Activities of the European Navigation Support Office
By Werner Enderle
The European Space Operations Centre (ESOC) in Darmstadt, Germany operates spacecraft on behalf of the European Space Agency (ESA) and maintains the ground facilities and expertise for ESA and other institutional and commercial customers. ESOC is composed of two departments: the Mission Operations Department and the Ground Systems Engineering Department, of which the Navigation Support Office is an integral part. The main objectives of the Navigation Support Office (NSO)are the provision of expertise for high-accuracy navigation, satellite geodesy, and the generation of related products and services for all ESA missions and for third-party customers, as well as supporting the European GNSS Programmes: Galileo and EGNOS.
In 2013, the NSO will conduct a number of projects and activities, described here.
The Navigation Office provides support in the area of data processing and analysis, performance analysis. It performs operational orbit predictions for the International Satellite Laser Ranging Service (ILRS), operational precise/rapid orbit and clock determination, computation of antenna patterns, and provides support to Galileo Sensor Stations (GSS) site deployment and to Ranging and Integrity Monitoring Station (RIMS) deployment. It also provides consultancy on modeling and data processing, mission analysis for the constellation, orbit validation activities for orbits and clocks, ionosphere, group delays, and intersystem biases, and is involved in the generation of the Galileo Geodetic Reference Frame. Furthermore, the Office participated in European Commission studies for the Galileo Commercial Service.
Earth Observation Missions
A number of European and American missions have been equipped with radar altimeter instruments that observe the level of the sea surface from space. To do this, the height component of the satellite orbits needs to be determined with centimeter-accuracy, matching the accuracy of the altimeter observations. The NSO provides support to Precise Orbit Determination (POD), evaluation, analysis and improvement of models and standards, as well as instrument calibration (radar altimeter and GNSS antenna).
Examples of missions already supported include ERS, Envisat, Cryosat, GOCE and also non-ESA missions JASON 1&2. Solutions with multiple simultaneous data types (GNSS, SLR, DORIS, altimetry, S-band range, Doppler, and angle tracking) are typically performed, allowing the alignment of different reference frames and estimation of inter-system and instrument biases. Based on all these capabilities, the NSO is one of the leading institutions for low-Earth orbiting (LEO) satellite POD activities and very well suited for supporting the upcoming European programme for Earth Observation, called Global Monitoring for Environment and Security (GMES) and its related Sentinel satellite missions.
Automated Transfer Vehicle
The Automated Transfer Vehicle (ATV) is part of the European contribution to the International Space Station (ISS) program. The main tasks of the ATV are to provide logistics supply, station re-boost and ISS waste retrieval. The rendezvous of the ATV and ISS is based on a real-time on-board relative navigation concept, using GPS data from receivers of ISS and ATV. The NSO conducts in this context simulations before the flight and also post facto performance analysis of the relative orbit determination accuracy to support the ATV missions.
Space Situation Awareness
An important atmospheric application of GNSS data is the monitoring of ionospheric activity (total electron content or TEC). Dual-frequency GNSS signals enable direct measurement of this parameter, and by merging the data from hundreds of globally distributed GPS receivers, detailed maps of the TEC and its evolution as a function of time can be constructed. Such maps have been computed routinely for many years. FIGURE 1 shows an example. The importance of these products lies in the fact that high solar activity leads to high TEC values, which can seriously disturb satellite communications. The NSO provides ionospheric TEC maps to the scientific community.
International GNSS Services
ESA/ESOC was one of the founding members of the IGS, and at the time the NSO was implemented at ESOC, all of the IGS activities were transferred to the NSO. ESA Analysis Centre products are among the best products available from the individual IGS analysis centres. Secondly, the ESA products are among the few multi-constellation GNSS products. ESA was the first IGS analysis centre to provide a consistent set of orbit and clock products for all available GNSS satellites. These products constituted the very first products that have been used for true GNSS precise point positioning.
The sampling rate of the ESA final GPS+GLONASS clock product is 30 seconds. FIGURE 2 shows the statistics of a kinematic PPP analysis using the ESA GNSS clocks for three different cases. The ESA/ESOC IGS Analysis centre contributes to all of the core IGS analysis centre products: Final GNSS (GPS+GLONASS) products provided weekly based on 24-hour solutions using 150 stations from true GNSS solutions simultaneously and fully consistently processing GPS and GLONASS measurements for a total of around 55 satellites, consisting of orbits, clocks, coordinates, ionosphere, and Earth-orientation parameters (EOPs). Also Rapid GNSS (GPS+GLONASS) products (available within 3 hours after the end of the observation day) and Ultra-Rapid GNSS (GPS+GLONASS) products (4 times per day, available within 3 hours after the end of the observation interval) are provided. These products are publicly available to the scientific community, being published at several data servers, such as the CDDIS at NASA’s Goddard Space Flight Center. They are also finding very frequent application in testing of experimental and commercial applications, and have become the standard reference for all high-precision GNSS applications.
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.
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.
Participation in the IGS Real Time activities has stimulated ESOC’s involvement in the development of standards and formats for GNSS data and products. ESOC has been instrumental in the decision of the IGS to join the Radio Technical Commission for Maritime Services (RTCM), which is the primary standards setting organisation for real-time GNSS services. ESOC is now one of two agencies that represent the IGS at the RTCM meetings.
