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BBC director general Mark Thompson and ITV executive chairman Michael Grade said the service had an important role in the development of UK digital television.
"With the launch of Freesat, all the BBC's digital channels, including BBC HD, will now be available to all licence fee payers wherever they live and however they choose to access them," said Thompson.
Grade said: "The launch of Freesat is the final piece in the digital jigsaw and will ensure that all viewers in the UK have access to free-to-air digital and high definition television."
"Now everyone in the UK can experience the benefits of digital television: more choice of channels, interactivity and the extraordinary picture quality of HD."
Freesat's EPG currently carries about 80 channels - Five is a notable absence - though the company says 200 are expected to be available by Christmas.
> Spy Guide: What Is Freesat?
Managing director Emma Scott and commercial director Richard Lindsay-Davies used the launch to emphasise the strongest message in Freesat's marketing material - the absence of a subscription charge.
Freesat officials said that, though open platform principles were important to the service, they had developed a managed and licenced specification to make sure there was a consistent consumer experience.
"We have worked extremely hard, and so have our partners, to achieve a set of standards," said Lindsay-Davies. "Freesat is a managed platform. Though certainly open standards have been used for elements, it is very much about Freesat owning that platform and ensuring we are able to develop that platform."
Freesat has already distanced itself from one personal computer satellite receiver which was linked to the service in promotion material.
Officials said that unlicensed free-to-air digital satellite receivers may receive some elements of the Freesat electronic programme guide but that the "consumer experience" would not be complete.
They cited plans for developing interactivity and IPTV applications, using Freesat receivers' Ethernet connections, for which existing licensed products were equipped.
Freesat said it was developing such services - making iPlayer catch-up programmes available, for example - and suggested they would be "open" rather than linked to a particular set of internet service providers.
