Scalable Space Inertial Reference Unit (SSIRU™) Family of Products

SSIRU™ Family of Products

The SSIRU™ family of products combines innovation and high reliability—with 100 percent mission success in more than 70 million on-orbit hours—for precision performance on space missions.

The Hemispherical Resonator Gyro (HRG) sensor on the SSIRU™ is composed of three simple machined quartz parts—an outer shell, a high-Q vibrating hemispherical resonator and an inner shell. The quartz construction of the HRG is inherently stable, impervious to aging effects, and naturally radiation hardened. Operating in a completely evacuated hermetically sealed case, the HRG sensor is the ideal gyro for space applications.

The SSIRU™ is the spacecraft manufacturer’s choice for sensor pointing/stabilization and spacecraft attitude control on demanding long-term space missions, including:

Earth orbits at all altitudes/angles
Interplanetary missions
Deep space missions

hemispherical resonator gyro (HRG) sensor

SSIRU™ Advantages

The HRG-based SSIRU™ incorporates unmatched features:

Proven high reliability and long life, due to the simple HRG sensor design
Ultra-low angle random walk and inherent freedom from dead band
Fault-tolerant, redundant system architecture with cross-strapped HRGs and electronics, helping to ensure the success of missions lasting 15 or more years
Dual bus architecture (RS-422, 1553), including RS-422 rates up to 1000Hz with the potential to feed both the satellite bus and payload, resulting in significant weight and power savings compared with a two-inertial measurement unit (IMU) design
Demonstrated gyro capability, providing an inertially stable, low noise, wide bandwidth reference
Radiation hardened sensors and electronics that operate reliably when subjected to long-term radiation exposure in space

SSIRU™ Performance

The SSIRU™ features precision inertial rate measurements, characterized by extremely stable gyro bias, low noise and precise scale factor. Additionally, the efficient and redundant architecture of the SIRU™ helps ensure high performance and minimizes subassembly count. It contains:

Four HRGs and four (optional) accelerometers, with independent associated loop control/readout/ thermal control electronics, and sensing along the octahedral-tetrad axes
Highly reliable n+1 design that allows the IMU to be reconfigured to achieve probability of mission success of 0.997 for a 15-year mission
In a compact 15.7 lb. standard package, the internally redundant SSIRU™ provides significant size, weight and power savings over competitive systems

Radiation Hardness

Radiation hardness can be scaled by selecting specific EEE parts and variable thickness bolt-on shielding for low total dose, high total dose, high dose rate [2E8 rad (Si/s)] and SEU/SEL tolerance.

SSIRU™ Heritage

The space-qualified SSIRU™ evolved from the heritage Core SSIRU™. Together, these systems have operated in space since 1996 for over 50 million hours with 100 percent mission success. Our heritage systems have been launched more than 225 times for NASA, Department of Defense, Civil, Commercial and International space applications.

The SSIRU™ includes low latency software, which was first introduced in 2012 and is now a standard implementation. This variant has the same overall hardware configuration as the standard SSIRU™ but includes a specific RS422 message software implementation, which is driven by the user’s timing reference. Sample rate and anti-aliasing filter is available in 100Hz and 200Hz options, but can also be tailored to user specs up to 1000Hz.