Keywords

1 Introduction and History

The Metsähovi Geodetic Research Station (MGRS) at the village Kylmälä in the Kirkkonummi municipality, Finland, was established in 1975. In the same area, there are also the Metsähovi Radio Observatory of Aalto University and small optical telescopes of the University of Helsinki. MGRS is managed by the Finnish Geospatial Research Institute (FGI) of the National Land Survey (NLS) (until 2015 the Finnish Geodetic Institute, FGI). It is one of the northernmost geodetic stations in the core network of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), located 60° north.

Fig. 1
figure 1

Aerial photo of the MGRS. The gravimetric laboratory is at the left and the old main building is in the centre with the new VGOS telescope in the background. The SLR observatory is on the right and behind it is the new main building

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The first geodetic measurements at MGRS, as part of the global International Satellite Laser Ranging Service (ILRS) network, started in 1978 with an in-house made Satellite Laser Ranging (SLR) system consisting of a 63 cm telescope and a Ruby laser capable of transmitting four pulses per minute. In 1993 the system was replaced by a 100 cm telescope and a 1 Hz-capable Nd:YAG laser. In 2005 it had to be abandoned because the maintenance of the laser, in particular, became impossible and the telescope was unsuitable for modern kHz or faster operation. Due to the lack of funding, the acquisition of a modern kHz-capable SLR system could only be started in 2012. For further details of the SLR history, see Raja-Halli et al. (2019).

The first permanent GPS station, now identified as METS00FIN, was established in 1992. First, data were sent to the Cooperative International GPS Network (CIGNET) and since 1994 to the International GNSS Service (IGS), and from 1995 also to the EUREF Permanent GNSS Network (EPN). Because the antenna is placed on a 25 m tall mast, the thermal expansion is eliminated with an invar wire-based compensator (Koivula et al. 1998). Additionally, a NASA/UNAVCO GNSS receiver is sharing the antenna of METS00FIN.

MGRS has hosted and provided local support to the Détermination d’Orbite et Radiopositionnement Intégré par Satellite (DORIS) radio beacon since 1990 (Koivula et al. 1998). The DORIS station is located about three kilometres from the Metsähovi main station to minimise radio frequency interference. It is operated and maintained by the French Space Agency (Centre national d’études spatiales, CNES). The DORIS system has been upgraded four times since the initial installation. METG00FIN GNSS station is located at the DORIS station and it is part of the Réseau GNSS pour l’IGS et la NAvigation (REGINA) infrastructure maintained by CNES and Institut national de l’information géographique et forestière (IGN). METG00FIN submits data also to EPN and IGS networks.

FGI procured its first absolute gravimeter (AG), JILAg-5, in 1986. In 2003 the JILAg-5 was replaced by the FG5-221, which was subsequently updated to FG5X-221 in 2014. In Finland, the FG5X-221 is used to, e.g., monitor the gravity change at MGRS and other locations due to postglacial land uplift, and to maintain the national gravity network. It is also the national standard for free-fall acceleration of gravity. FGI AG has been used in several places abroad, from Svalbard to Antarctica, and many AGs have visited the MGRS (Arnautov et al. 1982; Gitlein 2009; Bilker-Koivula et al. 2021).

In 1994 a dedicated gravity building was built at MGRS (Fig. 1). The building has two laboratory rooms, one for absolute gravimeters and one for superconductive gravimeters. In 1994 FGI procured a superconductive gravimeter (SG) GWR T020, which was continuously operated until 2016. It was then replaced with more modern GWR iOSG-022 and iGRAV-013 superconductive gravimeters (Virtanen and Raja-Halli 2018).

Fig. 2
figure 2

MGRS equipment timeline and contribution to the IAG services. 2025 timeline is an estimation

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The geodetic Very Long Baseline Interferometry (VLBI) observations have been made since 2004 with the 13.7 m radio telescope of the Aalto University Metsähovi Radio Observatory in the same area (Poutanen and Koivula 2007). An S/X receiver and Mk5A data acquisition system had been acquired for the purpose. Later the system was upgraded to Mk5B and DBBC2. Other equipment, like the hydrogen maser for timing, were already available. Observing time for a few geodetic VLBI sessions per year was purchased from Aalto University until 2021. Observations will continue with the new dedicated geodetic VLBI system at MGRS after the commissioning phase.

