Conversely, software-based receivers typically implement these two functions through software. In comparison to a hardware-based receiver, a software receiver provides more flexibility and supplies more complex signal processing algorithms. Therefore, software receivers are increasingly popular for research and development.
The frequency coverage range, amplifier performance, filters, and mixer properties of the RF front-end will determine the whole realization of the GNSS receiver. A variety of RF front-end implementations have emerged during the past decade. Real down-conversion multi-stage IF front-end architecture typically amplifies filters and mixes RF signals through several stages in order to get the baseband signals. However, real down-conversion can bring image-folding and rejection. To avoid these drawbacks, complex down-conversion appears to resolve much of these problems. Therefore, a complex down-conversion multi-stage IF front-end has been developed. But it requires a high-cost, high-power supply, and is larger for a multi-stage IF front-end. This shortcoming is overcome by a direct down-conversion architecture. This front-end has lower cost; but there are several disadvantages with direct down-conversion, such as DC offset and I/Q mismatch. DC offset is caused by local oscillation (LO) leakage reflected from the front-end circuit, the antenna, and the receiver external environment.
A comparison of current traditional RF front-ends and different RF front-end implementation types led us to the conclusion that one model of a universal software radio peripheral, the USRP N210, would make an appropriate RF front end option. USRP N210 utilizes a low-IF complex direct down-conversion architecture that has several favorable properties, enabling developers to build a wide range of RF reception systems with relatively low cost and effort. It also offers high-speed signal processing. Most importantly, the source code of USRP firmware is open to all users, enabling researchers to rapidly design and implement powerful, flexible, reconfigurable software radio systems. Therefore, we chose the USRP N210 as our reception device to develop our wide-band multi-constellation GNSS receiver, shown in Figure 1.
Figure 1. Custom wide-band multi-constellation software receiver architecture based on universal software radio peripheral (USRP).
USRP Front-End Architecture
The USRP N210 front-end has wider band-width and radio frequency coverage in contrast with other traditional front-ends as shown by the comparison in Table 1. It has the potential to implement multiple frequencies and multiple-constellation GNSS signal reception. Moreover, it performs higher quantization, and the onboard Ethernet interface offers high-speed data transfer.
Table 1. GNSS front-ends comparison.
USRP N210 is based on the direct low-IF complex down-conversion receiver architecture that is a combination of the traditional analog complex down-conversion implemented on daughter boards and the digital signal conditioning conducted in the motherboard. Some studies have shown that the low-IF complex down-conversion receiver architecture overcomes some of the well-known issues associated with real down-conversion super heterodyne receiver architecture and direct IF down-conversion receiver architecture, such as high cost, image-folding, DC offset, and I/Q mismatch.
The low-IF receiver architecture effectively lessens the DC offset by having an LO frequency after analog complex down-conversion. The first step uses a direct complex down-conversion scheme to transform the input RF signal into a low-IF signal. The filters located after the mixer are centered at the low-IF to filter out the unwanted signals. The second step is to further down-covert the low-IF signal to baseband, or digital complex down-conversion.
Similar to the first stage, a digital half band filter has been developed to filter out-of-band interference. Therefore, direct down-conversion instead of multi-stage IF down-conversion overcomes the cost problem; in the meantime, the signal is down-converted to low-IF instead of base-band frequency as in the direct down-conversion receiver, so the problem of the DC offset is also avoided in the low-IF receiver. These advantages make the USRP N210 platform an attractive option as GNSS receiver front-end.
Figure 2 shows an example GNSS signal-streaming path schematic on a USRP N210 platform with a DBSRX2 daughter board. Figure 3 shows a photograph of internal structure of a USRP N210 platform.
Figure 2 GNSS signal streaming on USRP N210 + DBSRX2 circuit.Figure 3. USRP N210 internal structure.
The USRP N210 platform includes a main board and a daughterboard. In the main board, 14-bit high precision analog-digital converters (ADCs) and digital-analog converters (DACs) permit wide-band signals covering a high dynamic range. The core of the main board is a high-speed field-programmable gate array (FPGA) that allows high-speed signal processing. The FPGA configuration implements down-conversion of the baseband signals to a zero center frequency, decimates the sampled signals, filtering out-of-band components, and finally transmits them through a packet router to the Ethernet port. The onboard numerically controlled oscillator generates the digital sinusoid used by the digital down-conversion process. A cascaded integrator-comb (CIC) filter serves as decimator to down-sample the signal.
The signals are filtered by a half pass filter for rejecting the out-of-band signals. A Gigabit Ethernet interface effectively enables the delivery of signals out of the USRP N210, up to 25MHz of RF bandwidth. In the daughterboard, first the RF signals are amplified, then the signals are mixed by a local onboard oscillator according to a complex down-conversion scheme. Finally, a band-pass filter is used remove the out-of-band signals.
Several available daughter boards can perform signal conditioning and tuning implementation. It is important to choose an appropriate daughter board, given the requirements for the data collection.
A support driver called Universal Hardware Driver (UHD) for the USRP hardware, under Linux, Windows and Mac OS X, is an open-source driver that contains many convenient assembly tools. To boot and configure the whole system, the on-board microprocessor digital signal processor (DSP) needs firmware, and the FPGA requires images. Firmware and FPGA images are downloaded into the USRP platform based on utilizations provided by the UHD. Regarding the source of firmware and FPGA images, there are two methods to obtain them:
directly use the binary release firmware and images posted on the web site of the company;
build (and potentially modify) the provided source code.
USRP Testing and Implementation
Some essential testing based on the original configuration of the USRP N210 platform provided an understanding of its architecture, which was necessary to reconfigure its firmware and to set up the wide-band, multi-constellation GNSS receiver. We collected some real GPS L1 data with the USRP N210 as RF front-end. When we processed these GPS L1 data using a software-defined radio (SDR), we encountered a major issue related to tracking, described in the following section.
Onboard Oscillator Testing. A major problem with the USRP N210 is that its internal temperature-controlled crystal oscillator (TCXO) is not stable in terms of frequency. To evaluate this issue, we recorded some real GPS L1 data and processed the data with our software receiver. As shown in Figure 4, this issue results in the loss of GPS carrier tracking loop at 3.18 seconds, when the carrier loop bandwidth is 25Hz.
Figure 4. GPS carrier loop loss of lock.
Consequently, we adjusted the carrier loop bandwidth up to 100Hz; then GPS carrier tracking is locked at the same timing (3.18s), shown in Figure 5, but there is an almost 200 Hz jump in less than 5 milliseconds.
Figure 5. GPS carrier loop lock tracking.
As noted earlier, the daughter card of the USRP N210 platform utilizes direct IF complex down-conversion to tune GNSS RF signals. The oscillator of the daughter board generates a sinusoid signal that serves as mixer to down-convert input GNSS RF signals to a low IF signal. Figure 6 illustrates the daughter card implementation. The drawback of this architecture is that it may bring in an extra frequency shift by the unstable oscillator. The configuration of the daughter-card oscillator is implemented by an internal TCXO clock, which is on the motherboard. Unfortunately, the internal TCXO clock has coarse resolution in terms of frequency adjustments. This extra frequency offset multiplies the corresponding factor that eventually provides mixer functionality to the daughter card. This approach can directly lead to a large frequency offset to the mixer, which is brought into the IF signals.
