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Fiber-Optic Gyros: The Vision Realized

Dr. George A. Pavlath

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Dr. George A. Pavlath Fiber-Optic Gyro Director Northrop Grumman, Electronic Systems, Navigation Systems Division

Dr. Pavlath is a world–recognized expert with 20 years of experience in fiber optics gyros (FOGs) and their components. He has led Northrop Grumman’s development effort in fiber-optic gyros from research and development, through product development, production transfer, and production support. His directorate has successfully completed numerous contracts with the United States Army, United States Navy, and the United States Air Force.

Dr. Pavlath has worked on fiber-optic gyros and their components since 1975. At Northrop Grumman, he is responsible for all fiber-optic gyro pro-grams including research and development, product development, factory process and test equipment development, production transfer, and support of the ongoing production activity.

He has led his directorate in the development of low accuracy FOGs for can-non–launched applications, medium accuracy FOGs for attitude and heading reference systems (AHRS) and tactical missile applications, and the LN–200 family – which is in production. His directorate led in the development of high accuracy FOGs for inertial navigation of aircraft and missiles and also for shipboard gyrocompass and fire control applications. He has also led in the development of fiber optic components, integrated optic components, light sources, photodetectors, assembly techniques, and automatic manufac-turing technology for component fabrication and fiber-optic gyro assembly.

Dr. Pavlath received his Doctorate degree in Applied Physics from Stanford University and his Bachelor of Science degree in Physics from the California Institute of Technology.

Dr. Pavlath has 21 issued patents on fiber-optic gyros and their components and assembly processes. Ten additional patents are pending. He has also published 15 papers on fiber-optic gyros.

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ABSTRACT

Over thirty five years have elapsed since the fiber-optic gyro was proposed by Vali and Shorthill. In those decades, fiber-optic gyros have matured. They are competing head to head in tactical, navigation and strategic applications with existing technologies such as mechanical gyros and ring laser gyros and they are winning. Northrop Grumman has produced the majority of fiber-optic gyros and fiber-optic-gyro-based inertial products in the world. This paper will cover the various Northrop Grumman fiber-optic gyro products, the platforms they are used on, and it will provide production and top level system data.

INTRODUCTION

It has been nearly 100 years since the discovery of the Sagnac effect1, which enabled optical rotation sensors. The Sagnac effect lay fallow for six decades awaiting the development of technology to permit the design and manufacture of products that could use it. The first application of the Sagnac effect was the ring laser gyro that was first demonstrated in the 1960s by Macek and Davis2 and entered production in the late 1970s. The enabling technology for this was the laser.

The development of the fiber-optic gyro required its own enabling technology, namely low loss, single mode, optical fibers that became available in the mid 1970s. Vali and Shorthill3 first proposed the fiber-optic gyro in 1975. It took researchers around the world about a dozen years to resolve the many technical issues with the fiber-optic gyro. The reader is referred to the following paper4 by this author for an overview of these issues and their resolutions.

Over thirty years have now elapsed since Vali and Shorthill proposed the fiber-optic gyro. Fiber-optic gyros and inertial products utilizing them have been in production for over a decade. Inertial products utilizing fiber-optic gyros are in high volume production and are used everywhere from under the sea to outer space and in most places in between. The vision of Vali and Shorthill has been realized.

Northrop Grumman Navigation Systems Division has manufactured well in excess of 70,000 fiber-optic gyros used in a wide variety of inertial products. These products include rate gyros, inertial measurement units (IMU), and inertial navigation systems (INS).

NAVIGATION SYSTEMS DIVISION HERITAGE

Northrop Grumman’s Navigation System Division was formed from several predecessor entities that have a rich and varied heritage in inertial products and in fiber-optic gyros. Figure 1 shows the time lines of the various predecessor entities beginning in 1980.

