Guide to ETPOS
With the ETPOS subscription service and your own GNSS measurements, you can recalculate measurements, thus achieving centimetre-level accuracy or better.
This is a service consisting of data files from the previous 3 months that include GNSS observations from the Norwegian Mapping Authority’s permanent geodetic base stations, as well as accurate coordinates in EUREF89 at these stations.
How to order
To gain access, a subscription to the service must be purchased for NOK 8,000 + VAT per year. If you already have a CPOS subscription, ETPOS is included. ETPOS data is downloaded via the web, log in with CPOS or ETPOS username and password.
Data is available in RINEX format as 24-hour files with 1-second and 30-second data rate, as well as 1-second data rate hour files.
The files are organised as follows:
/rnx3/<duration>/<rate>/<year>/<doy>/<name>
where <duration> is the duration of the file, <rate> is the time resolution of the observations in the file, <year> is the year and <doy> is the day number of the year (1-366). For an overview of stations and their abbreviated names, see Norgeskart.
RINEX3 file name is defined in RINEX3 standard (pdf)
The file names are denoted as follows: VIKE00NOR_S_20211530000_01D_30S_MO.rnx.gz
| Field | Description | Comments |
|---|---|---|
| VIKE | Station name | |
| 00 | Serial number | |
| NOR | Country code | |
| _S_ | Generator | Defines whether the file is generated directly on the GNSS receiver (_R_) or based on a real-time receiver data stream (_S_) |
| 2021 | Year | |
| 153 | Day-Of-Year | |
| 0000 | Time when file starts | GPS/UTC time |
| _01D_ | File duration | Hour files (_01H_) or 24-hour files (_01D_) |
| _30S_ | Date rate | Observations logged every second (_01S_) or every 30 seconds (_30S_) |
| MO | Data type | MO: Mixed observation, i.e. observations from several satellite systems. Ephemeris files: GN (GPS), RN (GLONASS), EN (Galileo) and CN (Beidou) |
|
.rnx |
File type | Identifies the file as a RINEX file |
| .gz | Compression | File is compressed using the gzip method |
CPOS also includes Swedish and Finnish stations along the national border. The Norwegian Mapping Authority is not allowed to resell historical data from these. Customers seeking data from these stations must contact the data owner directly: Lantmäteriet in Sweden and Geotrim in Finland.
Areas of use/application
ETPOS is suitable for use where real-time correction is not appropriate or not possible, or where higher precision is desired. Typical use of ETPOS includes the kinematic location of airborne sensors, such as aerial photography, laser scanning and drones. In such cases, the user does not have to set up their own base station as a correction source.
Other uses may be when establishing an accurate starting point for further measurement, for example with a total station. If the GNSS measurement of the point is done over time (static measurement), a higher degree of accuracy may be achieved than with real-time measurement. The Norwegian Mapping Authority and other geodetic environments use similar data to look at earth crust movements such as land elevation and tectonic plate movements.
For example, the service may also serve as a backup solution in areas/cases where distribution of real-time data over radio or mobile phone is not possible.
Accuracy
The GNSS receiver calculation technology can be roughly divided into two groups: Code and phase. For code measurement, the achievable accuracy is metre level, for phase measurement it is centimetre level. The result depends on measurement and calculation technique, measurement time, distance to base station, choice of GNSS receiver and the general measurement conditions for GNSS at the location and time in question.
With code measurement, correction can be made for a single GNSS measurement. However, in phase measurement, it is necessary to measure over some time. Within the CPOS coverage range, a shorter measurement time is normally required to achieve centimetre-level accuracy, but this is again dependent on the chosen calculation technique. See the “Satellite-Based Position Determination” standard for guidance and guidelines for static measurement/post-calculation.
Equipment and software requirements
The GNSS measurements are corrected or co-processed with ETPOS in the user’s calculation programme. Most suppliers of GNSS equipment provide packages for post-calculation of positions.
ETPOS is supplied in RINEX format, an international standard exchange format for GNSS raw data. RINEX is an abbreviation for Receiver INdependent EXchange format, i.e. the data format is independent of the type of GNSS receiver used. The calculation programme used must be able to read and apply RINEX. The exact coordinates of the base station are in the header of the RINEX file. The user's GNSS receiver must be able to log raw data in the format that the calculation software is able to read and apply.
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