1

Achieving High Navigation Accuracy Using Inertial Navigation Systems

in Autonomous Underwater Vehicles

Robert Panish and Mikell Taylor Bluefin Robotics Corporation

553 South Street Quincy, MA 02169 USA

Abstract-This paper presents a summary of the current state-of-the-art in INS-based navigation systems in AUVs manufactured by Bluefin Robotics Corporation. A detailed description of the successful integrations of the Kearfott T-24 Ring Laser Gyro and the IXSEA PHINS III Fiber Optic Gyro into recent Bluefin Robotics AUVs is presented. Both systems provide excellent navigation accuracy for high quality data acquisition. This paper provides a comprehensive assessment of the primary advantages and disadvantages of each INS, paying particular attention to navigation accuracy, power draw, physical size, and acoustic radiated noise. Additionally, a brief presentation of a recently integrated Synthetic Aperture Sonar system will be used to highlight how critical a high-performance INS is to hydrographic, mine countermeasures, and other SAS applications.

Keywords—inertial navigation system, autonomous underwater vehicles, AUVs, unmanned underwater vehicles, UUVs, Bluefin Robotics, Kearfott, IXSEA, ring laser gyro, RLG, fiber optic gyro, FOG

I. INTRODUCTION

The Bluefin Robotics Autonomous Underwater Vehicle (AUV) is an extremely versatile system that can be utilized for a wide range of missions. For shallow or deep survey applications the Bluefin AUV is a highly capable, extensively configurable tool that can be used to obtain high quality data from a wide array of oceanographic sensors. An AUV is able to maintain sensor positioning at an ideal height above the seafloor during surveys, is unaffected by surface sea states, and can follow a rough terrain to produce the best possible images.

Bluefin Robotics offers a wide range of products to meet the needs of a variety of customers, including propeller-driven torpedo form-factor AUVs, a hovering AUV, and a glider. Bluefin’s torpedo form-factor AUVs come in three different diameters – 9”, 12”, and 21”. A typical Bluefin 12” vehicle is shown in Fig. 1. The length of the 12” and 21” vehicles is

variable to meet the demands of the specific application – from one to eight meters. The vehicles make use of free-flooded modularity to create a robust system that is field maintainable, easily expandable and customizable, and some are equipped with field-swappable payload sections. Inside the free-flooding hull are a number of subsystems protected within their own pressure vessels or oil-filled, pressure-tolerant assemblies. The most critical of these subsystems is the Main Electronics Housing (MEH), which contains the main vehicle computer, power distribution network, Doppler Velocity Log (DVL), navigation system, and a variety of other subsystems.

A wide array of payloads has been successfully integrated into Bluefin AUVs, including side scan sonar (SSS), synthetic aperture sonar (SAS), multibeam echosounders, sub-bottom profilers, cameras, magnetometers, water sampling systems, fluorometers, conductivity and temperature sensors, and sound velocity sensors, to name a few. Collection of high quality data with these payloads requires a combination of high accuracy navigation data and stable vehicle dynamics.

Stable vehicle dynamics are achieved through closed loop control of vehicle motion in trackline or trackcircle modes, while operating at any depth or altitude above the seafloor. Vehicle propulsion, as well as horizontal and vertical control, is achieved by an articulated, ducted thruster known as the tailcone, shown in Fig. 2. The system is designed to be passively stable in roll, due to a proper separation of the center of buoyancy and the center of gravity. The tailcone is designed

Figure 1 - A 12" diameter Bluefin Robotics AUV.

Figure 2 - A 12" Bluefin Tailcone.

© 2011 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

2

to be torque-neutral over a wide range of speeds, and thus roll is minimally affected by propeller speed.

Navigation on a Bluefin AUV involves a fusion of data from several sensors. Selection of an appropriate navigation option depends on the vehicle configuration, operating depth, navigation accuracy specifications, and budget. Shallow water systems with moderate navigation accuracy requirements will often use a compass-based navigation solution. These systems achieve a quality navigation solution through the integration of a tactical-grade Inertial Measurement Unit (IMU), digital magnetic compass, DVL, GPS receiver, pressure/depth sensor, and a sound speed sensor.