Figure 1. Northrop Grumman Navigation

Systems Division’s Heritage

A portion of Navigation Systems Division comes from the purchase of Litton Industries by Northrop Grumman in 2001. These legacy Litton entities were Guidance and Control Systems (USA), Aero Products (USA), Litef (Germany), and Lital (Italy). Another portion of the division started as the Northrop Grumman Precision Products Division which became the Fibersense Technology Corporation in 1994 and was subsequently purchased by Northrop Grumman.

FIBER-OPTIC GYRO PRODUCT DEVELOPMENT AT NAVIGATION SYSTEMS DIVISION

Guidance and Control Systems, Litef, Lital, and Precision Products/Fibersense have been in the inertial product business since the 1960s. The legacy Litton divisions manufactured INS and IMUs. The Northrop Grumman Precision Products divisions manufactured inertial instruments and IMUs.

The development of the fiber-optic gyro at Navigation Systems Division begins with the acquisition of the Stanford fiber optics program from Atlantic Richfield Corporation by Guidance and Control Systems in 1982. Figure 2 depicts the timelines for the various fiber-optic-gyro-based inertial products.

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1980 1990 2000 2010

IMU

ACINS

Com

mer

cial

LandNav

ShipNav

GyrosMOG

uFORS Dev

Dev

Dev

FOG 200Atlas V

uFORS

LFK-95

LCR-92/93Development

LN-25xDevelopment

LTN-101eDevelopment

LISA 200

LN-270

LTR-97

LLN-G1/GX/GY

LN-260

LN-25x

EKV

ATFLIR

LTN-101e

LCR-92/93

Dev

LN-200Development LN-200

FOG R&DG&CS

Dev

FOG R&DLitef

Figure 2. Fiber-Optic-Gyro-Based Inertial Product Timelines Grouped by Product Category

In 1985, development of resonant fiber-optic gyros began at Litef in Germany and was followed by the development of interferometric fiber-optic gyros in 1987.

Development of fiber optic IMUs began simultaneously at Guidance and Control Systems and at Litef in 1988. Guidance and Control developed the LN-200 and Litef developed the LCR-92/93 family of IMUs. Both were in production by 1992. In 1995, Lital began using the LN-200 in its LISA 200 family of Attitude and Heading Reference Systems (AHRS).

In 1998, Fibersense started the development of the IMU 600 for precision pointing applications and the IMU 200 for missile defense applications.

Development of fiber optic inertial navigation systems started at Guidance and Control Systems in

1995 on the LN-251 INS product. The LN-251 entered production in 2001 and is now in high rate production.

Navigation Systems Division began development of the LN-260 product in 2005. This fiber-optic gyro inertial navigation system was specifically designed for the F-16 fighter aircraft and it was recently selected for this aircraft by the U.S. Air Force.

In 2003, Navigation Systems Division began development of the LTN-101E INS product for the commercial airline market using fiber-optic gyros. The LTN-101E is scheduled to be certified on Airbus aircraft in 2008.

Litef started the development of the LTR-97 in 1997. This unit is a fiber-optic gyro vertical gyro/directional gyro replacement for the commercial airline market, which began production in 1999.

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A land navigation version of the LN-251, designated the LN-270, began development in 2003. The LN-270 provides position and pointing information to land based artillery and to forward observation vehicles.

Litef began the development of its fiber optic land navigation systems in 1994 is producing three versions, the LLN-G1, the LLN-GX, and the LLN-GY. Production of these units began in 1995.

Litef started development of the LFK-95 in 1995. It is a commercial ship’s navigator product that was first produced in 1997.

Fibersense developed a single axis FOG 200 product and also single axis gyros for space programs starting in 1998.

Litef started development of its uFORS line of single axis, fiber-optic gyro rate sensors in 1990. Four major subfamilies exist with performance ranging from 1 to 36° per hour.

FIBER GYRO ARCHITECTURE

The fiber-optic gyros used in Northrop Grumman fiber optic inertial products share many design characteristics. Figure 3 is a generic schematic of the optical circuit of a typical Northrop fiber-optic gyro. The gyro optical circuit is reciprocally configured5 and utilizes a multifunction integrated optic chip (MIOC) manufactured in LiNbO3 using proton exchange technology6 to create the polarizing waveguides. The MIOC contains a polarizer, a Y junction coupler, and an electro-optic phase modulator.

