The main methods of GNSS processing are (1) the classical differential correction with a permanent base station, and (2) the Precise Point Positionning (PPP) method that does not require a base station and uses precises clock and ephemeris data.
The tools used can be softwares or web-services. Among software we can distinguish between commercial softwares (e.g. Trimble Business Center TBC), open-source softwares (e.g. RTKLib), and scientific softwares (Bernese, Gamit/Globk). We will introduce RTKLib and web-services.
RTKLib is the main open-source library and software to process GNSS data. It can do conversion, post-processing, navigation, plotting, and so on.
Note : Several versions of the software exist and they behave somewhat differently. I’ll try to describe them one by one but you’d better check carefully your results compared to another “official” reference in order to validate your process.
Versions of RTKLib :
- Official software, with actual version 2.4.2. Does not work for me in post-processing as it is not possible to load base station Rinex data.
- Emlid Fork here. Should be best for conversion of UBX (Emlid raw format) to RinexFor. For what I can say, it worked for conversions, for post-processing of the base station, but it was not stable for post-processing of rover.
- RTKLib_Explorer fork here. Located on the really rich blog about GNSS and RTKLib named RTKLibExplorer, this tool works well and is well documented. The setting of parameters for static processing and obtain a single position is hard (set ON for “Output Single for Sol outage” and set a value for Max Sol Std)
Complementary tools for dealing with POS files from RTKLib and CSV files from ReachView :
- Tool to sync RTKLib POS file with CSV file from ReachView called posprocessor or PPK point extractor software : https://community.emlid.com/t/ppk-point-extractor-software/12822/103
- There is also a bit of Python code to do this job https://github.com/dshean/sfm_tools/blob/master/emlid_survey_update.py , just take care to have corresponding timestamps, because GPST and UTC are shifted (by 18s today in 2020)
- Canada NRCAN https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php , works fine with Emlid Reach RS2 device
- IGN RGP service http://rgp.ign.fr/SERVICES/ppp_online.php
- Trimble RTX https://www.trimblertx.com/ , accepts also T02 files from Trimble receivers.
Static GNSS correction with web-service
This processing is done with static GNSS correction using a network of permanent stations.
RGP IGN http://rgp.ign.fr/SERVICES/calcul_online.php , good for France, gives results in many coordinate systems including RGF93-Lambert93
GNSS complentary data and tools
GPS and Glonass ephemeris data, along with atmospheric data at https://kb.igs.org/hc/en-us/articles/115003935351
and also here (search more simple) https://webigs.ign.fr/gdc/fr/product/format#ephem
and also here (search simple too) https://cddis.nasa.gov/Data_and_Derived_Products/GNSS/orbit_products.html
IGS Antenna calibration file (does not contain Emlid Reach RS2) : igs14.atx
List of IGS stations : here
Offset between GPST (GPS Time) and UTC : here
- Review of GNSS surveys mode : http://www.aftopo.org/FR/REVUES/revue-4-140.html , pdf downloadable here
- BOSSER P.“Evaluation d’une solution de positionnement ponctuel précis temps réel” Revue XYZ n°148, p 33, septembre 2016. PDF here
- GNSS course (french) by ENS : http://www.geologie.ens.fr/~ecalais/teaching/geopotential/cours_gnss_stagem1_2014.pdf
- Invaluable reference about RTKLIB : https://rtklibexplorer.wordpress.com
- Post about processing of GNSS survey with Emlid Reach RS2 : https://sigeo.cerege.fr/?p=354
- Post about coordinate reference system transformation : https://sigeo.cerege.fr/?p=467