The Cooperative Network for GIOVE Observation (CONGO) has been established by DLR's German Space Operations Center and the Federal Agency for Cartography and Geodesy (BKG) as an early test bed for experimentation with new GNSS signals. Further partners include the Technische Universität München with its Institute of Astronomical and Physical Geodesy (TUM/IAPG) and DLR's Institute of Communication and Navigation (DLR/IKN). Key as-pects that distinguish CONGO from other GNSS networks include the capability of GIOVE (Galileo) signal tracking, the fully global coverage and the real-time capability.
As of February 2010, CONGO comprises a total of ten different sites around the world (Fig.1). The distribution of sites has been 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. Two additional stations (Stanford, O'Higgins) were subsequently added to improve the availability in case of station failures or communication gaps and to improve the common-view statistics. Various CONGO stations are co-located with stations from other networks (such as IGS, EUREF, or EVNet) and common antennas are used where feasible.
In the current configuration, GIOVE satellites can simultaneously be observed from 2-3 CONGO stations for most of their orbit (Fig 2). An even higher depth of coverage is achieved for roughly 25% of the possible sub-satellite locations, while, on the other hand, three small re-gions with single-site visibility remain in southern Africa, the Black sea area, and near New Zealand.
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 because 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.
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).
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 A. Boer (BKG), L. Combrinck (HartRAO), U. Hessels (BKG), R. Langley (UNB), P. Mumford (UNSW), N. Nagarajan (NTU), T. Noack (DLR), D. O'Gara (IfA), Ch. Rizos (UNSW) 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.
Hugentobler U., Steigenberger P., Montenbruck O., Hauschild A., Weber G., Hessels U.;
Evaluation of GIOVE Satellite Clocks using the CONGO Network;
24th European Frequency and Time Forum; 13-16 April 2010, Noordwijk, The Netherlands (2010).
Cao W., Hauschild A., Steigenberger P., Langley R.B., Santos M., Montenbruck O.;
GPS/GIOVE Integrated Precise Point Positioning Performance Evaluation;
ION International Technical Meeting, 25-27 Jan. 2010, San Diego, California (2010).
Montenbruck O., Hauschild A., Hessels U., Steigenberger P., Hugentobler U.;
CONGO - First GPS/GIOVE Tracking Network for Science, Research;
GPS World, September 2009, 36-41 (2009).
Hauschild A., Montenbruck O.;
Kalman-Filter-Based GPS Clock Estimation for Near Real-Time Positioning;
GPS Solutions 13(3):173-182 (2009).