Recent conflicts have demonstrated the decisive role of precision guided weapons in modern warfare. With their improved accuracy over conventional weapon systems, PGMs increase the percentage of enemy targets being destroyed, while reducing collateral damage. Therefore, guided munitions are currently the weapon of choice, and ongoing DOD projects are aimed at developing a new generation of smaller weapons having improved accuracy.

To home in on the target, guided munitions contain a number of advanced electronics modules such as a global positioning system (GPS) and an inertial measurement unit (IMU). Based on the processed flight information, these guidance units can adjust the aerodynamic fins of the munition to change its flight path. As the electronics modules become smaller and lower in cost, their application to precision guided munitions (PGMs) will become more widespread. The EMPF, in conjunction with BAE Systems, is executing a Navy ManTech program to produce low cost IMUs for PGMs, such as the Extended Range Guided Munition (ERGM) shown in Figure 1-1, which are to be fired from Navy ships.

IMUs complement the munitions guidance that is provided by the GPS unit. The IMU is initially used after launch, prior to activation of the GPS. However, it can also be valuable during the final stages of a mission. For instance, when final approach to the target is through an area of high electromagnetic interference, or the GPS signal is purposely jammed by the enemy, the IMU can guide the PGM to its target. An IMU contains three gyroscopes and three accelerometers. Traditional IMUs are often too large for most munitions, and often cannot survive the high-g environment of a gun launch. The advent of micro electro-mechanical systems (MEMS) has significantly aided the miniaturization of the sensors required by an IMU. Due to their extremely low mass, properly designed MEMS devices can readily withstand the high-g environment associated with a gun launch (<20,000 g). BAE Systems, among other manufacturers, have already demonstrated IMUs that meet the specifications for PGMs. The goal of this ManTech Program is to reduce the cost of this IMU to below $2500 in high volume production.

Low cost manufacturing is enabled by high volume production demand. One way to increase demand is to have the same IMU design for multiple PGM programs. SiIMU01 has been shown to meet current form, fit, and function requirements for Army and Navy guided Munitions. BAE Systems is a candidate supplier for the US Army XM-982 Excalibur program. Excalibur is the highest volume, near term munitions program.

One of the barriers to using a single IMU across multiple PGM applications arises from fact that different munitions typically have custom mounting flange designs for the IMU, as well as their own individual I/O cable connector locations. To overcome this barrier, BAE Systems has designed the SiIMU02 IMU shown in Figure 2 to have a central pod that contains all of the electronics, which is then mounted in a custom shell. The shell includes all of the customized features required for each PGM. However, the central pod provides the electronics unit, and it has been demonstrated to be gun hard. This design partition minimizes the level of effort for design and demonstration of a gun hard IMU for each PGM. The central electronics pod will provide the demand for high volume manufacturing, in order to minimize the cost of an IMU.

The SiIMU02 also seeks to cost reduce the SiIMU01 design by incorporating COTS MEMS-based accelerometers into the IMU. The COTS accelerometer is based on a capacitive MEMS sensor that meets the ERGM performance specifications, and at the same time is less expensive than the more complex vibrating quartz sensor based accelerometers. Furthermore, this low mass capacitive MEMS sensor easily meets the high shock requirements (20,000 g) of a device used in munitions. However, an accelerometer is more complex than a simple sensing element as it also contains a low-power ASIC for signal conditioning, a micro-controller for storage of compensation values, as well as a temperature sensor. For munitions use, all of these components must be assembled into a gun hardened ceramic package. Commercial MEMS-based accelerometers are utilized in a wide variety of civilian measurement and control markets such as oil and gas exploration/drilling, civilian aeronautical guidance, and industrial tilt/control systems. Low cost, bulk machined capacitive sensing elements meet the performance requirements of these markets as well as the DOD market, providing a high volume demand for these MEMS devices. The PGM application then employs gun hard ceramic packaging and interconnections to manufacture the complete accelerometer in a very cost-effective fashion.

In the present ManTech program the SiIMU02 design modifications have been completed for incorporation of the COTS accelerometer, and the circuit boards are currently being fabricated. Then the SiIMU02 modules will be assembled and undergo the following test program:

• Calibration and Performance
• Gun Hard Validation
• Environmental Testing
• High-g survivability after accelerated aging

This Navy ManTech Program will assist in the cost reduction of the SiIMU02 module, and evaluate its insertion readiness in accordance with the ERGM program requirements. The EMPF has developed an accelerated aging plan that will simulate the effects of 20 years of storage on munitions. After aging, the IMUs will subjected to realistic high-g shock conditions that simulate the firing of munitions, and then the cost reduced SiIMU02 will be tested to current Navy guided munitions specifications.

A successful test will demonstrate that a cost reduced, high reliability IMU design containing COTS accelerometers has been developed. In addition, the SiIMU02 can be used in other DOD Programs such as the Army's XM-982 Excalibur and Advanced Precision Kill Weapons System (APKWS). The level of investment from multiple DOD branches demonstrates the high level of interest in precision guided munitions. Through the efforts of the Navy ManTech program, manufacturing technology is being developed to advance the level of electronic packaging of high-g electronics, and is expected to lead to increased availability and affordability of precision guided munitions.