The first thing to understand about American Mobile Satellite Corp. of Reston is that "mobile" is the key word in the company's name, not "satellite."

Satellite communications is an important segment of the business of the company -- and its spinoff, XM Satellite Radio -- but mobility is the message that matters, not the medium.

Terrestrial towers -- ordinary, on-the-ground antennas -- work just as well as orbiting satellites for much of the mobile data communications work that is the core business of American Mobile, as the company has taken to calling itself lately.

The down-to-earth approach has become a major part of American Mobile's business strategy since its acquisition last year of Ardis Co., a nationwide network of wireless data links formerly owned by Motorola Inc.

Combined with the company's single satellite, the Ardis network of 1,900 antennas has made American Mobile a major player in one of this year's buzzword businesses: mobile wireless data communications.

Only somewhat self-explanatory, that phrase has a multitude of meanings for American Mobile.

At United Parcel Service, it's how the company keeps track of packages from the time they're picked up until the time they're delivered. UPS drivers carry a book-sized box with a computer keyboard, a wireless modem and a two-way radio inside. When they pick up a package, they punch in the address of the sender and receiver. The data is transmitted to headquarters via American Mobile's Ardis network.

The same network, working off a wireless modem in a special laptop computer, links Sears repair technicians to customer call centers, routing them to their next service call. IBM, NCR and several other companies also keep in touch with big field service work forces the same way.

The satellite and ground systems work together in a wireless data communications network called MultiMode that is installed in thousands of semi-trailer trucks. Picking up the nearest antenna the same way a cellular phone does, the system automatically switches to satellite communication when the rig rolls into rural areas.

Communicating through a dome the size of a Tupperware lettuce keeper that's mounted atop the cab, the system helps trucking companies keep track of their fleets. Using a small screen and keyboard on the dash, drivers can swap messages with dispatchers.

Targeting consumers for the first time, American Mobile has just introduced eLink, a cigarette pack-size wireless data terminal that sends and receives e-mail. The company has started selling the device itself, but the major marketing push will come in January in a partnership with SkyTel, the nationwide paging company owned by MCI WorldCom Inc.

The service now costs $59 a month for unlimited messages, plus $359 for the machine.

The tiny keyboard looks toylike, an apparently tricky test of one-finger typing skill, until American Mobile chief executive Walt Purnell picks one up with both hands and begins tap-tapping away with two thumbs.

"Some people can type 40 words a minute with their thumbs," he said, whipping off a sample. Millions of people have learned to cradle their cellular phone in their palm and dial with their thumb, and Purnell hopes they will just as easily adapt to two-thumb e-mailing.

Manufactured by a company called Research in Motion Ltd., the "RIM pager" is a thumb-to-thumb competitor with the Internet cell phones introduced recently by several wireless phone companies.

Purnell contends that the RIM pager -- and other machines using the eLink network -- will be formidable competitors for the new Net phones, which also can be used to send and receive e-mail.

"The Net phone is fine for the 30 percent of the population who are casual users," he said, "but heavy users will want something more convenient" than drafting e-mails on a phone dial, which can require two or three keystrokes for a single letter.

A top priority for American Mobile is making its eLink network accessible from hardware such as the wildly popular Palm hand-held computer and its fast-proliferating competitors.

Unlike cellular and satellite phones, the system works fine inside buildings, Purnell said. He ran Ardis when it was part of Motorola and joined American Mobile as president and CEO after the acquisition -- a sign of how important the deal was to the company. Motorola got 6.5 million shares of American Mobile stock in the Ardis deal, making it the company's second-largest shareholder after Hughes Communications, which has 11.5 million shares.

As Purnell explains it, cell phone signals bounce back and forth inside buildings, creating multiple signals that often get all muddled together. Rather than try to eliminate those indoor echoes, Motorola engineered the system to use them to create a clearer signal.

(That trick would be pretty useful to another company with facilities in the same office park off the Dulles Toll Road as American Mobile. That's Iridium, the global satellite phone system, which went into bankruptcy for a lot of reasons, a key one being that its phones work anywhere in the world -- except inside buildings.)

American Mobile actually offers satellite phone service, but data is the company's concentration. It's the biggest player in the mobile wireless data business, but has only a bit part in sat-phones. About 20 percent of the company's revenue last year was generated from voice communications, and that share is projected to dwindle as other lines of business take off.

The growth will come from wireless messaging and telemetry -- tasks such as remote reading of utility meters.

The company also is experimenting with wireless point-of-sale terminals that would allow cab drivers to take credit cards.

Purnell projects that revenue, which grew 25 percent last year, will grow 30 percent this year and continue to accelerate.

Profit? Not soon. Losses are still growing along with revenue, with a net loss in the third quarter of $66 million ($1.45 a share), up from $39.4 million ($1.24 a share) a year earlier.

Nonetheless, all five of the securities analysts who follow the company have a "buy" rating on the stock. Riding Wall Street's infatuation with all wireless communications, the shares are up from around $4 a year ago to $14 as of Friday. The shares peaked at $22.43 3/4 in July, but have suffered two big selloffs.

The stock dropped to $14 from $17.87 1/2 last week, apparently because of selling by one of the company's big investors. Singapore Telecommunications Ltd., which once owned 12 percent of the stock, has announced plans to sell all its shares.

At the current price, American Mobile has a modest market capitalization of $550 million. Buried in that number is the value of company's 17 million-share controlling stake in its spinoff, XM Satellite Radio Holdings Inc.

XM went public Oct. 5 at $12 a share and the stock has jumped to $26.68 3/4, making American Mobile's holdings worth $459 million.

XM is one of two competing firms that are building nationwide satellite networks to broadcast compact disc-quality radio signals on a pay-to-listen basis.

Aimed primarily at car radios, XM and its New York rival CD Radio will make it possible to drive cross-country and keep the same station. Each will have 100 different channels but will require special new digital radios, which the big car companies have agreed to make an option on their vehicles.

XM is a money-losing start-up, and accounting rules require American Mobile to include part of XM's losses in its quarterly financial reports. As a result, the spinoff will continue to undercut the parent's effort to become profitable.

XM's stock price is expected to rise as it prepares to go on the air in the first half of 2001. By then, Purnell projects, American Mobile's stake in XM could be worth well more than $1 billion.