Real-Time and RTS

Real-Time

Single Frequency vs. Dual Frequency

IGS Real-Time Working Group Charter

Established: 2001

Chair: André Hauschild

Website: Real Time Working Group Website

Knowledge Base: Real Time Working Group Knowledge Base

Mailing List: Real Time Working Group Mailing List

Charter

Many very demanding applications and systems now require GPS raw data and products with greatly reduced delays. The requirement for real-time data and products is inevitable and it now seems prudent for the IGS to establish real-time systems.  Non-IGS near/real-time networks are emerging without standards to insure compatibility, integration, access, and sharing. To fully serve the multi-disciplinary scientific user community, the IGS must work towards enhancing its standards for infrastructure and data and product availability.  This will place the IGS in a position to, as soon as practical, serve real-time user’s needs.

The IGS Real-time Working Group (RTWG) will assess and address issues that pertain to the IGS developing real-time infrastructure and processes.  In addressing these issues the RTWG will cooperatively work towards a functional and scaleable model, which demonstrates the real-time delivery of raw data and the dissemination of products to real-time analysis centers and simulated or actual real-time users, respectively.  The activities of the RTWG will consist of the required planning, designing and implementing stages necessary for a prototype infrastructure and processes. A pilot project will be recommended following the completion of activities.

The RTWG will plan, design, and implement a prototype system for the support of precise real-time positioning guided by the principles of robustness, sustainability, and acceptance. The primary products of such a system will be GPS/GNSS station data and satellite orbits and clocks, made available to the user by Internet and other economical and available streaming technologies. Potential user groups include from among others; geodetic agencies mandated to provide access to a globally consistent reference frame for all position applications, precision navigation users (LEO), agencies involved in natural hazards monitoring, prediction, warning and response, structural engineering monitoring, near/real-time atmospheric monitoring for weather prediction, real-time earthquake seismology (simultaneously with seismological analysis), and time transfer and dissemination.

The requirements for the system will impact all components of the IGS and it will therefore be imperative that the RTWG receive cooperation and participation from all components with frequent and ongoing communication and meetings as required.

With the goal to maximize potential global acceptance and effectiveness the RTWG will:

In Phase 1:

  • Consider user requirements through consultation with real-time user representatives. These consultations will be used as a guide for developments and to maximize wide acceptance of the standards and specifications that are developed.
  • Research globally the existing Internet infrastructures and report on the suitability for real-time data delivery in various regions of the globe.
  • Investigate and adopt a real-time data format.
  • Investigate, adopt, and demonstrate a communications strategy for real-time data acquisition and dissemination.
  • Work with interested members who wish to be involved in the refinement of phase 1 deliverables.
  • Assist in coordinating and contribute to a “Towards Real-time” workshop aimed at developing plans to accommodate the changes required for areal-time component of the IGS.

In Phase 2: (Parts may occur in parallel with Phase 1)

  • Investigate, adopt and demonstrate a real-time communications strategy for product creation, combination, validation and dissemination.
  • Develop the real-time combination of orbit, clock and ionosphere products.
  • Develop a real-time robustness and reliability/integrity monitoring methodology.
  • Work with interested members who wish to be involved in the demonstration and refinement of phase 2 deliverables.

Ongoing: 

  • Communicate plans and seek input from beneficiary or complementary agency representatives, including IGS WGs, GPS/Met, Timing, Space Weather etc.

Expertise within the RTWG membership reflects the necessary background to deal with the issues in both phases 1 and 2. (Additional representation will be sought as required to help achieve the goals)

Real-Time Service (RTS) Pilot Project

The International GNSS Service (IGS) has ensured the availability of open access, high-quality GNSS data products since 1994. These products enable access to the definitive global reference frame for scientific, educational, and commercial applications – a tremendous benefit to the public.

Through the Real-time Service (RTS), the IGS extends its capability to support applications requiring real-time access to IGS products. RTS is a GNSS orbit and clock correction service that enables precise point positioning (PPP) and related applications, such as time synchronization and disaster monitoring, at worldwide scales. RTS is based on the IGS global infrastructure of network stations, data centers and analysis centers that provide world standard high-precision GNSS data products.

