The benefits of multi-constellation GNSS: reaching up even to single constellation

GNSS users

B. Bonet1, I. Alcantarilla1, D. Flament2, C. Rodriguez2, N. Zarraoa1; (1) GMV Aerospace, (2) European Space Agency

BIOGRAPHIES Beatriz Bonet received her M. Sc. in Physics from the ‘Universidad de Oviedo’. She is currently the technical responsible of the CPFPS Product Support Service and has been involved in the Satellite Navigation domain for more than 4 years. Ignacio Alcantarilla M. Sc. in Aerospace Engineering and has been involved in the Satellite Navigation domain for more than 10 years. He has been the CPFPS project and technical manager for the last years. He is currently in charge of the development and commercialization of the magicSBAS product Didier Flament graduated from Ecole Centrale de Lille Engineering school in 1983 and holds a Ph.D in Automatics (1987). He has been System Engineering Manager for the 10 years EGNOS development contract at prime contractor Thales Alenia Space. From 2004 to 2008, he was deputy CTO of the Navigation Business Unit of Thales Alenia Space. He joined ESA in March, 2008 to lead all system engineering activities for EGNOS new release development and implementation and also for the ESA GNSS Evolution Program. He represents ESA in the SBAS Interoperability Working Group and in the US-EU WG-C. He has now more that 20 years of experience in GNSS system development Catalina Rodríguez graduated as an electronic engineer from CNAM in Paris. She worked at Thales Alenia Space as a Navigation System Engineer on EGNOS and joined CNES in 2005. Today integrated within ESA's EGNOS project team, she is involved in many activities linked to Regional Augmentation Systems performance and integrity. Néstor Zarraoa holds a Ph.D. in Mathematics. He has more than 20 years of experience in Space Geodesy and since joining GMV in 1997, he has played an active role in the development of the Processing elements of both EGNOS and Galileo. Currently he is the Head of the Operational Systems Division of the GNSS business unit of GMV Aerospace.

ABSTRACT The scenario of GNSS is speeding up on a no-return track towards a multi-constellation and multi-frequency GNSS world. On top of the good-ole-boy GPS, the re-born GLONASS system is steadily improving. The exciting perspectives of new global systems as Galileo or Compass are getting closer to reality. Regional systems as QZSS or IRNSS are also on the starting line. Augmentation systems flourish worldwide: WAAS, EGNOS, MSAS, GAGAN… Within this seemingly chaotic scenario of possibilities, some initiatives are actively working on the definition of efficient, pragmatic and user-oriented approaches to take advantage of this new challenging scenario into a maximum-benefit scheme for all GNSS users. In particular Europe has launched, under ESA’s European GNSS Evolution Programme, the MRS initiative. MRS standing for Multi-constellation/multi-frequency Regional System, the initiative is putting different teams of experts into the exciting goal of defining the paths for the most successful GNSS usage on a future of multiple choices. Following this approach and based on the promising study results obtained so far by ESA (see ref [1]) MRS is increasingly becoming a serious and natural candidate for the evolution of the current EGNOS System which is qualified and in operations since 3 years and which delivers both an Open Service and a Safety Of Life aeronautical service (same as WAAS). EGNOS is on the way to certification planned for mid 2010. But MRS is not just looking far in the wonderful future of several matured GNSS systems. Also the present and short term future are being taken into account, based on what we have today and aiming to improve the services that we are already providing to ourselves today, as GNSS users. Today we do have already a multi-constellation scenario, with GLONASS getting close to having the nominal constellation in operation. Furthermore, processing platforms as that of EGNOS, magicSBAS or SPEED [2]

already implement the ability of processing GLONASS data into an SBAS solution. Based on these available tools, ESA has prepared a detailed experimentation plan aimed to demonstrate the benefits that the multi-constellation approach can provide to today’s GNSS users. This plan covers a wide range of objectives covering different user domains (aeronautical, land mobile, maritime) equipped with different types of receivers. This set of experimentations will rely on the deployment of several Test Beds combining multi constellation multi-frequency processing and multi broadcast channels (GEO, MEO, terrestrial). Among the first experimentations planned, there are some devoted to experiment and validate the performance benefits for SBAS users (GPS L1 only first then GPS/GLONASS or GPS/Galileo – L1/E1 mono frequency or dual frequency E1/E5 SBAS users). This paper presents the outcome of the first step of this experimentation campaign, which has been performed based on the magicSBAS tool, a flexible SBAS processing platform, able to acquire single and dual frequency GLONASS data, in addition to GPS, in order to compute and provide both standards SBAS corrections and integrity, as well as augmentation to GLONASS. The objectives of this first step are twofold: 1. Analyse the benefits for single frequency users of

using both GPS and GLONASS augmentation, in particular for uncontrolled environments, where the visibility of additional satellites play a key role on the reliability of the GNSS services, as for instance for urban applications.