The light source is a broadband light source7,8,9,10 the wavelength of which varies from product to product. Typical wavelengths are in the 800 nm and the 1500 nm bands where optical fibers and optical components are readily available. Some products use a single light source per gyro axis, while other products share a single light source between two or three axes. A semiconductor photodetector is used to convert the light exiting the gyro into an electrical signal so it can be processed to measure the rotation rate.

All of Northrop’s fiber-optic gyros are operated in a closed loop mode using variants of the dual loop, digital serrodyne technique pioneered by Arditty and Lefevre11.

Figure 3. Optical Schematic of a Typical Northrop Fiber-Optic Gyro

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IMU PRODUCTS

Figure 4 shows the LN-200 IMU which consists of three fiber-optic gyroscopes, three micromachined silicon accelerometers and a microprocessor. It senses acceleration and rotation about three orthogonal axes and outputs temperature compensated incremental angles and incremental velocities.

The IMU is 3.5 inches in diameter and is 3.35 inches high. It weighs approximately 750 grams. It utilizes an RS-485 digital bus to interface with the using platform. This product family has been in production for over 14 years and over 13,000 IMUs have been sold.

Figure 5 shows typical fiber-optic gyro bias and scale factor yields from the production line.

Figure 4. LN-200 IMU

80 70 60

50

40 30

20

10 0

Freq

uenc

y

Gyro SF Residuals (ppm)- 90 - 60 - 30 0 30 60

80 70 60

50

40 30

20

10 0

Freq

uenc

y

Figure 5. Typical LN-200 Fiber-Optic Gyro Bias and Scale Factor Yields

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The LN-200 is used in a wide variety of applications including torpedoes, missiles, fixed and rotary wing aircraft, radars, targeting pods, spacecraft, surveying systems, camera stabilization, and in many other applications.

The LN-200 is also used as a building block for other inertial systems. One such system is the LISA 200 AHRS manufactured in Europe. It is shown in Figure 6 below. The unit is approximately 7 × 4 × 4.5 inches in size and weighs 4.5 pounds. It contains both digital input/out (ARINC 429, Military Standard 1553, and RS-422/485) along with an analog synchro output (ARINC 407). Approximately 300 systems have been delivered to customers.

Figure 7 shows the LCR 92 and LCR 93 AHRS units also manufactured in Europe. The two systems are externally identical and they both utilize three fiber-optic gyros. The LCR 92 uses a bubble level for sensing local level while the LCR 93 uses three silicon MEMS accelerometers. Both digital and synchro outputs are provided. Over 5,500 LCR systems have been delivered to many customers and production is ongoing.

Figure 6. LISA 200 AHRS system

Figure 7. LCR 92/93 AHRS Products

The IMU 600 is a high performance unit which is used for precision pointing applications. It uses three fiber-optic gyros and three quartz accelerometers in a non-orthogonal configuration to reduce the volume occupied. The IMU is 5.4 × 7.5 × 2.7 inches and weighs approximately 3.25 pounds. It uses an RS-485 serial interface and outputs uncompensated incremental angles and velocities. Compensation for thermal sensitivity is performed in the computer of its host vehicle. Figure 8 shows the IMU 600. Over 200 IMU 600s have been delivered.

Figure 9 shows scale factor data from the calibration verification for approximately 140 IMU 600s. The scale factor is well within the specification.

The last IMU product that will be described is the IMU 200, which is shown in Figure 10. It utilizes three fiber-optic gyros and three quartz accelerometers. The IMU weighs approximately three pounds and utilizes an RS-485 serial interface. Approximately 30 have been delivered. It is primarily used in very high reliability missile applications.

