The Reagan administration plan to spin off the government's remote-sensing satellites to private enterprise has set off a commercial space race among companies ranging from tiny entrepreneurs to giant Communications Satellite Corp.

Hurtling 400 miles above the Earth, Landsat IV, the most sophisticated of the remote-sensing satellites, can span and scan the globe with a powerful variety of spectral sensors that gather data important to agriculture, mining, oil services, resource management and other fields.

Thus, operators could enjoy significant profits selling the information; some experts project a billion-dollar market in remote-sensing data and analyses by the end of this decade.

But the companies also may encounter diplomatic and military obstacles on their way into space. Private ownership of the data may conflict with international treaties on the sharing of information collected by satellite. And some U.S. defense officials are nervous about the idea of companies being capable of collecting sensitive information.

American Science and Technology, a fledgling Bethesda company scrounging for venture capital, wants to build and launch a set of custom-designed, remote-sensing satellites by early 1986 to feed the anticipated demand for satellite data. The key to its approach is the low projected cost of the satellites--between $25 million and $50 million each--says Diana Josephson, president of AS&T and a former official of the National Oceanic and Atmospheric Administration.

AS&T's approach contrasts with the recent proposal from Comsat, which would have the government turn over custody of the existing satellite-sensing system for roughly $350 million and guarantee Comsat a contract for its services.

Although the Reagan administration plan, which would require the approval of Congress, includes disposal by the government of its weather-sensing satellites, most experts predict that any vital private market will be built around the Landsat-type, earth-sensing satellite.

With infrared and microwave scanners that collect volumes of data every hour, Landsat's commercial potential has yet to be tapped, some experts say. "The technology is way ahead of its applications," observes Frederick Henderson, chairman of Geosat, a cooperative venture of companies like Mobil Oil Corp. and Bechtel Corp. that now use Landsat data.

In fact, says Henderson, the business probably will spawn two separate and distinct activities. One is the operation of the satellites themselves--the area American Science and Technology and Comsat want to enter. The other would be the "value-added" process of analyzing and "massaging" the satellite data into more valuable form for such customers as oil companies and agricultural firms.

Indeed, a rough sort of private market already exists for "value-added" companies that purchase raw Landsat data from the Earth Resources and Observation Service data center of Sioux Falls, S.D., a Commerce Department agency. EROS sold approximately $4 million of Landsat data tapes last year.

But government operation of the satellite system is discouraging development of a significant private market, according to Donn Walklet, president of TerraMar, a California consulting firm specializing in high technology..

"We're just beginning to get the data that is useful; we're at the beginning of the commercial threshold," Walklet said. He projects that the market for value-added products and services will grow to more than $200 million by 1985.

The creation of a vital private sector in remote-sensing satellites may depend less on money than on the question of who owns the view from outer space. "There's only one reason to fly a satellite, and that's for global data," said Geosat's Henderson. That raises diplomatic issues.

Many countries--especially those in the Third World--see remote-sensing satellites as spies in the sky and insist that they must give prior consent to the scrutiny of their territory from space. Partly to ease such concerns, the United States is signatory to several international agreements that created an Open Skies satellite policy, which means that data collected by satellite are accessible to all.

While one Commerce official says the Open Skies policy precludes creation of private remote-sensing satellites, Michael Halbouti, who was chairman of a Landsat advisory committee, disagrees, saying, "If you want to launch a private satellite, you can do so."

Still, "The issue of the proprietary nature of the data is important," according to Walklet. "Just because it's an Open Skies policy doesn't mean it can't be proprietary." Walklet, who consults for several value-added companies, argues that a competitive private marketplace will, in effect, maintain the Open Skies policy because vendors will try to sell their information to any and all who will buy it. "Commercialization will be a boon to the Third World," he said.

However, several experts concede that the formal Open Skies policy might have to be abandoned if a truly competitive market is to develop. If the private sector is called upon to replace the Landsat system with one of its own, it probably will insist on the right to market resulting data on a proprietary and exclusive basis. It is uncertain how the international community would respond to such a move.

The fledgling private market for such satellites faces other problems as well. Remote-sensing satellites that can scan the globe in pieces as fine as 10-meter increments enter the realm of defense intelligence data. "You can identify tank movements in the 10-meter resolution," Henderson of Geosat points out.

Walklet says that defense planners are more concerned with "site-specific" satellite data than Landsat-type data, but he concedes that the right blend of private satellite and value-added vendor could produce useful military intelligence. As an example, the use of such data could have alerted the British to the Argentinian naval build-up prior to the Falklands invasion.

For this reason, the Defense Department, which has its own remote-sensing satellite network, reportedly has expressed concern about the possibility of multiple private remote-sensing satellite systems..

However, Henderson, Walklet and others argue that the United States must continue its involvement in the remote-sensing field--either by renewing a commitment to a Landsat-type program or by aggressively encouraging a private market.

The Japanese and the French are in the process of launching remote-sensing satellite systems. And Americans knowledgable about remote-sensing satellites fear that the U.S. lead in them will fade and put the country at a disadvantage in the quest to identify and acquire vital natural resources.

"The real poker game isn't the satellites; they're just a tool," Henderson said. "The real game is resource information. And satellites will be the best way to find them."