Elon Musk's SpaceX is poised to launch a new spy satellite for the US military, marking its first designated national security mission for the United States.

SpaceX's Falcon 9 rocket, carrying a roughly $US500 million GPS satellite built by Lockheed Martin, is scheduled for lift off from Florida's Cape Canaveral shortly after 9 am local time (Wednesday AEDT), the US Air Force said.

Tuesday's launch, if successful, would be a victory for Musk, a billionaire entrepreneur who has tried for years to break into the market for lucrative military space launches long-dominated by incumbents Lockheed and Boeing.

SpaceX sued the US Air Force in 2014 in protest over the military's award of a multi-billion-dollar, non-compete contract for 36 rocket launches to United Launch Alliance, a partnership between Boeing and Lockheed.

SpaceX dropped the suit in 2015 after the Air Force agreed to open up competition, according to SpaceX's website.

The next year, SpaceX won a $US83 million Air Force contract to launch the GPS satellite, which will have a lifespan of 15 years, Air Force spokesman William Russell said by phone.

Tuesday's launch is the first of 32 satellites in production by Lockheed under contracts worth a combined $US12.6 billion for the Air Force's GPS III program.

US Vice President Mike Pence tweeted that he would travel to Florida to attend the launch, which he called "an important step forward as we seek to secure American leadership in space."

The next GPS III satellite is scheduled for launch in mid-2019, while subsequent satellites undergo testing in the company's Colorado processing facility.

The launch marks SpaceX's first so-called National Security Space mission as defined by the US military, SpaceX said.

The Hawthorne, California-based company has previously launched payloads for the Department of Defence in 2017 that were not designated as a National Security Space missions.