Figure 8. The High Performance IMU 600

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Gyro 1Gyro 2Gyro 3

100 110 120 130 140 150 160 170 180 190 200 210

IMU SNE

0.10

0.08

0.06

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Bia

s Er

ror (

deg/

hr)

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Gyro 2

Gyro 3

40 60 80 100 120 140 160 180

IMU Serial Number

100

80

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40

20

0

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e Fa

ctor

Err

or (P

PM)

Figure 9. FOG-600 IMU Bias and Scale Factor Error at Calibration Verification

for Over 100 IMUs

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Figure 10. The IMU 200

INS PRODUCTS Inertial navigation systems use their gyros and

accelerometers to compute current position and velocity from a given starting position. Northrop Grumman’s Navigations Systems Division has built inertial navigation systems since the 1960s. Its early systems used mechanical platforms with floated and dry tuned mechanical gyros. Strapdown systems, utilizing ring laser gyros (RLGs) which required mechanical dithering, started production in the 1980s. Zero lock gyros required no mechanical dither and they replaced RLGs at Navigation Systems Division in the 1990s.

Development of inertial grade fiber-optic gyros started in the mid-1980s and production of fiber-optic gyro based inertial navigations systems started in 2001. The first fiber-optic-gyro-based INS product was the LN-251 system shown in Figure 11. Its officially designation is the AN/ZSN-1.

Figure 11. LN-251 Fiber Optic INS

The LN-251 contains a 12 channel Selective Availability Anti-Spoofing Module (SAASM) GPS receiver in a tightly coupled configuration. It propagates three position solutions: free inertial, GPS only, and blended INS/GPS. It utilizes digital interface: RS-422/485 and dual Military Standard 1553 digital buses.

Over 400 LN-251 and LN-270 systems have been produced and delivered. Figure 12 shows the distribution of free inertial performance at Acceptance Test Procedure in the factory. The mean of the distribution is 0.7 nautical miles per hour (nmph) with a standard deviation of 0.3 nmph. The systems are available in 0.8, one, two, and five nmph navigation performance ranges.

Figure 12. Free Inertial Navigation

Performance at ATP for the LN-251/LN-270 Systems

Figure 13 depicts the LN-260, which is a variant of the LN-251, developed for use on the F-l6. It implements the full interface requirements of SNU-84 and has the required analog and mechanical interfaces for the F-16.

The LN-260 system has been flight tested on a Northrop Grumman Sabreliner, where it was used to stabilize the Synthetic Aperture Radar (SAR) antenna. Figure 14 shows the SAR image of Fort McHenry taken during the flight test. The system satisfies all SAR requirements.

Figure 15 depicts the flight test data from a F-16 aircraft. The aircraft was pulling nine gravities in tight turns. The radial position error rate for this flight was 0.44 nmph, well under the 0.8 nmph specification.

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Figure 13. LN-260 INS/GPS

Figure 14. Synthetic Aperture Radar Image of Fort McHenry Obtained Using the LN-260 to

Stabilize and Navigate the SAR Antenna

Latit

ude

(deg

)

F16[1] C015.1539.00015.BNG016.bd01.mat33.05

33

32.95

32.9

32.85

32.8

32.75

32.7

32.65

32.6

32.55-99 -98.8 -98.6 -98.4 -98.2 -98 -97.8 -97.6 -97.4

Longitude (deg) Figure 15. LN-260 Flight Test Trajectory (left) on an F-16 and

Radial Position Error Rate Versus Time (right) COMMERCIAL PRODUCTS

The LTN-101E (Figure 16) uses fiber-optic gyros in a four MCU package for use on commercial transport aircraft. The LTN-101E also uses navigation grade silicon MEMS accelerometers. It performs both a navigation function and an air data function and integrates with an external GPS receiver.

The graph on the left of Figure 17 shows the free inertial position error during a 15 hour static laboratory navigation test. The green line indicates the two nautical mile per hour (nmph) performance requirement and the red line is the radial position error rate (1.69 nmph) for the first hour of testing. The graph on the right in Figure 17 shows the free inertial

position error for a flight test that lasts over 10 hours. The LTN-101E readily meets its free inertial performance requirements.

The LTN-101E is in development and qualification and will start production in 2007. It will be DO-178 certified.

