Longevity Of Missile Warning Satellites Unclear

It is now nearly a year since the 23rd and last Defense Support Program (DSP) satellite failed in orbit.  DSP-23 was the most advanced in a series of satellites designed to detect ballistic missile launches anywhere in the world using infrared sensors.  A more capable replacement constellation called the Space Based Infrared System (SBIRS) is running years late, which raises the question of just how robust the legacy DSP birds are.  The government isn’t talking, but military space insiders say that some functions are now being performed by “single-string” systems, meaning there is no redundancy to fall back on if current components fail.  With the newest DSP spacecraft apparently a total loss and some of the legacy satellites operating beyond their design lives, it’s really important to get the geosynchronous SBIRS satellites into orbit soon.  One bright spot: initial SBIRS sensors piggybacked on intelligence satellites flying over polar regions are working very well.