Two more Galileo satellites have reached Europe’s Spaceport in French Guiana, joining the first pair of navigation satellites and the Ariane 5 rocket due to haul the quartet to orbit this December.
Inside the 747. (Photo: ESA)
Galileos 21 and 22 left Luxembourg Airport on a Boeing 747 cargo jet on the morning of Oct. 17, arriving at Cayenne-Félix Eboué Airport in French Guiana on the same day.
Resting within distinctive white air-conditioned containers, the satellites were driven to the cleanroom environment of the preparation building within the space centre.
Waiting for them there were Galileos 19 and 20, which arrived in September.
The four satellites will be launched together in mid-December by a customised Ariane 5, the elements of which reached French Guiana last month by sea.
Galileos 21 and 22 being unloaded from their 747 cargo aircraft at Cayenne – Félix Eboué Airport in French Guiana on Oct. 17. (Photo: ESA)
Galileo is Europe’s own satellite navigation system, providing an array of positioning, navigation and timing services to Europe and the world.
A further eight Galileo Batch 3 satellites were ordered last June, to supplement the 26 built so far.
With 18 satellites now in orbit, Galileo began initial services on Dec. 15, 2016, the first step towards full operations.
Further launches will continue to build the constellation, which will gradually improve performance and availability worldwide.
Europe’s next two Galileo navigation satellites have touched down in Europe’s Spaceport in French Guiana ahead of the launch of a quartet by Ariane 5 at the end of this year (scheduled for Dec. 12).
Galileos 19 and 20 left Luxembourg Airport on a Boeing 747 cargo jet on the morning of Sept. 18, arriving at Cayenne — Félix Eboué Airport in French Guiana that evening.
Safely cocooned within protective air-conditioned containers, the pair were offloaded and driven to the cleanroom environment of the preparation building within the space centre.
A Galileo satellite in its protective container is unloaded from its cargo plane after landing in French Guiana Sept. 18. (Photo: ESA)
This building will remain their home as preparations for their launch proceeds, with the next two Galileos due to join them later this month.
The satellites join the first elements of their customised Ariane 5 at the centre — including its cryogenic main stage and half-shell payload fairing — which were delivered by ship the week before.
Galileo is Europe’s own satellite navigation system, providing an array of positioning, navigation and timing services to Europe and the world.
A further eight Galileo “Batch 3” satellites were ordered last June, to supplement the 26 built so far.
A Galileo satellite is driven to the Guiana Space Centre following its arrival on Sept. 18. (Photo: ESA)
With 18 satellites now in orbit, Galileo began initial services on Dec. 15, 2016, the first step towards full operations.
Further launches will continue to build the constellation, which will gradually improve performance and availability worldwide.
By Carmela Ruta, Francesco Paggi and Monica Gotta, Thales Alenia Space-Italia; D. Oskam, Airbus Defence and Space; and Rafael Lucas Rodriguez and Igor Stojkovic; European Space Agency / Presented at the European Navigation Conference, Switzerland, May 2017
The European Space Agency, Thales Alenia Space-Italy and Airbus Defence and Space contributed to the Search and Rescue/Galileo Forward Link system deployment and performance evaluation with a full-scale System Performance Validation test campaign, aimed at evaluating the performances of the SAR/Galileo system, in terms of distress detection rate, localization probability and localization accuracy.
Forward Link Message Detection probability in 10 minutes.
The paper describes SAR/Galileo principles and the COSPAS-SARSAT MEOSAR concept (detection and localisation of distress events based on MEO satellites). It presents the space and ground segments of the Galileo infrastructure that enables the SAR/Galileo Forward Link Service provision and the main inherent performances of the system. The SPV test campaign is described in terms of objectives and organization; the main results are presented, and the foreseen milestones for SAR/Galileo deployment are summarized.
Global availability of 5 km beacon localisation accuracy (95%) in 10 minutes.
At Intergeo 2017, Septentrio debuted the Altus NR3: a multi-frequency, quad-constellation (GPS, GLONASS, BeiDou and Galileo) RTK receiver for survey and GIS applications.
The Altus NR3 features Septentrio’s AIM+ interference mitigation and monitoring system, allowing continued operation in the presence of both intentional and non-intentional interference. According to the company, it combines advanced GNSS features with a robust communications suite in a compact, low-power and easy-to-use unit.
The Altus NR3 is configurable as either a rover or a base station. It offers one-touch logging and Septentrio’s on-board web interface so users can monitor and configure the unit as well as collect data using any Wi-Fi-capable device.
Data collection is done using either SurvCE or Septentrio’s PinPoint Data Collector with data updating to the cloud. Septentrio’s open interface and fully documented data formats are widely supported, making the Altus NR3 easy to integrate into any existing workflow, the company said.
“We’ve built on the flexibility, reliability and ease-of-use that our Altus line is famous for, and we’ve added all-in-view RTK and the most the most advanced interference mitigation system on the market today,” said Gustavo Lopez, product manager at Septentrio. “Locations with bad visibility or at risk of interference that were previously off limits can now benefit from high-precision GNSS positioning, saving both time and cost.”
Arianespace will launch four new satellites for the Galileo constellation, using two Ariane 62 versions of the next-generation Ariane 6 rocket from the Guiana Space Center in French Guiana.
The Ariane 62 rocket. (Image: Arianespace)
The contract will be conducted by the European Space Agency (ESA) on behalf of the European Commission (DG Growth) and the European Union.
This is the first ESA first contract to use the company’s new rocket.
Stéphane Israël, Arianespace chief executive officer, and Paul Verhoef, director of Navigation at the European Space Agency (ESA), signed the launch contract for four new satellites to join the European satellite navigation system Galileo. The contract will be conducted by ESA on behalf of the European Commission (DG Growth).