Another fiber-optic gyro product for commercial transport aircraft is the LTR-97. This a replacement for an old mechanical direction gyro/vertical gyro (DG/VG) system used on older transport aircraft. It provides synchro outputs for pitch, roll, and heading, analog outputs for pitch and roll, and discrete outputs. It is DO-178A certified. Over 300 systems have been delivered to date and production is ongoing.

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Figure 16. LTN-101E Navigation and Air Data

System

LAND NAVIGATION PRODUCTS Figure 19 shows the land navigation version of the

LN-251 designated the LN-270. The US Army designation for it is the AN/VSN-12. It is designed for use in wheeled or tracked vehicles and adds odometer aiding, the ability to use a Precision Lightweight GPS Receiver (PLGR) external GPS receiver instead of the embedded SAASM receiver and software unique to land navigation. It meets all requirements of Military Performance Specification 71185 including specifications for gunfire shock. These systems are typically mounted on the trunnion of the cannon and see the full recoil of the gun firing.

Figure 17. LTN-101E Free Inertial Position Error Versus Time.

The graph on the left is a static lab test. The graph on the right is a flight test on a commercial aircraft.

Figure 18. LTR-97 DG/VG Replacement

Figure 19. LN-270 Land Navigation INS

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Figure 20 shows a van test of an LN-270 system in the San Fernando Valley area of Los Angeles. The top graph shows the path and elevation, while the bottom graph shows the position and altitude errors versus distance traveled. The error limits on the graphs are from Military Performance Specification 71185.

Figure 21 shows the LLN-GX, LLN-G1, and the LLN-GY land navigation systems manufactured by Northrop Grumman in Europe. The GX and the G1 are

similar and they both have three fiber-optic gyros. The GX uses two level sensors for reduced cost while the G1 uses three accelerometers for higher accuracy. The LLN-GY is the lowest cost of the three. It uses a single fiber-optic gyro sensing rotation about the vertical axis and two accelerometers. Over 1,700 of these systems have been manufactured and delivered. Production is ongoing.

Figure 20. LN-270 Van Test Position and Altitude (top) and North, East, and Horizontal Position

Errors (bottom). The lines on the top graph show the performance limits in Military Performance Specification 71185.

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Figure 21. From left to right, LLN-GX, LLN-G1, and LLN-GY Land Navigation Systems

SHIP’S NAVIGATOR PRODUCTS

Northrop Grumman manufactures the LFK-95 gyrocompass and reference system for commercial shipping. This was the world’s first marine fiber-optic gyro based gyrocompass and was designed specifically for high speed craft such as hydrofoils. It supports a variety of digital and synchro interfaces for compatibility with a broad range of watercraft. To date 650 systems have been sold to a wide range of customers and production is ongoing. Figure 22 shows the system. Vessels using the LFK-95 system include ships, hydrofoils, and unmanned underwater vehicles.

Figure 22. LFK-95 Marine Gyro Compass

FIBER OPTIC RATE SENSOR PRODUCTS

Northrop Grumman also sells many single axis fiber-optic gyro rate sensors. These gyros are modular and sold either standalone or combined into an IMU.

The uFORS line has been in production the longest of any fiber optic rate sensor. To date over 7500 rate sensors and IMUs have been delivered and the product line continues in production. Figure 23 shows the four major subfamilies in the uFORS product line.

Each uFORS is a fully contained, single axis, fiber-optic gyro rotation sensor. The uFORS-36 was the first product developed and it had 36° per hour bias accuracy. With the improvement of optical components, the bias accuracy was improved. The uFORS-6U was developed next with a six degrees per hour bias accuracy followed by the uFORS-1 with a one degree per hour bias accuracy. Scale factor repeatability is around 1000 ppm. The units draw two watts of power and have a digital output. The weight of the uFORS-36 and -uFORS-6U is 0.3 lb and their size is 0.8 × 2.5 × 4 inches. The uFORS-36m is miniaturized uFORS-36 with a 36° per hour bias accuracy for size critical applications.

