COoperative Network for GIOVE Observation (CONGO)

Overview

The Cooperative Network for GIOVE Observation (CONGO) has been established in 2008 by the German Space Operations Center (DLR/GSOC) and the Federal Agency for Cartography and Geodesy (BKG) as an early test bed for experimentation with new GNSS signals. The network was later joined by various other national and international partners, providing GNSS reference stations, network services, data archives and processing services.

Deutsches Zentrum für Luft- und Raumfahrt, German Space Operations Center (DLR/GSOC),
Federal Agency for Cartography and Geodesy (BKG)
Technische Universität München, Institute of Astronomical and Physical Geodesy (TUM/IAPG),
Deutsches Zentrum für Luft- und Raumfahrt, Institute of Communication and Navigation (DLR/IKN),
Deutsches GeoForschungsZentrum Potsdam, Section GPS/Galileo Earth Observation (GFZ/Sect.1.1)
Geoscience Australia (GA)
Centre National des Etudes Spatiales (CNES)

Key aspects that distinguish CONGO from other GNSS networks include the capability of GIOVE-A/B (Galileo) and QZSS QZSS signal tracking, the support of multiple frequencies (L1/E1, L2, L5/E5), the fully global coverage and the real-time capability.

Fig. 1 CONGO network sites (Feb. 2012)

As of February 2012, CONGO comprises a total of 23 different sites around the world (Fig.1). The distribution of sites was originally chosen such as to achieve a continuous GIOVE tracking coverage with the minimum of eight stations (Maui, Concepcion, Fredericton, Wettzell, Hartebeesthoek, Singapore, Chofu, Sydney) deployed in the first design stage of Sept. 2009. Since then the number of stations has increased by more than a factor of two. In particular, a dense coverage in the African and Asian region was achieved through stations contributed by the GFZ real-time GNSS network. Besides improving the common-view statistics the large number of stations now offers an enhanced robustness against station failures and communication gaps. Various CONGO stations are co-located with stations from other networks (such as IGS, EUREF, or EVNet) and common antennas are used where feasible. QZSS tracking is presently suported by five CONGO stations in the Asian-Pacific region.

Fig. 2 Depth of coverage (DOC) for GIOVE observation with the CONGO network (10° elevation limit)

In the current configuration, GIOVE satellites can simultaneously be observed from more than 4 CONGO stations for most of their orbit (Fig 2).

Equipment

The CONGO network presently employs different types of multi-frequency multi-constellation receivers on a routine basis. These include the Septentrio GeNeRx1 receiver, the Javad Triumph Delta-G2T/G3TH receivers and the Leica GRX1200+GNSS receiver. A dual frequency (L1/L5) NovAtel Europak15a receiver is, furthermore, available at the Fredericton site for experimental purposes. It is not, however, used within the routine CONGO processing due the lacking support of GPS L2 signal tracking. The above receivers are used with different types of tri- or quad-band antennas. These include Leica's AR25 chokering antenna, the Leica AX1203+ survey antenna and Trimble's Zephyr Geodetic II.

Data Processing and Products

Measurements at a 1 s sampling interval as well as auxiliary data are transmitted in real-time from all receivers to a central server at BKG, Frankfurt, where they can be accessed by multiple concurrent users. The NTRIP transport protocol is used for transmission to and from the central server, but receiver-specific raw data formats are employed in the respective data streams. A transition to the RTCM3 format is foreseen once a standardized set of high precision messages becomes available for all signals and constellations. A permanent CONGO data archive is hosted by the Technische Universität München (TUM), where all data streams are received and decoded in real-time. Besides the original data transmitted by each receiver, RINEX3 observation files at 10 s sampling data and navigation files in SP3 format are generated and stored for off-line analyses on a daily basis (Fig. 3).

Fig. 3 CONGO processing and data flow

Reconstructed and predicted GIOVE orbits are generated on a daily basis at the Technische Universität München based on CONGO observations with a modified version of the BERNESE software. In a first step, station coordinates, receiver clocks, and troposphere parameters are estimated in a GPS-only PPP solution. These parameters are then kept fixed in the second step, the GIOVE orbit and clock estimation. Based on a comparison with Satellite Laser Ranging, a radial orbit of about 12 cm has been inferred for the GIOVE-B orbits. Making use of the predicted TUM orbits a real-time clock solution is obtained at DLR using the REal-TIme-CLock Estimation system (RETICLE). The combined orbit-clock product is publically made available for intereste users with a 5 min latency at ftp.gsoc.dlr.de (user: reticle, pw: getReticle, directory: pub/reticle/).

Acknowledgment

The setup and operation of the CONGO network would not have been possible without the help of many individuals and institutions. We would like to specifically thank J.-P. Barriot (UPF), A. Boer (BKG), J.-P. Cardaliaguet (CNES), L. Combrinck (HartRAO), U. Hessels (BKG), T. Humphreys (UT), R. Langley (UNB), P. Mumford (UNSW), N. Nagarajan (NTU), T. Noack (DLR/IKN), D. O'Gara (IfA), M. Ramatschi (GFZ), Ch. Rizos (UNSW), R. Ruddick (GA), P. Steigenberger (TUM) and G. Weber (BKG) for their selfless and outstanding support. Special thanks are due to S. Yudanov (JAVAD GNSS), N. Brown (Leica GeoSystems), and J. van Hees (Septentrio) for technical support and advice concerning the Triumph, GRX1200, and GeNeRx receivers.

COoperative Network for GIOVE Observation (CONGO)

Overview

Equipment

Data Processing and Products

Acknowledgment

Further Reading