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Marine Geomatics > Hydro-Acoustic Positioning (USBL & LBL) > Flashcards

Hydro-Acoustic Positioning (USBL & LBL) Flashcards

1
Q

What is the general principle of hydro-acoustic positioning?

A

The position of a towed vehicle (or surface vessel) can be positioned relative to a network of acoustic beacons on the seabed whose positions are known

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2
Q

What global demand is leading to increased development of hydro-acoustic positioning systems?

A

Deep Water Operations

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3
Q

Give 5 industries that use hydro-acoustic positioning

A
  1. Offshore
  2. Search and Recovery
  3. Ocean Science
  4. Defence
  5. Civil Engineering
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4
Q

What are the 3 main hydro-acoustic positioning systems?

A
  1. USBL
  2. LBL
  3. LUSBL
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5
Q

What is a Beacon?

A

An active device that sends out continues signals. It is placed on the seabed or an ROV and each one has a unique frequency

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6
Q

What is a Transponder?

A

An active and passive device, which can receive and transmit. It transmits different frequencies to the interrogation frequency

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7
Q

What is another name for an Intelligent Transponder?

A

Transceiver

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8
Q

What is a Transceiver?

A

A device that transmits and receives acoustic pulses and interrogates other intelligent transponders. They have the ability to change frequencies, to avoid interference and may also have environmental sensors for accurately setting SV

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9
Q

What is the Baseline?

A

The distance between two transponders or between a transport and transceiver

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10
Q

If you have 3 baselines, what can you use to determine position?

A

Trigonometry

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11
Q

What needs to be determined on a regular basis when using hydro-acoustic positioning?

A

Sound Velocity

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12
Q

What is LBL?

A

Long Baseline

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13
Q

What is USBL?

A

Ultrashort Baseline

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14
Q

What is LUSBL?

A

Long and Ultrashort Baseline

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15
Q

What is the typically sized baseline for LBL?

A

100s-1000s of metres

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16
Q

What is the typically sized baseline for USBL?

A

cm’s

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17
Q

LBL arrays can be used to position what type of subsea vehicle?

A

ROVs

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18
Q

Why are the 3 seabed transponders used in LBL?

A

To give an unambiguous 3D Position

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19
Q

How does LBL work?

A

Seabed transponders at known locations are interrogated by a common interrogation signal and each transponder replies on an individual reply signal to the interrogating vessel

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20
Q

What does carrying out navigation along the seabed remove?

A

Errors caused by acoustic pulses traveling through the water column

21
Q

What 2 things are used to determine range when using USBL?

A
  1. Timing

2. Sound Speed

22
Q

What 3 additional sensors are required when using USBL?

A
  1. Gyro (Heading)
  2. MRU (Motion)
  3. GNSS
23
Q

What is direction derived from when using USBL?

A

Differences in phase of the signal at the transceiver elements

24
Q

How does USBL work to get position?

A

Position is obtained by measuring the range and vertical and horizontal angles from a multi-element transducer to the transponder, which may be on a fixed or mobile target

25
Q

What type of hydro-acoustic positioning system would you use to position during pipeline operations and why?

A

USBL – need low accuracy and high update rates

26
Q

What type of hydro-acoustic positioning system would you use to position during ROV pipeline inspections and why?

A

High Precision USBL – need good accuracy and update rates

27
Q

What type of hydro-acoustic positioning system would you use to position a drilling vessel with DP and why?

A

LUSBL – need high update rates and accuracy and redundancy

28
Q

What type of hydro-acoustic positioning system would you use to position during a deepwater structure installation?

A

LBL – need high accuracy

29
Q

How does a LBL calibration work?

A

The vessel circles around the seabed transponder locations, whilst continuously recording LBL data (Vessel DGPS and Beacon Range)

30
Q

What method is used to get estimates of transponder positions and depth when doing a LBL calibration?

A

Least Squares Method

31
Q

Describe the USBL calibration process

A

One beacon is deployed onto the seabed, the vessel rotates 360degrees around the beacon at a distance of ½ the water depth away. DGPS, USBL and attitude data are obtained simultaneously and estimates for beacon position, heading and attitude can be determined

32
Q

What is the problem with inertial sensing?

A

Position Errors Drift

33
Q

What is the most successful way of conducing underwater navigation?

A

By using INS and DVL

34
Q

What is INS?

A

Intertial Navigation System – predicts position on the move

35
Q

What is DVL?

A

Doppler Velocity Log – sends out a 4-beam ping and measures the resulting response in terms of frequency/doppler shift. This is translated to a velocity relative to the seabed.

36
Q

What 3 directions is velocity measured?

A

X, Y, Z

37
Q

What does an INS system consist of?

A

3 perpendicular accelerometers and 3 perpendicular gyros

38
Q

Give 3 -ves of INS

A
  1. Requires regular position and velocity updates
  2. Need to correct for the Earth’s rotation
  3. Need to correct for the Earth’s gravity
39
Q

What is the doppler shift equation?

A

Fd = 2FS(V/C)Cos(A) where Fd is doppler shift, FS is frequency, V is ships velocity, C is SV and A is angle

40
Q

What is the main potential problem with using DVL?

A

The frequency depends on the local SV and if it is inaccurate you could have >200m error after one hour of navigation at 3knots

41
Q

The overall accuracy of a position obtained from a hydro-acoustic positing system depends on what 7 things?

A
  1. Array accuracy relative to a geodetic datum
  2. Determination and suppression of multi-path effects
  3. Accurate determination of SV
  4. System frequency
  5. Transpoder Fix Geometry
  6. Seabed Topography
  7. Signal to Noise Ratio
42
Q

Give 3 +ves of USBL

A
  1. Quick Deployment & Calibration
  2. High Position Update Rate
  3. Accuracy is dependant on all sensors (0.09-1% of slant range)
43
Q

Give 3 -ves of USBL

A
  1. Difficult to determine slant range
  2. Accuracy decreases with distances from the transceiver
  3. Typically no redundancy
44
Q

Give 3 +ves of LBL

A
  1. Redundancy
  2. High Accuracy (0.05-1m)
  3. Long Ranges to Cover Large Survey Areas
45
Q

Give 2 -ves of LBL

A
  1. Time consuming to deploy and calibrate

2. Slow position update rate

46
Q

How does LUSBL work?

A

Range and bearing are measured to the transponder as in USBL, but there are more giving a greater accuracy

47
Q

What 2 benefits do you get when using LUSBL?

A
  1. LBL Accuracy

2. USBL Update Rate

48
Q

Give 3 -ves of LUSBL

A
  1. Takes a long time to set-up and deploy
  2. Complex
  3. Often not required

Marine Geomatics (11 decks)

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