The ESA Navigation Office.
Navigation Support Office Provides Services for IGS and Users
By Werner Enderle, Loukis Agrotis, Rene Zandbergen, Mark van Kints, and Jens Martin
The European Space Operations Centre has taken on the roles of real-time analysis center, data provider, and analysis-center coordinator for the International GNSS Service’s Real-Time Service, providing a number of products combining data streams from multiple sources.
The Navigation Support Office of the European Space Agency’s Space Operations Centre (ESA/ESOC) in Darmstadt, Germany, has for the last decade been involved in activities related to the provision of real-time GNSS augmentation services. The motivation for these activities is to support a number of ESA objectives, including:
Orbit determination support for low-Earth orbit missions using GNSS;
Development and validation of operational capabilities, with an emphasis on Galileo;
GNSS infrastructure development, including advanced techniques for better exploitation of the European GNSSs, Galileo, and EGNOS;
Research, development, and support to European industry through technology transfer.
The concept adopted is the generation of precise GNSS orbits using state-of-the-art batch orbit-estimation software. The predicted orbits, accurate to a few centimeters, are used in a Kalman filter, operating in real time, to estimate precise corrections to the satellite clocks from GNSS observations received from a global real-time receiver network. The orbit and clock products can then be made available to users with a latency of 3–4 seconds from the observation epoch.
The software architecture is modeled after concepts used in satellite control centers with the real-time observation and product streams treated in the same way as satellite telemetry data. A concept of circular history files has been developed, combining seamless real-time processing and retrieval capabilities with the ability to archive data for historical playback. Extensive display and visualization capabilities are also available.
Participation in the International GNSS Service (IGS) Real-Time Pilot Project has enabled validation of the ESOC software, with continuous operation and monitoring of two solution chains, starting in 2008. As the IGS Real-Time Analysis Center coordinator, ESOC has developed and operates a real-time combination solution, combining streams from multiple sources, as an offering of the IGS Real-Time Service, formally launched in April 2013.
GNSS Infrastructure
The ESOC software infrastructure modeled after real-time satellite control systems 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. Additionally, it includes a highly sophisticated job scheduler for automating operations and an integrated events and alarms monitoring system.
The software subsystems belong to one of three functional categories:
Infrastructure. Software is written in C++. The main components are middleware elements for history filing and event logging and a job scheduling application. All middleware elements have C++, Java, and FORTRAN interfaces.
Algorithmic. Software is written in FORTRAN 90, C++ or Java. It incorporates applications for real-time and batch data processing and estimation and for generation of products and comparison statistics between results sets.
Visualization. Software is entirely written in Java for portability. It includes real-time graphical and alphanumeric display applications and the graphical user interface.
Figure 1 shows the integrated desktop that provides all the functions for software configuration, monitoring, and control. Also shown are examples of graphical and alphanumeric displays. The integrated desktop combines the job scheduler display (left side) with the events display (right), allowing the operator to easily monitor the status of all running batch and real-time applications.
Figure 1. Real-time processing desktop and sample displays.
The job scheduler is configured to submit all batch jobs at pre-defined times or intervals, and to monitor the real-time applications. The batch orbit determination function is typically executed every two hours and includes jobs for screening and processing observations from up to 80 stations. The predicted orbits from these runs are updated to provide the most recent information to the real-time estimation.
The job scheduler also acts as a watchdog to ensure that all real-time processes (resident tasks) are continuously running. Any abnormal termination is detected, and the relevant task is restarted automatically. This can also guard against hardware failures, because tasks can be configured to run on more than one hardware node and will be restarted on a backup node if the prime fails.
Resident tasks are used for processing and filing observation and broadcast ephemeris messages and for performing the real-time estimation. The real-time estimation processes phase and pseudorange observations arriving at the rate of 1 Hz and screens the data to detect outliers and cycle slips. It uses a Kalman filter to estimate multi-GNSS satellite and receiver clock corrections, tropospheric zenith delays at each observing site, and phase biases for each satellite-receiver link. The estimation interval is user-configurable and is currently set at 5 seconds. The estimated satellite clock corrections and predicted orbit information are sent to an output stream and disseminated to users in the form of RTCM SSR messages.
The software capabilities were originally designed to support the GPS constellation. These capabilities have now been extended to support all the available GNSS constellations, with emphasis on Galileo. In addition to multi-constellation, the capability of multi-frequency processing has been added.
A network status monitoring display in the form of a world map (see Figure 2) gives the operator an overview of the network data flow. Station and satellite icons are color-coded to reflect the health of the live data links. It is also possible to see the number of live links to each station or from each satellite and the data latency and percentage availability of the observations from each station.
Figure 2. GNSS network status monitoring display (GPS-only).
To supplement the investment in software, ESOC has maintained and expanded the capabilities of its receiver network. This takes advantage of the existence of a number of ESA-operated satellite tracking sites with the necessary infrastructure (power, communications, atomic frequency standards, concrete pillar for mounting of the GNSS antennas) to host GNSS equipment with minimal additional operating costs. All ESA sites are now equipped with multi-GNSS capability receivers and associated antennas. Additional sites are also being procured with the objective of creating an independent network of around 30 sites with global coverage.
Real-Time Activities, Projects
The investment in GNSS software, equipment, and infrastructure has enabled ESA to participate in a number of projects with institutional and commercial partners.
