PTSS Kill Leaves Hole In Missile Defense Sensor Plan

By Amy Butler
Source: Aviation Week & Space Technology

This is key for an overarching missile defense, which by nature requires global engagements that far exceed the abilities of any one set of sensors or interceptors.

STSS demonstrated the “art of the possible” of tracking ballistic missile targets from space, Air Force Space Command chief Gen. William Shelton told Aviation Week this month. “If you have got the capability to provide [infrared] tracking from space, you can do really cold-body tracking [and] you can rely less on ground-based radars” for midcourse discrimination.

STSS was designed with both acquisition and tracking sensors—separate systems optimized to acquire the target as it is boosting and track it against the cold backdrop of space in its midcourse of flight. In an effort to reduce the price of the program, officials at the APL suggested a design that allows for Space-Based Infrared System (Sbirs) High satellites to manage the target acquisition role, leaving only the infrared bands on a PTSS sensor required for cold-body tracking in the midcourse of flight. And, PTSS was to prioritize a larger telescope than STSS and rely on subtle movements in space for tracking, not a sophisticated, gimbaled design.

MDA had planned to launch two PTSS demonstration satellites in 2017, which is now off the table. They were to be a follow-on demonstration to Northrop Grumman's STSS spacecraft that placed two satellites in orbit in 2009.

The PTSS termination raises a question of whether the Pentagon is risking a midcourse tracking gap. The STSS satellites were designed for a two-year in-orbit life, which has already been exceeded. And the spacecraft, which carry gimbaled infrared sensors, still have redundant systems. “If you don't get a failure early on in [a satellite's] life, it could go on for a long time,” says one industry official. Northrop is working to extend their service lives. “Current trends suggest that these satellites should be available for several more years,” says Bob Bishop, a company spokesman. However, program officials have not said just how long the satellites could last in orbit. Furthermore, STSS was intended to be a demonstration and, in its inclined position in low Earth orbit, provide only limited coverage with two satellites operating in tandem.

The ultimate goal from more than a decade ago was near-persistent coverage, a concept unlikely to be palatable in today's budget environment, and one that is being reevaluated. Officials are said to be once-again reviewing architecture concepts. Still on the table are hosted payloads, though operators are loath to consign missile-tracking sensors to a backseat when it comes to a primary spacecraft's mission, which is typically the case for a hosted payload. Radar advocates are also pushing for the use of such capabilities as the AN/TPY-2, multiples of which can be “stacked” for improved range. Radars, however, require a protected area from which to operate, which can be tricky when dealing with some host nations.

At issue in these studies is striking a balance between the infrared and radar phenomonologies. “Radar does its job by reflecting off of objects and IR does its job by detecting differences in temperature” between a missile and the cold backdrop of space, the industry source says. “Having two phenomonologies look at something makes it easier to do . . . discrimination” of a warhead and countermeasures.

The STSS satellites were crafted from parts fabricated during the defunct Air Force Sbirs-Low program and later put in storage once that project was terminated. Sbirs Low was intended to be a low-Earth-orbiting complement to the service's Sbirs-High infrared missile-detection satellites. Once resurrected as STSS, the program was transferred to MDA for management, and the Air Force focused on Sbirs High, but dropped the “high” portion of the moniker.

Sbirs has billions of dollars worth of overruns relating to technical problems and years of delay in service. But, after all of this scar tissue, commanders crow about its performance.

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