Oftentimes a vehicle will require a higher degree of navigation accuracy to achieve its mission. In these cases, an Inertial Navigation System (INS) will be integrated with a DVL, GPS receiver, pressure/depth sensor, and a sound speed sensor. This type of navigation solution is most valuable in deep water applications, when surfacing for GPS is infeasible, or when the payload requires high navigational accuracy. SAS processing, in particular, requires extremely accurate navigation information. Another application that cannot be dependent on a compass-based solution is operation in the polar-regions, where compasses are not useful for navigation. While INSs also suffer degraded performance at high latitudes, an INS can still provide some level of navigation whereas a compass-based solution may be rendered useless.

This paper will present information on the integration and performance of two different exceptional INS units into Bluefin AUVs - the T-24 Ring Laser Gyro (RLG) manufactured by Kearfott and the PHINS III Fiber Optic Gyro (FOG) manufactured by IXSEA. Data from hundreds of kilometers of submerged survey will be presented to compare the navigation accuracy of the two units in comparable vehicle platforms. In addition to the navigation accuracy, a number of other factors may influence the selection of an INS. These factors include physical size, power draw, and acoustic noise.

This paper will also highlight how navigation accuracy is a key contributor to obtaining high quality sonar data with favorable contact localization accuracy. High quality sonar data will be presented that could only be obtained with a high accuracy INS. Overall, this paper provides a summary of the state-of-the-art inertial navigation technologies for AUVs and their impact on data quality. Readers will find important information on the performance of INS technology and will take away a better understanding of the intricacies of INS integration on AUVs.

II. INSS ON BLUEFIN AUVS

Bluefin has integrated a number of different INSs in AUVs over the years. The two systems most commonly used are the Kearfott T-24 RLG and the IXSEA PHINS III FOG. Each of these systems makes use of DVL, pressure/depth sensor, and sound speed sensor inputs while submerged to achieve the desired navigation accuracy.

The differences between the T-24 and the PHINS III are much more than simply the brand and implementation details. Both units measure rotations using laser-based interferometric techniques. However, the fundamental technology behind the gyroscopes is different between the two units. The T-24 uses RLGs, which means the beam path is created by a set of mirrors redirecting the laser into a loop. To prevent a phenomenon known as injection locking, where under slow rotations the gyroscopes will not accurately measure rotation rates, the mirrors in the RLG are mechanically dithered. This dithering in the T-24 produces an audible tone at the dithering frequency. In a FOG-based INS, such as the PHINS III, the laser beam travels through a long optical fiber to create the beam path. One advantage of using optical fibers over dithered mirrors is that there are no moving parts in the system, and it is therefore acoustically silent.

The standard measure for evaluating a system’s navigation accuracy is to state the position drift as a function of distance travelled since the last GPS fix. Both the T-24 [1] and the PHINS III [2] have a stated position drift of less than 0.1% of distance travelled, CEP. CEP refers to the Circular Error Probability, or a circle about a mean value which includes 50% of the population. For instance, if a system with a drift of 0.1% of distance travelled CEP travels five kilometers between GPS fixes, it is expected that 50% of the time the position drift will be less than five meters.

Partially due to the differences in the type of gyroscopes, the T-24 and the PHINS III have a number of differing properties that provide advantages for the use of each unit. When choosing an INS for an AUV there are a number of device specifications that a system integrator may consider. The required navigation accuracy is certainly one of the most important considerations. However, a proper design process should also pay attention to the physical size and weight of the units, the power draw, and acoustic noise. In this comparison, the T-24 is significantly smaller than the PHINS III, whereas the PHINS III draws less power and is acoustically silent.

