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Session B11: Inertial Navigation Technologies

Advanced Inertial Measurement Unit (IMU) Hypersonic Flight Test Results
Simon Berman, Michael Chavez, Youngmin Choi, Phil Clark, Farzin Dinyarian, Jorge Gutierrez, Aaron Hofman, Lawrence Linick, Daniel Rampacek, Cole Umemura, Alexander Trusov, Northrop Grumman; Brian Irelan, Stephen Pethel, Scott David, NTA; Patrick Renfroe, Army AvMC
Location: Ballroom E
Date/Time: Wednesday, Jun. 4, 4:45 p.m.

In late 2024, we successfully conducted the first ever hypersonic flight testing of the Advanced Inertial Measurement Unit (IMU). This technology is in development by Northrop Grumman (NGC), in partnership with the U.S. Army Combat Capabilities Development Command (DEVCOM) Aviation & Missile Center (AvMC) and NTA Inc. for demanding GPS denied applications. The next generation IMU is based on in-house milli-Hemispherical Resonator Gyroscope (mHRG) and Silicon Accelerometer (SiAc) sensors. The sensor technologies are integrated with modern electronics and firmware in a self-contained and rugged unit with cutting edge Size, Weight, and Power (SWAP). The main purpose of the completed round of flight testing was to objectively validate a technology maturity milestone for the new IMU in representatively extreme flight regimes and environments using a reusable hypersonic vehicle.
Key attributes of the successfully completed flight test campaign include:
• Dynamics from zero velocity on the ground through take off and subsonic captive carry flight to hypersonic velocity and ultimately deceleration and landing.
• Hypersonic boost glide trajectory and associated mechanical and power separation, ignition, and deceleration events.
• Rapid altitude changes from ground to high altitude and back to ground without any sealing around the IMU installation within the vehicle.
• Additionally, tens of hours of captive carry flight tests have been conducted.
Inertial and internal diagnostics data was collected throughout and analyzed to enable the next round of continued improvement. Many challenges inherent to integration of a new technology onto a third-party test vehicle were encountered and resolved with lessons learned and are likely of value to the community at large. To further diversify test cases and performance data points, flight testing using a different hypersonic platform is planned later this year.
This presentation will discuss, for the first time, our results during the three phases of third-party flight test integration, including post-delivery checkout, benchtop testing, vehicle integration, flight event, and recovery. Background on sensor hardware technology choices and their enablement of advanced software capabilities will also be reviewed.
The rapid maturation results during the current Phase 3 of the effort are a culmination of the previous Phase 2, which focused on compact form factor integration of the Phase 1 developed solid state inertial sensor technologies developed with all support electronics and mechanical hardware in a self-contained, internally isolated, hypersonic mission robust, all altitude hermetically sealed package. The Flight Test IMU (FT-IMU) operates on a single selectable voltage supply, at low power, and provides a serial bus and high bandwidth Ethernet I/O through a single connector. The FT-IMU has been designed for and successfully tested at high levels of random vibration, shock, and operational temperature ranges. The FT-IMU covers a wide range of platform dynamics with the full-scale range of +/-1,000 deg/s and +/-60 g for gyros and accelerometers respectively.
Flight regimes of emerging hypersonic platforms, long range weapons, and unmanned vehicles (UxV) represent novel challenges for navigation and guidance in GPS/GNSS challenged and denied missions. For this domain, ever increasing flight maneuvering dynamics are coupled with the need for beyond navigation-grade performance in a reduced Size, Weight, Power and Cost (SWaP-C) strapdown IMU package based on inherently reliable sensing technologies with proven manufacturing readiness and hardening against relevant environments. To address this need, NGC has partnered with DEVCOM AvMC and NTA Inc. to develop an Advanced IMU based on mHRG and SiAc technologies. The end goal of the mHRG/SiAc Advanced IMU development is strapdown near-strategic grade performance in a sub-navigation grade system SWaP-C. The development was approached in a systematically phased structure to (1) mature sensor technologies, (2) validate a compact and rugged IMU in hypersonic flight environments, (3) finalize inertial performance optimization within the IMU by leveraging a unique dynamic self-calibration capability of the mHRG, and (4) leverage an Application Specific Integrated Circuit (ASIC) developed for the SiAc in parallel on NGC internal funding.
Phase 1 of the effort, focused on the development and realization of an extended dynamic range mHRG as a missionized derivative of the NGC spacecraft pointing product HRG. Engineering gyros have been built and prototype mHRG systems were delivered to and tested by DEVCOM AvMC. The gyro demonstrated a rate range that was significantly increased from that typical of the space HRG, while maintaining a beyond navigation grade Angle Random Walk (ARW) in a small sensor package just a few cu-inch in volume comprising a small number of simple parts (representing a significant reduction from the space product HRG). The demonstrated dynamic range, defined as the measured ratio between the full-scale rate range and the ARW of the gyro, is a factor of 3.5 higher than that of a commonly used navigation grade gyro using legacy laser technology. At the same time the sensor package is smaller in volume. In this configuration, rate measurement in a complete IMU prototype achieves Allan deviation bias stability which is attractive for high accuracy alignment and pointing. The rate range and ARW can be scaled up and down through real time software control maintaining the same dynamic range window for optimizing noise performance to mission specifics. The HRG and mHRG do not suffer from wear out mechanisms like lasers, plasma, mirrors, bearings, etc. of legacy gyros as demonstrated by the 70 million operational hours to date without a single mission compromise for the HRG space product.
Phase 2 of the development started with updating the gyro design and designing a new high-performance MEMS based accelerometer module. The accelerometer module builds on the heritage of the NGC SiAc which entered production in the early 2000s. Through continued improvements to design and manufacturing the SiAc remains, to this day, the highest performing silicon MEMS based accelerometer field-proven across many DoD and space domains. In the context of the LN-200 IMU production, SiAc electronics have been implemented as standalone modules based on hybrid or discrete electronics components integrated on a relatively small printed circuit card.
During recent years NGC has applied resources to pursue and realize the vision of miniaturizing the SiAc module by developing an ASIC while preserving, and potentially further improving, performance characteristics. Several iterations of the ASIC have been designed, built, characterized, and improved upon leading to a final version SiAc module much smaller in area than the production configuration that uses the same MEMS chip and signal processing and control scheme. The latest configuration’s measured performance, not previously reported, shows an outstanding dynamic range. Acceleration measurement Allan deviation achieves 8-9 micro-g precision at just 1 s and 1 micro-g instability at 1 min without any sacrifice in dynamic range or bandwidth. The new implementation maintains high scale factor linearity and low vibration rectification.
In the recent year of the effort the team has focused on performing and incorporating the results of several rounds of environmental (temperature, vibe, shock, vacuum) and performance testing of fully integrated IMUs to rapidly mature the capability. In addition to the accelerometer performance indicated above, the IMUs feature the Gen 3 mHRG with measured at system level ARW of 0.001 deg/rt-hr in a +/-1,000 deg/s rate range configuration.
With the FT-IMUs delivered and independently tested by the Army partners, the team has worked closely with the rapidly evolving hypersonic flight test community starting in early 2024. Many valuable lessons were learned and applied to the final preparation and integration of the units that highlighted some common themes as well as key differences across the diversity of hypersonic flight platforms. The presentation will include flight test results previously completed as of this writing as well as a second test flight campaign if available in time. With equal emphasis, the last technology performance improvement results as relates to SiAc, mHRG, compensation, and self-calibration will also be presented.

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