Techniques and facilities for GPS local tie measurements between the METS00FIN and the legacy telescope, simultaneously with the VLBI sessions, were developed (Kallio and Poutanen 2012). Observations started in 2008 and continued until 2015. The validation of the GPS local tie system was made with the automatic robot tachymeter monitoring during two VLBI sessions in the EMRP SIB60 project in 2015. The local tie vector from METS00FIN to legacy telescope METSAHOV in the ITRF2020 solution is based on the terrestrial measurements within the same project (Pollinger et al. 2015; Jokela et al. 2016; Kallio et al. 2016). Terrestrial techniques were further used to measure the consistent set of local tie vectors between all space geodetic instruments at MGRS.

All instruments and reference benchmarks and pillars are established on solid granite bedrock, except the DORIS/REGINA station. Even though the implication is that a minimal danger for movements of the benchmarks or instruments exists, regular control of the benchmarks (local tie measurements) is necessary and performed.

The reference points of the national height (N2000) and gravity (FOGN) systems are also located at Metsähovi. MGRS is also an International Height Reference Frame (IHRF) core site.

At the beginning of the millennium, the equipment of MGRS was partly outdated and did not meet the new requirements set for IAG’s services. The modernization of MGRS started in 2012 with special funding from the Ministry of Agriculture and Forestry and later on also by the internal funding of the NLS. The modernisation includes the renewal or updating of all major measurement systems, as well as the construction of a new VLBI Global Observing System (VGOS) system, SLR system and improvement of the station infrastructure and working facilities (Fig. 1). The MGRS data contribute through IAG services to various global geodetic and Earth-exploring purposes (Fig. 2). From 2023, the newly established Geodetic Infrastructure Unit of the NLS will be responsible for the daily operation and maintenance of instruments and facilities and the Ministry of Agriculture and Forestry has allocated special funding for the operation of MGRS.

2 Renewal of Major Instruments

2.1 SLR

The SLR technique is used to determine satellite orbits, and the observations from the northern location of MGRS are important for it. The design and integration of the cost-effective and modern third-generation SLR system was done in-house because there were no commercial solutions available for procuring a complete turnkey system. The system was designed to fulfil data quality and quantity requirements for a modern SLR system with the ILRS network (ILRS 2023). Most of the major subsystems, such as the observatory dome (Baader Planetarium GmbH), the telescope (Cybioms Corp.), and the operating system (DiGOS GmbH) were procured from commercial companies. A modern laser, built by High Q Laser Production GmbH, had been procured earlier for the modernisation attempt of the second-generation SLR system around the time the system failed and it was decided to move the laser into the third-generation system as legacy instrumentation.

The MGRS SLR system is bi-static, i.e. it has a twin telescope mounted on a shared gimbal. A ten cm refractor with a Coudé focus is used to transmit the laser pulses and a 50 cm reflector with the receiver system at the Cassegrain focus is used to capture the return pulses. Coudé focus allows for the laser to be situated downstairs in a separate and climate-controlled laser room which is a requirement for stable laser operation.

The laser is a 2 kHz repetition rate Nd:YVO4 regenerative amplifier laser which has a native wavelength of 1062 nm that is frequency-doubled to 532 nm for SLR operation. The laser pulse length is approximately 12 ps. The receiver detector is a time-walk compensated single-photon avalanche diode built by PESO Consulting that captures only a single photon per expected return pulse. The quantum efficiency of the receiver is better than 40% and it has a time resolution better than 20 ps. The system is triggered with an FPGA-based range-gate generator, built by DiGOS GmbH that can handle all foreseeable operating scenarios. Timing is based on an Eventech A033-ET event timer with a non-linearity better than 2 ps. The time and frequency base is provided by a GNSS-controlled oven-controlled crystal oscillator (Meinberg GmbH) with short-term stability of 5 ps. The system is operated via a dedicated operation control system written in C++ and real-time operations are handled via a dedicated Linux Ubuntu operating system with a low latency kernel.

Expected system performance, based on radar link budget calculations (Degnan 1993), system component specifications and experiences of SLR stations with similar instrumentation, has the normal point accuracy of ~1 mm at a distance of 300 km and 1 cm at a distance of 20,000 km. The system is capable of daytime observations up to GNSS orbits. This is especially important considering that statistically most clear skies occur mainly between March and September when there is almost no dark time at the MGRS latitudes. Based on the observations made with a local cloud sensor (Boltwood CloudSensor II ) we have estimated that Metsähovi SLR will have suitable weather for observations approximately 30–40% of the time (Del Pino et al. 2017).