These launches are planned between the end of 2020 and mid-2021, using two Ariane 62 launchers — the configuration of Europe’s new-generation launch vehicle that is best suited for the targeted orbit. The contract also provides for the possibility of using the Soyuz launch vehicle from the Guiana Space Center, if needed.
Both missions will carry a pair of Galileo spacecraft to continue the constellation deployment for Europe’s satellite-based navigation system. The satellites, each weighing approximately 750 kg, will be placed in medium earth orbit (MEO) at an altitude of 23,222 kilometers and be part of the Galileo satellite navigation constellation.
An ESA video about Ariane 6 is below:
Galileo is the first joint infrastructure financed by the European Union, which also will be the owner. The Galileo system incorporates innovative technologies developed in Europe for the greater benefit of citizens worldwide.
A total of 18 Galileo satellites already are in orbit. Fourteen of these satellites were launched two at a time by Soyuz launchers, with the last four orbited on a single Ariane 5 ES mission in November 2016. Two more Ariane 5 ES missions are planned on December 12, 2017 and in the summer of 2018.
Following the signing of this latest contract, Stéphane Israël, CEO of Arianespace, issued this statement:
“Arianespace is especially proud to have won this first launch contract for the Ariane 6 from its loyal customers and partners, the European Commission (DG Growth) and ESA. We are very pleased to have earned this expression of trust from the European Commission; by choosing to continue the deployment of the Galileo constellation with two Ariane 62 launches, they become the first confirmed customer for our next-generation heavy launcher, which is slated to make its initial flight in the summer of 2020. Through this decision, which adds two additional launches to follow the already-scheduled Ariane 5 ES flights, the European Commission and ESA are clearly indicating a key commitment to Arianespace’s next generation of launchers, which reaffirms more than ever its mission to ensure Europe’s autonomous access to space.”
Enclosed in its protective container, Galileo Full Operational Capability (FOC) Flight Model 21 (FM21) is seen departing ESA’s ESTEC Test Centre on Aug. 24. Photos courtesy of the European Space Agency
News from the European Space Agency
The last of 22 Galileo satellites has departed the European Space Agency’s (ESA) Test Centre in the Netherlands. This concludes the single longest and largest scale test campaign in the establishment’s history, ESA said.
Cocooned in a protective container for its journey — equipped with air conditioning, temperature control and shock absorbers — the final Galileo satellite left the establishment by lorry on Aug. 24.
ESA’s Test Centre at ESTEC in Noordwijk, the Netherlands, houses a collection of test equipment to simulate all aspects of spaceflight. It is operated for ESA by private company European Test Services (ETS) B.V.
In May 2013, the Test Centre began testing the first of 22 Galileo “Full Operational Capability” (FOC) satellites, having previously performed the same function for the very first Galileo “In-Orbit Validation” satellite under a separate contract.
Pictured is a Galileo Full Operational Capability satellite being removed from the Phenix thermal vacuum chamber after a fortnight-long “hot and cold” vacuum test.
The Galileo FOC satellites had their platforms built by OHB System AG in Germany, incorporating navigation payloads coming from Surrey Satellite Technology Ltd. in the United Kingdom. They then traveled on to ESTEC to be subjected to the equivalent vibration, acoustic noise, vacuum and temperature extremes that they will experience for real during their launch and orbit, plus testing of their radio systems.
With a steady stream of satellites coming off the production line, the challenge for the combined ETS and OHB team overseeing Galileo testing was to put them through all necessary tests on a rapid and efficient basis, while also keeping the Test Centre accessible to other European missions requiring its unique services.
A total of 14 FOC satellites have since joined the first four IOV satellites in orbit, forming an 18-strong constellation that began Initial Services to global users on Dec. 15, 2016. The next four FOC satellites are scheduled for launch on an Ariane on Dec. 5.
Europe’s Galileo navigation satellites orbit 23 222 km above Earth to provide positioning, navigation and timing information all across the globe.
“For the first time in more than four years, there are no Galileo satellites in the Test Centre, but hopefully this will not be the end of our association with the programme,” said Jörg Selle, managing director for ETS. “The contract for making the next eight Galileo satellites — known as Batch 3 — was also awarded to OHB last June, and ETS will be bidding for the contract to test these satellites too.”
“The availability of the ETS facilities in ESTEC have substantially contributed to the programme,” said Paul Verhoef, ESA director of the Galileo Programme and navigation-related activities. “We thank ETS for their professionalism and support over this extended period.”
The final Galileo travelled back to OHB in Germany for some final refurbishment ahead of its launch together with another three satellites in December.
QZS-2 L-band spectra, July 18, 2017, Weilheim, Germany. (Courtesy DLR)
Second QZSS Signal on Air
The successful launch of the Michibiki No. 2 satellite of the Quasi-Zenith Satellite System (QZSS) on June 1 has been followed by broadcast initiation. Researchers at the German Aerospace Center, Deutsches Zentrum für Luft- und Raumfahrt (DLR), have been observing the satellite from their ground station in Weilheim. They will provide a written analysis in the September issue.
The Japan Aerospace Exploration Agency launched first Michibiki satellite of the anticipated four-satellite constellation in September 2010.
Air Force to Recompete GPS III Follow-on
The U.S. Air Force will launch multibillion-dollar competition between current GPS III contractor Lockheed Martin Corp. and former GPS Block I and Block II contractor Boeing Co. for as many as 22 new GPS III satellites. At press time, an industry day in was scheduled for July 20 in El Segundo, California, to solicit company input, according to a new draft Request For Proposals.
In 2015 the Air Force undertook the first phase of a now two-year process to determine whether to put the next block of satellites up for competition. An initial review “has determined that viable, low-risk, high-confidence sources exist to conduct a full and open competition” for a second phase starting in fiscal 2018, according to the draft.