2. Analyse the potential benefits that can be achieved by improving the standards GPS L1 SBAS augmentation service, when GLONASS data are used within the processing facility to improve the overall accuracy, integrity and continuity of the service.

In particular we will analyse how such a dual-frequency/dual constellation processing can improve ionospheric information, by increasing: - accuracy, with more visible satellites - robustness, for instance against scintillation effects - service coverage, by offering improved (more

accurate and more available) Ionospheric Grid Point monitoring to users located at the service area boarder;

- reliability with better geometry for integrity monitorisation

The results of the analysis will be used as a key proof of concept for the definition of the short term evolutions of current European GNSS initiatives, which goes beyond the usage of GLONASS towards the incorporation of

Galileo in a truly System of Systems, where all users, including legacy users will be able to exploit the advantages stemming from optimized performances, higher reliability, maximum integrity and seamless navigation. MRS is one of several worldwide initiatives, but it is marching steadily and actively towards its goals, and there is no better way to march than by experimenting, trying and learning from practice. INTRODUCTION In a few years, several GNSS constellation will share Earth’s outer space, giving chances for innovative and improved navigation solutions, but also creating challenges that need to be addressed to ensure the optimal use of all the available navigation signals by the end users. ESA has launched the Multi-constellation/multi-frequency Regional System (MRS) study, in order to convey different experts to support the assessment and definition of the most optimal GNSS usage, not only involving potential innovative applications, but also taking into consideration the interests of legacy users that are already today relying on GNSS for many applications. The initiative for the specific study that gives basis for this paper, is aimed to try to demonstrate with the existing GLONASS system in combination with GPS, the potential added value of a multiconstellation SBAS system, both from the perspective of a truly multi-constellation augmentation, and from the perspective of enhancing the quality of the SBAS signal in space without modifying the standards (SBAS MOPS, [3]). The work has been organised in several activities mainly: GLONASS single frequency augmentation

assessment, aimed to assess the performances of a dual-constellation / single frequency GPS+ GLONASS augmentation.

Theoretical study on the potential usage of

GLONASS single or dual frequency data to enhance current single-frequency GPS augmentation

Experimentation on the specific usage of current GLONASS dual frequency data to enhance the ionospheric monitoring.

Experimentation on other potential usages of GLONASS data to enhance other aspects of the SBAS Data Processing (e.g. SBAS Network Time generation)

Experimentation of the usage of GPS and

GLONASS dual frequency data to provide SBAS

augmentation to single frequency users based on EGNOS processing design

The work is based on the usage of the magicSBAS tool [4], which is part of the global magicGNSS suite developed by GMV (magicGNSS.gmv.com). magicSBAS is a tool that has been developed by GMV through an internal R&D project, supported by the Regional Madrid Government, aimed to provide a light and versatile SBAS platform for demonstrations, pre-operational services and R&D. magicSBAS can operate in real-time by acquiring reference receivers data from the Internet through the NTRIP protocol. The tool can also operate offline for detailed investigation. It has been adapted to the latest evolutions of the GLONASS ICD, and has been tested performance-wise against the latest release of the processing facility of EGNOS (deployed for EGNOS V2.3 release). Also early tests have been done on regular operation with GPS/GLONASS dual receivers, providing a test “signal in space” augmenting both GPS and GLONASS, available through ESA’s SISNeT protocol [5]. In this paper we will describe the experiments performed using GLONASS data, using single and dual frequency capable GLONASS reference stations, in order to assess the added value of using a second constellation as complement to the current GPS one in order to improve the SBAS augmentation services. GLONASS AUGMENTATION The Satellite Based Augmentation Systems (SBAS) provide augmentation to GPS positioning system, by means of pseudorange corrections, as well as signal integrity information broadcast to the user. This leads to a notable improvement on the positioning system performances, highly increasing the accuracy of the user position estimation, and also the reliability and integrity of the system. EGNOS is the European satellite-based augmentation system, currently on the way to certification planned for mid 2010. It provides accurate real time corrections to GPS ephemeris data, a precise ionosphere model to be applied by the user, along with the integrity information on the service provided. Currently EGNOS is capable of acquiring and processing single-frequency GLONASS data in addition to dual frequency GPS data. This data processing would allow providing SBAS corrections and integrity for GLONASS satellites.