A. DVL-INS Calibration

When an INS is integrated into an AUV it must be calibrated with the DVL. This calibration is necessary to account for mechanical misalignment in the installations of the INS and DVL, as well as for potential errors in the velocity estimates of the units. The calibration procedure need only be performed once, unless the INS or the DVL is removed from the vehicle. The calibration procedures are different for each INS, but the basic procedure involves driving the vehicle in straight lines to compare the INS and DVL estimated motion with GPS truth data. To ensure a consistent calibration it is critical that the INS and DVL are both mounted to the same rigid structure and that the lever arms and orientation between the devices remain constant.

Calibration of the IXSEA PHINS III INS involves the use of the built-in calibration tool to put the INS into calibration mode. In calibration mode the INS uses its inertial sensors and GPS to

3

determine its motion, but does not make use of the DVL velocities. It records the DVL velocities and automatically determines the calibration parameters based on the difference between the known motion (from the INS and GPS) and the measured DVL motion. To perform the calibration, the AUV is sent on a single five kilometer trackline while remaining on the surface to maintain continuous GPS contact. During this procedure the various calibration parameters can be monitored to watch for convergence. These parameters are the roll, pitch, and heading misalignment angles, as well as the scale factor for velocity magnitude adjustment.

In practice on a Bluefin vehicle, where the DVL and the INS are rigidly mounted in the MEH, it has been observed that the pitch and roll misalignment angles converge rapidly to a stable value. The heading misalignment angle takes a bit longer to converge, as this direction of misalignment is slightly more difficult to measure. This value usually converges to a steady value within the first kilometer and becomes refined over the duration of the calibration. The scale factor is refined throughout the calibration and, for well functioning PHINS III and Teledyne RDI DVL units, should converge to a value near zero. Once the calibration procedure for the PHINS III has been completed, the unit must be power cycled and realigned. It is then ready for normal operations.

The INS-DVL calibration procedure for the T-24 INS is markedly different from that of the PHINS III. To calibrate the T-24, the INS is put into calibration mode and the vehicle is sent on a submerged dive to determine the calibration parameters. The dive consists of a pair of boxes with GPS surfacings at each corner. Each side of the box must be about fifteen minutes in duration, and should be completed with the vehicle submerged. The vehicle should have DVL bottom lock for the entire duration of the calibration. During the calibration procedure the INS is using the GPS fixes at each corner to determine its internal biases, scale factors, and misalignment angles. Once the vehicle has completed this dive, the INS is taken out of calibration mode, power cycled, and is ready for a verification dive or normal operations.

Operationally speaking, the T-24 calibration procedure is simpler to perform than the PHINS III’s. Although it takes significantly longer to complete, the T-24 procedure involves the vehicle operating submerged. This is certainly preferable to operating the vehicle on the surface, where additional care must be taken to ensure there is no boat traffic in the area and that the sea state is low enough for stable surface operations. B. INS Alignment

Upon power-up, an INS must perform an alignment. The primary purpose of the alignment is to determine the initial attitude of the INS. Roll and pitch are determined from a simple measurement of the direction of the gravity vector from the accelerometers, which must first be filtered to remove accelerations due to motion. Heading is determined by tracking the time derivative of the gravity vector, which lies in the East direction. Once the gravity (down) vector and the East

vector are known, the North direction (and thus, heading) can be determined through vector algebra. [2] Alignment of the INS may be performed on land, on the deck of a ship, or on the surface of the water. The alignment procedures are different for each INS.

Alignment of the IXSEA PHINS III INS requires GPS to determine the initial position and altitude of the vehicle. It does not use any inputs from the DVL or other sensors. The alignment proceeds in three phases – coarse alignment, fine alignment, and aligned. The primary value that can be monitored to determine the status of the alignment is the standard deviation of heading. During the coarse alignment phase, the INS should be kept fairly stationary. Minor motion while on the surface of the water or on the deck of a ship is acceptable, but the ship should not be moving at more than about five knots. This phase lasts for about five minutes, at which point the PHINS III proceeds to the fine alignment phase.

The fine alignment phase is marked by a decreasing heading standard deviation as the INS refines its heading estimate. During this phase the INS must be rotated through a few heading changes of at least ninety degrees. IXSEA recommends either a stair-step pattern or a box pattern. There is no restriction on the velocity of the system during this phase, however, without heading changes the alignment will not complete.

