ESA’s recently opened Navigation Facility has fast become a world-class provider of highly accurate navigation information, significantly enhancing data from cornerstone systems including GPS, EGNOS and – soon – Galileo.

In full operation since February 2006, ESA’s Navigation Facility, located at ESOC, the European Space Operations Centre, looks like any other modest computer centre. The room is quiet and the rows of workstations and large LCD displays are often unattended. But looks belie activity, and the facility and its team of expert geoscientists and engineers are anything but tranquil.

ESA’s Navigation Facility is producing a growing series of processed data products providing some of the world’s most accurate orbit and clock calculations related to GNSS, or global navigation satellite systems. These consist of America’s well-known GPS (Global Positioning Satellite) system, Russia’s GLONASS system and, soon, Europe’s own Galileo system.

Underlining the sophistication and complex customer requirements of this work, the facility’s servers are fully backed up and redundant, physical access is security controlled, and the facility is connected to ESOC’s operations LAN, or local area network, meaning on-duty engineers can communicate twenty-four hours per day with other ESA operations facilities.

Direct connection to a worldwide network of GPS receivers

The facility is directly connected to a network of 44 GPS signal receivers located worldwide, and can receive data from several hundred others.

These receivers monitor signals from GPS satellites and relay them to just a handful of highly specialised processing centres, including ESOC’s Navigation Facility. These in turn process the raw data into valuable atmospheric and geoscience information sets.

The ESOC Navigation Facility cornerstone geoscience service consists of calculating and predicting GPS satellite orbits in near real-time every three hours.

Based on these orbit solutions, highly accurate clock offsets, or timing corrections, are computed every 15 minutes for all active GPS satellites and for all ground receivers. This enhanced data can then be used to boost the accuracy of the original location data sent by the satellites, leading to enhanced GNSS applications for scientific studies, climate monitoring and tracking large-scale, long-term changes in the Earth’s geology.

Navigation Facility consolidates ongoing work

While the facility is new, this work has been ongoing for over a decade within ESOC’s Navigation Support Office through participation in the International GNSS Service (IGS, formerly the International GPS Service).

IGS analysis centres, which include ESOC, Germany’s GeoForschungsZentrum (GFZ) and the University of Bern, among others, make their processed data sets available to a growing user community, including national weather offices, satellite operators, universities, geoscience institutes and researchers worldwide.

The most precise orbit solutions, including those from ESOC, are on the order of 3 cm, with a corresponding clock offset accuracy on the order of 0.1 ns (nanosecond). ESOC’s ongoing efforts have been coordinated by Prof. John Dow, head of the Navigation Support Office, which is part of ESA’s Operations directorate.

The facility inherits and consolidates ESOC’s experience as one of the best producers of high-precision GPS orbit and clock offset data, and is now enabling a new series of geo computational services to be developed for an expanding list of customers.

“Demand is strong. We’re receiving requests for processed GNSS data from several sources, including EUMETSAT, the Galileo Project Office and the EGNOS office,” says Rene Zandbergen, day-to-day manager of the facility, which has a staff of four supported by nine contractors and one ESA YGT (young graduate trainee).

Atmospheric sounding supports EUMETSAT weather forecasting

One of the facility’s most challenging services will enable the new EUMETSAT (the European Organisation for the Exploitation of Meteorological Satellite) MetOp-series of weather satellites to perform ‘atmospheric sounding’ experiments.

Atmospheric sounding makes use of processed GNSS data sets to correct GPS radio signals received by a specially adapted receiver mounted on the satellite. This GRAS receiver (GPS Receiver for Atmospheric Sounding) receives signals that have travelled through a thick slice of the Earth’s atmosphere just before the Earth occultates, or blocks, the line of sight between the GPS satellite and MetOp.

Since the atmosphere affects the signal, it can be processed to extract valuable data on the ionosphere and troposphere, including the concentration of charged ions and physical measurements like temperature, pressure and humidity. “This can create a near-real-time map of the upper atmosphere, but you need very accurate processed GNSS data to support the process,” says Zandbergen.

Weather forecasters and climatologists can also use such studies to track atmospheric and climate change over time.

“Operating GRAS for MetOp will be an interesting challenge. For the first time, we’ll have to provide 24-hour-per-day on-call manning,” says Prof. Dow.

The GRAS support services are being provided on a commercial basis, and similar services can be provided to other satellite operator customers in the future.

Well-defined reference frame key to Galileo

Another key project is helping determine and define the fundamental reference frame to be used by the Galileo navigation system.

The frame can be thought of as a grid, similar to the grid lines shown on a city map to locate the positions of buildings and roads. Without a highly accurate frame, Galileo position data would be essentially useless.

In this role, the Navigation Support Office is developing, in consortium with GFZ and a small group of leading European partners, a prototype service to the Galileo Joint Undertaking (GJU), as a Galileo Geodetic Service Provider. The GJU was set up by the European Commission and ESA mainly to select the concessionaire which will be responsible for operating the Galileo system.

The Navigation Facility is also providing independent data validation services and verifying the accuracy of related work done for ESA by third-party outside contractors.

Further, and in partnership with GFZ, the Navigation Support Office is involved in setting up a network of ground-based Galileo signal receivers on behalf of Galileo Industries, the consortium of industrial partners building the system. The receivers are initially being used to test and validate GIOVE-A and, later, GIOVE-B, the first two Galileo satellites.

EGNOS and GLONASS

Another current project is the monitoring and validation of data handled by the EGNOS (European Geostationary Navigation Overlay Service) system. EGNOS is a predecessor service for Galileo, and provides augmented and enhanced data from both GPS and (later) GLONASS satellites. This boost makes these systems’ data suitable for safety critical applications such as flying aircraft and navigating ships through narrow channels.

Additionally, the facility is able to deliver processed data products using GLONASS satellite data.

Ready for the future

The wide and growing range of navigation-related work being done by the Navigation Support Office was more than sufficient to justify the investment to build the Navigation Facility. With ongoing upgrades, the facility will be capable of providing real-time orbit-and-time-correction data sets by the end of 2006, which will enable an even wider range of products and services.

“It made sense to build the Navigation Facility and consolidate a number of third-party revenue-generating projects. We gave it a lot of thought. It was clear that many activities were going to continue and grow,” says Prof. Dow.

He says the future will see more customers interested in multi-GNSS solutions, ultra-high-accuracy processed data sets that integrate source data from more than one satellite system, such as Galileo and GLONASS, or GPS and GLONASS.

“The main focus now is on GPS data processing. But once Galileo data becomes available, that will be a major boost to what we can do.”