The largest risk to the Air Force Mk21A program is the acquisition of a qualified inertial guidance system, known as the Path Length Module (PLM). This linear accelerometer measures the acceleration of the RV and this data is used to accurately predict the distance that the RV has traveled. This system is required to maintain a high degree of accuracy while being subjected to a combination of high acceleration, shocks, vibrations, and electromagnetic radiation due to nuclear detonations. Finally, the package is expected to have a 20-to-30-year long maintenance free lifespan. The accelerometer design currently used for strategic missiles has not been significantly redesigned for several decades. Over this time, technology advancements in MEMS and piezoelectric accelerometers have resulted in significant improvements in performance while minimizing size and cost. MEMS accelerometer designs based on Silicone Resonant concepts show great promise for the PLM application. One new MEMS sensor that has recently gone into production from MEI is a 6-degree-of-freedome (6-DOF) inertial measurement unit sensor. The currently available package comes in +/-40g sensitivity, responds from DC to 3 kHz, is rated for mg resolution, and can survive 10,000g shocks. The MEI package, which comes on a single 7-mm chip, is a combination of a 3-axis accelerometer and 3-axis gyro. The gyro is simply a system of four 3-axis accelerometers with a smaller test mass which can be repurposed as additional accelerometers. The gyro accelerometers in the commercial package operate at +/-500g and have a 10 kHz bandwidth, thus extending the dynamic range of the base device. Furthermore, MEI can manufacture custom packages based off of the current design. For example, each of the four gyro accelerometers could be modified to operate over a specific range of acceleration or frequencies. This will allow specific operating parameters to be addressed with a custom chip design in the Phase II program. The custom MEI accelerometer could be combined with strategic radiation hardened electronics and packaged in a radiation hardened shell to form the basis of a new PLM. The design solution must be able to survive mechanical shock loads, system generated electromagnetic pulse (SGEMP) and radiation environments while delivering reliable and accurate measurements of exceedingly small accelerations. The availability of a PLM with sufficient accuracy built from off-the-shelf components would represent a significant risk mitigation and cost savings to MK21A and future ICBM programs.
