WASHINGTON — Northrop Grumman and Boeing say they have responded to a U.S. Air Force call for contractors interested in building the service’s next batch of GPS position, navigation and timing satellites.

Lockheed Martin holds a multibillion-dollar contract to build eight next-generation GPS 3 satellites, the first of which is slated to launch in 2016. With plans to keep flying the GPS 3 series of spacecraft well beyond the next decade, the U.S. Air Force’s space acquisition arm is looking for possible challengers to Lockheed Martin to build the next batch of satellites.

“Boeing continues to believe there are affordable low-risk alternate GPS solutions, and looks forward to supporting the Air Force in the Sources Sought process to best meet the future warfighter needs,” said Paula Shawa, a Boeing spokeswoman, in a July 1 email.

Lon Rains, a Northrop Grumman spokesman, also confirmed in a July 1 email that the company responded to the Air Force notice.

An industry day event is planned for July 9, according to the Federal Business Opportunities website.

Some senior industry officials have privately questioned whether the Air Force will seriously consider an alternate bid on the GPS satellites or if the request is strictly a motivational tool to force Lockheed Martin to meet deadlines.

In the request, challengers must offer an alternative to the Exelis-provided navigation payload that will fly on the eight GPS 3 satellites Lockheed Martin Space Systems is building in Sunnyvale, California.

Exelis Geospatial Systems’ struggles developing that payload prompted Lockheed Martin to consider switching suppliers beginning with the ninth satellite in the GPS 3 series. Lockheed Martin has said five companies responded to its solicitation last year seeking information on alternate suppliers.

In its June 4 “sources sought” notice, the Air Force Space and Missile Systems Center said it plans to award up to two Production Readiness Firm Fixed Price contracts in 2015. The selected contractors would be given a couple of years and around $100 million to $200 million to come up with production-ready spacecraft and navigation payload designs to bid against Lockheed Martin’s design for a contract to build as many as 22 GPS 3 follow-on satellites, the first of which would need to be ready for launch by 2023. The Air Force said a formal competition for the fixed-price production contract could happen in late 2017 or early 2018.

Boeing built the platform and major payload components for the GPS 2F satellites and has been steadily reinvesting in its GPS technology in recent years.

“We’re going to continue to make the appropriate investments because we believe our technology and the expertise of our people in our electronic product is something ultimately that the government is going to want,” Craig Cooning,  president of Boeing’s Network and Space Systems, told reporters in August.

Boeing also is one of three companies that received contracts in January 2013 to study how to improve the accuracy, coverage and efficiency of GPS using smaller satellites.

Northrop Grumman Aerospace of Redondo Beach, California, has already delivered deployable antenna sets for the first six satellites in the GPS 3 constellation. The antennas are being manufactured by Northrop Grumman’s Astro Aerospace division, which specializes in unique deployable antennas.

The division has delivered more than 1,000 antennas for previous generations of GPS spacecraft, Northrop Grumman said.

Northrop Grumman also builds inertial navigation systems for various aircraft and navigational gyroscopes.

Chuck Cynamon, vice president of business development at SSL Federal, the government-focused division of commercial satellite builder Space Systems/Loral, said the company did not respond to the Air Force request, but SSL would be interested in providing “a low cost commercial bus solution.

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.