Figure 6. Daughter-card tuning implementation.
Finally, when we conduct the tracking operation through the software receiver, this large frequency offset is beyond the lock range of a narrow, typically desirable, GNSS carrier tracking loop, as shown in Figure 4.
In general, a TCXO is preferred when size and power are critical to the application. An oven-controlled crystal oscillator (OCXO) is a more robust product in terms of frequency stability with varying temperature. Therefore, for the USRP N210 onboard oscillator issue, it is favorable to use a high-quality external OCXO as the basic reference clock when using USRP N210 for GNSS applications.
Front-End Daughter-Card Options. A variety of daughter-card options exist to amplify, mix, and filter RF signals. Table 2 lists comparison results of three daughter cards (BURX, DBSRX and DBSRX2) to supply some guidance to researchers when they are faced with choosing the correct daughter-board.
Table 2. Front-end daughter-card options.
The three daughter cards have diverse properties, such as the primary ASIC, frequency coverage range, filter bandwidth and adjustable gain. BURX gives wider radio frequency coverage than DBSRX and DBSRX2. DBSRX2 offers the widest filter bandwidth among the three options.
To better compare the performance of the three daughter cards, we conducted another three experiments. In the first, we directly connected the RF port with a terminator on the USRP N210 platform to evaluate the noise figure on the three daughter cards. From Figure 7, we can draw some conclusions:
BURX has a better sensitivity than DBSRX and DBSRX2 when the gain is set below 30dB.
DBSRX2 observes feedback oscillation when the gain set is higher than 70dB.
Figure 7. Noise performance comparisons of three daughter cards.
The second experimental setup configuration used a USRP N210 platform, an external OCXO oscillator to provide stable reference clock, and a GPS simulator to evaluate the C/N0 performance of the three daughter boards. The input RF signals are identical, as they come from the same configuration of the GPS simulator. Figure 8 illustrates the C/N0 performance comparison based on this experimental configuration. The figure shows that BURX performs best, with DBSRX2 just slightly behind, while DBSRX has a noise figure penalty of 4dB.
Figure 8. C/N0 performance comparisons of three daughter cards.
In the third experiment, we added an external amplifier to increase the signal-to-noise ratio (SNR). From Figure 9, we see that the BURX, DBSRX and DBSRX2 have the same C/N0 performance, effectively validating the above conclusion. Thus, an external amplifier is recommended when using the DBSRX or DBSRX2 daughter boards.
Figure 9. C/N0 performance comparisons of three daughter cards with an external amplifier.
The purpose of these experiments was to find a suitable daughter board for collecting wide-band multi-constellation GNSS RF signals. The important qualities of an appropriate wide-band multi-constellation GNSS receiver are:
high sensitivity;
wide filter bandwidth; and
wide frequency range.
After a comparison of the three daughter boards, we found that the BURX has a better noise figure than the DBSRX or DBSRX2. The overall performance of the BURX and DBSRX2 are similar however. Using an external amplifier effectively decreases the required gain on all three daughter cards, which correspondingly reduces the effect of the internal thermal noise and enhances the signal noise ratio. As a result, when collecting real wide-band multi-constellation GNSS RF signals, it is preferable to use an external amplifier.
To consider recording GNSS signals across a 50MHz band, DBSRX2 provides the wider filter bandwidth among the three daughter-card options, and thus we selected it as a suitable daughter card.
Custom Wide-band Firmware Development. When initially implementing the wideband multi-constellation GNSS reception devices based on the USRP N210 platform, we found a shortcoming in the default configuration of this architecture, whose maximum bandwidth is 25MHz. It is not wide enough to record 50MHz multi-constellation GNSS signals (BeiDou E2, GPS L1, Galileo E1, and GlonassG1). A 50MHz sampling rate (in some cases as much as 80 MHz) is needed to demodulate the GNSS satellites’ signals.
Meanwhile since the initiation of the research, the USRP manufacturer developed and released a 50MHz firmware. To highlight our efforts, we further modified the USRP N210 default configuration to increase the bandwidth up to 100MHz, which has the potential to synchronously record multi-constellation multi-frequency GNSS signals (Galileo E5a and E5b, GPS L5 and L2) for further investigation of other multi-constellation applications, such as ionospheric dispersion within wideband GNSS signals, or multi-constellation GNSS radio frequency compatibility and interoperability.
Apart from reprogramming the host driver, we focused on reconfiguring the FPGA firmware. With the aid of anatomizing signal flow in the FPGA, we obtained a particular realization method of augmenting its bandwidth. Figure 10 shows the signal flow in the FPGA of the USRP N210 architecture.
Figure 10. Signal flow in the FPGA of the USRP N210 platform.
The ADC produces 14-bit sampled data. After the digital down-conversion implementation in the FPGA, 16-bit complex I/Q sample data are available for the packet transmitting step. According to the induction document of the USRP N210 platform, VITA Radio Transport Protocol functions as an overall framework in the FPGA to provide data transmission and to implement an infrastructure that maintains sample-accurate alignment of signal data. After significant processing in the VITA chain, 36-bit data is finally given to the packet router. The main function of the packet router is to transfer sample data without any data transformation. Finally, through the Gigabit Ethernet port, the host PC receives the complex sample data.
In an effort to widen the bandwidth of the USRP N210 platform, the bit depth needs to be reduced, which cuts 16-bit complex I/Q sample data to a smaller length, such as 8-bit, 4-bit, or even 2-bit, to solve the problem. By analyzing Figure 10, to fulfill the project’s demanding requirements, modification to the data should be performed after ADC sampling, but before the digital down-conversion. We directly extract the 4-bit most significant bits (MSBs) from the ADC sampling data and combined eight 4-bit MSB into a new 16-bit complex I/Q sample, and gave this custom sample data to the packet router, increasing the bandwidth to 100 MHz.
Wide-Band Receiver Performance Analysis. The custom USRP N210-based wide-band multi-constellation GNSS data reception experiment is set up as shown in Figure 11.
Figure 11. Wide-band multi-constellation GNSS data recording system.
A wide-band antenna collected the raw GNSS data, including GPS, GLONASS, Galileo, and BeiDou. An external amplifier was included to decrease the overall noise figure. An OCXO clock was used as the reference clock of the USRP N210 system. After we found the times when Galileo and BeiDou satellites were visible from our location, we first tested the antenna and external amplifier using a commercial receiver, which provided a reference position. Then we used 1582MHz as the reception center frequency and issued the corresponding command on the host computer to start collecting the raw wide-band GNSS signals. By processing the raw wide-band GNSS data through our software receiver, we obtained the acquisition results from all constellations shown in Figure 12; and tracking results displayed in Figure 13.
Figure 12. Acquisition results for all constellations.Figure 13. Tracking results for all constellations.
We could not do the full-constellation position solution because Galileo was not broadcasting navigation data at the time of the collection and the ICD for BeiDou had not yet been released. Therefore, respectively using GPS and GLONASS tracking results, we provided the position solution and timing information that are illustrated in Figure 14 and in Figure 15.
Figure 14. GPS position solution and timing information.Figure 15. GLONASS position solution.
