SKPOS®

Slovak real-time positioning service (SKPOS) is a multifunctional tool for precise object and phenomena positioning by global navigational satellite systems (GNSS). SKPOS consists of the GNSS network of permanent reference stations connected by a private virtual network to the National Service Centre established at the Geodetic and Cartographic Institute in Bratislava (GKU Bratislava). The National Service Centre is equipped with the service control software, which administers satellite observations from the permanent stations network and simultaneously generates so-called network corrections for the users of the real-time service and data serving for post-processing for users equipped with post-processing software.

At present, SKPOS represents the most widely used service of the Geodetic Control Department of GKU Bratislava, available to its users 24 hours a day. With its permanent GNSS reference stations infrastructure, represents active geodetic control of Slovakia and most of its points constitute the highest A class of the National Spatial Network points. The service enables to the users to work on-line (real-time kinematic method using a network solution) or in a post-processing way in mandatory geodetic reference systems ETRS89 and S-JTSK (in JTSK03 frame). In case of interest in the coordinates of JTSK frame it is recommended to use freely available official transformation service or software supporting the conversion of coordinates on the basis of a released JTSK03 ↔ JTSK interpolation table.

Legislation

In accordance with the Act No. 215/1995 Coll. of the National Council of the Slovak Republic on Geodesy and Cartography as amended by later regulations (215/1995 Act), a permanent location service, which uses global navigation satellite systems, represents the network of cooperating stations processing and providing real-time geocentric coordinates in real time for the precise location of objects and phenomena. Under §4 point 2n of the Act 215/1995, the task of the Geodesy, Cartography and Cadastre Authority of the Slovak Republic (UGKK SR) is to provide the creation and operation of the permanent positioning service using global navigation satellite systems. UGKK SR, by UGKK SR No. 300/2009 Coll. Regulation, implementing the Act 215/1995 (hereinafter referred to as 300/2009 UGKK SR Regulation), has charged with this task its organization, GKU Bratislava. GKU Bratislava as a legal entity established by UGKK SR under the UGKK SR 300/2009 Regulation provides, administers and operates the Slovak Spatial Observation Service (SKPOS) as a permanent positioning service. Under §3 point 2 of UGKK SR No. 300/2009 Regulation, SKPOS represents active geodetic control of SR, to which the results of all selected geodetic and cartographic activities included in state documentation and submitted to mandatory geodetic reference systems are mandatorily related.

Guideline: Usage of Slovak real-time determination system for surveying

Real-time Kinematic Method – RTK
Real-time Network – RTN

Real-time kinematic method (RTK) requires equipment consisting of one reference non-moving receiver referred to also as a base and the other moving receiver referred to as a rover. Both receivers have to carry out phase measurements to the GNSS satellites simultaneously. Between the receivers continual radio or internet connection must be working by which the transmission of measured data from the base to the rover is provided. The rover has to have inbuilt software for phase measurement processing, on the basis of which it is immediately after initialization able to, both from the received measurements and from the measurements of its own, create differences and to compute a relative position in relation to the base position. However, it is necessary to keep in mind the fact that the RTK accuracy decreases with increasing distance between the rover and the reference base. The method is reliable with the lengths of baselines up to ca 20 km, on account of different distribution of masses in atmosphere and different conditions between the reference base and the rover. To compensate this drawback the Real-Time Network (RTN) method has been developed. The RTN method uses the network of reference stations distributed in the area of interest in the recommended distance; the stations send their real-time observations to the control centre for processing. Using advanced algorithms, the software in the control centre generates a network solution from the data processed; the solution is transmitted to users via the internet and serves for the computation of differential corrections, which means relative positioning of the rover user.

