Taking global telecommunications a giant step forward, Motorola Inc. is expected to unveil plans today for a $2-billion network of orbiting satellites to provide cellular telephone service to remote stretches of the planet.

The new system, named Iridium, will rely on 77 small, low-orbiting satellites to send cellular radio signals from one caller's handset to another's, without the need for switching towers and relay stations on which current cellular telephone service is based--or poles and wires on which traditional phone service is based.

The Motorola network, projected for full operation by 1996, is designed to serve areas now without state-of-the-art telephone service--the primary markets are Third World nations and Eastern Europe--and is expected to connect with traditional land-line service of all types already available in industrialized nations.

"The network is designed to fill in the gaps of the worldwide telecommunications system," said Durrell Hillis, general manager of Motorola's satellite communications operations. "When you get to the end of the traditional service area, your call just gets handed up to the 'bird' orbiting the Earth." The system will be able to handle both voice and computer data.

The new network, which generated widespread interest long before its official unveiling this morning at the Hayden Planetarium in New York City, promises to deliver a service that experts have long predicted would become a reality before the end of the century: instantaneous communication and access to data banks, friends, family and fax machines from anywhere on the globe.

"This is the future," says Steve Sazeguri, a telecommunications analyst with Dataquest, a Silicon Valley technology marketing research firm.

The experts call it "personal communication," and they predict that it will expand the worldwide cellular market from about 7 million subscribers currently to 100 million by the year 2000.

Eventually, they say, nearly everyone will carry some sort of personal telephone, either on the wrist, in pockets or purses, or in a computer, that will allow individuals to make and receive calls wherever they are. Such personal phones already exist. Last year Motorola, already the world's largest supplier of cellular telephones, introduced a pocket-sized model weighing just 10 ounces.