Nevertheless the level of validation of these features was restricted to the Factory Qualification of separate elements and EGNOS is not qualified as a whole to use GLONASS operationally. This decision was aligned with the lack of an approved receiver MOPS for a potential GPS/GLONASS airborne receiver. It is worth on this context to recall that RTCA DO-229D MOPS does not provide sufficient information to allow implementing GLONASS augmentation. On the other hand, the ICAO SARPS [6] do define completely the means to provide that augmentation at Signal in Space level as well as in user algorithms. EGNOS primarily, and now magicSBAS, implement GLONASS augmentation according to ICAO SARPS. Notwithstanding these facts, the potential added value of GLONASS or any second constellation could be also obtained even if the user would ignore the existence of a second constellation. The use of multiconstellation data processing could eventually enhance the quality of the single frequency GPS augmentation. EGNOS actual design with respect to GLONASS is based on the systematic prevention of the combined use of GPS and GLONASS data for common functions in the SBAS data processing chain to prevent the potential degradation of GPS augmentation performances, due to any potential lower quality of the GLONASS data. But GLONASS constellation now is reaching a good maturity level, and moreover the ESA MRS studies have confirmed the possibility to offer a multiconstellation augmentation service which can remain robust to major failures of one of the constellations in use. These combined elements give sense to the objective of using GLONASS as a second constellation in order to test different approaches for a Multiconstellation Regional System (MRS). MULTICONSTELLATION SBAS AUGMENTATION (GPS + GLONASS) One of the straightforward added-value of a multiconstellation SBAS is to multiplicate the number of monitored satellites available to the user. On many circumstances this could be the key driver of the application. In addition, such a feature would enhance the added value of SBAS for applications other than Civil Aviation. The increment on available monitored satellites would improve significantly the availability of SBAS-related augmentation for applications as road transport, even in areas with lower visibility, as urban canyons, or mountain roads.

In order to analyze the benefits for single frequency users of receiving both GPS and GLONASS augmentation, in particular for environments where the visibility of additional satellites play a key role on the reliability of the GNSS services, a dedicated campaign of GPS/GLONASS single frequency augmentation has been performed and the performance improvements have been assessed in detail. Early works on this direction were already reported in [7]. The activities enclosed in this campaign consisted of evaluating the performances provided by EGNOS for a user receiving both GPS and GLONASS augmentation versus the performances obtained for a user receiving only GPS augmentation. With this purpose, different executions of the tool magicSBAS have been performed for a set of EGNOS real data, configuring the satellites mask for both GPS only and GPS+GLONASS, in order to compare the results obtained for each execution. The resulting performances have been evaluated first in terms of availability in different locations, in the centre of the ECAC and also near the border, in order to better appreciate the improvements obtained for the users in the border of the service area. Also the performances were evaluated for two different types of users in each location selected, considering a visibility mask angle of 5 degrees for one of them and 30 degrees for the other, to highlight the advantages that a second constellation would imply for users with reduced visibility (in case of degraded constellations, urban canyons, etc) compared with the advantages achievable on an all-in-view scenario. It may well be expected that a second constellation will show more significant benefits when the user visibility is impaired. The method selected to evaluate the performances in terms of availability is the GMV’s software utility eclayr (www.eclayr.com), which provides an SBAS performance assessment in both pseudorange and position domains. For this experimentation campaign, the analysis with eclayr was focused on the user availability mapping. The availability was evaluated in terms of the percentage of both HPL and VPL computed and lower than the HAL and VAL respectively versus all analyzed epochs, for APV-I requirements (HAL=40m and VAL=50m) for the different locations and visibility angles considered. Furthermore, a preliminary analysis of the availability achieved for LPV200 requirements for a user with 5 degree mask angle was also performed. The results obtained with the different analysis will be presented in the next sections.

MULTICONSTELLATION SBAS AUGMENTATION: NOMINAL SCENARIOS The first case that has been considered in terms of availability corresponds to a comparison against the nominal EGNOS performance for a user with 5 degrees elevation mask angle for both GLONASS and GPS satellites. For that purpose we have selected several days of real EGNOS reference stations (RIMS) data. EGNOS RIMS are capable of tracking GPS dual frequency data as well as GLONASS L1 data. The dates of reference, GLONASS constellation was composed of 14 operational satellites, which GPS had 30 healthy satellites in operation. A reference scenario has been processed with magicSBAS using only GPS data from the EGNOS network for comparison. Then a second scenario using both GPS and GLONASS has been also processed with magicSBAS. The following figure (figure 1) shows the differences obtained on the availability for a user using both GPS and GLONASS augmentation versus the performances obtained for a user using only GPS augmentation in this first case of analysis.