Lockheed Martin is assembling the first 10 satellites of the Block III program. Formal delivery of the first satellite was scheduled earlier this year, delayed by of a series of now-resolved problems with the navigation payload, cracked capacitors and a subcontractor gaffe last year that resulted in the wrong part being tested.
The satellite, which passed all of its qualification testing and verification, has been placed in storage pending the results of an unrelated review of the propulsion systems used to boost military satellites into orbit. The plan remains to launch the first GPS III satellite by spring of 2018.
“Lockheed Martin is working closely with the Air Force on resolving any concerns about the mission readiness of SV01’s Propulsion Subsystem,” Eschenfelder said in February. “We are confident that this review will not delay the Air Force’s planned spring 2018 Initial Launch Capability (ILC).”
NAVIC Clock Failures Resemble Galileo’s
The seven orbiting satellites of the Navigation Indian Constellation (NAVIC, formerly India’s Regional Navigation Satellite System, or IRNSS) have been hit by problems with some of their rubidium atomic clocks, similar to difficulties encountered earlier by Europe’s Galileo program.
NAVIC G-1 launch April 2017.
The Indian Space Research Organization (ISRO) had announced in July 2016 that all three atomic clocks on IRNSS-1A, launched in 2013, had malfunctioned, rendering that satellite ineffective.
Now, reports indicate that four more atomic clocks on the other six satellites launched more recently are not performing as required.
ISRO plans to launch a replacement satellite called IRNSS-1H in July-August to compensate for the loss of IRNSS-1A, although it is yet to announce the failure of more atomic clocks, which has not incapacitated the clock systems on the other six satellites.
The European Space Agency reported in January that anomalies had occurred in three of 36 Rubidium Atomic Frequency Standard (RAFS) clocks in the 18-satellite Galileo system, although none of the satellites were affected. ESA had said, “These failures all seem to have a consistent signature, linked to probable short circuits, and possibly a particular test procedure performed on the ground.”
ISRO has nine satellites indented for IRNSS. While seven satellites make up the Indian regional navigation constellation, the other two were indented as backup in the event of failure. Each satellite has three atomic clocks, one the primary timekeeper and the other two acting as backup.
“Measures are being taken to correct the problems caused by the clocks in the launch of future satellites. The atomic clocks to be used in the other satellites have been modified to prevent malfunction,” a senior official in the programme said.
ISRO chairman Kumar has indicated the number of satellites could go up from the originally envisaged seven to 11 but it is not clear if this is a consequence of the failing clocks. “We are set to launch more navigational satellites. They are in the process of approvals and clearances,” he said recently, and added efforts were on to revive the IRNSS-1A clocks.”
In Europe, the European Space Agency and an industrial partner-supplier have agreed that “some refurbishment is required on the remaining RAFS clocks” to be used in new Galileo satellites.
Look to GSA Service Centre for Galileo Advisories
In July, a wide transfer of responsibilities for the Galileo constellation took place, from the European Space Agency (ESA) to the European Global Navigation Satellite System Agency (GSA) of the European Union. Key among these was a handover of communications responsibilities to manufacturers, users and markets.
All parties can now find updates in the form of Notice Advisory to Galileo Users (NAGUs) at the GSA’s Galileo Service Centre, www.gsc-europa.eu/system-status/user-notifications.
NAGUs are issued as new satellites are launched and when satellites become ready for service provision, or to give advance warning of signal unavailability owing to planned maintenance or testing activities, or to notify users of unplanned outages and then to inform them when satellites become active again.
“Keeping our users in the picture on planned activities that might lead to satellite unavailabilityhas helped them to plan their own test activities and to prepare future products,” said Rafael Lucas Rodriguez, ESA’s Galileo services engineering manager.
A total of 189 NAGUs were issued under ESA oversight in the last four years, as the constellation grew to its current 18 satellites. The user base increased from 86 to 774 registered users on the European GNSS Service Centre website as companies worked to prepare Galileo-ready products. In December 2016, Galileo’s Initial Services began operating.
One regular consumer of Galileo NAGUs, Broadcom, uses them to organize engineering activities and tests as well as input them into its orbit prediction engine for its Long Term Orbits products.
Figure 1. Galileo constellation and occupation status of orbital slots (RAAN: right ascension of the ascending node, May 9, 2017). (Source: ESA)
What to Expect with the Current Constellation
This article demonstrates the benefits of Galileo integration for high-precision real-time kinematic (RTK) through representative case studies, considering baseline length, multipath impact and tree canopy.
The results confirm usability of the current Galileo constellation in high-precision RTK applications and show improved availability, accuracy, reliability and time-to-fix in difficult measuring environments.
Plus, Galileo-only RTK positions are compared with GPS-only and GLONASS-only solutions.
By Xiaoguang Luo, Jun Chen and Bernhard Richter, Leica Geosystems AG
Until now, based on simulated and observed data, the benefits of Galileo (FIGURE 1) for high-precision RTK have been investigated in single-base RTK and network RTK solutions. Building on the results of previous studies that frequently employed theoretic analysis and simulation, we present the benefits of Galileo for high-precision RTK based on real observations from the current Initial Operational Capability (IOC) satellite constellation. Using up-to-date real-time corrections including Galileo, we analyze the performance of network RTK under different measuring conditions with respect to availability, accuracy, reliability and time-to-fix.
To achieve the maximum inter-operability with other GNSS con-stellations, all the Galileo signals in the E1 and E5 band, i.e. E1, E5a, E5b and AltBOC (alternative binary offset carrier), are used for positioning in the latest proprietary firmware and receivers (see “Manufacturers” section for details).