Conclusions
By processing raw wide-band multi-constellation GNSS signals through our software receiver, we successfully acquired and tracked satellites from the four constellations. In addition, since we achieved 100MHz bandwidth, we can also simultaneously capture modernized GPS and Galileo signals (L5 and L2; E5a and E5b, 1105–1205 MHz).
In future work, a longer raw wide-band GNSS data set will be recorded and used to determine the user position leveraging all constellations. Also an urban collection test will be done to assess/demonstrate that multiple constellations can effectively improve the reliability and continuity of GNSS navigation.
Acknowledgment
The first author’s visiting stay to conduct her research at University of Colorado is funded by China Scholarship Council, File No. 2010602084.
This article is based on a paper presented at the Institute of Navigation International Technical Conference 2013 in San Diego, California.
Manufacturers
The USRP N210 is manufactured by Ettus Research. The core of the main board is a high-speed Xilinx Spartan 3A DSP FPGA. Ettus Research provides a support driver called Universal Hardware Driver (UHD) for the USRP hardware. A wide-band Trimble antenna was used in the final experiment.
Ningyan Guo is a Ph.D. candidate at Beihang University, China. She is currently a visiting scholar at the University of Colorado at Boulder.
Staffan Backén is a postdoctoral researcher at University of Colorado at Boulder. He received a Ph.D. in in electrical engineering from Luleå University of Technology, Sweden.
Dennis Akos completed a Ph.D. in electrical engineering at Ohio University. He is an associate professor in the Aerospace Engineering Sciences Department at the University of Colorado at Boulder with visiting appointments at Luleå University of Technology and Stanford University
New Organization Advocates for GPS Industry; Galileo Lives to Fly Another Day, Budget Passed; Safer Skies for EGNOS; and GLONASS in Brazil
New Organization Advocates for GPS Industry
A new group, the GPS Innovation Alliance, has formed and announced itself as the voice of the U.S. GPS industry and community of users, to “support the ever-increasing importance of GPS” in the U.S. capital, Washington, D.C. The organization subsumes and replaces both the U.S. GPS Industry Council, an entity of longstanding, and the Coalition to Save Our GPS, which arose in March 2011 in response to a Federal Communications Commission (FCC) conditional waiver granted to LightSquared.
The alliance appears to reflect a desire on the part of some industry members to take a more aggressive approach inside the Washington Beltway, a sign, it would seem, of the political times. Some of those involved spoke informally of a desire to take advantage of contacts made on Capitol Hill and in the media during the highly visible LightSquared combat, fought in the glare of media attention heretofore unknown in industry circles.
GPSv Innovation Alliance logo
Members of the Alliance are drawn from a variety of fields and businesses reliant on GPS, as well as leading manufacturers of GPS equipment. The former group includes, aviation, agriculture, construction, transportation, first responders, and surveying and mapping, and consumer organizations representing users of GPS for boating and other outdoor activities, and in automobiles, smartphones, and tablets.
Joining John Deere, Garmin, and Trimble — three lead drivers of the Coalition effort at the FCC — are NovAtel Inc. and Topcon Positioning Systems. All five were previously long-time members of the USGIC, and they appear as founding members of the alliance at www.gpsalliance.org.
Affiliate members listed on the website include the Association of Equipment Manufacturers, General Aviation Manufacturers Association, National Association of Manufacturers, Association for Unmanned Aerial Vehicles International, and Boat Owners Association of the United States.
The alliance plans to build on “the proud heritage and extensive expertise of the United States GPS Industry Council (USGIC), which was formed in 1991 to promote broader commercial applications of GPS and to expand global markets while assisting in safeguarding the technology’s military advantages. The council has a long history of highly effective advocacy on behalf of the GPS industry, as well as serving as a trusted source of objective information for policy makers, the media and the public both in the U.S. and around the world.” The alliance website gives a longer statement about the history and record of the USGIC, highlighting its role in international negotiations.
Michael Swiek, executive director of the USGIC, has transitioned to become the executive director, executive branch and international, of the Innovation Alliance. In addition to working closely with leading offices of executive branch departments of the U.S. government, he will continue well-established dialogs with governmental, private sector and academic entities in areas critical to GPS and satellite navigation among key players in Europe, Japan, Russia, Korea, China, and elsewhere.
Heather Hennessey, a principal of Innovative Federal Strategies LLC, a “comprehensive government relations firm,” has taken the position of executive director, legislative, at the alliance. Hennessey has seven years of service in the House of Representatives, including two years as chief of staff for Congressman Jack Kingston of Georgia.
An active voice in alliance representations on Capitol Hill will presumably be that of Jim Kirkland, vice president and general counsel for Trimble. Kirkland was the most prominent spokesperson for the coalition during the LightSquared battle, which appears to be either over or nearly so. “The alliance is committed to ensuring constructive, robust dialog between GPS users, manufacturers and policy makers on critical policy issues affecting GPS,” Kirkland said, “a commitment Trimble is pleased to be a part of as the industry continues to innovate and modernize.”
The alliance mission statement cites the importance of GPS to global economy and infrastructure; vows to aid further GPS innovation, creativity and entrepreneurship; and to protect, promote and enhance the use of GPS.
The GPS Innovation Alliance officially launched on February 13 with a reception on Capitol Hill, a traditional lobbying tactic that previous efforts had perhaps not envisioned. The organization has also hired a public relations firm, Prism Public Affairs, and commissioned a logo.
Galileo Lives to Fly Another Day, Budget Passed
European Union leaders approved a scaled-down budget in early February, with none of the cuts to the Galileo program that had been widely feared. The project, conducted by the European Space Agency (ESA) under close supervision of the European Commission (EC), will draw on funding of 6.3 billion euros (about $8.5 billion) from 2014 to 2020. The satellite navigation program held onto its requested revised budget of 6.3 billion euros, even as telecommunications research and broadband deployment projects, including another ESA pet project, the somewhat related Copernicus Global Monitoring for Environment and Security (GMES), underwent severe cuts. Galileo has already spent more than 3 billion euros ($4 billion), three times its original budget, to launch four of an envisioned 30-satellite constellation.
The EU deliberative system requires unanimous approval of budget decisions, so what smaller countries seek for their farmers or fishermen carries practically equal weight to the desire of industrial/aerospace giants like Germany, closely followed by France and the United Kingdom. Negotiation is a delicate matter indeed, and reached an impasse in November 2012; resolution came only after a 24-hour marathon session of talks. The total budget represents the first decrease in the European Union’s history; austerity is the watchword in a region beset with an ongoing bevy of international debt crises and serious recession in many of the smaller EU countries.
Galileo supporters within the European Commission, the EU’s policy-making arm, continued to maintain that Galileo will “open a whole new world” for business to develop applications, as Antonio Tajani, EC vice president stated recently. The program drew strong support, for once, from powerful backers in the EU administrative capital, Brussels, and among industrial and political interests in key member states: France, Germany, and for an exception Britain, often a proponent of deep cuts.
Negotiators helped Galileo’s chances by placing it in a research group labeled “Competitiveness for Growth and Jobs.” This category actually rose in budget allocation by nearly 40 percent over the last seven-year allotment.
The allocation should cover operational costs for EGNOS and Galileo, the completion of the initial Galileo constellation of 14, and early procurement stages of a full, or second-generation orbiting set of 30.