Figure 1: Delta Map HPL<HAL and VPL<VAL for GPS vs. GPS/GLONASS

The results show that significant benefits are obtained with respect to the nominal EGNOS APV-I performance in terms of availability in the borders of the service area (European Civil Aviation Conference). This is particularly relevant for the Mediterranean area and west of Great Britain. Although in the availability map it is not visible because EGNOS is already meeting availabilities in excess of 99,9%, in the central EGNOS regions, a systematic reduction in the protection levels achieved is noticed, as it is illustrated in the following histogram (figure 2) for the horizontal protection levels:

 

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Figure 2: HPL histogram for the nominal scenario (Blue for GPS and Green for GPS+GLONASS)

Please note that the values -1 correspond to the epochs for which the protection levels cannot be computed. As can be observed, the number of epochs for which the protection levels have been computed is increased when GLONASS is configured. The figures clearly show that the horizontal protection level values are significantly reduced when using GLONASS. The same behavior is observed for the vertical protection levels. A similar analysis has been performed using as reference a more demanding set of requirements, as those currently defined for the LPV200 operation (VAL of 35 meters instead of 50 in APV-I). In this case once more, the inclusion of GLONASS, show a significant increase in availability percentage for various boundary regions of the ECAC, leading to a much better level of compliance against the 99.9% requirement within ECAC. The following figure (figure 3) shows the availability results obtained for the LVP200 requirements.

Figure 3: Delta Map VPL<VAL for GPS vs. GPS/GLONASS for LPV200

In addition to availability, we have also evaluated the accuracy and integrity performances. In terms of 95% accuracy, there is a systematic though relatively small improvement in the values achieved. The following table (table 1) shows the 95% accuracy percentile, both horizontal and vertical, achieved in the all-in-view scenario for the RIMS stations located in Berlin (Denmark), Rome (Italy), Lapeenranta (Finland) and Santiago de Compostela (Spain): Table 1: Accuracy comparative for 5º user mask angle

for 4 analyzed RIMS stations GPS GPS/GLO RIMS

HPE VPE HPE VPE BRN 0.6642 1.2451 0.6454 1.1859

ROM 0.6553 1.2072 0.6039 1.2016

LAP 0.7553 2.2192 0.7918 1.8368

SDC 1.3275 0.9053 1.1489 0.8582

Finally, the performances were also analyzed in terms of integrity. The following figures show as example the Stanford diagrams achieved for a user in the border of the service area (Lappeenranta, Finland), comparing the results with GPS and with GPS+GLONASS for horizontal performances. As can be seen from both pictures (figures 4 and 5), there is no apparent degradation on the integrity level. The same behavior was observed for all analysed stations both in vertical and horizontal performances. To confirm this impression, the safety index, which is defined as the maximum ratio between the Position Error and Protection level among all analyzed epochs, has been computed for different locations in the service area.

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Figure 5: HPE vs. HPL (LAP, 5º) GPS+GLONASS The expected result on this integrity analysis would be that inclusion of GLONASS should not degrade the safety indexes achieved. It must be noted that the inclusion of GLONASS is not expected to improve the safety index in nominal situations. Any improvements brought by GLONASS in accuracy should be matched by equivalent reductions on the computed levels of protection. The following Table (table 2) shows the results achieved:

Table 2: Safety index for 5º user mask angle for 4 analyzed RIMS stations

Horizontal Safety Index

Vertical Safety Index

RIMS

GPS only

GPS+ GLO

GPS only

GPS+ GLO

BRN 0.1231 0.1343 0.1205 0.1361 ROM 0.1087 0.1079 0.1779 0.1721 LAP 0.0772 0.0911 0.1579 0.1476 SDC 0.1632 0.1552 0.0951 0.0935

The results obtained for the safety index are very similar, with or without GLONASS, in line with the expected behavior. This confirms that the level of integrity achieved by the system is preserved in an equivalent way when only GPS is used than when both GPS and GLONASS are considered. The results obtained demonstrate that the processing of combined GPS and GLONASS augmentation is not only feasible functionally, but that the level of quality of GLONASS corrections and integrity (UDREs) are comparable to GPS ones, allowing to take advantage of the improved geometry to obtained better accuracy and availability results than with GPS only solutions. It is important to recall that GLONASS data could be successfully used in the SBAS processing chain only if this does not degrade GPS augmentation performances and in particular in terms of integrity. The results shown on the integrity level preservation are very satisfying and

provide higher confidence on a potential further use of GLONASS data within EGNOS. MULTICONSTELLATION SBAS AUGMENTATION: DEGRADED SCENARIOS The second case that has been considered in terms of availability corresponds to a degraded scenario, constructed by limiting the mask angle of both reference stations and user positions to 30º elevation. The same input data as for the previous scenario were used, with the limitation described. Even if the selected scenario may be more relevant to other kind of applications, we decided to use the reference APV-I performances to perform the comparison between the GPS only and GPS+GLONASS executions. The following figure (figure 6) shows the differences obtained in availability using both GPS and GLONASS augmentation versus the performances obtained using only GPS augmentation in this second case of analysis.

