RINEX Format Research Articles

Assessment of TID activity from GPS phase data collected in a dense network of GPS receivers

GPS data from a dense network of receivers can be an important source of information on Travelling Ionospheric Disturbances (TIDs), especially for smaller scale fluctuations (small and medium scale TIDs). Since absolute TEC ``calibration'' is not necessary it is sufficient to work with GPS differential phase data. We are using data from a dense network of GPS receivers distributed over Austria to assess TID activities on a bi-hourly basis. The raw data are stored in the well-known RINEX format. The phase differences are extracted and after correction of ``cycle slips'' the data are subjected to band-pass filtering. Cycle slip correction is a tricky but necessary process. The use of ``cycle slip free'' time intervals would leave only a fraction of available data for TID activity assessment. We describe the data retrieval and preparation system and show first results (case studies and preliminary results).

REGAL; reseau GPS permanent dans les Alpes occidentales; configuration et premiers resultats

Abstract The kinematics of the present-day deformation in the western Alps is still poorly known, mostly because of a lack of direct measurements of block motion and internal deformation. Geodetic measurements have the potential to provide quantitative estimates of crustal strain and block motion in the Alps, but the low expected rates, close to the accuracy of the geodetic techniques, make such measurements challenging. Indeed, an analysis of 2.5 years of continuous GPS data at Torino (Italy), Grasse (France), and Zimmerwald (Switzerland), showed that the present-day differential motion across the western Alps does not exceed 3 mm/yr [Calais, 1999]. Continuous measurements performed at permanent GPS stations provide unique data sets for rigorously assessing crustal deformation in regions of low strain rates by reducing the amount of time necessary to detect a significant strain signal, minimizing systematic errors, providing continuous position time series, and possibly capturing co- and post-seismic motion. In 1997, we started the implementation of a network of permanent GPS stations in the western Alps and their surroundings (REGAL network). The REGAL network mostly operates dual frequency Ashtech Z12 CGRS GPS stations with choke-ring antennae. In most cases, the GPS antenna is installed on top of a 1.5 to 2.5 m high concrete pilar directly anchored into the bedrock. The data are currently downloaded once daily and sent to a data center located at Geosciences Azur, Sophia Antipolis where they are converted into RINEX format, quality checked, archived, and made available to users. Data are freely available in raw and RINEX format at http://kreiz.unice.fr/regal/. The GPS data from the REGAL network are routinely processed with the GAMIT software, together with 10 global IGS stations (KOSG, WZTR, NOTO, MATE, GRAZ, EBRE, VILL, CAGL, MEDI, UPAD) that serve as ties with the ITRF97. We also include the stations ZIMM, TORI, GRAS, TOUL, GENO, HFLK, OBER because of their tectonic interest. We obtain long term repeatabilities on the order of 2-3 mm for the horizontal components, 8-10 mm for the vertical component. Using a noise model that combines white and coloured noise (flicker noise, spectral index 1), we find uncertainties on the velocities ranging from 1 mm/yr for the oldest stations (ZIMM, GRAS, TOUL, TORI, SJDV) to 4-5 mm/yr for the most recently installed (CHAT, MTPL). Station velocities obtained in ITRF97 are rotated into a Eurasian reference by substracting the rigid rotation computed from ITRF97 velocities at 11 central European sites located away from major active tectonic structures (GOPE, JOZE, BOR1, LAMA, ZWEN, POTS, WETT, GRAZ, PENC, Effelsberg, ONSA). The resulting velocity field shows residual motions with respect to Eurasia lower than 3 mm/yr. We obtain at TORI, in the Po plain, a residual velocity of 2.3+ or -0.8 mm/yr to the SSW and a velocity of 1.9+ or -1.1 mm/yr at SJDV, on the Alpine foreland. These results indicate that the current kinematic boundary conditions across the western Alps are extensional, as also shown by the SJDV-TORI baseline time series. We obtain at MODA (internal zones) a residual velocity of 1.2+ or -1.2 mm/yr to the SSE. The MODA-FCLZ baseline show lengthening at a rate of 1.6+ or -0.8 mm/yr. These results are still marginally significant but suggest that the current deformation regime along the Lyon-Torino transect is extension, as also indicated by from recent seismotectonic data. It is in qualitative agreement with local geodetic measurements in the internal zones (Briancon area) but excludes more than 2.4 mm/yr of extension (FCLZ-MODA baseline, upper uncertainty limit at 95% confidence). Our results indicate a different tectonic regime in the southern part of the western Alps and Provence, with NW-SE to N-S compression. The GRAS-TORI baseline, for instance, shows shortening at a rate of 1.4+ or -1.0 mm/an. This result is consistent with seismotectonic data and local geodetic measurements in these areas. The Middle Durance fault zone, one of the main active faults in this area, is crossed by the GINA-MICH baseline, which shows shortening at a rate of 1.0+ or -0.8 mm/an. This result is only marginally significant, but confirms the upper bound of 2 mm/yr obtained from triangulation-GPS comparisons. The REGAL permanent GPS network has been operating since the end of 1997 for the oldest stations and will continue to be densified. Although they are still close to or within their associated uncertainties, preliminary results provide, for the first time, a direct estimate of crustal deformation across and within the western Alps.

Concatenation of hourly RINEX files

A subset of stations that are part of the International GPS Service (IGS) submit hourly RINEX observation files in addition to the daily files to be used for near real time applications. Ideally, concatenating the hourly files will result in the original 24 hour file. If this is indeed the case, there is no need for submitting every observation twice and the concatenated hourly files can replace the daily file. In order to compare such concatenated files with the original daily files, each data field has to be inspected according to the RINEX format definition. Differences have to be removed before any replacement could be performed. By using a RINEX inspection software tool on a subset of stations, the concatenated version of the daily file was in agreement with the original file in 94% of the conversions. This suggests that it may be possible for IGS to completely depend on the hourly files for the creation of the daily files in the future, thereby eliminating the need for transferring each observation twice.

A binary exchange format for GPS data

Near real time monitoring of the quality of GPS data in a “high rate & low latency” GPS ground tracking network sets new requirements for data formats. At GFZ a compact binary exchange format for GPS data was developed for the level-1 data products of the CHAMP mission. This new binary format preserves all the advantages of the widely accepted RINEX format, but avoids its limitations on the precision of the observables. It is receiver-independent, compact and can be easily extended to future observation types. A library for the GFZ-binex data handling, integrity checking and conversion from/to RINEX is available for public use. A more detailed description and up-to-date information on the development of this format standard can be found on the GFZ homepage