The program still faces an extremely unlikely date for the establishment of early services by the end of 2014. “Then, the market, as well as the governments of the Member States, will start increasing their interest and promoting further investments,” the ever-optimistic Tajani maintained.
The budget must still secure approval by the European Parliament. Its president, Martin Schulz of Germany has stated, “The further we step away from the Commission’s proposed figures, the more likely the proposal will be rejected. More and more tasks, and less and less money — the inevitable result is budget deficits. The Parliament will not go along with this.”
Parliament’s decision is forecast for the summer months. Parliament’s budget power consists of a direct yes-or-no vote to accept or reject the budget. The body cannot make modifications, and if rejecting would simply send it back to the EU ministers to begin all over again. The picture is further complicated somewhat by the 20-nation make-up of ESA, whereas the European Union and its executive commission have 27 national members.
Safer Skies for EGNOS
Results of a September 2012 flight test in the Galileo Test and Development Environment (GATE) near Berchtesgaden, Germany, the one place on Earth where Galileo services are already routinely available, show that adding Galileo signals to the European Geostationary Navigation Overlay Service (EGNOS) should boost accuracy significantly. EGNOS augments the accuracy and reliability of GPS signals over Europe, rendering satnav usable for safety-critical applications such as aircraft guidance, as well as more general precision uses.
Operational horizontal and vertical distance “protection levels” for safety were cut by half by combining use of GPS and Galileo within EGNOS. In addition, new integrity algorithms installed within the user receiver turned out to reliably detect and exclude reflected or otherwise faulty signals.
Next-generation EGNOS, planned for 2020, is envisaged to augment both constellations and dual frequencies at the same time, making the system much more robust.
GLONASS in Brazil
The first overseas GLONASS ground monitoring station for differential correction and monitoring outside Russian territory opened in Brasilia, Brazil, in mid-February. The station represents an early step in an initiative to modernize and significantly improve the accuracy of GLONASS signals.
Plans call for similar monitoring stations “in more than 30 countries of the world. Most of the countries that received the offers for the installation of the stations responded positively.However, the process is slow because of the need to conclude appropriate intergovernmental agreements. The documents with Brazil were signed in 2012. Agreements with Spain, Indonesia and Australia will be finalized soon,” according to a Pravda story.
European Union leaders approved a scaled-down budget in early February, with none of the cuts to the Galileo program that had been widely feared. The project, conducted by the European Space Agency (ESA) under close supervision of the European Commission (EC), will draw on funding of 6.3 billion euros (about $8.5 billion) from 2014 to 2020. The satellite navigation program held onto its requested revised budget of 6.3 billion euros, even as telecommunications research and broadband deployment projects, including another ESA pet project, the somewhat related Copernicus Global Monitoring for Environment and Security (GMES), underwent severe cuts. Galileo has already spent more than 3 billion euros ($4 billion), three times its original budget, to launch four of an envisioned 30-satellite constellation.
The EU deliberative system requires unanimous approval of budget decisions, so what smaller countries seek for their farmers or fishermen carries practically equal weight to the desire of industrial/aerospace giants like Germany, closely followed by France and the United Kingdom. Negotiation is a delicate matter indeed, and reached an impasse in November 2012; resolution came only after a 24-hour marathon session of talks. The total budget represents the first decrease in the European Union’s history; austerity is the watchword in a region beset with an ongoing bevy of international debt crises and serious recession in many of the smaller EU countries.
Galileo supporters within the European Commission, the EU’s policy-making arm, continued to maintain that Galileo will “open a whole new world” for business to develop applications, as Antonio Tajani, EC vice president stated recently. The program drew strong support, for once, from powerful backers in the EU administrative capital, Brussels, and among industrial and political interests in key member states: France, Germany, and for an exception Britain, often a proponent of deep cuts.
Negotiators helped Galileo’s chances by placing it in a research group labeled “Competitiveness for Growth and Jobs.” This category actually rose in budget allocation by nearly 40 percent over the last seven-year allotment.
The allocation should cover operational costs for EGNOS and Galileo, the completion of the initial Galileo constellation of 14, and early procurement stages of a full, or second-generation orbiting set of 30.
The program still faces an extremely unlikely date for the establishment of early services by the end of 2014. “Then, the market, as well as the governments of the Member States, will start increasing their interest and promoting further investments,” the ever-optimistic Tajani maintained.
The budget must still secure approval by the European Parliament. Its president, Martin Schulz of Germany has stated, “The further we step away from the Commission’s proposed figures, the more likely the proposal will be rejected. More and more tasks, and less and less money — the inevitable result is budget deficits. The Parliament will not go along with this.”
Parliament’s decision is forecast for the summer months. Parliament’s budget power consists of a direct yes-or-no vote to accept or reject the budget. The body cannot make modifications, and if rejecting would simply send it back to the EU ministers to begin all over again. The picture is further complicated somewhat by the 20-nation make-up of ESA, whereas the European Union and its executive commission have 27 national members.
Europe’s two satellite navigation systems could combine in the future for heightened performance, an airborne test has confirmed. A helicopter flight took place above an alpine valley in Germany, the one place on Earth where Galileo services are already routinely available.
The test receiver. The helicopter flew a variety of maneuvers, from fast loops to mid-air hovering, to see how satnav signals were received in practice.
Results of the flight test, conducted in September 2012, show that adding Galileo signals to the European Geostationary Navigation Overlay Service (EGNOS) should boost its accuracy significantly. EGNOS, which augments the accuracy and reliability of GPS signals over Europe, renders satnav usable for safety-critical applications such as aircraft guidance, as well as more general precision uses.
Operational horizontal and vertical distance “protection levels” for safety were cut by half by combining use of GPS and Galileo within EGNOS. In addition, new integrity algorithms installed within the user receiver turned out to reliably detect and exclude reflected or otherwise faulty signals.
The first test of real Galileo navigation fixes is scheduled for later this year from the four satellites already in orbit, with more satellites set to join them by the end of the year.
The Galileo Test and Development Environment – GATE – is a giant outdoor laboratory where prototype Galileo receivers can be used freely without any modifications.
As the constellation takes shape, satnav researchers and industrial developers can already try out Galileo services with prototype receivers at the German Galileo Test and Development Environment, or GATE, a giant outdoor laboratory. GATE, in and around the town of Berchtesgaden in the Bavarian Alps, is Europe’s go-to place for Galileo testing: transmitters atop eight neighbouring mountains cover 65 square kilometers of territory with simulated Galileo signals.
ESA’s Global Navigation Satellite System Evolution program carried out helicopter-based testing here on September 24–26. The results will help to guide the development of next-generation satnav systems.
The helicopter flew a variety of maneuvers, from fast loops to mid-air hovering, to see how satnav signals were received in practice. The test relied on ESA’s SPEED platform — Support Platform for EGNOS Evolutions & Demonstrations, co-funded by French space agency CNES and operated by Thales Alenia Space France — which enabled the receiver to receive simultaneous realtime augmentation for both GPS and Galileo.
Europe’s next-generation EGNOS, planned for around 2020, is envisaged to operate in the same way, with augmentation of both constellations and dual-frequencies at the same time making the system much more robust.
