The first test satellites for SpaceX’s global internet constellation are being prepped for launch — three years after SpaceX CEO Elon Musk unveiled the project in Seattle.

The prototype spacecraft, known as Microsat 2a and 2b, are reportedly to be included as secondary payloads on a Falcon 9 rocket from Vandenberg Air Force Base in California, due for launch on Feb. 21. The primary payload is a 3,000-pound Spanish radar observation satellite called Paz.

SpaceX conducted a static-fire test of the Falcon 9, which makes use of a previously flown first-stage booster, at Vandenberg today. The test involved briefly firing up the booster’s rocket engines as a rehearsal for liftoff.

Paz is due to be launched into a 319-mile, nearly pole-to-pole orbit — but SpaceX’s satellites could eventually go higher to test a Ku-band radio communication system in concert with an array of ground stations.

SpaceX has given the lead role for development of the satellite network to a team headquartered in Redmond, Wash.

Documents filed with the Federal Communications Commission indicate that ground stations will be housed at SpaceX facilities in Redmond and Brewster, Wash., as well as at the company’s headquarters in Hawthorne, Calif., and facilities in McGregor and Brownsville in Texas. Another ground station is to be placed at Tesla’s headquarters in Fremont, Calif., which Musk heads as CEO.

SpaceX says it will also be testing satellite communications with receiving terminals built into mobile vans.

The company’s business plan calls for putting thousands of communication satellites in orbit, with limited service starting by 2020. The satellite constellation, informally known as Starlink, eventually would provide low-cost internet access on a global scale.

SpaceX has kept mum about many of the details relating to Starlink. Outside observers have had to piece together most of the information from required government filings and from insider forums such as NASASpaceflight.com and Reddit r/SpaceX.

It’s no secret why: In a 2016 FCC filing, Patricia Cooper, SpaceX’s vice president for satellite government affairs, explained that the process of building a satellite system is “highly proprietary and may take several years to finalize, during which time the operators hold details as highly confidential for obvious competitive reasons.”

SpaceX’s main competitor is OneWeb, which has built up alliances with Airbus and other heavy-hitters to put its own satellite internet constellation in orbit. There are still more rivals on the horizon, ranging from Boeing and SES O3b to ViaSat, Telesat and LeoSat.

OneWeb expects to launch its first test satellites later this year, and go into operation on a limited basis next year to satisfy the terms of a license it has with the International Telecommunications Union. Its launch partners include Amazon billionaire Jeff Bezos’ Blue Origin, British billionaire Richard Branson’s Virgin Orbit, and Europe’s Arianespace.

Having an ITU license serves as an advantage for OneWeb, but SpaceX is catching up quickly, on the development front as well as the regulatory front. And Musk has said that a profitable satellite operation is crucial to SpaceX’s Mars settlement vision.

Here’s what Musk said in Seattle, back in 2015:

“Over time, to build a full version of the system, we’re talking about something that would be $10 or $15 billion to create, maybe more, and then the user terminals, would be at least $100 to $300 depending on which type of terminal. This is intended to generate a significant amount of revenue, and help fund a city on Mars. So in looking in the long term, and saying what’s needed to create a city on Mars, well, one thing’s for sure: a lot of money. So we need things that will generate a lot of money.”

Update for 10:43 a.m. PT Feb. 17: This report has been updated to reflect SpaceX’s decision to reschedule launch from Feb. 17 to Feb. 21. An initial postponement to Feb. 18 was made “to allow for additional time for pre-launch systems checks,” SpaceX said in a tweet. An additional postponement now calls for launch to take place no earlier than 6:17 a.m. PT Feb. 21: