Cablevision Systems, a US cable TV group, has ordered five new direct broadcast satellites from Lockheed Martin to boost the capacity of Voom, its fledgling high-definition satellite TV service.

The deal, disclosed in a filing with the Securities and Exchange Commission yesterday, is worth about $740m and underscores Cablevision's determination to push ahead with the Voom service in spite of continuing doubts on Wall Street about its costs and future viability.