There are several RTN concepts such as MAX, FKP, VRS, and other concepts. SKPOS service provides to its users corrections in the virtual reference station concept (VRS) and nereast reference station concept. VRS concept is based on generating corrections for VRS station, which is situated near the site of the rover (only a few metres distance). The rover uses and interprets data from VRS in the same way as if these data had originated in the real reference station situated in the rover vicinity. In practice it is carried out in such a way that the user's rover after successful authorization sends its approximate position in the NMEA GGA message form to the SKPOS control centre via the internet. The SKPOS control centre software accepts this position as a location for a new VRS, computes corrections for this VRS and sends them back to the rover in RTCM standard or in some other proprietary format. When the rover receives the corrections, it classifies them as data from the reference station and determines the relative position by the same processing as that used with the RTK method. The rover takes the coordinate system, framework and epoch of the measurement from VRS that is from the SKPOS service setting.
Nereast reference station concept is based on sending corrections directly from the nearest reference station. Network corrections are not generated, but data are sent directly from the nearest reference station. The moving rover, after successful authorization, sends its approximate position in NMEA GGA message to the SKPOS control center via the Internet. Based on this approximate location, the software identifies the nearest reference station and starts sending data directly from that reference station. Due to the different distribution of masses in the atmosphere and the different conditions between the location of the reference station and the moving receiver, the method is reliable up to a distance of 20 km from the reference station. The advantage of the method is that a larger number of satellites are sent than with the VRS method. More information about the concept of the nearest reference station can be found in the tutorial section.

SKPOS Infrastructure

• Reference stations network
• National service centre
• Virtual private network

Reference stations network (reference stations list)

SKPOS represents active geodetic control of Slovakia. Physical geodetic points, on which antennas of SKPOS receivers are mounted, make up the reference stations network of global navigation satellite systems, essential part of which forms the highest class of geodetic control points, i.e. the A class of the National Spatial Network. The network of SKPOS reference stations consists of the reference stations located on the territory of Slovakia, but also of the reference stations located in adjacent foreign countries on the basis of an agreement on data exchange from foreign stations with the neighbouring administrators of national services.

The reference stations located on the territory of Slovakia are established on the supporting parts of the roofs of the buildings of predominantly cadastral departments of district councils, or, as the case may be, they are monumented by a reinforced concrete pillar, or by a special deep drilled braced monument. Positions of the coordinates of reference stations are computed using the scientific Bernese GNSS software. Reference stations located on the territory of Slovakia are also connected by precise levelling to the national level network, which has allowed the determination of their elevations in the Bpv datum. All reference stations located on the territory of Slovakia are equipped with the Trimble brand instruments. 21 antennas have precise position parameters and phase centre variations determined by individual absolute robotic calibration. On other antennas, during computation the values from so-called type absolute calibrations are used.

All reference stations located on the territory of Slovakia receive signals from the GPS, GLONASS, Galileo and BeiDou satellite systems. With regard to the monumentation type, (monumentation by a reinforced concrete pillar, or by a special deep drilled braced monumentation), as many as 18 permanent stations located in Slovakia fulfil also high requirements on the geodynamic research quality; therefore they are incorporated also into the network used for diagnosing deformations of physical earth surface on the territory of Slovakia.

National service centre

National service centre of SKPOS is established at the Geodetic and Cartographic Institute in Bratislava and provides all activities connected with its routine operation and development. The national centre provides the administration of reference stations and their data monitoring, collection and back-up, supervision of the running of the control software that provides generation of corrections for individual services, user registration and contact and monitoring of the quality of services provided. Major part of routine work is provided using the control and processing software, which from the beginning is of the same brand as the infrastructure of the reference stations. In pursuance of the service development the national service centre provides various analyses of the data obtained and stored by the service control software, as well as the creation and administration of applications using the service data. Collaboration with partner foreign positioning services and various international organizations is a matter of course, an also the service promotion and its presentation in professional events and journals.

WPS-WAN virtual private network

As it is necessary to have high-quality and stable communication between SKPOS reference stations and the national service centre, most reference stations are located in the vicinity of the cadastral departments of district councils, which are connected to the UGKK SR virtual private network (VPS). VPS to other reference stations located on the territory of Slovakia is established especially via direct connections. VPS serves for data transmissions from the reference stations to the national service centre and for user internet communication. Foreign stations of partner national positioning services are connected to SKPOS via the external WAN network.