Once the heading standard deviation has fallen to an acceptable level, the INS is considered aligned. At this point it is ready for operation. Further heading changes can refine the alignment further to achieve even greater sensor performance. While not particularly difficult, the PHINS III alignment procedure does require some operator attention to ensure that the heading changes are performed.

Alignment of the Kearfott T-24 INS also requires GPS to determine the initial position of the vehicle, and does not use any other sensors during the alignment. The alignment proceeds in three phases – coarse alignment, fine alignment, and aligned. The coarse alignment phase is very similar to that of the PHINS III. As with the PHINS III, the vehicle must be kept at a low velocity. Coarse alignment lasts for about five minutes, at which point the INS proceeds to the fine alignment phase.

During fine alignment, the INS further refines its heading estimate. Unlike with the PHINS III, no heading changes are required to complete the alignment. When the T-24 reaches the aligned state, it is ready for operation.

III. ASSESSING NAVIGATION ACCURACY

The navigation accuracy of an INS is an important measure of the quality of the system. The accuracy of a system is presented as the position drift as a function of distance travelled since the last GPS fix. When evaluating navigation position drift, it is important to consider over what submerged distances it can be discerned whether there is INS drift, relative

4

to GPS uncertainty. Standard, commercially-rated GPS has an uncertainty of five meters. Since the published drift of the T-24 and the PHINS III is 0.1% of distance travelled, a true test of navigation accuracy should involve dives with submerged distances of at least five kilometers. In practice, shorter submerged distances may be used, but with the caveat that GPS uncertainty may be corrupting the calculations.

The best type of mission to determine the navigation accuracy is a long, straight trackline. The longer the trackline, the more certainty there is that the measured position drift is due to INS drift and not GPS uncertainty. And since the performance specification is given as a statistical value (CEP), it is important to have a statistically significant number of data points for analysis. Since obtaining a statistically significant number of sufficiently long tracklines is not always practical within the concept of operations (CONOPS) of a vehicle, it is often necessary to include different types of dives in the analysis. This may include shorter tracklines or even circular tracks. While these types of dives are not ideal for testing the navigation accuracy, they are useful because they provide a measure of the navigation accuracy that a system will achieve during its normal operations.

A. Calculating Navigation Accuracy

Calculation of the navigation accuracy from a data set is more complicated than simply comparing the INS position to the first GPS position fix. Since GPS can often be erroneous during the first few position fixes after surfacing, it is important to extrapolate the INS position forward in time based on measured DVL velocities on the surface. Once the GPS has reached an acceptable number of satellites and quality, the reported positions are then compared.

The measure of distance travelled is not simply the distance between initial and final GPS positions. Since it is likely that the AUV did not travel a perfectly straight path between the two points, a simple GPS position difference would result in an underestimate of distance travelled. The AUV may have traversed various transits to and from the surface locations, which may not have been collinear with the trackline. Additionally, normal dynamic variations about the nominal trackline should be included in the calculation of distance travelled. The distance travelled is the length of the trajectory calculated by the INS when submerged plus the length of the trajectory extrapolated on the surface.

B. Types of Drift

An INS will experience position drift while submerged, and this drift may be decomposed into two types – along-track and cross-track error, as shown in Fig. 3. Along-track error is the component of the drift in the direction parallel to the nominal AUV motion. Cross-track error is the component of drift in the direction perpendicular to the nominal motion. In general, along-track error is the result of a poorly calibrated scale factor or errors in sound speed, and cross-track error is the result of a poorly calibrated heading misalignment angle or residual errors

in heading after alignment. Depending on the type of payload, different types of drift are often more acceptable than others. If the concern is with maintaining a constant range to a target, for instance, then a low cross-track error is of greater importance.

