MAUI, Hawaii — The U.S. Air Force is planning a three-satellite constellation to replace an existing space surveillance satellite and hopes to launch sooner than the 2021 timeline previously expected.

Gen. John Hyten, the commander of Air Force Space Command, said in a speech here at the Advanced Maui Optical and Space Surveillance Technologies Conference that the Air Force spent the past year defining a follow-on program to the Space Based Space Surveillance (SBSS) Block 10 pathfinder satellite launched into a 630-kilometer, sun-synchronous low Earth orbit in 2010 aboard an Orbital Sciences Corp. Minotaur 4 rocket. The next iteration, he said, would likely use three smaller satellites in low Earth orbit to keep tabs on objects in the geosynchronous belt. Located 36,000 kilometers above the equator, geostationary orbit is home to critical U.S. communications and missile warning satellites.

“What do we need from SBSS that we really can’t get anywhere else? We need that real-time, all-weather access,” he said. “You can do that from an equatorial orbit looking up. You don’t need a big satellite to do that. You can do that with small satellites. You can do that fairly affordably.”

Air Force leaders have made space situational awareness a top priority in recent years, saying they need to ensure they can keep tabs on geostationary orbit. The service has asked Congress for $251 million for the SBSS follow-on program between 2016 and 2019.

Built by Boeing Space and Intelligence Systems of El Segundo, California, the SBSS Block 10 satellite is part of a space situational awareness architecture that also includes ground-based radar and optical sensors. Ball Aerospace & Technologies Corp. of Boulder, Colorado, was a major subcontractor on the SBSS satellite.

Hyten said the satellite is “doing great things.” But the Air Force may face a lengthy gap in capability if the Block 10 satellite, built to operate into 2017, does not last longer than that.

The Air Force has long cited the need for a follow-on program for broader coverage, but budget pressures have forced the service to defer the mission, which is now viewed as more of a replacement than a complement to the existing satellite.

In 2012, the Air Force said it planned to award a contract for a follow-on SBSS satellite during the 2015 fiscal year — which starts Oct. 1 — and launch the spacecraft in 2020. That launch timeline had since appeared to have slipped to about 2022, industry sources had said, but Hyten said here that he hopes to launch the first of the follow-on satellites before 2021, depending on the budget process.

As a temporary fix, Air Force officials have said the Operationally Responsive Space (ORS)-5 satellite would serve as a pathfinder for technologies to be used in a follow-on to the SBSS program.

“I don’t like to use the term gap-filler because it’s not going to provide the capability you really need,” Hyten said in an interview. “It’s not going to fill the gap. Nonetheless, it’s the pathfinder to the follow-on and it will provide us a little bit of insurance when SBSS may die.”

In addition, he said, the ORS-5 mission will prove “whether a small satellite can do what we think it will do.” Those satellites in the follow-on program “will look more like ORS-5 than it does SBSS.”

The Air Force said in late July that its Operationally Responsive Space Office at Kirtland Air Force Base in New Mexico would tap the Massachusetts Institute of Technology’s Lincoln Laboratory to build the ORS-5 satellite, also known as SensorSat. Development is expected to start in 2016; launch is planned for 2017.

The Air Force has said the ORS-5 satellite should be small enough to launch on several small U.S. rockets including Orbital Sciences’ Pegasus XL and Minotaur 1, and Lockheed Martin and ATK’s Athena 1C, a variant of a vehicle that last flew more than a decade ago.

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.