The Galileo E1 signal is overlapped with the GPS L1 signal at a center frequency of 1575.420 MHz, whereas the Galileo E5a and GPS L5 signals are overlapped at 1176.450 MHz. As far as BeiDou is concerned, the E5b frequency of Galileo corresponds to the B2 frequency of BeiDou-2 at 1207.140 MHz.
The AltBOC signal is also supported in order to benefit from its superior performance in multipath suppression. The availability of more than two frequencies is beneficial for ionospheric modeling, which plays an important role in ambiguity resolution on the fly.
In addition, multi-frequency RTK provides more immunity to temporary interruption of GNSS signals caused by interference or by site-specific effects like multipath. When forming linear combinations, the incorporation of multi-frequency signals enhances flexibility and robustness, where the mathematical correlations introduced by including the same signal in different linear combinations of the same type need to be handled properly in RTK algorithms.
By enabling the tracking of Galileo satellites in the aforementioned firmware, the Galileo signals will be used in different RTK position types by default, including navigation position, phase-aided differential code position, extended RTK (xRTK) position and RTK fixed position. When compared to a standard RTK fix, an xRTK fix is provided at a slightly lower accuracy level, but with higher availability in difficult environments such as urban canyons and dense canopy.
In terms of RTK correction data formats, Galileo is included in the standardized RTCM v3 MSM format and in the proprietary 4G format. To use Galileo in network RTK, the real-time products provided by network correction services need to include Galileo as well. In the latest version of a proprietary GNSS network software, Galileo is used in network processing to provide RTK corrections via the individualized master-auxiliary (iMAX) method and the virtual reference station (VRS) method in the RTCM 3.2 MSM formats.
RTK PERFORMANCE CHARACTERISTICS
Multi-constellation and multi-frequency GNSS RTK is a complex real-time process, aiming to provide cm-level positioning accuracy with as few as possible data epochs for variable user kinematics and even in difficult measuring environments. Therefore, RTK performance characteristics need to be carefully selected to be able to evaluate the system as a whole and to address users’ concerns in their applications.
The following parameters are used in this article to assess the benefits of Galileo for high-precision RTK:
Satellite usage. Number of satellites used in RTK fixed solutions with an elevation cut-off angle of 10°;
Availability. Percentage of RTK fixed positions relative to all positions obtained during a time period;
Accuracy. Deviation of RTK fixed positions from ground truth with a higher degree of accuracy, where the ground truth can be determined by means of a total station or by post-processing long-term GNSS data;
Reliability. Percentage that the position error (with respect to ground truth) is less than 3 x coordinate quality (CQ) indicator;
Time to Fix. Time needed to regain an RTK fixed solution after losing ambiguity fix provided that GNSS signal tracking is not interrupted.
OPEN-SKY CASE STUDY
The open-sky case study was performed in the Heerbrugg testbed. Two receivers were connected to a single antenna via a four-way antenna splitter. One receiver received four-system iMAX corrections in the RTCM v3 MSM format over a short baseline of 2 km, whereas the other received RTK data of the same type over a long baseline of 116 km. By considering different baseline lengths, the open-sky experiment focused on the usability of the current Galileo constellation in GNSS RTK under normal conditions. Two days of 1-Hz GNSS data were investigated with respect to satellite usage and positioning accuracy.
Using different combinations of GNSS to analyze the short baseline data — GPS+GLO (GG), GPS+GLO+BDS (GGB) and GPS+GLO+GAL+BDS (GGGB) — the mean numbers of used satellites are 15, 17 and 20, respectively, where the elevation cut-off angle was set to 10°. On average, three Galileo satellites contribute to RTK fixed solutions.
For the four-system combination GGGB, Figure 2 shows the satellite usage for each individual system over the two-day period. It can be seen that for a short baseline of 2 km, a maximum number of four Galileo satellites can be used for positioning. In fact, during 80.3% of the whole test period, the number of Galileo satellites used in RTK fixed solutions is equal to or greater than the number of BeiDou satellites used.
Figure 2. Number of satellites used in RTK fixed positions with GGGB under open sky (iMAX, RTCM v3 MSM, baseline length: 2 km, GGGB: GPS+GLO+GAL+BDS, DOY: day of year).
Table 1 provides statistics on Galileo satellite usage in case of GGGB for different baseline lengths. As would be expected, the number of Galileo satellites used decreases with an increasing baseline length. In approximately 41% of the cases, three Galileo satellites are used in the short baseline test, whereas two Galileo satellites are used in the long baseline test.
Moreover, the probability that no Galileo satellites are involved in a four-system combined solution grows significantly from 1.9% to 15.0% as the baseline length increases from 2 km to 116 km. The probability that only one Galileo satellite is used under open sky is relatively small, amounting to around 0.5%. This is reasonable since no benefits for high-precision RTK are expected in this particular situation. Regarding the short baseline case, there is a 97.7% probability that at least two Galileo satellites are used for positioning, whereas this probability decreases to 84.4% in the long baseline case.
Table 1. Probability [%] that n Galileo satellites are used in RTK fixed positions with GGGB during the two-day period of the open-sky experiment (iMAX, RTCM v3 MSM, GGGB: GPS+GLO+GAL+BDS).In terms of positioning accuracy, Figure 3 compares the 3D errors from analyzing the long baseline data with different GNSS constellations. Regarding the entire two-day period illustrated in Figure 3a, the integration of BeiDou (GG vs. GGB) and Galileo (GGB vs. GGGB) results in higher position repeatability with more consistent errors. For a selected period of 12 hours, Figure 3b highlights the advantages of Galileo in reducing large 3D errors from 6–8 cm to 3–4 cm, where two or three Galileo satellites are used in case of GGGB.
