Fiber Optic Gyroscopes (FOGs) for INS

Dec. 11, 2021 – By Hywel Rhys Curtis / Satsearch

Fiber Optic Gyroscopes (FOGs) are increasingly being viewed as highly suitable sub-systems for inertial guidance and navigation systems, for small satellite applications that require a medium to high range of performance.

What are satellite Fiber Optic Gyroscopes?

Fiber Optic Gyroscopes (FOGs) are widely used in car, ship, and aircraft navigation systems, and the technology has now matured for use in space-based applications.
Due to their low mass and power consumption, and the limited use of mechanically movable parts, they can have a significant performance advantage over traditional gyroscopes.
They measure the change in the angular rotation of any single axis using the ‘Sagnac effect’. An FOG includes closely-wound optic fibers through which two different beams of light in opposite directions are transmitted. The amount of time taken by each beam
to return to the starting point is measured in order to quantify the rate of change of rotation.
When there is no rotation of the coil, the two beams reach the starting point at the same time, hence there is no delay or phase shift.
When there is rotation of the coil, there will be a phase shift measured between the light beams. This phase shift is measured in terms of the rotational change of the vehicle.

The Inertial Measurement Unit (IMU)
Fiber Optic Gyroscopes are used in the Inertial Measuring Unit (IMU) of many civilian and military aerospace applications.
In order to perform the measurement of rotation rates and accelerations without external reference, the Inertial Measurement Unit (IMU) uses an Inertial Sensor
Assembly (ISA), which is a combination of accelerometers and gyroscopes in a defined axis.
Accelerometers are used for measuring linear acceleration while the gyroscopes are used for measuring rotational changes along an axis.
For example, GranStal’s GS-IMU3000TA Fiber Optic Gyroscopes Inertial Measurement Unit uses a three-axis, precision closed-loop FOG and three precision Quartz pendulous accelerometers.

When to use a Fiber Optic Gyroscope
When GPS is unavailable and the flight stability and orientation are mission-critical, a FOG is capable of providing hundreds of measurements every second as output, with a high level of precision.
It is a proven technology used in applications such as naval defence, maritime navigation, land defence and air navigation – areas where high levels of accuracy, responsiveness, and shock resistance are required.
Fiber Optic Gyroscopes have been found to be highly reliable and require little to no operational maintenance, as they do not have any physically moving parts. In addition, as the measurement does not rely on inertial resistance to movement, as is the case with alternative systems, FOGs can remain robust and stable for long timescales.
The systems have also been designed for ease of installation and integration. FOGs have a compact design and plug-and-play interface so they can be added to a satellite with relatively little additional testing and qualification.
FOGs are also highly resistant to vibrations, shock, thermal transients, and changes in operational temperature.

The FOG alignment time is relatively short, for responsive control. For example, GranStal Solutions’ Fiber Optic Gyroscope alignment time is typically only 6 minutes.
GranStal’s GS-FOG70A is an example of a closed loop Fiber Optic Gyroscope, designed to be small and lightweight, that is used in systems requiring high-performance motion sensing and control.
The GS-FOG70A is suitable for stabilization and positioning systems for platforms, such as high-speed gimbals, antennas, laser pointing, high-definition cameras, and other optical and sensor systems.
GranStal adopts an innovative sensor fusion algorithm to integrate different sensors and provide continuous precision positioning and accurate attitude data in all conditions.
In addition, GranStal’s FOG also undergoes a series of robustness and long-term stability tests to meet the stringent demands for Guidance and Stabilization Operations. This helps to ensure that the system is capable of accurately measuring rotation rates and accelerations, even in GNSS-denied or deficient environments.

Use of FOG technology in space
Adapting FOGs for space applications requires engineering sub-systems with high accuracy and high bandwidth, as well as line-of-sight stabilization pointing, and Radiation Hardness Assurance (RHA).
For the past decade GranStal has designed navigation systems for harsh and demanding environments, on Earth and in space.
The systems are able to deliver fully-compensated angular rates for a variety of satellite applications. Ultra-high-performance fiber optic systems with higher levels of accuracy are also available.
Multiple performance and size options are available to satisfy unique satellite packing, bias instability, noise performance, and dynamic range requirements.

About GranStal solutions

GranStal Solutions is a China-based manufacturer of guidance components and systems for both space-based and terrestrial applications.
Established in 1997, GranStal Solutions was founded with a vision to develop cutting-edge Global Navigation Satellite System (GNSS), and inertial navigation and stabilization systems.
Today the company has two modernized manufacturing facilities, one in Changsha, Hunan province, and one in Baoding, Hebei province. The headquarters span 70,000 square feet and the firm has a headcount of 165 employees, including 79 members in the research and development (R&D) section.
GranStal’s facilities include temperature controlled precision and velocity/position rotary tables, testing systems, sensor assembly, and Class-10000 cleanrooms, a CNC machining workshop, and an electrical assembly facility.
The company testing facilities include single-axis rotary tables control systems, precision centrifugal test systems, multiplex data collect systems, and automatic accelerometer test systems.
The company’s portfolio consists of multi-frequency GNSS receivers, accelerometers, FOGs, and navigation software systems for civilian and military purposes.
GranStal is a certified supplier to prime satellite manufacturers with various ongoing production deliveries and is ITAR-Free – all products are entirely designed and developed in-house.

Sagnac effect-based gyroscopes are gaining traction to be used as part of the IMU, as they exhibit intrinsic reliability, require low maintenance and have longer lifetimes.
Fiber Optic Gyroscopes (FOG) have demonstrated navigation-grade performance and are suitable for wide adoption in the industry.
The performance scaling of the FOG can be achieved by varying the fiber coil length and the FOG also offers significant resistance against vibration, shock, and acoustic noise due to the lack of moving parts in the system.

In addition, another of the advantages of FOGs is the higher bandwidth that is achievable, however, the interface sampling frequency can pose some limitations on performance, depending on the application.
In use cases with fast control loops, such as military applications, FOG is an ideal choice to be part of an IMU system.