This user guide provides users of the Norwegian Mapping Authority’s positioning services with an overview of equipment setup, technical information and special conditions that apply when measuring/surveying with CPOS and DPOS.
To connect to the service, do the following:
- Connect via the internet with your connection address (NTRIP caster IP and port): 18.104.22.168:2101
- Enter your username and password, and specify the mountpoint you wish to use from the table below.
Choose from available services:
|Mountpoint||GNSS system||Correction format||Additional information|
|CPOSRTCM32||GPS+GLO+GAL+BDS||RTCM 3.2 MSM4||HREF NN2000 available|
|HREFNN1954||GPS+GLO||RTCM 3.1||HREF NN1954 available|
|HREFNN2000||GPS+GLO||RTCM 3.1||HREF NN2000 available|
|SVALBARD||GPS+GLO||RTCM 3.1||Nearest base, Svalbard|
- If it needs to be specified, transmission of the NMEA position to CPOS takes place every 10 seconds.
- If possible, specify VRS (Virtual Reference Station) as the method.
- RMS, elevation angle, fixing level, etc. can be set to the same values as with traditional RTK measurement.
- Coordinate system: EUREF89 and ellipsoidal height.
For further information regarding the setup of rover equipment, please contact the equipment supplier.
Troubleshooting when connecting
If you experience problems when connecting, the following should be checked:
- Check that you are using the correct IP address and IP port number.
- Check that your username and password are correct.
- Check that the correct correction format is selected (RTCM version).
- Consider whether poor mobile coverage could be the cause of the problem.
- Check operational messages from the Norwegian Mapping Authority’s control centre.
If all these points have been checked and the system still doesn’t work, call your CPOS distributor while you are out in the field. You can also call the Norwegian Mapping Authority’s customer centre on +47 32 11 80 00.
Height reference model (measurement in NN2000)
The HREF model can be entered into the equipment as a file or received along with the corrections. For more information on the implementation and use of HREF, please contact your equipment supplier.
CPOS and DPOS are network systems that use virtual reference stations (VRS). VRS takes care of all base functions; this means that the customer does not have to establish a separate base station and check that it is set up correctly, or check that it is functioning correctly at all times. On the rover side of things, the work must be quality assured in the same way as when using RTK with a separate base station.
In order to obtain valid data in the area being measured, the rover must specify its position. The first received position that is sent to the control centre forms the basis. It is recommended that the GNSS receiver has been given sufficient time to determine its position (uncorrected) before connecting.
Virtual GNSS observation data is then calculated for the point at which the GNSS receiver is located, and these are then sent to the user. A virtual reference station (VRS) is then created at this point. The user then carries out vector measurements in relation to this point instead of using a physical base. When the user has moved 5 km from the virtual reference station, a new one is automatically generated. Therefore, you will never be more than 5 km from the point to which the reference data refers, providing you with the same amount of accuracy as long as you are within the coverage range. Note that a new VRS is created if you disconnect and then reconnect.
In order to measure with a high degree of accuracy, a correct fix solution is required in CPOS. The GNSS receiver creates a fix solution based on its own observations that are combined with reference data from CPOS. A fix solution means that the unknown number of whole wavelengths in the distances to each satellite is determined. The time it takes to calculate the fix solution is called initialisation time. The same factors that affect accuracy will also affect the initialisation time.
Among other things, a lot of ionospheric disturbance can lead to a long initialisation time or that a fix solution is not achieved. If you have waited 2 minutes or more after connecting without achieving a fix solution, it is recommended that you disconnect and then reconnect.
In areas with poor mobile coverage, the weak connection may affect the initialisation time. Therefore, if possible, monitoring the signal strength is recommended. At low signal strengths, data transmission may be interrupted.
Conditions affecting accuracy
In all forms of GNSS measurement, accuracy may be reduced by certain external conditions. This also applies to correction services such as CPOS and DPOS.
PDOP (Position Dilution of Precision) provides users with an indication of the satellite geometry of the available satellites. When good geometry is achieved (i.e. a good spread of satellites over the sky), an observational error is less likely to cause errors in determining the point’s position compared to poor geometry. Poor satellite geometry can be detected through a high PDOP, while a low PDOP represents good satellite geometry. As a general rule, a PDOP of less than 4 is satisfactory. On most GNSS receivers, users can set an upper limit for PDOP so that GNSS receivers don’t register positions if this limit is exceeded. In cases where there are few satellites available, there may be a need to use an elevation mask lower than 10-15° in order to detect as many satellites as possible. However, if one chooses to reduce the elevation mask, one must be aware that the signals from the lower satellites may include somewhat larger errors.
Ionospheric disturbances – targets with a high elevation mask:
When receiving correction data, they include corrections for the delay that the GNSS signals are subject to during the journey through the ionosphere. Although the calculation of the corrections is based on an ionospheric model optimised for Nordic conditions, today’s technology is unable to detect all the changes that occur in this layer of atmosphere. Therefore, GNSS users should be aware of days with high levels of ionospheric activity and take precautions during these periods. During periods when the ionosphere is unstable, it is important to be critical of signals from satellites located low on the horizon. The signals from these satellites have to travel a very long way through the ionosphere, and will therefore contribute to a lot of interference during calculations.
Multipath interference - be aware of reflective surfaces:
Multipath interference occurs when the antenna receives a satellite signal that is reflected from a certain object located near the antenna, instead of the signal coming directly from the satellite. These interfering surfaces can be things like a building, a car, a water or snow surface, or other reflective objects. Multipath interference can also be a problem in forests. When a GNSS antenna is affected by multipath interference, the GNSS receiver does not register the correct distance between the satellite and the antenna. Instead, it registers the distance from the satellite, via the reflective surface, to the antenna. Therefore, when planning the measurements, the operator must pay attention to possible sources of reflection that are in the vicinity of the point being measured. Currently, most GNSS receivers have advanced algorithms that reduce the impact of multipath interference. Nevertheless, the operator must be aware that measurements carried out near reflective surfaces will still be affected by this type of error to a certain degree.
Documentation and quality assurance of measurements
As with all surveying, it is important to check all measurements and ensure consistency in order to document accuracy and avoid errors. This means that one must ensure that the rover is set up and used correctly. If possible, a test measurement of existing fixed marks will give an indication of the measurement accuracy in the area during the relevant period of time.
If the measurements taken require analysis and documentation, it is recommended that this should be carried out at the coordinate level rather than at the vector level. The rover must store coordinates during the measurement work, which can then be imported into an adjustment and analysis tool. There are several such tools on the market that have modules to adjust and analyse at the coordinate level.
In addition, we refer to the latest version of the “Satellite-Based Positioning (in Norwegian)” standard, which provides a recommendation on how satellite-based positioning should be carried out and where documentation of measurements is discussed. For measurement requirements, etc., please refer to the standards: