I thought I’s put together a set of links that may be of use to our EU and UK members, and many apply globally. These are resources I regularly use for accurate GNSS surveying and planning.
A Guide to Coordinate Systems in Great Britain, and excellent resource for UK GNSS surveyors (PDF)
Ordnance Survey RINEX download, access 30 second RINEX 2.11 data for over 130 UK stations
NASA CDDIS product holdings website
ETRS89 definitions, parameters and realisations
WGS84 to UTM transformation tool
GPS Time calendar, find any GPS date
Simple DMS to Decimal converter tool
GPS and GNSS for Geospatial Professionals, excellent free course from Penn State University
Enjoy!