Figure 6: Delta Map HPL<HAL and VPL<VAL for GPS vs. GPS/GLONASS with 30º Mask Angle

As it can be observed in the results, in this scenario the addition of GLONASS yields to a notable improvement on availability levels. In this case the improvement is noticeable in the entire ECAC region, improving the availability levels by an average of approximately 15-30%.. This is because the reference availability obtained with GPS only is far lower than in the nominal case where availabilities were already close to 100% (hence, with no margin for improvement in terms of availability). We have also analysed in this scenario the accuracy obtained, by looking at the 95% accuracy value. It must be noted however, that the 95% value has been computed only upon epochs with a valid solution. The following table shows the 95% accuracy percentile, both horizontal and vertical, achieved for each station in the degraded scenario:

Table 3: Accuracy comparative for 30º user mask angle for 4 analyzed RIMS stations

GPS GPS/GLO RIMS

HPE VPE HPE VPE BRN 2.2177 4.1489 2.2151 4.3377

ROM 1.6139 2.9097 1.5662 2.7029

LAP 2.3408 4.9664 1.7785 3.8286

SDC 1.8467 2.8059 1.2749 2.8198

In this degraded scenario the results are apparently misleading because when using GPS + GLONASS, what has happened is that the number of epochs with valid solutions is much larger than in the case of GPS only, so the 95% values are computed upon different sampler and thus, are not entirely comparable. The following histogram (figure 7) shows the overall improvement in a more visual manner. It can be observed that apart of having more epochs with valid data, the position errors values are significantly decreased also. This behavior is observed both for horizontal and vertical.

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Figure 7: HPE histogram for the degraded scenario.

Integrity was also evaluated in this case and, as seen for the nominal case, the safety indexes achieved are equivalent in both cases (GPS or GPS+GLONASS). The following table (table 4) shows the results achieved:

Table 4: Safety index for 30º user mask angle for 4 analyzed RIMS stations

Horizontal Safety Index

Vertical Safety Index RIMS

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GPS+GLO GPS only

GPS+GLO

BRN 0.0909 0.0955 0.0913 0.0990 ROM 0.0941 0.0901 0.1119 0.1037 LAP 0.0954 0.0896 0.1108 0.1130 SDC 0.1024 0.1526 0.1231 0.1176

A MULTICONSTELLATION APPROACH TO ENHANCE CURRENT SBAS SIGNAL IN SPACE Even if no GLONASS augmentation is provided to the user, the use of multiconstellation data in the EGNOS processing chain could potentially enhance the quality of the single frequency GPS augmentation. After a detailed analysis of the EGNOS design, the conclusions were that the main aspects where the combined use of more than one constellation (GPS and GLONASS in our study) could have a positive influence on the GPS augmentation information were: The Ionospheric Corrections and Integrity function.

Having input data from two constellations could improve very significantly the monitorisation of the ionosphere over the service area, providing additional redundancy to improve the quality of the ionospheric corrections and could even allow to, reduce the GIVE values without loss of integrity.

The potential added value of using two constellations for the realization of the EGNOS Network Time (ENT), and the estimation of precise clocks for the reference stations.

Other potential benefits, for instance on orbit determination or integrity assessment could be worth to consider, but they were given lower priority than the other two aspects. In spite of the potential benefits, one of the objectives of the study was focused on assessing carefully potential detrimental aspects that mixing constellation may bring into the processing. We should not forget the EGNOS maxima that “GLONASS processing shall not be detrimental to the GPS service levels”. Hence the following step in the experimentation campaign developed was focused on the assessment of the benefits and potential drawbacks obtained when GLONASS data, single and dual frequency, are used in the processing chain for the computation of the SBAS information. The following sections outline the main findings of this activity. USE GPS AND GLONASS TO IMPROVE THE EGNOS NETWORK TIME REALISATION EGNOS actually does not permit to use GPS and GLONAS on the ground clock estimation and EGNOS Network Time (ENT) realization, hence the first step was to adapt, magicSBAS to include the GLONASS constellation in the RIMS clock synchronization process in the EGNOS algorithm for the clock corrections computation.