A new observatory building was inaugurated in 2014 and all of the major system components were in place in late 2015. However, there have been unforeseen difficulties with the commissioning of the telescope subcomponent by the manufacturer. We anticipate test observations in 2024.

2.2 VGOS

Geodetic VLBI is the only technique capable of observing all Earth Orientation Parameters (EOP), especially UT1-UTC, and therefore invaluable e.g. for GNSS. Development towards the VGOS concept (Petrachenko et al. 2009) requires dedicated, relatively small (~12 m) and fast slewing radio telescopes, using broadband receivers. To meet the requirements, the capability of the legacy telescope is not sufficient.

The VGOS performances should respond to the high demands for accuracy, reliability, and timeliness of the global reference frame. This in turn requires an order-of-magnitude improvement in the geodetic VLBI measurement accuracy. The ambitious goals of VGOS are 1 mm accuracy for positioning, 0.1 mm/yr for velocity, continuous operation, and extremely fast processing of the data to obtain the geodetic products, such as EOPs, in near real-time.

The new MGRS VGOS telescope has been installed and commissioned during 2018–2020. The telescope has been designed by MT Mechatronics GmbH. The technical characteristics of the telescope meet the VLBI2010 antenna specifications described in (Petrachenko et al. 2009).

The telescope dish is mounted on a steel pedestal embedded in a large, heavily reinforced concrete block which is firmly attached to the solid bedrock. To ensure the stability of the telescope reference point (intersection of the azimuth and elevation axis of the telescope), the pedestal was additionally layered with extra insulation and a shell at the end of 2022. Twelve temperature sensors have been installed inside the pedestal to monitor its temperature.

The telescope has a ring-focus antenna with a 13.2 m diameter main reflector. It is a high-speed slewing antenna with 12°/s in azimuth and 6°/s in elevation axes, the acceleration on both axes is 2.5°/s2. The surface accuracy of the main and secondary reflectors is better than 0.3 mm RMS and 0.1 mm RMS, which has a good margin to work within the VGOS frequencies and above.

The telescope is equipped with a broadband receiver manufactured by the IGN-Yebes (Spain) technology development centre in October 2019. The receiver has a quad-ridge feed horn (QRFH), designed to measure both linear polarisations at a frequency range of 2.1–14.1 GHz. The signal from the receiver is passed through the filtering and pre-amplifier modules, and each polarization component is divided into low (2.1–5.6 GHz) and high (3.6–14.1 GHz) frequency bands. Initial RFI-background evaluation has been carried out. Substantial interference sources were identified, especially in the 2–3 GHz range, thus limiting the use of the lower band. The vertical and horizontal polarisation components of the signal are distributed into 4 (5) channels by the filter bank module and further transferred over a fibre link to the backend located in the instrumentation room of the station’s main building. At the backend, the signal is digitised with a Digital Baseband Converter DBBC3, produced by Hat-Lab (Tuccari et al. 2014). A FlexBuff system will be used for signal recording. The first light with the receiving system was obtained at the end of 2019. The finalisation of the VGOS signal chain is in progress with an expected test observation period of the whole system beginning in 2024.

Fig. 3
figure 3

Detrended position time series of MET300FIN computed by the NKG GNSS Analysis Centre (Lahtinen et al. 2018). The vertical dashed line shows the epoch of the receiver change at the station

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2.3 GNSS

A new GNSS station, MET300FIN, was established as part of the renewal of MGRS, which was done in parallel with the modernisation of the Finnish permanent GNSS network FinnRef. All the new FinnRef stations (47 stations in total) have the same design and the same type of instruments. The choke ring antenna has been installed on the top of a steel grid mast, which has been attached to the bedrock whenever possible. The steel grid masts were preferred to concrete pillars because they were much easier to install in remote locations. We had experience with both concrete and steel grid masts, but we did not notice any difference. The setup at MGRS and other FinnRef stations allows operating other receivers parallel to the official stations. The technical details of the instrumentation are available in the site logs, e.g. by EPOS, EPN, or IGS websites (https://gnss-metadata.eu/, https://epncb.eu/, https://igs.org/). The MET300FIN was included in the EPN and IGS networks in 2017.

The MET300FIN station has achieved the same high quality as the METS00FIN. The RMS of the position time series are approximately 1.0 and 3.2 mm in north/east and up, respectively, without any strong seasonal signals. The receivers of METS00FIN and MET300FIN were both upgraded from Javad TRE_G3TH DELTA to Javad TRE_3 DELTA in spring 2018 to enable new tracking features. However, the change caused a small but significant jump in the time series as shown in Fig. 3. The same discontinuity was observed in the METS00FIN time series. Lahtinen (2022) computed a zero baseline between parallel-tracked data of the two receiver types and found a difference of 0.7 mm in the east component that corresponds to the detected change in the position time series. However, as the effect is on a one mm level, it may not appear in the analysis with a higher noise level.

