The U.S. ballistic missile defense system relies on a global suite of sensors to detect and track a hostile, incoming missile. Improving those assets, which today are mostly land- and sea-based radar, is a priority for military leaders, who want to take advantage of new technology and stay ahead of evolving threats.

The U.S. Missile Defense Agency (MDA) hoped that the Precision Tracking Space System (PTSS), a proposed missile-tracking satellite system, would provide the next big leap in sensor capabilities. But earlier this year, MDA announced it was canceling the program, saying “concurrency in the development schedule and uncertainty in the cost estimates put in doubt long-term fiscal sustainability.”

MDA has now gone back to the drawing board to determine its future sensor needs.

“A study has been initiated to determine how best to support future sensor requirements and we are exploring technologies to improve the capabilities of ground, air and space sensors,” said Navy Vice. Adm. James Syring, MDA’s director, who testified July 17 before the Senate Appropriations Committee’s defense panel. “We believe we need to be in space for infrared (IR) discrimination capability, but for now we can address the threat with other land-based sensors in key locations, which will allow us to provide support to the warfighter in the near term and assume less acquisition risk.”

According to a July 25 letter to Congress from the Government Accountability Office, the Department of Defense actually has several reviews of missile defense sensors underway, and “DOD expects to complete some of these efforts later this year in order to inform internal DOD fiscal year 2015 budgetary decisions.”

In 2009, MDA launched two Northrop Grumman-built Space Tracking and Surveillance System Demonstrators (STSS-D) to demonstrate the viability of space-based missile-tracking sensors. Until PTSS was targeted for termination, it was supposed to provide an STSS-like operational capability. The STSS-D satellites remain in use and have participated in a variety of missile tests, including a February test of the ship-based Aegis Ballistic Missile Defense system in which STSS-D tracking data helped guide an interceptor to its target.

After the February test, Doug Young, vice president of missile defense and warning at Northrop Grumman Aerospace Systems, said, “The mature technologies onboard both STSS-D satellites are demonstrating capabilities like continuous missile tracking that are possible only from the high ground of space.”

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Marc Selinger is a journalist based in the Washington, D.C., area. He can be reached at Follow him on Twitter at @marcselinger.