Equipment required for SKPOS user

GNSS receiver with ability:

• work with RTK/RTN method,
• mobile internet connection (GSM/GPRS/3G/...),
• data reception using the NTRIP protocol and support of some of RTCM 2.1, RTCM 2.3, RTCM 3.1, RTCM 3.4, CMRx, CMR+ formats,
• transmitting position in the NMEA GGA string.

For correct transformation of resulting coordinates from ETRS89 into S-JTSK (JTSK03), in the GNSS receiver parameters must be set as follows:

• correct parameters of Bessel ellipsoid:
  • length of main semi-axis: 6377397.155 m
  • inverse flattening: 299.1528128
• correct parameters of a global transformation key:
  • direction of transformation S-JTSK(JTSK03) → ETRS89:
    • Translation in the direction of axis X: tx = 485.021 m
    • Translation in the direction of axis Y: ty = 169.465 m
    • Translation in the direction of axis Z: tz = 483.839 m
    • Rotation of axis X: rx = –7.786342"
    • Rotation of axis Y: ry = –4.397554"
    • Rotation of axis Z: rz = –4.102655"
    • Parameter of scale change: m = 0.000000 ppm
  • Direction of transformation ETRS89→S-JTSK(JTSK03):
    • Translation in the direction of axis X: tx = –485.014055 m
    • Translation in the direction of axis Y: ty = –169.473618 m
    • Translation in the direction of axis Z: tz = –483.842943 m
    • Rotation of axis X: rx = 7.78625453"
    • Rotation of axis Y: ry = 4.39770887"
    • Rotation of axis Z: rz = 4.10248899"
    • Parameter of scale change: m = 0.000000 ppm

Note. The above-given parameters of the global transformation key are transmitted via RTCM3.x message No. 1021 (using mountpoints SKPOS_CM_31, SKPOS_CM_32, SKPOS_CM_32_MSM7, SKPOS_CM_NS_34_MSM7).

For conversion of S-JTSK coordinates from JTSK03 frame to the JTSK frame it is recommended to use transformation service, or the use of JTSK03 ↔ JTSK interpolation talbe implemented to the GNSS receiver.

For the transfer of ellipsoidal heights from the ETRS89 geodetic reference system to elevations in the Bpv vertical system it is possible to use also the transmitted RTCM3.x message No. 1023 (using mountpoints SKPOS_CM_31, SKPOS_CM_32, SKPOS_CM_32_MSM7, SKPOS_CM_NS_34_MSM7) or using the implementation of Digital Elevation Reference Model (DVRM) to the receiver. Various types of DVRM formats you can download here Geodetic control – for download, or the use of the transformation service is also possible.

User manuals

• Reference Data Shop
• SKPOS Online Postprocesing
• Use of the concept of the nearest reference station

SKPOS as part of EUPOS

EUPOS (European Position Determination System) is an international non-profit initiative of public institutions mainly from Central and East European countries, providing GNSS services on their territories. Its aim is to facilitate the creation of and support the operation of multifunctional ground-based augmentation systems making GNSS measurements more accurate on unified basis, to act as a European organization representing the voice of public positioning services representatives and to collaborate with scientific and research organizations as well as with international organizations such as RTCM. The Initiative was established in the year 2002 in order to establish GNSS positioning services on the territories of member states, following example of the German service SAPOS, which initiated also the later establishment of SKPOS. Currently EUPOS comprises 14 member states. Positioning services of member organizations (countries) are established and operated on the basis of unified standards, which results in the interoperability of national positioning services and in simple collaboration in data exchange. Slovakia, or GKU Bratislava with the SKPOS service has been from the beginning of the Initiative establishment its integral part, having observed its standards.

SKPOS as a part of EPN Densification

In addition to the SKPOS service operator, GKÚ Bratislava is also an operational centre (OC) of the EUREF permanent network (EPN), which performs routine processing and computation of precise SKPOS station’s coordinates to densify the EPN network in Europe and to implement ETRS89 geodetic datum in Slovakia. Daily and weekly (combined) SKPOS coordinate solutions in SINEX format are calculated weekly using the scientific GNSS software Bernese 5.2 and then sent to EPN Combination Centers where they are combined with other national solutions (the so-called EPN Densification project).