2.4 DORIS

With the global coverage of DORIS stations, the International DORIS Service (IDS) provides the geodetic community with various space geodetic products that are independent of, e.g., Global Navigation Satellite Systems (GNSS). DORIS is thus considered one of the major geodetic techniques of a geodetic core station. The latest (4th) generation DORIS system was installed at MGRS in 2021. The infrastructure for the system has also been updated within the overall MGRS upgrade, with a new instrument shelter installed in 2022.

The coordinates between the main station instrument reference points and the DORIS beacon are regularly verified that they are not moving locally relative to each other. The height of the REGINA point (originally a mobile VLBI monument from 1988) has been levelled relative to a nearby bedrock point. During the first decade, there has been a small subsidence, which has been stabilised (Fig. 4). Locally the height difference between the DORIS beacon and the REGINA pillar has been monitored regularly. There are no relative height changes between these two instruments. Because the heights are relative to a bedrock benchmark, the postglacial uplift, about 4 mm/yr, is not visible in the time series.

Fig. 4
figure 4

Height of DORIS and REGINA pillars in the national N2000 height system. REGINA antenna mast is placed on the old pillar which was originally built for 1988 Mobile VLBI. The time series to the current DORIS pillar is shorter

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As the REGINA station METG00FIN is not established on bedrock, we prefer not to constrain its velocity to the METS00FIN and MET300FIN station velocities in the reference frame-related works.

2.5 Gravity Instruments

Absolute (AG) and superconducting (SG) gravimeters are prerequisites for creating and maintaining national gravity systems and for researching temporal and spatial variations of gravity. They also contribute to global and regional gravity networks. The absolute gravimeter FG5X-221 is also the Finnish national standard for the free-fall acceleration of gravity.

The FG5X-221 absolute gravimeter provides measurements traceable to the SI. Additionally, the superconducting gravimeters monitor the gravity changes in time. Due to the metrological traceability and detailed time series of the gravity change, the laboratory is suitable for absolute gravimeter comparisons. The MGRS contributes also to the realisation of the International Terrestrial Gravity Reference System, ITGRS (Wziontek et al. 2021).

Both gravity laboratories at MGRS have multiple concrete pillars connected directly to the bedrock and mechanically separated from the building to minimize disturbance to the instruments. Thermal stability better than 0.2 °C can be achieved in the laboratories during measurements. Bilateral AG comparisons have regularly taken place on two pillars in the AG laboratory. These include comparisons with the FG5-233 (Lantmäteriet), FG5-220 (IfE, Univ. Hannover), FG5-301 (BKG), FG5-101 (BKG) and the GBL-P-1 (TsNIIGAiK). The FG5X-221 routinely participates in international comparisons of absolute gravimeters (e.g. Wu et al. 2020).

The new SG’s, GWR iOSG-022 and iGRAV-013, replacing the first superconducting gravimeter (GWR SG-T020) are located in the same laboratory room on separate concrete pillars (Virtanen and Raja-Halli 2018). The iGrav is a transportable model and may be taken out of the laboratory for off-site measurements.

SGs are the most sensitive relative gravity instruments with an accuracy in the order of 10−11 m/s2. One of the largest un-modelled gravity signals comes through the direct attraction from the changes in groundwater level as well as changes in the local hydrology (e.g., soil moisture, surface water, snow, Mäkinen et al. 2014). To understand the water-related gravimetric signals, a suite of environmental sensors has been installed at MGRS. These include boreholes with pressure gauges, soil moisture sensors, a gamma ray spectrometer for snow water content evaluation, and meteorological sensors including a snow depth sensor. Data from SGs is available through the International Geodynamics and Earth Tide Service (IGETS) database. The high-quality SG time series of MGRS is one of the longest in the world and has been used in many studies of e.g., hydrology (Boy and Hinderer 2006) solid Earth and ocean tides (Boy and Lyard 2008), and Earth’s free core nutation (Rosat and Lambert 2009).

The scale and drift of the SGs are routinely determined using measurements from the FG5X-221. In turn, the time series of the SGs are used to correct absolute gravity measurements for temporal gravity changes and possibly detect malfunctioning of the absolute gravimeter (Fig. 5).