IV. COMPARISON OF T-24 AND PHINS III NAVIGATION ACCURACIES

To present a comparison between the T-24 and the PHINS III, data taken throughout 2010 on a pair of Bluefin vehicles has been analyzed. Each of these systems was delivered in 2010 and completed an extensive set of missions during Engineering Sea Trials in Boston and after delivery to the customers. Both vehicles utilize a recent build of the Bluefin Huxley operating system, a Teledyne RDI DVL, and a Valeport sound velocity sensor for navigation aiding. Although they have different payloads, both systems require high performance navigation to aid their SAS payloads.

The two vehicles have different CONOPS, so the library of dives available is not identical. The vehicle with a Kearfott T-24 is most often used to conduct long, linear surveys. Thus, most of the data available is from dives where the vehicle conducted a ‘lawnmower’ pattern of a series of back and forth lines, acquiring GPS position information either after every leg or after every pair of legs. The vehicle with an IXSEA PHINS III is used for a combination of tracklines and circles. The standard missions for this vehicle could include one and a half orbit circles with GPS at the start and end, single straight lines with GPS at each end, or a large box with GPS at convenient locations. Although the two systems operate with different types of dives, it is important to realize that the navigation accuracies presented represent the achieved accuracy of each AUV system in its normal CONOPS.

Fig. 4 shows the cumulative probability curves of the achieved navigation accuracies for the two systems, for submerged distances greater than five kilometers. The plot shows the position drift (percent of distance travelled) as the

Figure 3 – Position drift can occur in the along-track or cross-track direction.

5

independent variable on the horizontal axis, and the percent of the time the system achieves that level of drift or better (percent occurrence) on the vertical axis.

Recall that the specification is for each system to achieve a position drift of 0.1% of distance travelled (CEP), which means that 50% of the runs should have a drift of less than 0.1% of distance travelled. In analyzing the results for the two navigation systems, it is acceptable to present either the achieved position drift CEP or to state the percentage of dives that achieved the specification of 0.1% of distance travelled. The data presented in Fig. 4 is summarized in each of these ways in Table I. It is clear from this table that both the Kearfott T-24 and the IXSEA PHINS III are superb navigation systems that can be successfully integrated into a Bluefin vehicle. Both systems exceed the published drift specifications, achieving high accuracy navigation in an AUV.

Another way of looking at the data, which shows the importance of analyzing tracks longer than five kilometers, is to examine a scatter plot of all data points as a function of distance travelled. This type of plot, shown in Fig. 5, highlights the greater distribution of results for shorter submerged distances due to the effects of GPS uncertainty that dominate the error budget.

TABLE I

POSITION DRIFT

Specification Kearfott T-24

IXSEA PHINS III

CEP Drift

0.1% dt 0.05% dt 0.07% dt

Percentage of Dives with Drift < 0.1% dt

50% 81% 83%

V. CONSIDERATIONS FOR SELECTION OF AN INS

Section IV has demonstrated that the navigation accuracies of both the Kearfott T-24 and the IXSEA PHINS III are superb and both can be successfully integrated into a Bluefin AUV. Selection of an INS between these two units should not be based on navigation accuracy alone, as they are both excellent. However, each unit has certain advantages over the other that deserve to be presented and considered.

An important consideration with any underwater system is power draw of components. Due to the differences in architecture, the PHINS III draws significantly less power than the T-24. The PHINS III draws only 15 Watts, whereas the T-24 draws 30 Watts. While neither of these devices will be a major tax on the system in comparison with the propulsion systems, for low energy density or long endurance platforms every Watt can impact the total endurance of the AUV.

The physical size of each system is also an important consideration. At present, the PHINS III is a box with dimensions 180mm x 180mm x 160mm. The T-24, which is installed as an OEM system, consists of a cylinder of diameter 115mm and length 155mm and a board set that is about 135mm x 115mm x 65mm. The smaller size and flexible form factor of the T-24 enables it to be fit more easily into a smaller diameter vehicle.

Another differentiator that may be important to some customers is the acoustic noise generated by the devices. Since the T-24 uses an RLG, which has mechanically dithered components, it makes noise. The FOG of the PHINS III has no moving parts, making it acoustically silent. Acoustic radiated noise may be an important consideration for certain types of sonar and should be considered in the selection of a navigation system.

Figure 4 – Achieved navigation accuracies for submerged distances greater than five kilometers.

Figure 5 – Scatter plot showing position drift as a function of trackline distance. Note that the drift appears much higher for shorter tracklines as

a result of GPS error being a non-negligible contribution.

6

VI. APPLICATIONS OF HIGH QUALITY NAVIGATION

High quality navigation is critical for many underwater applications. In particular, the expanding application of SAS is dependent on high accuracy and high precision navigation.

Side scan sonar is widely used throughout the AUV industry, but SAS offers constant high-resolution imagery across a greater range than a comparably sized SSS. In order to generate this imagery, SAS payloads perform complex processing that requires a number of inputs from the platform with which the SAS is integrated – in this case, the AUV. High-accuracy motion data from an INS – which has already processed inputs from the DVL and other aiding sensors – is fed directly to the SAS computer to allow processing to compensate for vehicle motion. Low drift and high accuracy are critical to proper beamforming.

While high-accuracy motion data is needed for SAS functionality, high-accuracy navigation is required for accurate absolute positioning of sonar targets. In mine warfare applications, for example, contact localization accuracy is the driving indicator of sensor and navigation performance for any minehunting technology.

In 2010, Bluefin delivered a Bluefin-12 vehicle outfitted with a PROSAS Surveyor sonar, manufactured by Applied Signal Technology, Inc. aided by a Kearfott T-24 INS. This vehicle regularly obtained seafloor imagery at constant 1x1-inch resolution across 200 meter range on each side, with an estimated contact localization accuracy of ten meters. This was observed during both deep and shallow water surveys using long trackline missions. Overall this provided the operators with a six fold improvement in operational efficiency over a comparable SSS AUV. Sample SAS data from this

system is presented in Fig. 6. It is clear that aided by enabling technology such as high-performance tactical INS, SAS systems integrated on Bluefin AUV platforms yield favorable improvements both in data quality and in the efficiency of data collection.

VII. CONCLUSIONS AND ONGOING WORK

Integration of these navigation systems is an ongoing effort, with continuous improvements being made on both the Bluefin side and the INS manufacturer side. The data presented in this paper for INS-based navigation accuracy was taken using recently completed Bluefin AUVs. The data demonstrates that the navigation accuracies of both the Kearfott T-24 and the IXSEA PHINS III exceed the published specifications when integrated into a Bluefin AUV. Each of these systems provides exceptional navigation accuracy that can be used to collect high quality oceanographic data.

Bluefin is continually expanding its library of dives that can be processed to create plots like these. Additionally, Bluefin is refining its processes to facilitate faster and more accurate INS-DVL calibrations to produce even greater navigation accuracy. In addition, as high-performance INS units shrink in mechanical size and power consumption, cutting-edge payloads such as SAS will become feasible for integration onto smaller and lower-power AUV platforms.

ACKNOWLEDGMENT

The authors would like to acknowledge the entire Bluefin Robotics team for their work in integrating both of these

Figure 6 – Post-processed SAS image of a shipwreck, taken with an AST PROSAS Surveyor sonar on a Bluefin-12 AUV. Note the constant resolution across the entire range.

7

excellent navigation systems. Bluefin’s strong relationships with Kearfott and IXSEA have enabled the creation of well-integrated systems that are capable of meeting the customer’s needs.

REFERENCES [1] Kearfott Corporation, Seaborne Navigation System (SEANAV), KN-5050

Family. Data Sheet. 2010. [2] IXSEA, PHINS User Guide. 2009.

ABOUT BLUEFIN ROBOTICS

Bluefin Robotics manufactures and develops Autonomous Underwater Vehicle (AUV) systems and technology. Founded in 1997, the company has grown to become a world leader in AUV products designed for defense, commercial, and scientific applications. Bluefin Robotics is a wholly-owned subsidiary of Battelle. For more information, please visit www.bluefinrobotics.com.