Figure 3. 3D errors of RTK fixed positions under open sky (iMAX, RTCM v3 MSM, baseline length: 116 km, GG: GPS+GLO in green, GGB: GPS+GLO+BDS in blue, GGGB: GPS+GLO+GAL+BDS in red, DOY: day of year) (a) Entire two-day period, (b) Selected 12-hour period (28–40 h).
MULTIPATH CASE STUDY
In this case study, a GNSS smart antenna was set up in a location with strong multipath effects, where GNSS signals were obstructed and reflected by the surrounding buildings (Figure 4). This test setup simulates the use case that a user measures a point near a building with degraded GNSS signal reception, even at high elevation angels.
Figure 4. Test setup in a strong multipath environment in Heerbrugg (rover: GS16, antenna height: 1.8 m) (a) View from the south, (b) View from the north.
The default elevation cut-off angle of 10° was applied. The receiver received four-system VRS corrections in the RTCM v3 MSM format, where the distance to the physical reference station was approximately 200 m. Three hours of 1-Hz GNSS data were analyzed with respect to accuracy, reliability and time to fix.
Figure 5 illustrates the 3D errors from multi-GNSS RTK with and without Galileo (GGGB vs. GGB), along with the number of used satellites. Regarding the periods marked with dashed rectangles, the inclusion of two or three Galileo satellites (Figure 5b) leads to significant improvements in positioning accuracy at the few cm to dm level (Figure 5a). By comparing the empirical cumulative distribution function (CDF) of the 3D errors, the probability that 3D error is within 5 cm increases from 70% to 85% if Galileo is used, even with a maximum number of three satellites.
Figure 5. Impact of Galileo integration on RTK positioning accuracy under strong multipath (VRS, RTCM v3 MSM, GGB: GPS+GLO+BDS in blue, GGGB: GPS+GLO+GAL+BDS in red, DOY: day of year) (a) 3D errors of RTK fixed positions, (b) Number of used satellites (Galileo in green).
Tables 2 and 3 provide the root mean square (RMS) errors and reliability of RTK fixed positions from the multipath experiment, respectively. By using Galileo in high-precision RTK, the 3D RMS error is significantly reduced by 56.3% in this case study, from 0.080 m (GGB) to 0.035 m (GGGB). When compared to the horizontal components, the height RMS error shows a larger relative improvement of 58.7% due to Galileo integration. The reliability reflects the consistency between the actual position error with respect to ground truth and the CQ indicator estimated based on mathematical models in RTK algorithms. As shown in Table 3, the 3D reliability improves by 7.3%, from 88.2% (GGB) to 95.5% (GGGB), where the increases for the horizontal components and height are comparable.
Table 2. Root mean square errors [m] of RTK fixed positions under strong multipath (VRS, RTCM v3 MSM, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).Table 3. Reliability [%] of RTK fixed positions under strong multipath (VRS, RTCM v3 MSM, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).The time to fix (TTF) was determined by constantly re-initializing RTK once an ambiguity fix was gained. During the whole period of repeatedly resetting the RTK filter, the GNSS signals were tracked continuously without interruption. A total of 765 TTF values were obtained with GGB, whereas 1,128 TTF estimates were available with GGGB. The significantly larger number of the TTF samples from GGGB indicates higher availability of RTK fix if Galileo is used.
Figure 6 shows the statistical distribution of TTF with respect to Galileo integration. As can be seen in the empirical CDF in Figure 6a, it takes shorter time for GGGB to regain an ambiguity fix. As an example, GGGB allows ambiguity resolution within 5 s (10 s) with 46% (87%) probability, which is 29% (16%) higher than GGB. Regarding the boxplots of TTF in Figure 6b, GGGB shows a smaller median (by 25% from 8 s to 6 s) and a smaller interquartile range (IQR; by 50% from 4 s to 2 s) than GGB, where the IQR is the length of the box. This indicates that the integration of Galileo enables a faster ambiguity resolution with more consistent fixing performance.
Figure 6. Impact of Galileo integration on time to fix (TTF) statistics under strong multipath (VRS, RTCM v3 MSM) (a) Empirical cumulative distribution function (CDF) of TTF, (b) Boxplot of TTF with median and interquartile range (IQR).
CANOPY CASE STUDY
In this case study, a receiver was connected to an antenna under tree canopy (Figure 7), where GNSS signals are blocked, attenuated and reflected, leading to decreased number of observations, low data quality and degraded RTK performance.
Under these circumstances, the inclusion of Galileo satellites transmitting multi-frequency signals could be particularly beneficial for high-precision RTK. Using an elevation cut-off angle of 10°, the receiver received four-system iMAX corrections in the RTCM v3 MSM format, where the baseline length was 116 km. A long baseline was intentionally selected as an additional challenge for the RTK system. About seven hours of 1-Hz GNSS data were investigated regarding availability, accuracy and reliability.
Figure 7. Test setup under canopy in Heerbrugg (rover: GS10, antenna: AS10).
Figure 8 illustrates the impact of Galileo integration on RTK availability and accuracy under canopy, along with the number of used satellites. As can be seen in Figure 8a, the inclusion of Galileo improves the availability of RTK fixed positions by 12.2%, from 65.7% (GGB) to 77.9% (GGGB). Moreover, dm-level position errors are largely reduced, as shown in FigURE 8c. The improvements in availability and accuracy are achieved by using up to three Galileo satellites (Figure 8b). This demonstrates that the current Galileo constellation in the IOC phase brings considerable benefits to high-precision RTK under canopy conditions.
Figure 8. Impact of Galileo integration on RTK availability and accuracy under canopy (iMAX, RTCM v3 MSM, baseline length: 116 km, GGB: GPS+GLO+BDS in blue, GGGB: GPS+GLO+GAL+BDS in red, DOY: day of year) (a) Availability of RTK fixed positions over time, (b) Number of used satellites (Galileo in green), (c) 3D errors of RTK fixed positions.