A helicopter flies over the Galileo Test and Development Environment – GATE – in Berchtesgaden, Germany, gathering data on how EGNOS and Galileo will work together. The promising results from the testing are now being analyzed.
BeiDou ICD: Signal Specs Are Free At Last; First Demonstration of Galileo-Only Positioning (By Peter Steigenberger, Urs Hugentobler, and Oliver Montenbruck)
BeiDou ICD: Signal Specs Are Free At Last
The interface control document (ICD) describing the details of the BeiDou B1I open service signal on 1561.098 MHz was released December 27 at a news conference held in Beijing by the Chinese State Council Information Office. The ICD includes details of the navigation message, parameters of the satellite almanacs, and ephemerides that did not appear an earlier, incomplete version of the ICD released at the end of 2011.
Beidou
An English version is available for download courtesy of the University of New Brunswick. The ICD specifies the relations of the signal in space interface between BeiDou Navigation Satellite System and users’ terminal receivers. It is the essential technical document to develop and make receivers and chips.
Anyone who has questions about the ICD is invited to submit them to [email protected].
The document, BeiDou Navigation Satellite System Signal In Space Interface Control Document — Open Service Signal B1I (Version 1.0), includes a system introduction, signal standards and navigation message, which defines the related contents of the open-service signal B1I between the BeiDou Navigation Satellite System and users’ terminals.
In a previous presentation given at the Seventh Meeting of the International Committee on Global Navigation Satellite Systems (ICG) in November, 2012, BeiDou officials stated that by 2020 there will be five GEO and 30 non-GEO satellites. The number of IGSO and MEO satellites was not specified, but previous presentations have said three IGSOs and 27 MEOs. These numbers are also stated in the official ICD.
“The GEO satellites are operating in orbit at an altitude of 35,786 kilometers and positioned at 58.75°E, 80°E, 110.5°E, 140°E and 160°E respectively. The MEO satellites are operating in orbit at an altitude of 21,528 kilometers and an inclination of 55° to the equatorial plane. The IGSO satellites are operating in orbit at an altitude of 35,786 kilometers and an inclination of 55° to the equatorial plane.”
The China Satellite Navigation Office presented a new official logo for the BeiDou system, with a yin/yang symbol representing Chinese culture, dark and light blue for space and Earth, and the Big Dipper constellation, symbolizing a long tradition of Chinese navigation since ancient times.
A spokesperson said the English name for China’s GNSS will be BeiDou Navigation Satellite System, abbreviated as BDS. The name Compass, which first designated the prototype regional system and has been employed in conjunction with the name BeiDou, will apparently now be discontinued.
Other salient details from the ICD include:
Signal Structure. “The B1 signal is the sum of channel I and Q which are in phase quadrature of each other. The ranging code and NAV message are modulated on carrier. The signal is composed of the carrier frequency, ranging code and NAV message.
“The B1 signal is expressed as follows:
S j (t) = A I C I j (t) D I j (t) cos (2 π f0 t φ j) + A Q C Q j (t) D Q j (t) sin (2 π f0 t + φ j)
where superscript j is the satellite number; subscript I equals channel I; subscript Q is channel Q; A is the signal amplitude; C the ranging code; D the data modulated on ranging code; f0 represents the carrier frequency; and φ the carrier initial phase.”
The nominal frequency of the B1I signal is 1561.098 MHz.
As is the norm with most other GNSSs, BeiDou’s transmitted signal is modulated by quadrature phase shift keying (QPSK). The transmitted signal will be right-handed circularly polarized (RHCP), and its multiplexing mode is code-division multiple-access (CDMA).
User-Received Signal Power Level. “The minimum user-received signal power level is specified to be -163 dBW for B1I, which is measured at the output of a 0 dB RHCP receiving antenna (located near ground), when the satellite’s elevation angle is higher than 5 degree.”
Bandwidth and Suppression. “Bandwidth (1 dB): 4.092 MHz (centered at carrier frequency of B1I); Bandwidth (3 dB): 16 MHz (centered at carrier frequency of B1I). Out-band suppression: no less than 15 dB on f0±30 MHz, where f0 is the carrier frequency of B1I signal.”
Ranging Code on B1I. “The chip rate of the B1I ranging code is 2.046 Mcps, and the length is 2,046 chips. The B1I ranging code (hereinafter referred to as CB1I) is a balanced Gold code truncated with the last one chip. The Gold code is generated by means of Modulo-2 addition of G1 and G2 sequences which are respectively derived from two 11-bit linear shift registers.”
NAV Message. “NAV messages are formatted in D1 and D2 based on their rate and structure. The rate of D1 NAV message which is modulated with 1 kbps secondary code is 50 bps. D1 NAV message contains basic NAV information (fundamental NAV information of the broadcasting satellites, almanac information for all satellites as well as the time offsets from other systems); while D2 NAV message contains basic NAV and augmentation service information (the BDS integrity, differential and ionospheric grid information) and its rate is 500 bps.
“The NAV message broadcast by MEO/IGSO and GEO satellites is D1 and D2 respectively.” The adjacent table from the BeiDou ICD gives information on nav message contents.
First Demonstration of Galileo-Only Positioning
By Peter Steigenberger, Urs Hugentobler, and Oliver Montenbruck
The European satellite navigation system, Galileo, is currently in its in-orbit validation (IOV) phase with a constellation of four satellites. The satellites, launched in pairs on October 21, 2011, and October 12, 2012, are representative of the full 30-satellite constellation. The IOV satellites will demonstrate that the satellites and the ground segment meet the system’s requirements and will validate the system’s design before completion of the rest of the constellation.
The IOV satellites have already started transmitting signals, and short periods of four-satellite visibility have allowed us to demonstrate, for the first time, absolute and relative positioning using measurements from Galileo operational satellites only. This follows the positioning demonstration last year with the signals from the Galileo IOV Element (GIOVE) test satellites and the first two IOV satellites. As in that earlier work, external orbit and clock information is necessary, since the IOV satellites were not transmitting valid navigation messages at the time of our study.
Three Javad GNSS Triumph-VS receivers with external antennas were set up at Technische Universität München (TUM) in Munich, Germany. The reference station TUME is equipped with a Javad GNSS RingAntG3T choke-ring antenna whereas the stations TUMW and TUMO are equipped with Javad GNSS GrAntG3T antennas. Unfortunately, all antennas are mounted near metal surfaces introducing pronounced multipath effects. The resulting baseline lengths are approximately 19.4 meters for TUME-TUMW and 101.7 meters for TUME-TUMO. Galileo satellite orbit and clock information was determined from stations of the Cooperative Network for GNSS Observation (CONGO) and the Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS). For GPS satellites, the rapid products of the Center for Orbit Determination in Europe (CODE) were used. All computations were performed with a modified version of the Bernese GPS Software 5.0.
Figure 1. Single-point positioning results for the TUME reference station based on E1/E5a dual-frequency pseudorange measurements of the four Galileo IOV satellites. The standard deviations in the north, east, and up directions are given. Note the different scale of the north component.
At a cutoff angle of 10 degrees, the four IOV satellites were jointly visible from TUM on January 6, 2013, for about two hours – from 04:16 to 06:09 UTC. Using an ionosphere-free dual-frequency linear combination of pseudorange measurements on the Galileo E1 and E5a frequencies, the position of the TUME reference station could be determined with a 3D position error of less than 1.5 meters (see Figure 1).