As a major contributor to the IGS, ESOC has been a strong supporter of the IGS Real-Time Pilot Project. Since the original call for participation, and through to the establishment of the recently launched (April 2013) IGS Real-Time Service (RTS), ESA has played a leading role by assuming the roles of real-time analysis center, data provider, and analysis-center coordinator. In the latter role, ESOC is responsible for the generation of the RTS products and has been generating and disseminating IGS real-time combination streams after processing the real-time solutions from up to 10 analysis centers. Included in these solutions are two streams generated by the ESOC Real-Time Analysis Center. One of these uses orbit information generated by the NAPEOS software (ESOC’s Navigation Support Office standard software package for precise orbit determination), which provides orbit updates every 2 hours. The second ESOC solution stream uses the IGS rapid orbit product, which is updated every 6 hours.
Stemming from the recognition that real-time services rely on the development of standards and data formats, ESOC has been instrumental in aligning the interests of the IGS community with those of the Radio Technical Commission for Maritime Services (RTCM). ESOC, along with NRCan, represents the IGS at RTCM meetings. Over the last 4–5 years, this forum, which brings together GNSS service providers, users, and receiver manufacturers, has made significant progress in agreeing on standards for:
real-time orbit and clock correction messages in state space representation (SSR) format;
new multi-GNSS standards for real-time high-precision observations and for broadcast ephemeris dissemination.
ESOC also represents the RTCM at the Galileo Geodetic Reference Interface Working Group, a group of experts advising the EC on exploitation of Galileo services for the geodetic community.
In its mandate to assist European industry, ESOC has been working with Fugro for software development related to the implementation of high-precision augmentation services. The Fugro G2 service, providing augmentation products for GPS and GLONASS, uses software developed by ESA and has been operational since early 2009. The service is being extended to include Galileo, with successful trials already demonstrated by Fugro.
Capabilities and Performance
In terms of the IGS RTS, Figure 3 shows the performance of the combination solution produced by ESOC from the results of the contributing analysis centers. The plots show daily clock standard deviations and 1-D RMS orbit differences between the combination solution and the IGS rapid solution. It can be seen that the clock results are of the order of 0.1 nanosecond and the orbit differences at the level of 30–40 millimeters. The advantage of the combination is the ability to identify and eliminate outliers, by examining the differences between the contributing analysis-center solutions. It can be seen that the outliers affecting the early results have been eliminated, with very stable results since around GPS week 1650.
Figure 3. Real-time service orbit and clock comparisons against IGS rapid products.
The monitoring of the RTS clock solutions in the precise point positioning (PPP) domain is performed by BKG. Figure 4 shows the kinematic PPP performance of one of the ESOC solutions over an interval of 24 hours. It can be seen that accuracies at the decimeter level can be achieved.
Figure 4. Example of kinematic PPP performance of ESOC solution.
To highlight the importance of combining computational and visualization capabilities, the plot in Figure 5 shows the estimated satellite clock behavior of GPS satellite G01. Since the middle of January 2013, the satellite clock started exhibiting a series of clock jumps with a magnitude of 3 nanoseconds. This pattern was observed once per orbit, with clock jump events every 12 hours. The problem was resolved on February 6, with the satellite being taken out of service and reconfigured. The ESOC capabilities allow for the detection and monitoring of such events in real time, creating the possibilities for a timely response (for example, by suppressing the problematic satellite) to ensure the service is not degraded.
Figure 5. GPS PRN-1 anomalous clock behavior.
The software visualization capabilities also allow the possibility to identify and visualize signal problems with the satellites. In the example in Figure 6, GPS satellite G30 is seen to be tracked by 14 receivers at 19:43:19 on April 11, 2009. The live links are identified by the light blue lines radiating from the satellite. In the next snapshot, at 19:44:35, all 14 receivers appear to have lost the measurements from this satellite, as the grey lines indicate geometric visibility but no measurements arriving at the stations. At the same times, the receivers are continuing to track other satellites. This behavior has been observed a number of times and is known to affect only the Block IIA range of GPS satellites. A loss of measurements for a period of 1–2 minutes is typically observed.
Figure 6. Signal drop from Block IIA GPS satellite.
Conclusions
The latest improvements of ESOC’s Navigation Support Office software provide full multi-frequency and multi-constellation processing capability. The IGS Real-Time Service is provided as a routine operational service since April 2013, enabling a kinematic precise point position solution at accuracy levels in the 10–20 centimeter range. Existing ESOC real-time capabilities are also ready for potential use within Galileo.
Acknowledgements
ESOC is working with a large number of partners and real-time analysis centers. In particular we would like to thank BKG, NRCan, GFZ, CNES, DLR, GMV, JPL, IGS Governing Board, Fugro, GEO++, TUW, WHU, Geoscience Australia, NGS, UPC.
Werner Enderle is the head of the Navigation Support Office at ESA\ESOC. Previously, he worked at the European GNSS Authority and 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.
Loukis Agrotis, with his company Symban, is a contractor for ESA working on the development of ESOC’s Real-Time GNSS infrastructure. He is also the Analysis Centre Coordinator for the IGS Real-Time Pilot Project and represents the IGS at the Radio Technical Commission for Maritime Services (RTCM). He holds a Ph.D. in satellite orbits and the Global Positioning System from the University of Nottingham, UK.
René Zandbergen is a navigation engineer in ESA’s Navigation Support Office, based at ESOC in Darmstadt, Germany. He is involved in running operational activities related to high-precision and high-availability navigation support services in near-real time and real time. He holds a Ph.D. in satellite altimeter data processing from the Delft University of Technology in the Netherlands.