Missile Defense 101: Sensors (Pt I)

Sensors – Intro

A layered system such as the BMDS requires a multitude of sensors with differeing characteristics.  Why you ask?  For several reasons.  One is the nature of the threat – be it short, medium or intercontinental in range, there will be one or more differing sets of sensors that will come into play.  Another lies in the reason for a multi-layered system – redundancy.  But the primary reason is tied to the characterisitcs of the ballistic profile.  recall that here we broke it into three phases – boost, mid-course and terminal.  Each phase has certain characterisitcs that lend themselves to one form of surveillance and tracking over another.  For example, the boost phase is charcterized by a (relatively) short, intense discharge of energy that yields a large thermal signature which is ideal for an IR seeker.  Conversely, during the midcourse phase, there is relatively little energy expended by the RV bus and thus a cold body against a cold background is a less than ideal target for an IR seeker, but a better target for radar. 

Because the BMDS is also a global system, its sensors are found on a variety of fixed- and mobile platforms, both land- and sea-based as well as on orbit.  There are also airborne sensors, but they are mostly used in the course of the testing program and as such, will be passed on for now.  In discussing the BMDS sensors our taxonomy will be to first categorize by use in one of the three flight phases, and then further categorize by sensor type – e.g., IR or radar.  For this post, we begin with the boost phase.

A final comment before proceeding to the more granular discussions.  A number of the sensors integrated into the BMDS are legacy systems and as such, have a dual, or in some cases, multiple missions.    Most of these will be self evident as the discussion proceeds, but also understand that there is a great deal that perforce will not be able to be covered in this forum.  What can and is covered is derived strictly from open sources and available to one and all.

 

 
  DSP

 Sensors – Boost Phase

 As mentioned, the boost phase is characterized by the appearance of a strong IR signature against a relatively cooler background. This, coupled with the fact that most threat launches will be from well within a threat country’s territory, advocates for an on-orbit, IR sensor that is geo-stationary and providing constant surveillance of a potential threat sector.  In this mode, the sensor provides initial launch detection, type of vehicle, location of launch and other data that can be used to "cue" other sensors.  The primary platform used in this mission is the Defense Support Program (DSP) constellation of satellites.

IR:

Defense Support Program (DSP): The US was investigating the us of IR sensors to detect missiles and rockets in flight as early as 1948.  In a study conducted by the Naval Research Lab, it was determied feasible and worth further investigation that the flame from rockets burning a compound of nitric acid and aniline (used in the WAC Corporal)  and missiles burning a compound of alcohol and oxygen (used in the V-2 and Bell X-1).   By 1955, a study by a pair of RAND staff members generated sufficient interest that investigative work began on a satelite that could detect missile launches from orbit.  The subsequent launch of Sputnik the following year accelerated efforts, beginning with selection by the Air Force of  Lockheed Corporation to build a photographic reconnaissance satellite.   In concert with thtat selection, Lockheed proposed a number of additional systems, including a satellite equipped with an infrared radiometer and telescope to detect both the hot exhaust gases emitted by long-range jet bombers and large rockets as they climbed through the atmosphere. As a result, before the end of 1957, Lockheed’s proposal became Subsystem G of Weapons System 117 (WS-117L), the overall Defense Department space-based reconnaissance and surveillance program.  By early November 1958, Subsystem G had become MIDAS – the Missile Defense Alarm System (MIDAS). Throughout 1959 and for a number of years afterwards plans for a future MIDAS constellation were drawn up and revised-with the number of satellites and their orbital characteristics changing.  A January 59 plan recommended an operational constellation of twenty spacecraft operating at 1,000 miles and was follwed by a revision later that year for a constellation of twelve spacecraft at 2,000-mile altitudes.

     Improved MIDAS – ca. 1966

After several failed attempts (booster failures, communication failures, etc.) two MIDAS satellites evetually were placed on orbit and as early as 1963, were demonstrating the ability to detect launches of Polaris and Minuteman launches. By 1966, after demonstrating the ability to detect not only ICBM, but SLBM and MRBM launches (to include the detection of the SS-N-6) a decision was made to go ahead with the construction and deployment of an operational constellation of early warning satellites.

     Early DSP Satellite – ca. 1973

The program to produce an operational constellation eventually came to be first designated the Defense Support Program (DSP). In contrast to the satellites associated with the MIDAS and 461 programs, which orbited about 2,000 miles above the earth, the DSP satellites would be launched into a geostationary orbit allowing them to maintain a constant view of a third of the earth that their sensor could monitor. While the first of those satellites, launched on November 5, 1970, would fail to attain the proper orbit, the second did, placing it a position over the equator that allowed it to monitor Soviet and Chinese missile launches.

  Projected coverage of DSP

After two more successful launches, the U.S. established a three-satellite DSP constellation-with satellite stations over the Atlantic, Pacific, and Eurasia. With the launch of DSP-14 in June 1989, a four-satellite operational constellation was established, with the creation of a European station. That constellation has been maintained since that time. Over the life of the program, the satellites have detected thousands of strategic and tactical missile launches, as well as French and Chinese atmospheric nuclear detonations-the later via its infrared sensor and the nuclear detonation detection sensors also carried on the spacecraft.

  Launch if DSP from Shuttle

Today, the existing Defense Support Program (DSP) satellites, provide global coverage for early warning, tracking and identification. Besides warning of a ballistic missile launch, satellite sensors can develop an early estimate of where the hostile missile is headed.  DSP alerts are also used to cue otehr elements of the BMDS as to where to begin their searches.  As good as DSP has been for early warning of ballistic missile launches, it suffers in other areas and it is in those areas that STSS is looking to address.
 

  STSS

STSS: The Missile Defense Agency is pursuing the Space Tracking and Surveillance System program as a space-based sensor component of the Ballistic Missile Defense System. The program uses sensors capable of detecting visible and infrared light. STSS will consist of  two research and development satellites to be launched into low earth orbit in 2008, a ground segment to operate the satellites and with subsequent research and development satellites.  It’s primary mission will be to demonstrate the key functions of a space based sensor, passing missile tracking data to missile defense interceptors with the accuracy and timeliness necessary to enable them to successfully intercept missile targets. 

Radar

Even in the boost phase, as a missile beins to go exoatmospheric it will start to loose a significant portion of its thermal image due to lessening of airframe heating and the like.  Here is where radar systems can take over, cued by DSP and continue to track the missile, feeding the information into the BMDS fire control architecture (which we will duiscuss later).   There are three principle systems of note – Aegis, Forward-based X-band and COBRA DANE which are the "first responders" if you will where radar is concerned.  All three will be covered in the next installment.