The offer follows the case of Malaysia Airlines flight MH370, which disappeared without trace on 8 March.
It was very brief electronic "pings" from Inmarsat equipment on the lost plane that prompted investigators to look for wreckage in the Indian Ocean.
Inmarsat says the free service it is offering would carry definitive positional information.
It would see a plane determine its location using GPS and then transmit that data - together with a heading, speed and altitude - over Inmarsat's global network of satellites every 15 minutes.