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Activities of the Navigation Support Office

European GNSS

The Navigation Office provides support in the area of data processing and analysis, performance analysis. It performs operational orbit predictions for the International Satellite Laser Ranging Service (ILRS), operational precise/rapid orbit and clock determination, computation of antenna patterns, and provides support to Galileo Sensor Stations (GSS) site deployment and to Ranging and Integrity Monitoring Station (RIMS) deployment. It also provides consultancy on modeling and data processing, mission analysis for the constellation, orbit validation activities for orbits and clocks, ionosphere, group delays, and intersystem biases, and is involved in the generation of the Galileo Geodetic Reference Frame. Furthermore, the Office participated in European Commission studies for the Galileo Commercial Service.

Earth Observation Missions

A number of European and American missions have been equipped with radar altimeter instruments that observe the level of the sea surface from space. To do this, the height component of the satellite orbits needs to be determined with centimetre-accuracy, matching the accuracy of the altimeter observations.  The Navigation Support Office provides support to Precise Orbit Determination (POD), evaluation, analysis and improvement of models and standards, as well as instrument calibration (radar altimeter and GNSS antenna).  Examples of missions already supported include ERS, Envisat, Cryosat, GOCE and also non-ESA missions JASON 1&2. Solutions with multiple simultaneous data types (GNSS, SLR, DORIS, altimetry, S-band range, Doppler, and angle tracking) are typically performed, allowing the alignment of different reference frames and estimation of inter-system and instrument biases. Based on all these capabilities, the Navigation Support Office is one of the leading institutions for Low Earth Orbiting (LEO) satellite POD activities and very well suited for supporting the upcoming European programme for Earth Observation, called Global Monitoring for Environment and Security (GMES) and its related Sentinel satellite missions.

Automated Transfer Vehicle

The Automated Transfer Vehicle (ATV) is part of the European contribution to the International Space Station (ISS) program. The main tasks of the ATV are to provide logistics supply, station re-boost and ISS waste retrieval. The rendezvous of the ATV and ISS is based on a real-time on-board relative navigation concept, using GPS data from receivers of ISS and ATV. The Navigation Support Office conducts in this context simulations before the flight and also post facto performance analysis of the relative orbit determination accuracy in order to support the ATV missions.

Space Situation Awareness

An important atmospheric application of GNSS data is the monitoring of ionospheric activity (total electron content or TEC). Dual-frequency GNSS signals enable direct measurement of this parameter, and by merging the data from hundreds of globally distributed GPS receivers, detailed maps of the TEC and its evolution as a function of time can be constructed. Such maps have been computed routinely for many years. Figure 1 shows an example. The importance of these products lies in the fact that high solar activity leads to high TEC values, which can seriously disturb satellite communications. The Navigation Support Office provides ionospheric TEC maps to the scientific community

International GNSS Services

ESA/ESOC was one of the founding members of the IGS, and at the time the Navigation Support Office was implemented at ESOC, all of the IGS activities were transferred to the Navigation Support Office. ESA Analysis Centre products are among the best products available from the individual IGS analysis centres. Secondly, the ESA products are among the few multi-constellation GNSS products. ESA was the first IGS analysis centre to provide a consistent set of orbit and clock products for all available GNSS satellites. These products constituted the very first products that have been used for true GNSS precise point positioning.

The sampling rate of the ESA final GPS+GLONASS clock product is 30 seconds. Figure 2 shows the statistics of a kinematic PPP analysis using the ESA GNSS clocks for three different cases. The ESA/ESOC IGS Analysis centre contributes to all of the core IGS analysis centre products: Final GNSS (GPS+GLONASS) products provided weekly based on 24-hour solutions using 150 stations from true GNSS solutions simultaneously and fully consistently processing GPS and GLONASS measurements for a total of around 55 satellites, consisting of orbits, clocks, coordinates, ionosphere, and Earth-orientation parameters (EOPs). Also Rapid GNSS (GPS+GLONASS) products (available within 3 hours after the end of the observation day) and Ultra-Rapid GNSS (GPS+GLONASS) products (4 times per day, available within 3 hours after the end of the observation interval) are provided. These products are publicly available to the scientific community, being published at several data servers, such as the CDDIS at NASA’s Goddard Space Flight Center. They are also finding very frequent application in testing of experimental and commercial applications, and have become the standard reference for all high-precision GNSS applications.