For the synchronization process, it is necessary to select and form the satellite common-view measurements and the covariance matrices from smoothed L1 pseudo-ranges data for the reference ENT ensemble and non-ensemble clocks. As mentioned before, in the present EGNOS algorithm, only GPS satellites are used for this RIMS synchronization process. The modification implemented in the algorithm was focused on using also GLONASS satellites. As the algorithm does not act differently whether the input GLONASS data would be originated from single or dual frequency data, the only change to further use GLONASS was to use it also for the ground clocks estimation by including it into the computation of the common-view observations. For the analysis campaign, EGNOS data were used, that is, they contained only single frequency GLONASS data. Different executions of magicSBAS were performed for the chosen set of EGNOS Real data. The potential benefits and drawbacks obtained with the modifications in the accuracy of the RIMS clock estimation were analyzed by comparison of the results of the different magicSBAS executions.

The main potential benefit expected from the inclusion of GLONASS in the algorithm for the RIMS clock estimation, was the improvement on the stability and accuracy of the estimation, due to the increment of the number of common views. Certainly, the number of valid common views available is highly increased when GLONASS satellites are considered. In the following figure (figure 8) it is shown as an example the comparison of the number of valid common views for the RIMS in Kirkenes (Norway) versus time for each magicSBAS execution.

Figure 8: KIR clock offset validity evolution in time

Moreover, it has been observed that this increment on the available common-views, leads to an improvement on the availability of clock offset estimation for the time intervals when the estimation is not available using only GPS satellites. Therefore, the availability of RIMS clock offset estimation could be improved with the use of GLONASS in the process, especially in northern stations. Although the availability was indeed improve, in the analyzed cases no significant impact on the accuracy of the estimation is observed, neither improvement nor degradation, when GLONASS satellites are used for the RIMS synchronization process. The GLONASS measurement data used in this first step were only single frequency. These measurements are corrected by the ionosphere by using the EGNOS broadcast ionospheric model. This may lead to slightly less accurate iono-free measurements, compared with dual frequency iono-free combinations as used for GPS. Therefore, the inclusion of GLONASS constellation in the computing of the set of satellite common-view observations for use within the RIMS clock synchronization process could be affecting the algorithm performance in terms of overall estimation noise. If dual frequency GLONASS data would be used, then it is expected that the advantage of the additional number of common views, will be also complemented by an improved accuracy. In addition to the clock specific parameters, the signal in space performances at user level were also analyzed. As expected, no sensible impact was observed when GLONASS is used in the RIMS synchronization process. Yet this result obtained with a limited GLONASS constellation (only 14 SV) would remain to be confirmed for a complete constellation, in case of dual frequency processing and in scenarios of service area extension in which the RIMS synchronization issue might be more delicate to handle in GPS only mode. At this stage, until further studies are done as indicated above, it can only be concluded that the level of integrity, accuracy and availability is preserved by the system when GLONASS single frequency data is introduced in the RIMS clock offset computation and do not degrade GPS augmentation performances, even when only single frequency GLONAS data is available (as it is currently the case in EGNOS). USE GPS AND GLONASS TO IMPROVE THE IONOSPHERIC MODELLING The last step in the experimentation campaign was to analyze the potential benefits obtained when GLONASS dual frequency data are used together with GPS data within the EGNOS processing chain for ionosphere monitoring.

The main source of error when estimating ionospheric delays from GNSS, are the HW delays. Being the HW delays frequency dependent and taking into account that each GLONASS satellite transmits a different pair of frequencies the problem of the HW biases was expected to be potentially more relevant in the case of GLONASS data than for GPS.

One of the first objectives of the study was aimed to compare the ionospheric estimations obtained with each constellation separately on one hand, and with the combination of the two and to assess the main technical drawbacks that the combination of GPS and GLONASS data for ionosphere monitoring could present, like the interfrequency and intersystem hardware biases. The tool selected for this experimentation was GILION. GILION [8] is a software package to estimate the GPS/GLONASS satellite and receiver differential instrumental biases and the ionospheric total electron content (TEC) at GPS/GLONASS stations. GILION uses dual-frequency observations, both carrier phases and pseudoranges. Cycle-slips are detected and corrected for the ionospheric combination (L1-L2), so that the basic observables are the pseudorange-leveled carrier phases. This pre-process of data is done independently for each station and satellite, so that for each processed epoch, the biased slant ionospheric delay (containing the contribution of the inter-frequency biases) is obtained. In order to separate the contribution of the ionosphere from the inter-frequency biases, the biased ionospheric term is modeled as:

sr kkxtFeStI ),()()(

where t is the observation time; F represents the vertical TEC at the subionospheric point of the observation, and it is modeled as a second-order polynomial in x (spherical coordinates of the subionospheric point in a geocentric solar fixed reference system with the Z axis toward the solar center) around the zenith of the station; kr and ks are the receiver and satellite inter-frequency biases, respectively; and S is a mapping function relating the slant and the vertical TEC:

)cos(arcsincos

1)(

ehr

reS

where e is the elevation of the satellite, r is the mean radius of the Earth, and h is the height of the main ionospheric layer (350 km). For each day, the biased ionospheric terms from all the available stations are combined in a Kalman filter process, where the coefficients of the polynomial for each station are considered random walk stochastic processes and the biases are considered constant for the entire period (typically 24 hours).