The RTS is currently offered as a GPS-only operational service. The Russian GLONASS is initially provided as an experimental product for the development and testing of applications and will be included within the service when the IGS feels confident that it is supported by a sufficient number of Analysis Centres to ensure solution reliability and availability. Other GNSS constellations will be added as they become available.

This service is made possible through partnerships with Natural Resources Canada (NRCan), the German Federal Agency for Cartography and Geodesy (BKG), and the European Space Agency’s Space Operations Centre in Darmstadt, Germany (ESA/ESOC). Support is provided by 160 station operators, multiple data centers, and 10 analysis centers around the world.

The RTS is operated by the IGS as a public service. Users are offered open and readily available access through subscription.

RTS User Access

RTS product streams are available through designated product distribution centers around the world. To access RTS, users must accept the terms of service and complete the online subscriber registration. After a brief processing period, users will be contacted with login and further information for connecting to the RTS streams.

 

RTS Client Applications

RTS is broadcast using the open standard NTRIP protocol, which encodes the satellite orbit and clock correction streams as RTCM State Space Representation (SSR) messages. Users must obtain an NTRIP client application to access RTS. As this is a relatively new standard, it is currently supported by a limited number of client applications, for example:

  • BKG NTRIP Client (BNC) Open Source program. This is a client software allowing Precise Point Positioning (PPP) in real-time wherever mobile communication means are available.
  • Real-time Kinematic Library (RTKLIB) Open Source tool set, which has a similar functionality embedded in its RTKNAVI program.

After downloading and installing the client application, users may configure access to the RTS Products.

 

Information for GNSS Equipment Manufacturers

NTRIP-based streaming of GNSS correction data enables world-wide, highly accurate Precise Point Positioning (PPP) following the State Space Representation (SSR) approach wherever mobile communication is available. As NTRIP is an RTCM standard, no licensing is involved regarding stream transport or stream format, allowing GNSS equipment manufacturers to freely integrate this functionality into their products. To encourage this, IGS will consider on a case basis providing generic company accounts for the RTS broadcasters that allow direct access to the RTS correction streams without specific user registration. Once GNSS equipment manufactures incorporate this functionality into their receiver firmware, RTS will be directly accessed through receivers having a built-in PPP capability. Interested manufacturers are requested to contact [cb at igs.org] for further information.

Register for User Access for individual caster streams:

IGS Central Bureau | BKG | CDDIS

RTS Products

The RTS Products consist of GNSS satellite orbit and clock corrections to the broadcast ephemeris. The RTS correction streams are formatted according to the RTCM SSR standard for State Space Representation and are broadcast using the NTRIP protocol. RTS corrected orbits are expressed within the International Terrestrial Reference Frame 2014 (ITRF 2014).

The product streams available in the RTS are combination solutions generated by processing individual Real Time solutions from participating Real-time Analysis Centers (RTAC). The effect of combining the different RTAC results is a more reliable and stable performance than that of any single AC’s product. Operational responsibility for the official combination products lies with the IGS Real Time Analysis Center Coordinator (RTACC).

The official products currently include corrections to the GPS satellite orbits and clocks:

  • IGS01/IGC01: Single epoch GPS combination is a single-epoch combination solution produced using software developed by ESA/ESOC. The solution epochs in this product are completely independent of each other, which has the advantage that the full accuracy is available as soon as product generation starts. The combination process removes a common offset from all satellite clocks at each epoch, computed by processing pairs of Analysis Center solutions, in order to align the clocks in each contributing solution. The aligned clocks are then screened for outliers and combined by calculating a weighted average clock value for each satellite. The orbit states are combined by using the average value from all contributions. The product is offered with the orbit solution referred to the antenna phase center under the designation IGS01. A parallel product in center of mass coordinates is designated IGC01. Analysis Center RTCM stream decoding and combination solution encoding uses BKG’s BNC software.
  • IGS02: Kalman fIlter GPS combination is a Kalman filter combination produced using BKG’s BNC software. The Kalman filter solution requires a few minutes convergence time to reach full accuracy. Once converged, the accuracy is maintained unless there is a reason to restart the software. A mechanism is in place to avoid publishing results during the convergence period. The orbit information in IGS02 is extracted from one of the incoming ultra-rapid solutions. In the Kalman Filter approach satellite clocks estimated by individual Analyses Centers (ACs) are used as pseudo observations within the adjustment process. Each observation is modeled as a linear function of three estimated parameters: AC specific offset, satellite specific offset common to all ACs, and the actual satellite clock correction which represents the result of the combination. These three parameter types differ in their statistical properties. The satellite clock offsets are assumed to be static parameters while AC specific and satellite specific offsets are stochastic parameters with appropriate white noise. The solution is regularized by a set of minimal constraints. A recursive algorithm is used to detect orbit outliers. The greatest difference between AC specific and mean satellite positions is computed. If this is greater than a threshold, then corrections of the affiliated AC are ignored for the affected epoch.

The IGS is working on including solutions for additional constellations, starting with GLONASS. A third combination product, IGS03, containing GPS and GLONASS corrections is offered as an experimental product:

  • IGS03: Kalman filter GPS+GLONASS combination is a Kalman filter combination produced using BKG’s BNC software and follows the same approach as IGS02. The major difference from IGS02 is that GLONASS corrections are included in addition to GPS. While clocks are combined for both systems in a Kalman filter approach, orbits are extracted from one of the incoming correction streams and checked for gross errors. Contributing analysis centers use orbit information either from IGS Ultra Rapid products (IGV), or disseminate their own orbit estimates. Note that a major part of the real-time IGS reference stations nowadays supports GLONASS.

Content Description of the RTS Product Streams

The RTS products are disseminated in the form of RTCM SSR streams. The technical content of the RTS products is described in the Table below. The products, designated at IGS01/ICG01 and IGS02, contain corrections only for the GPS satellites. The experimental product, designated at IGS03, contains corrections for GPS and GLONASS.

Stream Name Description Ref Point RTCM Messages Provider / Solution ID Bandwidth kbits Software
IGS01 Orbit/Clock Correction, Single-Epoch Combination APC 1059 (5),1060 (5) 258 / 1 1.8/sec ESA/ESOC
IGC01 Orbit/Clock Correction, Single-Epoch Combination CoM 1059 (5),1060 (5) 258 / 9 1.8/sec ESA/ESOC
IGS02 Orbit/Clock Correction, Kalman Filter Combination APC 1057 (60), 1058 (10), 1059 (10) 258 / 2 0.6/sec BKG
IGS03 Orbit/Clock Correction, Kalman Filter Combination APC 1057(60), 1058(10), 1059(10), 1063(60), 1064(10), 1065(10) 258 / 3 0.8/sec BKG
APC: Antenna Phase Center CoM: Center of Mass, (not compliant with current RTCM-SSR standard). The figures in brackets next to each RTCM message ID denote the message sample interval in seconds. Additional analysis center product streams may be available through the IGS casters.

 

 

The RTCM v3 streams listed above may be used to support development and testing of real-time Precise Point Positioning (PPP) and related applications.

RTCM v3 Message Types
1019 GPS Broadcast Ephemeris
1020 GLONASS Broadcast Ephemeris
1045 Galileo Broadcast Ephemeris
1057 GPS orbit corrections to Broadcast Ephemeris
1058 GPS clock corrections to Broadcast Ephemeris
1059 GPS code biases
1060 Combined orbit and clock corrections to GPS Broadcast Ephemeris
1061 GPS User Range Accuracy
1062 High-rate GPS clock corrections to Broadcast Ephemeris
1063 GLONASS orbit corrections to Broadcast Ephemeris
1064 GLONASS clock corrections to Broadcast Ephemeris
1065 GLONASS code biases
1066 Combined orbit and clock corrections to GLONASS Broadcast Ephemeris
1067 GLONASS User Range Accuracy
1068 High-rate GLONASS clock corrections to Broadcast Ephemeris

 

Additional product streams including individual Analysis Center product streams, broadcast ephemeris and streamed GNSS observation data are available through the RT Product Distribution Centers.