Tables 4 and 5 provide the RMS errors and reliability of RTK fixed positions from the canopy experiment, respectively. The main factors degrading the RTK accuracy in this case study are not only the canopy environment, but also the long baseline length of 116 km. It can be seen in Table 4 that the integration of Galileo leads to a significant reduction of 3D RMS error by 23.7%, from 0.114 m (GGB) to 0.087 m (GGGB).
By comparing the 2D and 1D RMS errors, the benefits of Galileo for the height are more dominant than for the horizontal components, which was also observed in the multipath experiment (Table 2). In terms of reliability, only slight (below 2%) increases are visible in Table 5. 116km baseline length and heavy canopy are considered extreme conditions and beyond the standard conditions relevant for specifications. Considering reliability together with availability (Figure 8a), it is encouraging to see that both the RTK performance characteristics are improved in this case study.
Table 4. Root mean square errors [m] of RTK fixed positions under canopy (iMAX, RTCM v3 MSM, baseline length: 116 km, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).Table 5. Reliability [%] of RTK fixed positions under canopy (iMAX, RTCM v3 MSM, baseline length: 116 km, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).
GALILEO-ONLY RTK
To optimize the performance of multi-GNSS RTK positioning, the individual systems need to be fully understood and mastered. With a previous firmware release in August 2014, mass-market devices were able to perform GLONASS-only and BeiDou-only high-precision RTK. In 2014 tests, we compared the performance of GPS-only, GLONASS-only and BeiDou-only RTK at different accuracy levels. Considering that Galileo has reached the IOC phase, it is reasonable to assess the Galileo-only RTK performance with the latest firmware.
Due to the limited number of usable Galileo satellites, Galileo-only RTK positioning was carried out in the Heerbrugg open-sky testbed over a very short baseline of 1 m. In addition, the elevation cut-off angle was set to 0° in order to track as many Galileo satellites as possible simultaneously. Two receivers were connected to two choke-ring antennas with good low-elevation tracking ability. Single-base RTK positioning was performed with four-system corrections in the RTCM v3 MSM format. About one hour of 1-Hz GNSS data was analyzed with a special focus on positioning accuracy.
Figure 9 shows the 3D errors from GPS-only, GLONASS-only and Galileo-only RTK positioning, where the numbers of used satellites are 8–11, 7–9 and 5–6, respectively. During the test period, only three or four BeiDou satellites were tracked with poor geometry, making BeiDou-only RTK impossible. As the figure shows, the 3D errors from GPS-only and Galileo-only RTK are at a comparable level with similar RMS values, whereas the 3D RMS error from GLONASS-only RTK is almost twice as large as the GPS/Galileo-only case. Note that when compared to GPS-only RTK, almost half as many satellites are used in Galileo-only RTK.
Figure 9. 3D errors of RTK fixed positions from GPS-only, GLONASS-only and Galileo-only RTK under open sky (single-base RTK, baseline length: 1 m, RTCM v3 MSM, DOY: day of year, RMS: root mean square).
Figure 10 displays the statistical distribution of the 3D errors from GPS-only, GLONASS-only and Galileo-only RTK positioning. Regarding the empirical CDF in Figure 10a, GPS/Galileo-only RTK shows a clearly more favorable error distribution than the GLONASS-only case. Using only GPS or Galileo, the probability that 3D error is within 1 cm is above 80%, which is approximately 30% higher than using only GLONASS. For 3D errors ranging between 5 mm and 1.7 cm, Galileo-only RTK even provides a slightly higher cumulative probability than the GPS-only case. The 3D error boxplots in Figure 10b illustrate a similar pattern between GPS-only and Galileo-only RTK, which is superior to GLONASS-only RTK due to the significantly smaller median and IQR.
Figure 10. 3D error statistics from GPS-only, GLONASS-only and Galileo-only RTK under open sky (single-base RTK, baseline length: 1 m, RTCM v3 MSM). (a) Empirical cumulative distribution function (CDF) of 3D errors, (b) Boxplot of 3D errors (IQR: interquartile range).
CONCLUSIONS
With the declaration of Galileo Initial Services in December 2016, for the first time ever all GNSS users worldwide are able to use the positioning, navigation and timing information provided by Galileo’s global satellite constellation. Upon full system completion by 2020, Galileo will play an important role in high-precision GNSS applications for users around the world. This article showed representative case studies to understand the benefits of the current Galileo constellation for high-precision RTK. In addition to a multi-GNSS solution, the performance of Galileo-only RTK was presented. The main findings from the case studies can be summarized as follows:
In the open-sky test, with an elevation cut-off angle of 10°, on average three Galileo satellites can be used for high-precision multi-GNSS RTK. This leads to cm-level improvements in coordinate repeatability over a long baseline of 116 km.
In the multipath case study, the additional use of two or three Galileo satellites produces significant enhancements in positioning accuracy at the few cm to dm level, where the benefits for the height component are more significant. Moreover, the integration of Galileo increases the 3D reliability of RTK fixed positions by 7.3% and reduces the median time to fix by 2 s (25%).
In the canopy experiment, the inclusion of Galileo improves the availability of RTK fixed solutions by 12.2%. Furthermore, dm-level position errors are largely reduced.
When compared to GPS-only RTK, Galileo-only RTK provides a similar positioning accuracy over a 1-m baseline under open sky, where almost half as many satellites are used. The 3D RMS error from GLONASS-only RTK is approximately twice as large as the GPS/Galileo-only case.
The promising results achieved through Galileo integration already indicate the very important role of the European GNSS in high-precision, multi-frequency and multi-constellation RTK positioning. During the deployment of the Galileo system, more benefits can be expected in the near future.