Fig. 5
figure 5

Absolute and superconducting time series in Metsähovi. Both time series have been corrected for polar motion, tides and standard air pressure. A linear trend was removed from the superconductive gravimeter time series

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2.6 Local Tie

The local tie is said to be the fifth technique in the ITRF combination. It connects the systems of different techniques, and without the local ties common use of techniques would not be possible.

Two different techniques are used for local ties at MGRS, based on kinematic GNSS or terrestrial tachymetric monitoring measurements. Both techniques use an indirect method (Dawson et al. 2007) to determine the reference point. The GNSS antennas or the targets/prisms rotate along with the radio telescope and their positions are measured in hundreds of telescope orientations. By using the angle positions and the adjusted coordinates of the targets, the coordinates of the reference point and telescope axis orientation and offset can be estimated (Kallio and Poutanen 2012).

Terrestrial monitoring measurements are connected to the local survey network, which is aligned to the ITRF by constraining the horizontal rotation of the local GNSS network to zero. The vertical axes of the tachymeter station points are aligned with the ellipsoidal normal at the tachymeter station points by converting the tachymeter angle observations with deflection of vertical corrections to each direction separately. Components of the deflection of vertical are calculated using the gravimetric geoid (Saari and Bilker-Koivula 2018). No further transformations are needed because the 3D adjustment is performed in the global reference frame. The scale of the network is traceable to the definition of the metre through the calibration of the tachymeter in the Nummela standard baseline (Jokela 2014). The Metsähovi model (Kallio and Poutanen 2012) is used in reference point estimation.

The GNSS-based local ties do not need a local survey network. The post-processed kinematic solutions are calculated for the vectors between two rotating GNSS antennas attached at the side of the radio telescope dish and the permanent GNSS point. The GNSS antennas have individual absolute calibration tables and their phase centre and phase variation are corrected for every single phase observation in each antenna position in every observation epoch. Also, the influence of the difference in tropospheric delay due to the height differences is corrected before the final trajectory calculation by applying the troposphere model GMT3 for each phase observation in each epoch (Landskron and Böhm 2018).

The GPS-based local tie measurements were continued since 2008 until 2015 simultaneously with VLBI sessions between the METS00FIN GPS point and the legacy telescope (Jokela et al. 2016), while the terrestrial monitoring were conducted during two VLBI sessions in 2015 using eight Leica GPR1 prisms, which were installed onto the counterweights of the legacy telescope (Kallio et al. 2016).

The MGRS local tie network was expanded during the EMPIR GeoMetre project 2019–2022 (Kallio et al. 2022; Pollinger et al. 2022) to also include the GGOS telescope, the new SLR telescope, and the MET300FIN GNSS station (Fig. 6). In 2020, the new VGOS telescope was equipped with several Bohnenstingel spherical prisms on the counterweight of the telescope, and the GNSS antennas were installed onto the dish edges of the VGOS telescope. The terrestrial and GPS-based monitoring measurements to the new VGOS telescope were started in 2020 and repeated in 2021. The terrestrial and GPS-based local tie results were compared – the coordinates of reference points in the simultaneous GPS and tachymeter monitoring measurements, performed in the summer of 2021, differed by 0.9200, 0.3100, and 0.1200 mm in North, East and Up directions, respectively (Kallio et al. 2023). Furthermore, in 2022, the terrestrial indirect method was applied to the SLR telescope with two spherical prisms which rotated with the SLR telescope (Raja-Halli et al. 2022).

Fig. 6
figure 6

The local survey and monitoring networks at MGRS (black). The yellow contour lines show the geoid slope with a contour interval of 2 mm. The local ties, vectors between the space geodetic instruments, are connected to the local survey network through monitoring networks

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3 Infrastructure

Refurbishing and reconstruction of the MGRS infrastructure and working facilities improve or enable new functionality. A new satellite laser ranging observatory was built for the SLR system in 2014. The new VGOS system required additional infrastructure work to host the new VGOS telescope. A new main building was constructed with especially the needs of VGOS in mind (Fig. 1). It has sufficient lab space to allow maintenance work of the VGOS receiver as well as a temperature-controlled server/electronics room for the backend. The whole building is made as radio frequency interference (RFI) free as possible via the use of special RFI shield mesh in the walls, using as RF silent building electronics as possible. The RFI shield is installed on all outer walls, floor, and ceiling of the building to prevent RFI towards the telescope. Some rooms, such as the server room where the VLBI backend will be located, have also shielding in the internal walls.