Orbit corrections are provided as along-track, cross-track and radial offsets to the Broadcast Ephemeris in the Earth-centered, Earth-fixed reference frame. After applying corrections, the satellite position is referred to the ‘ionospheric free’ phase center of the antenna (for the APC streams) or to the satellite Center of Mass (CoM streams). Clock corrections are given as offsets to the Broadcast Ephemeris satellite clock corrections.

The SSR format provides for the dissemination of signal code biases. These would be the biases to apply to the pseudo ranges for the signals that are processed in generating the RTS solution. Because of the complexities of the combination, and the fact that different Analysis Centers could potentially use different signals, it was decided to set the signal biases to zero in the current product streams. The user is encouraged to use other sources for signal bias solutions, if these are needed.

 

Content Description of the Broadcast Ephemeris Streams

RTS also provides real-time access to broadcast ephemeris. Two streams are generated which carry only ephemeris data and no observations. Incoming ephemeris are checked for plausibility then merged, encoded and uploaded to NTRIP broadcasters with a high repetition rate.

  • RTCM3EPH: Broadcast ephemeris for GPS, GLONASS and Galileo satellites are produced using BKG’s BNC software. The data stream is derived from a major part of receivers in the real-time IGS global network and is encoded in RTCM Version 3 messages. The complete set of messages is repeated every five seconds.
  • RTCM3EPH01: Broadcast ephemeris for GPS using DLR’s RETICLE software. The data stream is also derived from the real-time IGS global network and encoded in RTCM Version 3 messages with a five seconds repetition rate.

 

Stream Name Description RTCM Messages Supported GNSS Bandwidth kbits Software
RTCM3EPH Broadcast Ephemeris 1019(5), 1020(5), 1045(5) GPS, GLONASS, Galileo 6.0/sec BKG/BNC
RTCM3EPH01 Broadcast Ephemeris 1019(5) GPS 4.0/sec DLR/RETICLE

 

Regional Reference Frame Information

Global IGS RTS product streams refer to the International Terrestrial Reference Frame 2014 (ITRF2014). Other product streams can refer to Regional Reference Frames such as ETRF2000, GDA94, NAD83, SIRGAS95, or SIRGAS2000. Applying orbit and clock corrections from regional product streams in a real-time PPP solution automatically leads to regional coordinates. The PPP client would not need to transform coordinates because that is already done on the server side. More information on Regional resources can be found at http://igs.bkg.bund.de/ntrip/orbits#Transformations.

RTS Monitoring

Product quality is monitored by the Real-time Analysis Center Coordinator. The IGS RTS products are a continuation of products developed during the span of the IGS Real Time Pilot Project (RTPP). Details of this project can be found at http://www.rtigs.net. The IGS01/IGC01 product has been available within the RTPP since the beginning of 2010. Prior to that, a batch product using the same techniques was generated from daily file submissions of the ACs since mid-2008.

The continuous history of the performance of the IGS01 product shows that since spring of 2011, when a new outlier detection algorithm was implemented, the product has been very stable with clock standard deviations (sigma) of 0.1-0.15 ns. Performance comparisons are performed daily against the IGS rapid solution.

Performance comparisons are performed daily against the IGS rapid solution. The comparison plots are available here:


IGS01 Clock SD

IGS01 orbit

IGS02 Clock SD

IGS02 orbit

IGS03 Clock SD

IGS03 Orbit

 

 

Typical results are shown in the plot below, which also indicates the impact on the statistics of certain satellite-specific events.