ACKNOWLEDGMENTS
The staffs of Leica Geosystems AG (Heerbrugg/Switzerland), Christian Waese and Youssef Tawk, are gratefully acknowledged for support in setting up the variety of RTK network streams.
MANUFACTURERS
SmartWorx 6.16 of Leica Viva GNSS is the latest firmware cited and used in these high-precision RTK tests. Leica GNSS Spider 7.0.0 furnished the GNSS real-time corrections. The open-sky case study used two Leica Viva GS10 units connected to a Leica Viva AS10 antenna via a four-way antenna splitter. The multipath case study used a Leica Viva GS16 GNSS smart antenna. The canopy case study used a Leica Viva GS10 receiver and a Leica Viva AS10 antenna. The Galileo-only RTK test used two Leica Viva GS10 receivers and two Leica AR25 choke ring antennas.
Each Galileo satellite must go through a rigorous test campaign to assure its readiness for the violence of launch, the vacuum of space, and temperature extremes of Earth orbit, reported the European Space Agency.
Each one is despatched to a unique location in Europe to ensure its readiness before launch: a 3,000-square-meter cleanroom complex nestled in sandy dunes along the Dutch coast, filled with test equipment to simulate all aspects of spaceflight.
The test centre in Noordwijk — Europe’s largest satellite test site — is part of ESA’s main technical centre, but it is maintained and operated on a commercial basis on behalf of the Agency by a private company created for the purpose: European Test Services (ETS) B.V.
“Our company was founded 2000 as a joint venture between two of Europe’s leading satellite environmental test companies, Intespace in France and IABG in Germany,” said Pierre Destaing, ETS test programme support manager for Galileo. “That business setup is a source of flexibility: there are 30–35 people working here throughout the year, but if extra specialists are needed for a given campaign, we can call on our parent companies.”
ETS has been responsible for supporting many historic test campaigns – including space-certifying Europe’s 20-tonne ATV space truck and Envisat, the world’s largest civilian Earth-observing mission. But in terms of scale alone, its work with Galileo is the company’s greatest challenge.
ETS is about to complete its contracts with OHB System AG, covering the environmental test of 22 ‘Full Operational Capability’ Galileo satellites, preceded by the testing of the very first of the first-generation ‘In-Orbit Validation’ Galileo satellites on a previous, separate contract.
A Galileo FOC satellite is slid out of its transport container into the clean room at ESTEC. (Photo: ESA)
The pressure has been steady to ensure satellites are available in time to meet Galileo’s launch schedule.
“Traffic management is a big part of the job – it’s like a game of Tetris,” Pierre said. “We have a steady stream of Galileo satellites to accommodate, along with other missions such as the BepiColombo Mercury orbiter, Solar Orbiter, the Cheops exoplanet detector and currently the latest MetOp weather satellite, with a fixed set of test facilities. The biggest challenge is definitely ensuring that every project can have the access to the facility they need at the right time, which demands complicated logistics and security adherence.”
ETS has built up to a steady rhythm with the OHB System team, typically accommodating multiple satellites in storage on site, at the same time as others undergo further active testing.
“When each new satellite arrives, it is first unpacked within the carefully filtered and air conditioned Test Centre environment,” Pierre said.
Moving a Galileo Full Operational Capability satellite between test facilities at ESA’s Test Centre in Noordwijk, the Netherlands. (Photo: ESA)
After four years of work, the European Space Agency (ESA) team tasked with keeping the world informed on the status of the Galileo satellite navigation system has formally passed on its responsibility to a European Union agency.
This shift is part of a wider transfer of responsibilities, as this month see the official handover of the running of the Galileo system from ESA to the European Global Navigation Satellite System Agency (GSA).
“Our job — working with the European Commission and GSA — has been to inform Galileo users in an official, transparent way of any system changes that could affect Galileo satellites,” explains Rafael Lucas Rodriguez, ESA’s Galileo services engineering manager.
“Keeping our users in the picture on planned activities that might lead to satellite unavailability, or any unplanned outages, has helped them to plan their own test activities around Galileo signals and to prepare future products.”
The very first Notice Advisory to Galileo Users (NAGU) was issued in June 2013, just three months after the first Galileo positioning fix was achieved, to a then small community of researchers and industrial users, interested in making tests with the newborn four-satellite constellation.
A total of 189 NAGUs were issued under ESA oversight in the last four years, as the constellation grew to its current 18 satellites. The user base increased dramatically from 86 to 774 registered users on the European GNSS Service Centre website as companies worked to prepare Galileo-ready products and then, on 15 December 2016, Galileo’s Initial Services began operating.
GSC web portal 2013.
Throughout this period, the NAGUs, published on the website of the European GNSS Service Centre and sent to subscribers via email, gave the user community a reliable overview of Galileo’s overall status and that of individual satellites.
NAGUs are issued as new satellites are launched and when satellites become ready for service provision, or to give advance warning of signal unavailability owing to planned maintenance or testing activities, or to notify users of unplanned outages and then to inform them when satellites become active again.
“Broadcom is a regular consumer of the NAGUs released by the Galileo Service Centre,” says Javier de Salas, R&D Director at GNSS receiver chipset manufacturer Broadcom.
“Not only do they help us to organise our engineering activities and tests but, more importantly, they are used as input into our orbit prediction engine for our Long Term Orbits products, which in turn are used by hundreds of millions of consumers worldwide.”
Rafael Lucas of the ESA team adds, “Around a dozen people at ESA worked to begin defining, setting up and operationalising the NAGU process, modelled after the well-established Notice Advisory to Navstar Users of GPS.
The Trimble VRS Now GNSS correction service is now available in France. The service is designed for a variety of geospatial and construction applications including surveying, cadastral, land administration, and urban and rural construction that would benefit from easy access to high-accuracy, centimeter-level positioning.