Third Party Activities

Different customers have different needs. One important customer for the Navigation Facility is the Metop mission operated by EUMETSAT. For the exploitation of its GNSS Receiver for Atmospheric Sounding (GRAS) payload, which delivers atmospheric profiles to the European Met offices, EUMETSAT requires GPS products with a guarantee on accuracy, availability and latency. To deliver this service, the Navigation Facility now hosts the operation of the GRAS Ground Support Network (GSN), which is a dedicated network of 45 stations. It has been operating successfully for five years, delivering products with a latency of only 45 minutes, and an availability of better than 99 percent. Based on these, EUMETSAT delivers a daily set of more than 500 atmospheric profiles (and double that number as soon as Metop-2 will be operational) to the European Met offices, a data set that has already become one of the key elements in numerical weather prediction.

As an example for Navigation Office activities related to Metop, see the basic principle below:

The Metop satellites can “see” the lower flying GPS satellite, they can capture the GPS navigation signals via an on-board GPS receiver. As these GPS signals travel through the upper atmosphere, they are affected in ways that, though specialized processing, the Navigation Support Office provide valuable data on the ionosphere and troposphere, including the concentration of charged ions, and temperature, pressure and humidity measurements.

Radio occultation sounding of the earth's atmosphere using GNSS signals

Real-Time Processing

Over the last 10 years, ESOC has embarked on a program to build a Real Time GNSS software infrastructure. The main justification for this effort is the realization that the delivery of precise GNSS products in real-time processing will become increasingly more important for the user community. ESOC needs to be at the forefront of these developments, particularly with respect to products related to Galileo. The system for REal TIme NAvigation (RETINA) has been modelled after ESOC’s experience in real-time satellite control systems and includes many of the elements for data processing, archiving, and visualization that are common to such systems. In particular, it implements a specially designed circular filing system for streaming data, allowing maintenance-free operations for processing and archiving of data and products, and seamless transitions from historical to live data processing.

The investment in GNSS software and receiver infrastructure has enabled ESOC to participate in the IGS Real Time Pilot Project, assuming the roles of Real Time Analysis Centre and Analysis Centre Coordinator. In the latter role, ESOC has been generating and disseminating the IGS Real Time Combination stream after processing the real-time solutions from up to ten analysis centres. Included in these solutions are two streams generated by the ESOC Real Time Analysis Centre.

Standardization Activities

Participation in the IGS Real Time activities has stimulated ESOC’s involvement in the development of standards and formats for GNSS data and products. ESOC has been instrumental in the decision of the IGS to join the Radio Technical Commission for Maritime Services (RTCM), which is the primary standards setting organisation for real-time GNSS services. ESOC is now one of two agencies that represent the IGS at the RTCM meetings.

Work with the RTCM focuses on:

  • Development of standards and formats for transmission of multi-constellation observations in real time (RTCM-MSM).
  • Development of standards and formats for the transmission of real-time orbit and clock products (RTCM-SSR).
  • Further development of the RINEX standard for generation of multi-GNSS batch observation files
Another key activity on the Navigation Support Office’s horizon includes support to Europe’s new GNSS system, Galileo. The Office, in conjunction with Germany’s GeoForschungsZentrum (GFZ) and a small group of European partners, will become a Galileo Geodetic Service Provider to the Galilieo Joint Undertaking (GJU) — the organization established by the European Commission and ESA to select the concessionaire responsible for operating Galileo. In this role, the Office will be responsible to determine the “fundamental reference frame,” used to map the location of the system’s satellites.

In addition, and in partnership wit the GFZ, the Navigation Support Office will be involved with the establishment of a network of ground-based Galileo signal receivers, used to test and validate the first two Galileo satellites. This service of the Office is conducted on behalf of Galileo Industries, the consortium of industrial partners responsible for the building of the Galileo system.