Daily comparison reports
Daily comparison reports of clocks from different Analysis Centers are made available for download from IGS Global Data Centers:

Availability of Service

Availability of Service
Availability of Service Figure

In order to ensure robust distribution of the IGS RT products, and to maximize availability, a redundancy concept has been designed and implemented by the RTACC and the IGS Working group. The concept is illustrated in the Availability of Service figure. It relies on the parallel generation of each combination product on LINUX servers at two or more independent combination centers. In addition, the Analysis Centers themselves make their solutions available at two or more NTRIP casters.

The combinations are performed independently at each combination center and are then sent out as RTCM streams to two or more dissemination casters. What is important is that each caster mount point is supplied by two or more combination centers at the same time. If one combination center is unavailable, stream distribution switches automatically to the second combination center, without the user being aware of the switch.

If one dissemination caster is unavailable, the user will see an interruption in service. It is up to the user to switch to a second dissemination caster to resume receiving the product stream. It is also up to the user to establish procedures and software to allow reception of the streams from two or more dissemination casters simultaneously, to enable seamless processing in case one of the casters is unavailable.

PPP Performance

Monitoring of Precise Point Positioning (PPP) performance which may be expected by users under different scenarios is carried out by BKG. The following charts show 24h time series’ of position displacements that may be expected when using the RTS products.

Recent PPP performance of product stream IGS01

Recent PPP performance of product stream IGS02


Recent PPP performance of product stream IGS03

 

 

The chart below shows an example 30/90 day time series of daily 2D-RMS position errors that can be expected from kinematic PPP using RTS at a station in Frankfurt Germany. Historical PPP performance information is available for download from ftp://igs.bkg.bund.de/NTRIP/ppp.

 

30 days’ time series’ of 2D-RMS

90 days’ time series’ of 2D-RMS

RTS Contributors

The RTS is overseen by the IGS Real-time Working Group. The Real-time Analysis Center Coordinator (RTACC), currently the European Space Agency’s Space Operations Centre in Darmstadt, Germany (ESA/ESOC), has operational responsibility for the generation of the official combination products. These are generated at several combination servers and combine products from up to ten Real-time Analysis Centers. RTS is distributed through two primary Product Distribution Centers and a number of secondary centers that provide a highly available service.

Analysis Centers

The IGS combination products can only exist because of the contributions of the individual solution streams from the participating Analysis Centers. Below is a short description of the contributing solutions from the participating ACs.

Center Description
BKG GPS RT orbits and clocks using IGU orbits
GPS + GLONASS RT orbits and clocks using IGV orbits
CNES GPS RT orbits and clocks based on IGU orbits
GPS+GLONASS orbits and clocks
DLR GPS RT orbits and clocks based on IGU orbits
GPS+GLONASS orbits and clocks
ESA/ESOC GPS RT orbits and clocks using NRT batch orbits from ESOC s/w running every 2 hours
GPS RT orbits and clocks using IGU orbits
GFZ GPS RT orbits and clocks and IGU orbits
GMV GPS RT orbits and clocks based on NRT orbit solution
GPS+GLONASS orbits and clocks
Geo++ Not contributing at present. Working on RTCM SSR Standard.
NRCan GPS RT orbits and clocks using NRT batch orbits every hour
TUW Not contributing at present
WUHAN GPS RT clocks based on IGU orbits

 

The following agencies have additional functions in the RTS:

  • NRCan – RT Working Group Chair
  • ESOC – Real Time Analysis Center Coordinator
  • BKG – Data Flow Coordination

Combination Centers

The RTS Combination Centers are hosted by:

  • European Space Agency’s Space Operations Centre in Darmstadt, Germany (ESA/ESOC)
  • German Federal Agency for Cartography and Geodesy (BKG)
  • Natural Resources Canada (NRCan)

 

Product Distribution Centers

The IGS Central Bureau (IGSCB) and the German Federal Agency for Cartography and Geodesy (BKG) are the two primary Product Distribution Centers. Users may be also referred to an alternate product distribution centers upon confirmation of registration.

RTS Station Data Providers

  • Addis Ababa University – Ethiopia – (2)
  • Agenzia Spaziale Italiana – Italy – (1)
  • Agricultural University of Wroclaw – Poland – (1)
  • Alfred Wegener Institut – Germany – (1)
  • Astrogeodynamical Observatory – Poland – (1)
  • Astronomy and Space Science Institute – Korea – (2)
  • Brazilian Institute of Geography and Statistics – Brazil – (9)
  • Bucharest Technical University of Civil Engineering – Romania – (1)
  • Bundesamt fuer Eich- und Vermessungswesen – Austria – (1)
  • Centre National d Etudes Spatiales – France – (2)
  • Centre National d´Etudes Spatiales – France – (4)
  • Clark Fortune McDonald & Associates – New Zealand – (1)
  • Croatian Geodetic Institute – Croatia – (2)
  • DIST Universita di Cagliari – Italy – (1)
  • ESA/ESOC – Germany – (9)
  • FOMI Satellite Geodetic Observatory – Hungary – (1)
  • Federal Agency for Cartography and Geodesy – Germany – (29)
  • Finnish Geodetic Institute – Finland – (1)
  • Fugro Seastar AS – Norway – (3)
  • GOP Research Institute of Geodesy Topography and Cartographie – Czech Republic – (1)
  • GPS Solutions Inc. – U.S.A. – (1)
  • GSOC/DLR German Space Operations Center – Germany – (1)
  • GeoForschungsZentrum Potsdam – Germany – (14)
  • GeoNet – New Zealand – (5)
  • Geodetic Institute University Warszawa – Poland – (1)
  • Geodetic and Cartographic Institute – Slovakia – (1)
  • Geoscience Australia – Australia – (22)
  • Geospatial Information Authority of Japan – Japan – (5)
  • Institut Geographique National – France – (5)
  • Instituto Cartografico de Catalunya – Spain – (1)
  • Instituto Geografico Nacional – Spain – (1)
  • Instituto Geografico Portugues – Portugal – (3)
  • Instytut Geodezji i Kartografii Warszawie – Poland – (1)
  • Istanbul Technical University – Turkey – (1)
  • Istituto Nazionale di Ricerca Metrologica I.N.RI.M – Italy – (1)
  • L’equipe du reseau Banian – New Caledonia – (2)
  • NASA Stennis Space Center – U.S.A. – (1)
  • NERC Space Geodesy Facility – United Kingdom – (1)
  • National Geographic Information Institute – Korea – (1)
  • National Land Survey – Sweden – (6)
  • National Oceanic and Atmospheric Administration – National Geodetic Survey – U.S.A. – (8)
  • Natural Resources – Canada – (18)
  • Naval Observatory – U.S.A. – (1)
  • Point Inc. – Canada – (2)
  • Puerto Rico Seismic Network – Puerto Rico – (1)
  • Puget Sound Reference Network – U.S.A. – (1)
  • Regional Centre for Mapping of Resources for Development – Kenya – (1)
  • Regional Centre for Training in Aerospace Surveys – Nigeria – (1)
  • Rocco V. D’Andrea Inc. – U.S.A. – (1)
  • Royal Observatory – Belgium – (1)
  • Singapore Land Authority – Singapore – (1)
  • Solucoes em Posicionamento Global SPG – Brazil – (1)
  • Surveys and Mapping – South Africa – (2)
  • SwissTopo – Switzerland – (1)
  • Technical University Delft -The Netherlands – (1)
  • Teodonivel – Brazil – (1)
  • Trimbase Ltda – Brazil – (1)
  • UNAVCO – U.S.A – (1)
  • Universidad de Cordoba – Argentina – (1)
  • Universidad de Rosario – Argentina – (1)
  • Universidad del Zulia – Venezuela – (1)
  • Universidade Estadual Paulista UNESP/FCT – Brazil – (4)
  • Universidade da Baira Interior UBI/CGUL/IDL – Portugal – (1)
  • University New South Wales – Australia – (1)
  • University Padova – Italy – (2)

For more information