Work with the RTCM focuses on:
- development of standards and formats for transmission of multi-constellation observations in real time (RTCM-MSM);
- development of standards and formats for the transmission of real-time orbit and clock products (RTCM-SSR);
- Further development of the RINEX standard for generation of multi-GNSS batch observation files.
Expertise and Areas of Activities
To comply with the main objectives of the NSO, the main pillars of expertise and areas of activities can be summarized as:
- Precise orbit determination at centimeter-level accuracy for satellites in low-Earth orbits such as Earth observation missions, and satellites in medium-Earth orbits, typically GNSS satellites.
- Development of state-of-the-art models and algorithms for high-precision orbit and clock determination, based on the capability to process all geodetic data types, namely GNSS, satellite laser ranging, Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), altimetry, and S-band tracking data.
- Realization of Geodetic Reference Frame.
- Operation of global distributed real-time sensor stations and networks, based on remote control of GNSS receivers.
- The capability to operate complex navigation software infrastructure to generate operational products and services for a wide variety of applications.
- Involvment in several international organized and coordinated activities. Besides being an IGS analysis center, ESOC’s NSO is also an analysis centre for the IDS and ILRS services.
ESOC’s ESOC’s Navigation Facility (see FIGURE 3) provides a fully operational environment, compliant with ESA’s ECSS ground segment standards. The Navigation Facility consists of a control room including secure operational LAN (ESACERT against intruders from outside) with two physically separated computer and data centres for redundancy purposes and a globally distributed operational real time sensor station network (see FIGURE 4). An operational system availability of more than 99.9 percent on a 24/7 basis measured over the last 5 years (products delivered every 15 minutes) has been demonstrated.
Currently the sensor station network consists of 12 sites, but ESOC is extending the global network to at least 25 sites. Negotiations with new sites are currently ongoing or near completion. The objective is to deploy a homogeneous (all sites will have the same receiver and same antenna type) sensor station network by the third quarter of 2013. The deployment of new equipment on existing sites began in April 2012, and first results are very promising. The new type of geodetic quality GNSS receiver has been chosen, based on an internal selection process, and deployment is under way. Each receiver has 264 physical channels, is capable of multi-signal, multi-frequency and multi-constellation tracking and will be remotely controlled from the Navigation Facility at ESOC.
The NSO develops, maintains and operates a range of software packages and tools for high-precision orbit- and clock determination and prediction. The software capability also includes the estimation of station coordinates, Earth-orientation parameters, model parameters (radiation pressure, drag, and so on), ionosphere, troposphere, instrument biases, intersystem biases, ambiguities and antenna phase-centre variations based on state-of-the-art models and standards (for example, IERS, ITRF).
The main software packages used within the NSO are:
- NAPEOS, which is the ESOC standard for high-precision navigation tasks. NAPEOS is used for almost all projects and is compliant with the highest navigation accuracy requirements, based on batch processing techniques with the capability to process different types of geodetic observations.
- RETINA, the NSO’s real time software package for GNSS based precise navigation. This software is based on Kalman Filter techniques and has a closely coordinated interface to NAPEOS.
- IONMON, processing GNSS data and producing ionosphere information and TEC map predictions.
In this context it is important to mention that ESA owns all the intellectual property rights to these software packages and that licences for operationally qualified software can be released on request to European companies, universities and R&D 0rganisations (currently only NAPEOS).
Summary and Outlook
The Navigation Support Office offers a combination of different capabilities, namely highest quality software, tools for real-time and batch processing ( the Office is the only analysis centre capable of processing three different geodetic techniques within a single software package), operation of own global GNSS sensor station network and demonstrated operational experience for mission support and provision of services. Operations are conducted in a controlled environment, fully in accordance with ESA safety and security standards.
The Navigation Support Office is ready for multi-frequency, multi-signal and multi constellation GNSS data processing. The Office is involved and strongly committed to support Galileo and EGNOS. In this context, the Office will soon become the consortium leader for the provision of the Galileo Geodetic Reference Frame.
Concerning the participation to international GNSS activities like IGS, ICG and GNSS standardisation aspects, the Navigation Support Office intends to continue its support for the foreseeable future.
In the area of LEO POD, the Navigation Support Office offers POD capability for all types of LEO satellites. For this reason, the Office intends to play a major role in the precise orbit determination activities for the European GMES Sentinel satellite missions.
Finally, the Navigation Support Office also intends to increase its capabilities related to navigation concepts for high-precision satellite formation flying and satellite constellations, via specific research and development activities. The aim is to maintain and expand its capabilities as a very attractive partner with cutting edge know-how and technology for the support of ESA activities and European industry.
Werner Enderle is the head of the Navigation Support Office at ESA\ESOC. Previously, he worked at the European GNSS Authority (GSA) as the Head of System Evolutions. He also worked for the European Commission, in charge of the procurement for the Galileo Ground Control Segment. He holds a doctoral degree in aerospace engineering from the Technical University of Berlin, Germany.
Co-authors: Rene Zandbergen, Tim Springer, and Loukis Agrotis.