The National Land Survey positioning service offers free DGNSS correction data for code-based positioning.
DGNSS service (Differential GNSS) is based on the precisely known positions of FinnRef reference stations. The positions are used to determine the deviation of the observed distances to the GNSS satellites from the expected. These deviations are called DGNSS corrections that are sent to the user. The user receiver first corrects its own observed distances by the amount of the corrections and then computes its position as usual.
The NLS positioning service offers differential correcions from single reference stations (mountpoints DGNSS or for example DG_HETT) or as corrections modelled to the user location (DGNSS_VERKKO). The positioning accuracy using differential corrections decreases when the distance to the reference stations increases.
The NLS positioning service currently offers differential corrections for GPS and Glonass satellites. In the near future, the service will send corrections also for the satellites of the European Galileo system.
Use of DGNSS service
The differential corrections are sent over the internet according to Ntrip protocol (Networked Transport of RTCM via Internet Protocol). The user receiver must have an internet connection and an Ntrip-ready software. The correction stream mountpoints DGNSS and DGNSS_VERKKO require the approximate location of the user receiver. This location is sent as an NMEA message (The National Maritime Electronics Association).
DGNSS corrections are sent in two versions of RTCM-standard, in RTCM 2.2 and in new RTCM 3.2 (MSM) sent in RTCM 2.2. -format. Sent message types for version 2.2 are:
|1||1 s||GPS differential correction|
|3||10 s||Reference station information|
|31||1 s||Glonass differential correction|
And message types for version 3.2 are:
|1006||10 s||Reference station information|
|1071||1 s||GPS code observations and differential correction|
|1081||1 s||Glonass code observations and differential correction|
DGNSS service can be used with these credentials:
The user also has to select the desired datastream's mountpoint from the following:
|Correction stream mountpoint||NMEA||Description|
The service connects the user automatically to the correction from the closest reference station
|DG_XXXX (XXXX is the four-letter station ID, see below)||No||User selects the desired reference station|
The service uses the error modeling of the whole FinnRef network and sends corrections individualized to the user location (RTCM 2.2)
|DGNSS_VERKKO3||Yes||The service uses the error modeling of the whole FinnRef network and sends corrections individualized to the user location (RTCM 3.2)|
Same as DGNSS_VERKKO, but correction is sent for maximum of 12 satellites. This is intended for older 12-channel receivers.
The available mountpoints are in the service sourcetable.
Below is an example how the user can connect to the mountpoint DG_JOE2 (differential correction from the Joensuu reference station). The example software is RTKLIB's strsvr .
 GNSMART-ohjelmiston tuottaja Geo++:n kotisivu http://www.geopp.de/gnsmart/
 BKG:n (Bundesamt für Kartographie und Geodäsie) tuottama epävirallinen Ntrip-dokumentaatio https://igs.bkg.bund.de/root_ftp/NTRIP/documentation/NtripDocumentation.pdf
 RTKLIB, http://www.rtklib.com/
- GNSS = Global Navigation Satellite System
- DGNSS = Differential GNSS
- RTK = Real Time Kinematic
- Ntrip = Networked Transport of RTCM via Internet Protocol
- NMEA = The National Maritime Electronics Association
- RTCM = Radio Technical Commission for Maritime Services