Strapdown INS Errors Most of the error sources that corrupt the navigation solution are sensor errors or random disturbances (Stovall, 2000). Most common errors in Strapdown INS are:
 Fig 1 Flow chart of Strapdown INS Types of Inertial Sensors Sensors are often compared on the basis of certain performance factors, such as bias and scale-factor stability and repeatability or noise (random walk). Sensor selection is made difficult by the fact that many different sensor technologies offer a range of advantages and disadvantages while offering similar performance. Nearly all new applications are strapdown (rather than gimbaled) and this places significant performance demands upon the gyroscope (specifically: gyro scale-factor stability, maximum angular rate capability, minimum g-sensitivity, high BW). For many applications, improved accuracy/performance is not necessarily the driving issue, but meeting performance at reduced cost and size is. (Barbour, 2003) Ring Laser Gyros (RLG) The RLG has excellent scale-factor stability and linearity, negligible sensitivity to acceleration, digital output, fast turn-on, excellent stability and repeatability across dormancy, and no moving parts. The RLG's performance is very repeatable under temperature variations so that a temperature compensation algorithm effectively eliminates temperature sensitivity errors. It is superior to spinning mass gyros in strapdown applications, and is an exceptional device for high-dynamic environments. (Barbour, 2003) Fiber Optic Gyros (FOG) Fiber Optic Gyros (FOG) were developed primarily as a lower-cost alternative to RLGs, with expectations of leveraging technology advances from the telecommunications industry. FOGs are now beginning to match and even beat RLGs in performance and cost, and are very competitive in many military and commercial applications. (Barbour, 2003) Inertial MEMS Sensors MEMS inertial sensors are expected to enable so many emerging military and commercial applications that are becoming too numerous to list. Apart from size reduction, MEMS technology offers many benefits such as batch production and cost reduction, power (voltage) reduction, ruggedization, and design flexibility, within limits. However, the reduction in size of the sensing elements creates challenges for attaining good performance. In general, as size decreases, then sensitivity (scale factor) decreases, noise increases, and driving force decreases. The evolution of the most important gyro technologies is shown with the plot of years against accuracies in Figure 2. The high accuracy end is still dedicated to RLG. The moderate accuracy range from 0.01°/h to 30°/h is mainly covered by FOG and MEMS gyros are coming up from the low-end accuracy. (Handrich, 2003)  Fig 2 Evolution of Gyro Technologies (Source: Handrich, 2003) |