In addition to absolute positioning, relative positions between pairs of receivers were computed from Galileo E1, E5a, E5b, and E5 AltBOC single-frequency carrier-phase observations. Two GPS solutions covering the same time interval serve for comparison purposes. The first solution utilizes all visible GPS satellites (9 to 12 per epoch) whereas the second solution is intentionally limited to four satellites (G06, G16, G27, G29) for best comparison with the Galileo case. So-called kinematic-style processing was used where the baseline is not constrained to be unchanging and a relative-position solution is computed for each epoch of measurements. 3D standard deviations of the different solutions are listed in Table 1. The overall accuracies are at the level of a few centimeters.
Tabe 1. 3D position errors (standard deviation) of carrier-phase-based kinematic-style Galileo and GPS baseline solutions.
A slightly degraded performance is achieved for the TUMO-TUME baseline, which can be attributed to both the larger separation and the inferior multipath environment compared to the TUMW-TUME baseline.
Comparing the individual Galileo signals, the best relative positioning results were obtained for the E1 carrier-phase measurements. Interestingly, the use of carrier-phase measurements from the E5 AltBOC tracking yielded a lower performance in our test than use of either the E5a or E5b observations. Apparently, the carrier-phase tracking benefits less from the ultra-wideband signal than the code tracking, where AltBOC usually offers notably reduced noise and multipath. Besides their good performance for Galileo-only positioning, the E1 and E5a carrier-phase measurements will be particularly relevant for future relative positioning applications due to the possibility of mixed-constellation ambiguity resolution with GPS L1 and L5 signals.
For illustration, Figure 2 shows the Galileo E1 solution as well as the GPS L1 solution computed from four satellites. For the north component, the scatter of the Galileo solution is larger by a factor of two compared to GPS whereas it is on almost the same level for the east and up components as a result of the specific geometry of the satellites employed.
Figure 2. Kinematic positioning results for the TUMW-TUME baseline based on Galileo E1 (left) and GPS L1 (right) carrier-phase observations of four satellites. The standard deviations in the north, east, and up directions are given. Note the different scale of the north component.
With the recent testing of navigation messages on the first pair of IOV satellites, Galileo-based positioning as described in this article will not be limited to post-processing, but will be available to real-time users as well.
Peter Steigenberger is a staff member in the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München (TUM) in Munich, Germany.
Urs Hugentobler is the head of the Fachgebiet Satellitengeodäsie (Department of Satellite Geodesy) and the Forschungseinrichtung Satellitengeodäsie (Research Facility for Satellite Geodesy) at TUM.
Oliver Montenbruck is the head of the GNSS Technology and Navigation Group in the German Space Operations Center in Oberpfaffenhofen, Germany, and a TUM associate faculty member.
Beating up the backstretch neck and neck, tied for third in the GNSS race, Galileo and Compass today offer some signals and some satellites to GNSS users — as long as those users are researchers. Galileo has more going for it in the way of signals, while Compass holds an edge in the number of satellites. Without an interface control document (ICD) to guide user/researchers and most importantly manufacturers in the employment of its signals, Compass satellites, however they may increase, are practically useless to anyone outside China. A Compass ICD has been rumored before and is now rumored again. Wait and see before placing your bets.
The fourth Galileo in-orbit validation (IOV) satellite, Flight Model 4 (FM4), began transmitting signals on December 12, joining its co-launched confrère FM3, which began airing navigation signals on December 1. The FM4 spacecraft uses PRN code E20. As of this writing, FM3 is broadcasting E1, E5, and E6 signals, and FM4 is broadcasting E1 and E5 signals; we don’t know if and when FM4 E6 signals start(ed) until ESA tells us.
GPS World authors Oliver Montenbruck (German Space Operations Center) and Richard Langley (University of New Brunswick) have written an early analysis of the signals from FM3; this account will appear in the January issue of the magazine. A few selected excerpts from that article, and one figure:
“Anyone with commonly available GNSS receivers can presently access the open signals in the E1, E5a, and E5b frequency bands as well as the wide-band E5 AltBOC signal.
Figure 1: Pseudorange errors of IOV-3 tracking at Tanegashima, Japan, using the E1 BOC(1,1) signal (top) and the E5 AltBOC signal (center). The elevation angle over time is shown in the bottom panel.
“According to an ESA statement, FM3will continue to use binary offset carrier modulation — specifically BOC(1,1) — on the E1 Open Service signals for the time being. In contrast to this, the first pair of IOV satellites has already started to use composite binary offset carrier modulation, which offers better multipath suppression in the received signal.
“The E5 AltBOC pseudorange measurements in particular exhibit an exceptionally low noise and multipath level of better than 10 centimeters at mid- and high-elevation angles.”
After discussing and displaying some carrier-phase measurements of the Galileo FM3 E1, E5, and E6 signals, Montenbruck and Langley conclude; “This level of performance highlights the potential benefit of Galileo signals in advanced triple-frequency techniques such as undifferenced ambiguity resolution and ionospheric monitoring.”
Theoretically, the total of four Galileo IOV satellites now in medium-Earth orbit yield the minimum number needed to perform a 3D navigation fix, although no statement of initial — or even sketchy — operating capability has been issued by the European Space Agency (ESA), nor is one expected.
Antonio Tajani, vice-president of the European Commission (EC) and head of the EC directorate-general responsible for industry and entrepreneurship, continues to publicly maintain a “political objective [of] the delivery of the first services before the end of 2014,” based on 18 orbiting satellites. In a December speech, he revised the basis for that position slightly to say the civil Open Service (OS) could be declared operational with as few as 12 satellites.
The system operators had announced three dual-satellite launches in 2013, two dual-satellite launches and one four-satellite launch in 2014, hypothetically producing an operable constellation of 18 satellites by the end of the promised 2014. However, unconfirmed reports from Europe suggest that problems with manufacture of the next set of 14 Galileo satellites mean that no launches at all will take place until Q4 of 2013. Whether this will push out the service delivery date beyond 2014 or not remains open to conjecture.
Compass
Another matter open to conjecture and much speculation is whether the world will soon — or ever — see an interface control document (ICD) for China’s Compass system. More than a year ago, I wrote that “The ICD has been rumored to be available previously to receiver manufacturers within China, creating some disgruntlement among companies outside the country . . . GPS/Compass chips and receivers are being actively developed by many Chinese manufacturers and research institutes.” Indeed, conference presentations, leading to a published article in this magazine’s October issue, “What Is Achievable with the Current Compass Constellation,“ confirm that this is so.
And yet, the rest of the world neither has nor holds a Compass ICD.
The end-of-year rumor mill has kicked into gear again, though. A GNSS industry representative stationed in Shanghai, China sent this message recently to a U.S. colleague: “Latest unofficial news said that the Compass Interface Control Document (ICD) will be released on 27th this month, and will be available on the internet on 28th.”
