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Aiding Sensors in Subsea INS Applications

There are several sensors commonly used in subsea INS applications. The following table summarizes these sensors, their measurements, their contributions to the INS solution, the transforms required to convert the sensor data to the navigational frame, and notes on each.

Sensor Measurement INS Contribution Notes
Compass

Φ, θ, Ψ

ECEF attitudes

RbCmps

Φ, θ, Ψ

ECEF attitudes
  • Provides absolute measurement of attitude.
  • Magnetically based measurement is easily corrupted, unreliable, and dependent on calibration and environment.
  • Short-term noise is usually significant.
  • Long-term stability is often highly dependent on vehicle location.
  • Moderate update rates (<10Hz typically).
Gyrocompass

Φ, θ, Ψ

ECEF attitudes

RbCmps

Φ, θ, Ψ

ECEF attitudes
  • True north seeking capability.
  • Non-magnetically based attitude solution unaffected by ferrous material.
  • Exceptional attitude measurement performance.
  • Typically a component of full AHRS system that also provides IMU data.
AHRS

Φ, θ, Ψ

(Φ, θ, Ψ used in IMU process.)

ECEF attitudes

RbAHRS

Φ, θ, Ψ

ECEF attitudes
  • Often more stable than a traditional compass because it has aiding gyroscopic data.
  • Provides absolute measurement of attitude.
  • Magnetically based measurement is easily corrupted, unreliable, and dependent on calibration and environment.
  • Short-term noise is usually significant.
  • Long-term stability is often highly dependent on vehicle location.
  • Moderate update rates (<10Hz typically).
DVL

xdot, ydot, zdot

Instrument frame velocities

TBdvl

Xdot, Ydot, Zdot

ECEF velocities through transform
  • Frequency of DVL usually determines accuracy of measurement as well as maximum available distance from seafloor in an inverse relationship.
  • Most accurate and most widely used velocity sensor for vehicles.
  • Typically provides highly accurate velocity measurements.
  • Measurement quality dependent on ambient acoustic noise, bottom conditions, bottom contours, and obstructions.
  • Measurement quality is usually difficult to determine.
  • Requires bottom lock and is typically effective only within 50m to 200m of bottom.
  • Slow update rates (<1Hz typically).
GPS

X, Y, Z

ECEF positions

TBgps

X, Y, Z

ECEF positions
  • Typically accurate on surface.
  • Accuracy is dependent on visible satellites and quality of receiver.
  • Not useful underwater.
  • Slow update rates (<5Hz typically).
USBL

X, Y, Z

Relative to topside transceiver

TBusbl

X, Y, Z

ECEF positions through transform
  • Highly dependent on environmental conditions and system setup.
  • Provides good, low-resolution, positioning.
  • Typically has relatively large amplitude noise.
  • Can contain significant measurement biases.
  • Large amplitude noise is pseudo-random at short time scales but Gaussian at large time scales.
  • Slow update rate (<1Hz typically).
  • Often is not used in the navigation solution if a DVL is present because the noise from the USBL will pollute the overall stability of the navigation solution. Instead, the USBL fusion is selectable by the operator.
LBL

X, Y, Z

Relative to transponders

TBlbl

X, Y, Z

ECEF positions through transform
  • Highly dependent on environmental conditions and system setup.
  • Provides good, low-resolution, positioning.
  • Typically has much better noise and overall performance than USBL.
  • Can contain significant measurement biases.
  • Large amplitude noise is pseudo-random at short time scales but Gaussian at large time scales.
  • Slow update rate (<1Hz typically).

Pressure/

Depth

Ambient pressure

TBdpt

Z

ECEF position through conversion formula
  • Accuracy is a function of scale.
  • Depth measurement is highly dependent on conversion formula used.
  • Signal has Gaussian noise error combined with possible thermal bias errors.
  • Fast update rate (>10Hz typically).
  • Greensea uses the UNESCO 1983 formula for converting pressure to depth in sea water.
Altimeter

Height off bottom

Zref

TBalt

Height off bottom

Zref

Not fused in INS
  • Frequency of altimeter usually determines accuracy of measurement as well as maximum available distance from seafloor in an inverse relationship.
  • Not fused in INS.
  • Simple filtering is usually used to produce a stable measurement.
Feature-based

xdot, ydot, zdot

Instrument frame velocities

TBinst

Xdot, Ydot, Zdot

ECEF velocities through transform
  • Driftless velocity estimate based on perceiving features with sonar or vision sensors.
  • Highly computationally complex.
  • Methods and processes are still in development.

 

Article ID: 
184