SpaceX is set to launch two prototype probes on Sunday to test the company’s proposed network of internet satellites.

The probe satellites, named Microsat-2a and Microsoft-2b, will hitch a ride on a Falcon 9 rocket sending up a Spanish Earth-observation satellite called Paz.

It is the first major step in SpaceX’s plan to create the Starlink network, comprising nearly 12,000 satellites in Earth’s orbit, which will provide coverage to almost any spot on Earth at all times. It aims to achieve this by beaming internet connectivity to antenna receivers on the planet’s surface.

Launch is scheduled from Vandenberg Air Force Base at 9:17am ET on Sunday, after a day’s delay to carry out extra checks on the rocket’s systems.

According to financial projections obtained by the Wall Street Journal, SpaceX expects to have over 40 million subscribers to its satellite internet service by 2025, providing a revenue of $30 billion that same year.

The company hopes to launch its first operational satellites in 2019, but there are many hurdles to overcome before then, such as how to coordinate so many satellites in a non-geostationary orbit at all times.

Recently, SpaceX received the endorsement of the Federal Communications Commission (FCC), which regulates how satellites use the radio spectrum, after filing numerous applications.

“Following careful review of this application by our International Bureau’s excellent satellite engineering experts, I have asked my colleagues to join me in supporting this application and moving to unleash the power of satellite constellations to provide high-speed internet to rural Americans,” said FCC chairman Ajut Pai in a statement.

“If adopted, it would be the first approval given to an American-based company to provide broadband services using a new generation of low-Earth orbit satellite technologies.”

Currently, there are around 14 million rural Americans and 1.2 million Americans on tribal lands who lack mobile broadband even at relatively slow speeds.

SpaceX is not the only competitor in the space communications race, with OneWeb, backed by SoftBank and Richard Branson, hoping to start providing global internet broadband service in 2019.

Sunday’s planned take-off follows SpaceX’s much-publicised launch of Falcon Heavy, which saw the world’s most powerful rocket send CEO Elon Musk’s Tesla roadster towards Mars.