Navigation and Positioning

The navigation and positioning accuracy required in an AUV will be determined by the mission requirements. Positional accuracy is the error which the AUV makes in determining its geographic position. Navigation accuracy is the precision with which the AUV can guide itself from one geographical point to another.

A very basic navigational error model for AUV navigation uses the following terms:

= initial vehicle position error	= along-track velocity error
= initial heading error	= cross-track velocity error
= sensor heading misalignment

The values for these terms are combined to give a root-sum-square estimate (1s ) of vehicle position error as a function of distance traveled (D). All of these error contributions are independent of vehicle speed and higher order terms such as heading drift rate are not included.

It should be stressed that this is a worst case scenario as it assumes the vehicle is on a constant heading throughout the mission. In many operations, the vehicle changes or reverses heading frequently, and in these cases, other error models are used to account for cancellation of navigational errors.

Watson Industries AHRS-E303 Unit

In the QuickDesign© standard options, we offer two navigation sensor packages. One is a low cost system which offers a basic navigation capability.

The other is a survey grade system providing high levels of accuracy over extended periods without positional updates.

The lower cost standard system consists of:

• Watson Industries, Inc (USA) E303 Attitude Heading Reference System (AHRS)

• SonTek Argonaut Doppler Velocity Log

• OMEGA PX5500LI strain gauge depth transducer

An AHRS transforms and integrates signals from three solid state angular rate sensors to determine the attitude and heading of the AUV. A velocity input from the Doppler Log compensates for errors within the unit, to improve overall stability and accuracy. The Doppler Velocity Log transmits two separate beams downward from the AUV to measure the speed along the AUV track and across it. The AUV controller then integrates the heading, attitude and velocity into a dead-reckoned position estimate.

For highest accuracy, the DVL must be in contact with the seabed, and consequently, lower frequency units can operate at higher altitudes above the seafloor. Typically, a 300 kHz system can operate at altitudes of up to 200 metres, while a 1200 kHz system will be limited to altitudes on the order of 30 metres. Finally, the Paroscientific strain gauge depth transducer measures the depth of the AUV from the surface.

The survey grade precision system consists of:

• Kearfott Guidance and Navigation Corp. (USA) SEANAV Inertial Navigation Unit

• RD Instruments (USA) Workhorse Doppler Velocity Log

• Paroscientific, Inc. Digiquartz® Strain guage depth transducer

Very basically, the Inertial Navigation Unit consists of an Inertial Measurement Unit (IMU) and a Kalman Filter. The IMU is comprised of 3 sensors which measure acceleration and angular velocity and integrate them to determine heading and speed. In the Kearfott SEANAV INU, three ring laser gyros are used for this.

Kearfott 5000 INU

A Kalman filter is a digital data processing algorithm which takes inputs from other sensors and based on the knowledge of the dynamics of these systems, generates an overall best estimate of position. In its standard configuration, the Kearfott SEANAV INU is capable of accepting inputs from Global Positioning System (GPS) receivers, Doppler Velocity Logs as well as propeller rpm.

A sophisticated analysis of this system has demonstrated that when it is used to survey a rectangular area such that the AUV reverses its heading frequently, navigational accuracies of ±15 metres can be maintained for periods of up to 10 hours without position update.

As part of our ongoing research, we are presently examining the benefits of fiber optic gyro (FOG) technology as a replacement for the ring laser technology used in our current INU.

Both GPS and acoustic positioning systems can be used to assist in determining AUV position. DGPS positions accurate to 1-2 metres are obtained on the surface. When the vehicle has dived, position has to be obtained acoustically. There are several methods for this including:

• Ultra Short Baseline System (USBL)
• Short Baseline systems (SBL)
• Long Baseline Systems (LBL)

Although ultrashort and short baseline systems do not require additional equipment to be deployed in the water or on the seafloor, they do require careful shipboard calibration. In the case of a short baseline system, two transducers need to be installed on the ship’s hull as far apart as possible. The ultrashort baseline system on the other hand requires only one transducer to be installed. This makes the ultrashort baseline system more portable and consequently the preferred approach in instances where a variety of surface ships may be used

ORCA Inverted LBL – GIB System

Ultra Short Baseline systems can provide excellent positional accuracy. In order to achieve these results, it is necessary to provide high quality motion sensing instrumentation on the support ship. This can include a Vertical Reference Unit and a gyro. The ultrashort baseline system must be also be accurately calibrated to the motion sensing reference. Another option is to invert the USBL and place the transceiver in the AUV. This enables the AUV to position itself with respect to a hull-mounted transducer or to "home" on transducers that are placed on the seabed.

In the past, Long Baseline systems have also required careful calibration and the deployment of equipment on the seafloor. However, an inverted LBL system, in which GPS receivers are used on the ocean surface, does not require any shipboard system calibration. Although equipment must be deployed in the water, there is no need for special shipboard motion sensing equipment. Thus, an inverted LBL system may be utilized with a ship of opportunity.

In QuickDesign©, we offer the Sonardyne International Ltd(UK) combined USBL/LBL AUV transponders (models 7656 and 7866 depending on depth) as well as the ORE 4326A. These can be used with a variety of Long and USBL systems as well as Simrad HPR systems. We also offer the ORCA Instrumentation(France) Inverted Long Baseline System known as GPS Intelligent Buoy or GIB.

In the case of the inverted long baseline GIB system, a basic model to assess the positional accuracy is given by:

where:

= total positioning error	= number of buoys
= pressure sensor depth error	= GPS error
= acoustic range error and clock drift

This error is depth ndependent. For a USBL system, a basic model to assess the positional accuracy is given by:

where:

= total positioning error	= VRU angular error
= USBL azimuth measurement error	= USBL range error estimate
= gyro angular error	= GPS error
= USBL elevation angle measurement error	= Range

With I/LBL, the dominant error is GPS. With USBL, the dominant error is GPS at short slant ranges and USBL system errors at longer ranges. Note that unlike the I/LBL, the positioning error is a function of the operating range and it increases with depth.