General Motors Corp’s Hughes Aircraft arm, McCaw Communications Cos and six other companies have come together in a consortium to invest $730m building a three-satellite US-wide mobile telephone and data system. American Mobile Satellite Consortium hopes to put up its first bird in 1993, but wants to start the service in January 1990, leasing transponders on existing satellites until its own three are in orbit. Cars and trucks would drive around with dish antennas on their roofs, connected to a dashboard telephone terminal, and the system would be linked in with the public switched telephone system. Seen as complementary to cellular systems, the satellite network would serve the 85% of the US that is outside the range of cellular transmitters, but cellular would remain the preferred medium within cities where the satellite signals would tend to get lost among the tall buildings. The consortium looks for Federal Communications Commission approval to proceed with its implementation plan to come through before the end of the year.