As it is not possible to determine unambiguously all the satellite and receiver biases, one of them (normally one station) is set as a reference, k0, and then GILION estimates the satellite and station biases relative to the reference station:

k k k

k k kos s

o

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Ionospheric delays and HW biases were estimated using the following modes in GILION:

GPS: only GPS data are used. GPS-only results are the reference to compare the other results.

GLONASS: only GLONASS data are used for each station.

MIXED: both GPS and GLONASS data (in the same RINEX file) are used simultaneously for each station. A single station (GPS and GLONASS) HW bias is forced.

BOTH: both GPS and GLONASS data are separately pre-processed for each station and the two sets of data are treated as if they corresponded to two different stations. Two station (GPS and GLONASS) HW biases are allowed; but all of them are referred to a single GPS station.

Two networks of stations from the IGS network containing both GPS and GLONASS data were considered, the first one with different type/version of receivers for each station and the second with the same type and version of receivers, to try to correlate the results with the nature of the receivers selected. A first round of analysis considered each station separately. The following figure shows as an example the results obtained for the Vertical Total Electron Content (VTEC) for one of the stations considered (CONZ). The VTEC values have been estimated using different approaches as explained above.

Figure 5: VTEC for CONZ

The results show clearly that as far as the GPS and GLONASS data are used as if coming from a different stations, the results are nearly equivalent. However, when both sources of data are mixed and a single station bias is assumed, the estimated ionosphere is clearly biased with respect to the other estimations. As expected, there exists at least an inter-system bias. Moreover, the inter-system bias between GPS and GLONASS seemed to be different for each station. The next step of the process was to mix all network data on the same processing. In this approach, the satellite HW biases are expected to be common for all stations, and as a single bias is estimated at stations level, it is expected that the station bias is identical for all satellites. This is true in the case of GPS, but we have seen in the previous picture that at least one inter-system bias exists between GPS and GLONASS data at station level. The question that remains to be assessed is whether a single bias can be assumed at station level for all GLONASS satellites. When using a single station, each satellite bias is estimated separately, so it would absorb station bias components. When mixing a network of receivers that would not be the case and any bias across different GLONASS satellites at receiver level would show up as degradation in the estimation process. The dependency of the receiver HW biases with the frequency pair can be seen in the biases estimated when processing a single station:

),(),(),( 212121 ffkffkffk rss

r

Subtracting the values estimated with data from two different stations, the satellite contribution is removed and only the stations biases remains:

),(),(),(),( 212211212211 ffkffkffkffk rrsr

sr

If there were not dependency of the station HW biases with the pair of frequencies, then those values should be the same for all satellites. The following figures show the results obtained for these values for all GPS stations. As we can see, the values are just biased by the station HW bias, but other than that remain very stable.

Figure 6: Comparison of GPS satellite+station HW biases (ns) wrt satellite+conz_station

On the other hand, the behavior for the GLONASS satellite+station HW biases is quite different, presenting

the ),( 21 ffk sr values a large dependency on the pair of

frequencies. This is shown in the next figure (satellites with no data present zero values in this figure).

Figure 7: Comparison of GLONASS satellite+station HW biases (ns) wrt satellite+conR_station Therefore, the conclusion of this analysis was that the used GLONASS data present a dependency of the receiver differential HW biases with the pair of frequencies considered. These biases, when ignored, induce indeed and as expected a degradation of the ionospheric estimation, as illustrated in the following figure:

Figure 8: VTEC for ANKA (Ankara) It is clear from the previous results that the effect of the GLONASS station HW differential biases has to be properly removed to finally assess the impact of using GLONASS data for ionospheric monitoring. In order to do so, the following approach was followed: 1. GLONASS satellite+station differential HW biases

are estimated independently per station (this allows to take into account the dependency of GLONASS stations biases on the frequency).

2. The estimated satellite+station HW biases are removed from the biased ionospheric term

3. The ionospheric term free of HW biases for all stations are combined in the Kalman filter and the VTEC values at each station are estimated.

The following figure shows as an example the results obtained for the VTEC after the HW biases removal for the station considered in Figure 5 (CONZ).

