GLONASS is the Russian equivalent of the American GPS (Global Positioning System). GLONASS is an acronym for "Globalnaya Navigatsionnaya Sputnikovaya Sistema" or "Global Navigation Satellite System". Further technical information can be found on the Russian Federal Space Agency web site:
https://www.glonass-ianc.rsa.ru/en/index.php
The current version of the GLONASS ICD (Interface Control Document) is 5.1 and use to be available from the web site given above but has since been removed, however a search on the internet should reveal other sources.