Trimble also now provides Galileo support for VRS Now. Powered by the Trimble Pivot Platform, VRS Now in Europe fully supports GPS, GLONASS, BeiDou, QZSS and the Galileo satellite system.
Galileo support improves network performance and reliability with access to additional satellites, particularly in urban canyons or other harsh environments. The increased number of visible satellites provides additional data observations that enhance positioning integrity to better mitigate errors.
“Trimble continues to aggressively expand its VRS Now footprint in Europe,” said Patricia Boothe, general manager of Trimble’s Advanced Positioning Division. “With the addition of correction services in France, Trimble VRS Now covers over 179 million square kilometers (732 million square miles) across 10 countries.”
VRS Now coverage is available throughout the majority of France as well as Belgium, The Czech Republic, Estonia, Germany, Great Britain, Ireland, Luxembourg, the Netherlands and Sweden using a compatible GNSS receiver or display.
Subscriptions are available through Trimble’s Authorized Business Partners or Trimble’s online store.
UK’s SSTL to build third batch of Galileo navigation payloads
News from the European Space Agency
Europe’s Galileo navigation constellation will gain an additional eight satellites, bringing it to completion, thanks to a contract signed at the Paris Air and Space Show.
The contract to build and test another eight Galileo satellites was awarded to a consortium led by prime contractor OHB, with Surrey Satellite Technology Ltd overseeing their navigation platforms.
This is the third such satellite signing: the first four In Orbit Validation satellites were built by a consortium led by Airbus Defence and Space, while production of the next 22 Full Operational Capability (FOC) satellites was led by OHB.
These new batch satellites are based on the already qualified design of the previous Galileo FOC satellites, except for changes on the unit level – such as improvements based on lessons learned and reacting to obsolescence of parts.
ESA’s Director of the Galileo Programme and Navigation-related Activities, Paul Verhoef, signed the contract with the CEO of OHB, Marco Fuchs and OHB Navigation Director Wolfgang Paetsch, in the presence of ESA Director General Jan Woerner and the EC’s Deputy Director-General for Internal Market, Industry, Entrepreneurship and SMEs, Pierre Delsaux.
“This procurement from OHB will enable the completion of the Galileo constellation and have reserves both in-orbit and on-ground,” said Director Verhoef. “This signing delivers the necessary infrastructure robustness that is essential for the provision of Galileo services worldwide.”
ESA signed the contract on behalf of the EU represented by the European Commission – Galileo’s owner. The Commission and ESA have a delegation agreement by which ESA acts as design and procurement agent on behalf of the Commission.
Signing Ceremony
Galileo is Europe’s own satellite navigation system, providing an array of positioning, navigation and timing services to Europe and the world.
With 18 satellites now in orbit, Galileo began Initial Services on Dec. 15, 2016, the first step towards full operational capability.
Further launches will continue to build the satellite constellation, which will gradually improve the system performance and availability worldwide. The launch by Ariane 5 of another four satellites is due to take place later this year.
The full Galileo constellation will consist of 24 operational satellites in three orbital planes plus orbital spares, intended to prevent any interruption in service.
These new eight satellites will provide the constellation with in-orbit and on-ground spares. ESA and the Commission are also in the process of developing an improved Galileo Second Generation for the next decade.
Galileo is now providing three service types, the availability of which will continue to be improved.
ESA’s Director of the Galileo Programme and Navigation-related Activities, Paul Verhoef (right), signing the contract of behalf of the European Commission, shakes hands with the CEO of OHB, Marco Fuchs beside OHB Navigation Director Wolfgang Paetsch, in the presence of ESA Director General Jan Woerner (in background) and the EC’s Deputy Director-General for Internal Market, Industry, Entrepreneurship and SMEs, Pierre Delsaux.
Galileo coverage
The Open Service is a free mass-market service for users with enabled chipsets in, for instance, smartphones and car navigation systems. Fully interoperable with GPS, combined coverage will deliver more accurate and reliable positioning for users.
Galileo’s Public Regulated Service is an encrypted, robust service for government-authorized users such as civil protection, fire brigades and the police.
The Search and Rescue Service is Europe’s contribution to the long-running Cospas–Sarsat international emergency beacon location. The time between someone locating a distress beacon when lost at sea or in the wilderness will be reduced from up to three hours to just 10 minutes, with its location determined to within 5 km, rather than the previous 10 km.
The public will begin benefiting as Galileo-capable devices enter the marketplace: 17 companies, representing more than 95% of global supply, now produce Galileo-ready chips.
SSTL continues Galileo work
“SSTL is delighted to have been selected to build the third batch of navigation payloads needed to complete the initial Galileo Constellation,” said Gary Lay, SSTL’s director of navigation. “I am confident that the OHB-SSTL solution offered the lowest risk and best value for money, and I believe that our selection as payload providers for the third time in succession demonstrates a high regard for our work.”
SSTL’s state-of-the-art Galileo FOC payload comprises different units including European sourced atomic clocks, navigation signal generators, high power traveling wave tube amplifiers and antennas. SSTL’s payload proposal for Batch 3 is for a recurrent build of the existing payload, with an evolution of the atomic clocks to incorporate advances made under the European GNSS Evolution Programme.
Fourteen of SSTL’s Galileo FOC navigation payloads are currently operational in orbit, with a further eight payloads already delivered to OHB for integration and test.
SSTL has been involved in the Galileo program since 2003 with the design and build of GIOVE-A, Galileo’s pathfinder mission. GIOVE-A was launched in 2005 and is still operational today, providing valuable data about the radiation environment in Medium Earth Orbit. An experimental GPS receiver on board GIOVE-A is also used to map out the antenna patterns of GPS satellites for use in planning navigation systems for future high altitude missions in Geostationary orbit, and beyond into deep space.