Figure 9: VTEC for CONZ from HW bias-free iono terms With this approach the GLONASS derived VTEC values are totally consistent with the GPS derived VTEC values.

The most relevant conclusions derived from these results are hence that: The used GLONASS receivers present a dependency

of the receiver differential HW biases with the pair of frequencies considered. This dependency has to be calibrated or removed in order to get results equivalent to the GPS ones.

In term of accuracy, the VTEC results obtained with

GPS+GLONASS data are equivalent at the level of centimeters with the results obtained with only GPS data, so indeed GPS+GLONASS data can be used together to improve the SBAS ionospheric monitoring, once the receiver biases are properly dealt with.

At the light of these conclusions, the following recommendations could be put in place when planning the use of dual GLONASS data or, for tat matter, any other constellation data, in SBAS systems and in EGNOS in particular: It is recommended that the reference receivers are

calibrated, in such a way, that at most a single inter-system bias appears (if possible this inter-system bias should also be removed).

It is recommended to further work out algorithmic implementations allowing estimating and removing any residual station HW bias across channels.

In this study we have taken advantage of the capacities of GILION and the post-processing of the data to be able to isolate any bias within the receiver that is dependent of the frequency channel of the observed satellite, but that capacity should be brought forward to real-time processing algorithms as those implemented in EGNOS. With the implementation of any or both of the recommendations above, the combined use of GPS + GLONASS would have a great potential of providing a significant added value to EGNOS users. DISCUSSION OF RESULTS AND EXTRAPOLATION TO GALILEO The results obtained from the study so far are very reassuring towards the feasibility and added value of implementing a multiconstellation approach on SBAS services, once a fully mature second constellation is deployed. On one hand the study has demonstrated that a multiconstellation data processing on SBAS systems is perfectly feasible without major modifications on existing software.

By implementing these improvements on a tool like magicSBAS routine processing of multiconstellation data is available and the results that can be obtained for the second constellation corrections and integrity, in this case, with GLONASS, are perfectly compatible with those obtained for GPS. This leads to a significant improvement in user performances when multiconstellation augmentations are broadcast. Such improvement is already noticeable under nominal conditions, where GPS-only solutions are already meeting Civil Aviation requirements, but it makes itself more evident under degraded scenarios. The results obtained with GLONASS can be immediately extrapolated to a multiconstellation scenario consisting of GPS plus Galileo. The Galileo system is expected to provide significant advantages with respect to GLONASS in aspects like the total size of the constellation (27+3 vs. 24) , the signal quality and the usage of a CDMA separation approach as opposed to the FDMA separation mode used in GLONASS. The most significant risk identified in the study is linked precisely to the presence of receiver HW biases which depend on the satellite frequency, leading to multiple biases for the same reference station when GLONASS is used. Eliminating those biases would imply an increased complexity of the receiver design or on the processing algorithms. Galileo will not be subject to those problems, so in practice, the processing algorithms would just require a minimal adaptation to be able to processes combined GPS and Galileo signals for instance, for improving the ionospheric monitoring of the SBAS system even for legacy users. GPS + Galileo together with a multiconstellation SBAS augmentation system are currently the backbone of the design of the European MRS (Multiconstellation/ Multifrequency Regional System), and this paper has shown that the concept is perfectly feasible and can be already demonstrated today making use of the existing GPS and GLONASS data. The next step of this study will be to further investigate the benefit of multi constellation in degraded conditions of operations (e.g. EGNOS reference station or network failure, reduced GPS constellation size, RF environment quality degradation). From the MRS backbone so defined, other added value services, target users and applications may be envisaged in the future, but they require the foundations to be solidly established. With this study, and other activities already on-going and planned in Europe, these foundations will be laid for the future (already present!) multi-GNSS world. To this aim, end-to-end testing and demonstrations

of the benefit from multiconstellation (and also from multifrequency and multibroadcast) will be done on the field over the period 2010-2011 using the ESA system test beds to be deployed across Europe. These system tests will be performed in the frame of the GNSS Evolution Programme. ACKNOWLEDGEMENTS This paper summarises the results of a study project (PS_FUTUR_GLO) developed with the financial support of ESA. Many individuals have contributed to the realisation of this project both at ESA and GMV and without their contribution the work would not have reached the status it has and they could rightfully be considered as co-authors to this paper. On this regard, special thanks must be given to Esther Sardón, Mark van Kints and Nuria Pérez from GMV and to Daniel Brocard and Juan Pedro Lam from ESA. DISCLAIMER The work presented in this paper has been funded under a contract of the European Space Agency. The views presented are those of the authors and do not necessarily represent those of the Agency. REFERENCES [1] D Lekaim, D Brocard, D Flament, JC Levy, A Cezo-

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