Globalnaya Navigatsionnaya Sputnikovaya Sistema (shortly GLONASS) is a satellite based positioning system similar to GPS, but owned and maintained by the Russian Space Forces. It is fully functional and available from 2011 and for this reason not many devices support it.
The benefit of supporting both GPS and GLONASS is truly visible in difficult locations like highly build urban areas and other places with low satellite coverage, as the combined use of both systems gains access to 55 satelites and gives more accurate location readings. For power consumption reasons, however, the GLONASS receiver is activated only when GPS coverage is insufficient for correct readings.
The combined use of GPS and GLONASS is supported by all preinstalled navigation software available on Sony Ericsson Xperia phones.