Ulla Kallio

Can We Really Promise a mm-Accuracy for the Local Ties on a Geo-VLBI Antenna

For the next-generation geodetic VLBI network a 1 mm positioning accuracy is anticipated. The accuracy should be site-independent consistent, reliably controlled, and traceable over long time periods. There are a number of remaining limitations. These include random and systematic components of the delay observable itself, various antenna-related errors, and especially a proper handling of local ties at multi-technique sites.

Finnish permanent GNSS network: From dual-frequency GPS to multi-satellite GNSS

The Finnish Geodetic Institute (FGI) is building a new multi-satellite GNSS network in Finland. The renewal of the old GPS-only network FinnRef is funded by the Ministry of Agriculture and Forestry. The new network will consist of 19 stations collecting data from GPS, Glonass, Galileo and later also from Compass satellites. The network stations will be build on stable bedrock sites, equipped with Javad Delta-G3T receivers and individually calibrated Javad Dorne Margolin choke ring antennas covered with SCIGN radomes. Data will be transferred both as hourly files and real-time streams to the FGI and distributed to the users, co-operation partners and research institutions.

Local Ties at Fundamental Stations

In this article we discuss and give an example of three different methods to measure the local tie between the IGS GPS antenna reference point (ARP) and the VLBI antenna reference point at the Metsahovi fundamental station. First we introduce traditional survey approach combined with a space intersection technique and then local tie based on kinematic GPS during geo-VLBI campaigns and finally local ties with static GPS measurements. We discuss the measurements and computations, problems and error sources encountered between the planning of the measurements and the end product, the local tie vector. Although millimetre precision can be achieved, our experience during tests at Metsahovi fundamental station shows that some techniques are very time consuming and impractical for a routine use. We aim for a more automated process in local tie measurement.

Improving Length and Scale Traceability in Local Geodynamical Measurements

Traceability is a feature that is required more frequently in local geodetic high-precision measurements. This basic term of metrology, a measurement science, describes the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty (BIPM International vocabulary of metrology – basic and general concepts and associated terms (VIM). JCGM 200:2008. Joint Committee for Guides in Metrology, 2008b).

Validating and comparing GNSS antenna calibrations

GNSS antennas have no fixed electrical reference point. The variation of the phase centre is modelled and tabulated in antenna calibration tables, which include the offset vector (PCO) and phase centre variation (PCV) for each frequency according to the elevations and azimuths of the incoming signal. Used together, PCV and PCO reduce the phase observations to the antenna reference point. The remaining biases, called the residual offsets, can be revealed by circulating and rotating the antennas on pillars. The residual offsets are estimated as additional parameters when combining the daily GNSS network solutions with full covariance matrix. We present a procedure for validating the antenna calibration tables. The dedicated test field, called Revolver, was constructed at Metsähovi. We used the procedure to validate the calibration tables of 17 antennas. Tables from the IGS and three different calibration institutions were used. The tests show that we were able to separate the residual offsets at the millimetre level. We also investigated the influence of the calibration tables from the different institutions on site coordinates by performing kinematic double-difference baseline processing of the data from one site with different antenna tables. We found small but significant differences between the tables.

Future and Development of the European Combined Geodetic Network ECGN

The European Combined Geodetic Network ECGN aims at the unification of time series of spatial/geometric observations obtained from GNSS, physical quantities from gravity field related observations, and other parameters including precise levelling, tide gauge records, and Earth and ocean tides. The objective of ECGN is to contribute to the maintenance of a stable terrestrial reference system for Europe, including 3D geometric parameters together with the gravity related height component. We discuss future, need and structure of the ECGN and a pilot project initiated for practical demonstration of the ECGN.