Activities of the European Navigation Support Office
Headshot: Werner Enderle
By Werner Enderle
The European Space Operations Centre (ESOC) in Darmstadt, Germany operates spacecraft on behalf of the European Space Agency (ESA) and maintains the ground facilities and expertise for ESA and other institutional and commercial customers. ESOC is composed of two departments: the Mission Operations Department and the Ground Systems Engineering Department, of which the Navigation Support Office is an integral part. The main objectives of the Navigation Support Office (NSO)are the provision of expertise for high-accuracy navigation, satellite geodesy, and the generation of related products and services for all ESA missions and for third-party customers, as well as supporting the European GNSS Programmes: Galileo and EGNOS.
In 2013, the NSO will conduct a number of projects and activities, described here.
European GNSS
The Navigation Office provides support in the area of data processing and analysis, performance analysis. It performs operational orbit predictions for the International Satellite Laser Ranging Service (ILRS), operational precise/rapid orbit and clock determination, computation of antenna patterns, and provides support to Galileo Sensor Stations (GSS) site deployment and to Ranging and Integrity Monitoring Station (RIMS) deployment. It also provides consultancy on modeling and data processing, mission analysis for the constellation, orbit validation activities for orbits and clocks, ionosphere, group delays, and intersystem biases, and is involved in the generation of the Galileo Geodetic Reference Frame. Furthermore, the Office participated in European Commission studies for the Galileo Commercial Service.
Earth Observation Missions
A number of European and American missions have been equipped with radar altimeter instruments that observe the level of the sea surface from space. To do this, the height component of the satellite orbits needs to be determined with centimeter-accuracy, matching the accuracy of the altimeter observations. The NSO provides support to Precise Orbit Determination (POD), evaluation, analysis and improvement of models and standards, as well as instrument calibration (radar altimeter and GNSS antenna).
Examples of missions already supported include ERS, Envisat, Cryosat, GOCE and also non-ESA missions JASON 1&2. Solutions with multiple simultaneous data types (GNSS, SLR, DORIS, altimetry, S-band range, Doppler, and angle tracking) are typically performed, allowing the alignment of different reference frames and estimation of inter-system and instrument biases. Based on all these capabilities, the NSO is one of the leading institutions for low-Earth orbiting (LEO) satellite POD activities and very well suited for supporting the upcoming European programme for Earth Observation, called Global Monitoring for Environment and Security (GMES) and its related Sentinel satellite missions.
Automated Transfer Vehicle
The Automated Transfer Vehicle (ATV) is part of the European contribution to the International Space Station (ISS) program. The main tasks of the ATV are to provide logistics supply, station re-boost and ISS waste retrieval. The rendezvous of the ATV and ISS is based on a real-time on-board relative navigation concept, using GPS data from receivers of ISS and ATV. The NSO conducts in this context simulations before the flight and also post facto performance analysis of the relative orbit determination accuracy to support the ATV missions.
Space Situation Awareness
An important atmospheric application of GNSS data is the monitoring of ionospheric activity (total electron content or TEC). Dual-frequency GNSS signals enable direct measurement of this parameter, and by merging the data from hundreds of globally distributed GPS receivers, detailed maps of the TEC and its evolution as a function of time can be constructed. Such maps have been computed routinely for many years. FIGURE 1 shows an example. The importance of these products lies in the fact that high solar activity leads to high TEC values, which can seriously disturb satellite communications. The NSO provides ionospheric TEC maps to the scientific community.
International GNSS Services
ESA/ESOC was one of the founding members of the IGS, and at the time the NSO was implemented at ESOC, all of the IGS activities were transferred to the NSO. ESA Analysis Centre products are among the best products available from the individual IGS analysis centres. Secondly, the ESA products are among the few multi-constellation GNSS products. ESA was the first IGS analysis centre to provide a consistent set of orbit and clock products for all available GNSS satellites. These products constituted the very first products that have been used for true GNSS precise point positioning.
The sampling rate of the ESA final GPS+GLONASS clock product is 30 seconds. FIGURE 2 shows the statistics of a kinematic PPP analysis using the ESA GNSS clocks for three different cases. The ESA/ESOC IGS Analysis centre contributes to all of the core IGS analysis centre products: Final GNSS (GPS+GLONASS) products provided weekly based on 24-hour solutions using 150 stations from true GNSS solutions simultaneously and fully consistently processing GPS and GLONASS measurements for a total of around 55 satellites, consisting of orbits, clocks, coordinates, ionosphere, and Earth-orientation parameters (EOPs). Also Rapid GNSS (GPS+GLONASS) products (available within 3 hours after the end of the observation day) and Ultra-Rapid GNSS (GPS+GLONASS) products (4 times per day, available within 3 hours after the end of the observation interval) are provided. These products are publicly available to the scientific community, being published at several data servers, such as the CDDIS at NASA’s Goddard Space Flight Center. They are also finding very frequent application in testing of experimental and commercial applications, and have become the standard reference for all high-precision GNSS applications.
Figure 2. Kinematic PPP analysis using ESA GNSS clocks: GLONASS-only PPP (red); GPS-only, (green), and a truee GNSS-PP (blue).
Third-Party Activities
Different customers have different needs. One important customer for the Navigation Facility is the Metop mission operated by EUMETSAT. For the exploitation of its GNSS Receiver for Atmospheric Sounding (GRAS) payload, which delivers atmospheric profiles to the European Met offices, EUMETSAT requires GPS products with a guarantee on accuracy, availability and latency. To deliver this service, the Navigation Facility now hosts the operation of the GRAS Ground Support Network (GSN), which is a dedicated network of 45 stations. It has been operating successfully for five years, delivering products with a latency of only 45 minutes, and an availability of better than 99 percent. Based on these, EUMETSAT delivers a daily set of more than 500 atmospheric profiles (and double that number as soon as Metop-2 will be operational) to the European Met offices, a data set that has already become one of the key elements in numerical weather prediction.
Real-Time Processing
Over the last 10 years, ESOC has embarked on a program to build a Real Time GNSS software infrastructure. The main justification for this effort is the realization that the delivery of precise GNSS products in real-time processing will become increasingly more important for the user community. ESOC needs to be at the forefront of these developments, particularly with respect to products related to Galileo. The system for REal TIme NAvigation (RETINA) has been modelled after ESOC’s experience in real-time satellite control systems and includes many of the elements for data processing, archiving, and visualization that are common to such systems. In particular, it implements a specially designed circular filing system for streaming data, allowing maintenance-free operations for processing and archiving of data and products, and seamless transitions from historical to live data processing.
The investment in GNSS software and receiver infrastructure has enabled ESOC to participate in the IGS Real Time Pilot Project, assuming the roles of Real Time Analysis Centre and Analysis Centre Coordinator. In the latter role, ESOC has been generating and disseminating the IGS Real Time Combination stream after processing the real-time solutions from up to ten analysis centres. Included in these solutions are two streams generated by the ESOC Real Time Analysis Centre.
Standardization Activities
Participation in the IGS Real Time activities has stimulated ESOC’s involvement in the development of standards and formats for GNSS data and products. ESOC has been instrumental in the decision of the IGS to join the Radio Technical Commission for Maritime Services (RTCM), which is the primary standards setting organisation for real-time GNSS services. ESOC is now one of two agencies that represent the IGS at the RTCM meetings.
