Accelerometer input axis angular acceleration sensitivity

The Lockheed Missiles and Space Division contracted Honeywell in 1991 to develop an instrumentation package for the US Navy Trident system reentry bodies. The reentry inertial measurement unit (RIMU) is the result of this work. RIMU is a strapdown system consisting of three Honeywell GC1320 Ring Laser Gyros (RLG) and three Bell XI accelerometers as the inertial sensors. The Honeywell RIMU has performed reliably and accurately on several Trident flights. The Trident missile employs a pyrotechnic deployment mechanism to impart an attitude stabilizing angular velocity to the reentry body (RB) to assure a proper atmosphere reentry angle. A linear velocity increment is also imparted to push the RB away from the bus. Since it cannot be measured by the bus mounted booster IMU, this incremental deployment velocity is modeled in the Trident guidance computation. Hence, a key requirement on RIMU is the accurate measurement of the deployment velocity increment. The measurement is to be accomplished with the accelerometer exposed to the full angular dynamics environment because of RIMU´s strapdown system mechanization. The sensitivity of this accelerometer is discussed, and confirmed by spin tests in the laboratory. A theory of the error mechanism was hypothesized. While there was no effort to physically prove the hypothesis, this sensitivity was calibrated by post flight reprocessing of spin test data archived for the RIMU units already flown. Flight data were reprocessed with compensation for this sensitivity. Effective improvement in the down range impact miss was accomplished in all RIMU units flown. This paper presents: (1) discussion of the process of RIMU flight data analysis, (2) anomalous deployment velocity measurements observed in flight tests, (3) investigative study leading to the discovery of the new sensitivity, (4) a hypothesis of the error mechanism, and (5) post flight calibration and flight data reprocessing