Another single axis rate sensor product is the FOG-200. It is available as a single axis assembly that is fully self-contained as shown in the picture on the left side of Figure 24. The rate sensing element can be packaged remotely from the electronics for special applications as shown in the middle picture of Figure 24 for a two axis rate sensor. The right-hand picture shows a three axis, remote sensing, rate sensor package.

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Figure 23. uFORS Product Line (left to right): uFORS-1, uFORS-36m, uFORS-6U, and uFORS-36

Figure 24. FOG-200 Product Family

From left to right: FOG-200 single axis, self contained rate sensor; two axes, remote sensing rate package; three axes, remote sensing package.

Figure 25 shows a variant of the FOG-200 that was developed for a space launch application in which the fiber-optic gyro had to operate through a 200 g rms vibration environment.

Figure 25. High Vibration Environment

Version of the FOG-200 Single Axis Rate Sensor

SUMMARY

In the last fifteen years, Northrop Grumman Navigation Systems Division has delivered approximately 22,000 fiber-optic gyro based inertial system products and 7,500 individual fiber-optic gyro rate sensors. This represents nearly 75,000 axes of fiber-optic gyros. The vision that Vali and Shorthill gave the world is no longer just a vision. The vision has been achieved. It is a reality.

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REFERENCES

1G. Sagnac, “L’ether lumineux demontre par l’effet du vent relatif d’ether dans un interferometre en rotation uniforme” C.R. Acad. Sci., vol. 95, pp 708-710, 1913.

2W. M. Macek and D. T. M. Davis, “Rotation rate sensing with traveling wave ring lasers”, Appl. Phys. Lett., vol. 2, pp 67-68, 1963.

3V. Vali and R. W. Shorthill, “Fiber Ring Interferometer”, Appl. Opt., vol. 15(5), 1099, 1976.

4G. Pavlath, “Challenges in the development of the IFOG”, AIAA-2003-5763, AIAA Guidance, Navigation and Control Conference, Austin, Texas, August 11-14, 2003.

5K. Bohm, P. Russer, E. Weidel, and R. Ulrich, “Low-noise fiber-optic rotation sensing”, Opt. Lett., vol. 6(2), pp 64-66, 1981.

6M. Papuchon and C. Puech, “Integrated Optics: a possible solution for the fiber gyroscope”, Proc. SPIE vol. 157, pp 218-219, 1978.

7K. Bohm, P. Marten, K. Petermann, E. Weidel, “Low-drift Fibre Gyro Using a Superluminscent Diode”, Electron. Lett., Vol 17(10), pp 352-353, 1981.

8R. A. Bergh, B. Culshaw, C. C. Cutler, H. C. Lefevre, and H. J. Shaw, “Source statistics and the Kerr effect in fiber-optic gyroscopes”, Opt. Lett., vol. 7(11), pp 561-565, 1982.

9W. K. Burns and R. P. Moeller, “Polarizer Requirements for Fiber Gyroscopes with High-Birefringence Fiber and Broad-Band Sources” J. Lightwave Tech., vol. LT-2(4), pp 430-435, 1984.

10M. J. Digonnet, P. F. Wysocki, B. Kim, H. J. Shaw, “Broadband fiber sources for gyros”, Proc. SPIE, vol 1585, pp 371-382, 1991.

11H. C. Lefevre, J. P. Bettini, S. Vatoux, and M. Papuchon, “Progress in Optical Fiber Gyroscopes using Integrated Optics”, AGARD CPP-383, 9A/1-l3, 1985.

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Strategic Programs & Business Development (SP&BD), Navigation Systems Division One of the main functions of the SP&BD organization is to sustain NSD’s competitive advantage in all of its product lines by providing cutting edge technologies that supply entirely new capabilities to the marketplace and also enhance its current products. Ike J. Song, Director of SP&BD ike.song@ngc.com For more information, please contact: Northrop Grumman Corporation Navigations Systems 21240 Burbank Boulevard Woodland Hills, CA 91367 USA 1-866-NGNAVSYS (646-2879) www.nsd.es.northropgrumman.com