Work with the RTCM focuses on:
development of standards and formats for transmission of multi-constellation observations in real time (RTCM-MSM);
development of standards and formats for the transmission of real-time orbit and clock products (RTCM-SSR);
Further development of the RINEX standard for generation of multi-GNSS batch observation files.
Expertise and Areas of Activities
To comply with the main objectives of the NSO, the main pillars of expertise and areas of activities can be summarized as:
Precise orbit determination at centimeter-level accuracy for satellites in low-Earth orbits such as Earth observation missions, and satellites in medium-Earth orbits, typically GNSS satellites.
Development of state-of-the-art models and algorithms for high-precision orbit and clock determination, based on the capability to process all geodetic data types, namely GNSS, satellite laser ranging, Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), altimetry, and S-band tracking data.
Realization of Geodetic Reference Frame.
Operation of global distributed real-time sensor stations and networks, based on remote control of GNSS receivers.
The capability to operate complex navigation software infrastructure to generate operational products and services for a wide variety of applications.
Involvment in several international organized and coordinated activities. Besides being an IGS analysis center, ESOC’s NSO is also an analysis centre for the IDS and ILRS services.
Operational Facility
ESOC’s ESOC’s Navigation Facility (see FIGURE 3) provides a fully operational environment, compliant with ESA’s ECSS ground segment standards. The Navigation Facility consists of a control room including secure operational LAN (ESACERT against intruders from outside) with two physically separated computer and data centres for redundancy purposes and a globally distributed operational real time sensor station network (see FIGURE 4). An operational system availability of more than 99.9 percent on a 24/7 basis measured over the last 5 years (products delivered every 15 minutes) has been demonstrated.
Currently the sensor station network consists of 12 sites, but ESOC is extending the global network to at least 25 sites. Negotiations with new sites are currently ongoing or near completion. The objective is to deploy a homogeneous (all sites will have the same receiver and same antenna type) sensor station network by the third quarter of 2013. The deployment of new equipment on existing sites began in April 2012, and first results are very promising. The new type of geodetic quality GNSS receiver has been chosen, based on an internal selection process, and deployment is under way. Each receiver has 264 physical channels, is capable of multi-signal, multi-frequency and multi-constellation tracking and will be remotely controlled from the Navigation Facility at ESOC.
Software Packages
The NSO develops, maintains and operates a range of software packages and tools for high-precision orbit- and clock determination and prediction. The software capability also includes the estimation of station coordinates, Earth-orientation parameters, model parameters (radiation pressure, drag, and so on), ionosphere, troposphere, instrument biases, intersystem biases, ambiguities and antenna phase-centre variations based on state-of-the-art models and standards (for example, IERS, ITRF).
The main software packages used within the NSO are:
NAPEOS, which is the ESOC standard for high-precision navigation tasks. NAPEOS is used for almost all projects and is compliant with the highest navigation accuracy requirements, based on batch processing techniques with the capability to process different types of geodetic observations.
RETINA, the NSO’s real time software package for GNSS based precise navigation. This software is based on Kalman Filter techniques and has a closely coordinated interface to NAPEOS.
IONMON, processing GNSS data and producing ionosphere information and TEC map predictions.
In this context it is important to mention that ESA owns all the intellectual property rights to these software packages and that licences for operationally qualified software can be released on request to European companies, universities and R&D 0rganisations (currently only NAPEOS).
Summary and Outlook
The Navigation Support Office offers a combination of different capabilities, namely highest quality software, tools for real-time and batch processing ( the Office is the only analysis centre capable of processing three different geodetic techniques within a single software package), operation of own global GNSS sensor station network and demonstrated operational experience for mission support and provision of services. Operations are conducted in a controlled environment, fully in accordance with ESA safety and security standards.
The Navigation Support Office is ready for multi-frequency, multi-signal and multi constellation GNSS data processing. The Office is involved and strongly committed to support Galileo and EGNOS. In this context, the Office will soon become the consortium leader for the provision of the Galileo Geodetic Reference Frame.
Concerning the participation to international GNSS activities like IGS, ICG and GNSS standardisation aspects, the Navigation Support Office intends to continue its support for the foreseeable future.
In the area of LEO POD, the Navigation Support Office offers POD capability for all types of LEO satellites. For this reason, the Office intends to play a major role in the precise orbit determination activities for the European GMES Sentinel satellite missions.
Finally, the Navigation Support Office also intends to increase its capabilities related to navigation concepts for high-precision satellite formation flying and satellite constellations, via specific research and development activities. The aim is to maintain and expand its capabilities as a very attractive partner with cutting edge know-how and technology for the support of ESA activities and European industry.
Werner Enderle is the head of the Navigation Support Office at ESA\ESOC. Previously, he worked at the European GNSS Authority (GSA) as the Head of System Evolutions. He also worked for the European Commission, in charge of the procurement for the Galileo Ground Control Segment. He holds a doctoral degree in aerospace engineering from the Technical University of Berlin, Germany.
Co-authors: Rene Zandbergen, Tim Springer, and Loukis Agrotis.
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.
The Soyuz ST-B launcher carrying the next two Galileo In-Orbit Validation satellites took off as scheduled on 18:15:00 GMT (11:15 PDT) October 12. Deployment of its twin satellites into orbit took place 3 hours 44 minutes after take-off. All the stages of the Soyuz vehicle performed as planned and the
Fregat-MT upper stage released the Galileo satellites into their targeted orbit at close to 23 200 km altitude.
CANSPACE Listserv reports, “NORAD/JSpOC are tracking three objects from the launch:
“Presumably, the first two (A and B) are the Galileo satellites. They are drifting towards their designated orbits.”
The European Space Agency (ESA) launched this second pair of Galileo IOV satellites from Europe’s Spaceport in French Guiana.
This flight is designated VS03 in Arianespace‘s mission numbering system, and it was the Spaceport’s third launch since Soyuz was introduced at this near-equatorial facility one year ago. Arianespace is the launch contractor.
The two Galileo satellites will join the first two spacecraft orbited by Arianespace’s historic VS01 flight on October 21, 2011, marking Soyuz’ introduction at the Spaceport. Once all four are operational in space, they will provide the minimum number of satellites required for navigational fixes — enabling system validation testing when all are visible in the sky.
As a European initiative, the Galileo satellite navigation system is being developed in a collaborative effort of the European Union and the European
Space Agency. The In-Orbit Validation (IOV) satellites weigh 700 kg. each and were built by a consortium led by the Astrium division of EADS — which
produced the platforms and has responsibility for the payloads, while Thales Alenia Space handled the assembly and testing tasks.
The European Union (EU) and China will be meeting in December in Paris to discuss overlapping radio frequencies both plan to use for their future encrypted government/military satellite navigation services, according to a joint statement from both parties, reports Space News.
The December meeting will be conducted under what the Joint Statement on Space Technology Cooperation specifies as the ITU Framework. ITU is the International Telecommunication Union of Geneva, a United Nations affiliate that regulates satellite orbital slots and frequencies.
The statement was signed as an annex to a broader EU-China summit held September 20 in Brussels. As Space News reports, the two sides are continuing collaboration on satellite navigation despite the signal conflict, which has been a subject of debate for at least two years. The 27-nation EU and China have agreed to continue the China-Europe GNSS Technology Training and Cooperation Center.