The new VGOS system will increase the data production volume of MGRS by several orders of magnitude. The data production of the system is more than 16 Gb/s while on target. As the data needs to be correlated with data from other systems, it must be transferred to correlation centres. To allow efficient data transfer, the MGRS internet connection has been upgraded to allow transfers of up to 100 Gb/s via the Finnish University and Research Network FUNET. MGRS has also been connected via the FUNET optical cable to the Finnish realisation of UTC (UTC MIKE), maintained by the Finnish metrological institute VTT MIKES. The MGRS time and frequency base can be compared to the UTC MIKE via commercial White Rabbit time and frequency transfer technology as well as dedicated in-house built frequency-only transfer technology. The latter is currently under development but preliminary results indicate better performance than White Rabbit. The ultimate aim would be to use UTC MIKE as the time and frequency base for many of the measurements at MGRS providing a direct link between the geodetic measurements and the SI unit of one second. However, currently, the feasibility of the link for such operational use is still under investigation and the link is used for research and technology development purposes only (Calvés et al. 2018).

MGRS also has a GPS-based Network Time Protocol (NTP) server that provides a time signal to the local station network allowing the measurements to be synchronised to the same time base. VLBI especially requires a very stable frequency source. For the commissioning phase, we are using reference frequency from the nearby hydrogen masers of Aalto University Metsähovi Radio Observatory. The cable length between the masers and the backend is approximately 100 m. For operational use and to provide the best possible data, the H-maser needs to be closer. To this end, we are in the process of procuring an H-maser that will be located in the same room as the VLBI backend.

MGRS has since 2013 hosted a triangular trihedral 1.5 m corner reflector (CR) for interferometric synthetic aperture radar (InSAR) satellites, owned by the German Aerospace Agency DLR (Gisinger et al. 2022). The CR is used for radiometric and geometric calibration of InSAR data of satellites such as TerraSAR-X and Sentinel-1. InSAR is also being intensively studied as a newly emerging technique for geodesy as it provides continuous data on heights and their changes over large geographical areas. MGRS is an ideal site as a testbed for such CRs as the bedrock provides a stable foundation and local surveys of the CR reference point position can be directly linked to the International Terrestrial Reference Frame through the IGS station(s) on-site.

Almost all measurements at MGRS require some kind of weather or environment data either directly in the processing and analysis or as metadata to be included as added information. With the station upgrade it became possible to create a more easily managed, maintained and calibrated central solution for providing the weather data. Additionally, there are detectors for soil moisture, groundwater and snow depth measurements. Calibration of sensors is done according to the manufacturer’s specifications. It is critical to use properly calibrated sensors as, e.g., false pressure readings would distort the SLR and gravity results in a way that might not be immediately obvious in the data analysis.

4 Conclusions and Discussion

During the last decade, most of the MGRS activities were concentrated on technical issues and construction work of new instrumentation. When the installation and commissioning of equipment are completed, the main attention will be shifted back to research.

MGRS measurements contribute to the global and regional geodetic reference frames and global geodesy as shown in Fig. 2. As an example, the GNSS receivers produce data for IGS and EPN networks, and together with other EPN stations provide the primary link of the national reference frame to global and European reference frames.

The long and continuous GNSS time series at MGRS with only a few equipment changes has made the station very valuable for reference frame realisations. However, it is noteworthy that REGINA station METG00FIN does not co-locate at the Metsähovi main station, and its station velocity should not be constrained to METS/MET300FIN in the reference frame related works.

Long gravity time series of AG and SG are equally valuable for regional and global research. The status as the National Standards Laboratory of the free fall acceleration enables participation in metrological research projects also in the future and mutual comparison of gravimeters at MGRS.

MGRS is also one of the northernmost GGOS Core stations, and therefore data are valuable for Arctic region research. Especially, SLR is crucial for improving the orbit information of both GNSS and Earth exploring satellites in the Arctic where climate change-induced phenomena, like ice sheet decay, are pronounced.

MGRS is Finland’s contribution to the UN General Assembly’s 2015 resolution 69/266 A “Global geodetic reference frame for sustainable development”. As a small country, Finland has limited resources for global geodetic research, but MGRS with its upgraded instrumentation and its long history will maintain the internationally recognized status of Finnish geodesy. Recent international evaluation of the FGI drew special attention to the Metsähovi Geodetic Research Station, which is “unique, at a national and international level in terms of its research infrastructure” (Hämäläinen et al. 2023).