Install Auxiliary Systems

Attitude sensor

Compass

Position

Height

Combined attitude, heading and position systems

Speed of Sound Sensors

Echosounders

Attitude system Ethernet interface

 

Attitude sensor

When mounting the transducers in the hull

  • Mount the attitude sensor as close to the transducers as possible.
  • Mount the attitude sensor on the fore-and-aft centre line of the vessel.
  • Align the attitude sensor's vertical axis with the vertical that applies when the vessel is afloat.
  • Strive to make the above alignments as exact as is practical. It should however, be noted that slight misalignments of 1 or 2 degrees can be compensated for in the patch-test calibration of the system.

Ensure that the attitude sensor's mount is completely rigid. The attitude sensor measures the angular location of the transducers to better than 0.05°. It must therefore not move relative to the transducers by more than this, when the vessel rolls and pitches. Prepare a mounting bracket for the attitude sensor, that:

  • is rigidly fixed to the structure of the vessel,
  • cannot move relative to the transducers,
  • does not need to be disturbed for any reason, and
  • is not in a walkway.

 

Some attitude sensors are taller than they are wide. When mounting these units, consider using a frame that holds the top of the unit to prevent it swaying relative to the vessel.

When pole-mounting

  • If possible, use a version of the attitude sensor that is supplied in a pressure-tight bottle, and fix it next to the transducers. Otherwise, the attitude sensor will not be able to correct for the motion of the end of the pole relative to the sensor.
  • Use the attitude sensor mounting bracket supplied by Bathyswath, appropriate to the unit being used.

In general

  • Refer to the attitude sensor’s own handbook for installation instructions that are specific to the particular model used.
  • Most attitude sensors are supplied with set-up software that runs on the PC computer. Run this software before using the sensor to survey.
  • Most sensors need some kind of calibration process before they give an accurate output. This might typically consist of a settling period, followed by a series of turns or pre-define manoeuvres. Consult the sensor’s own manual for details.
  • Ensure that the attitude sensor is mounted the correct way up, facing in the correct direction. Refer to the attitude sensor's manual for further details. Many models can be configured to work in different orientations. Make sure that the orientation is entered correctly into the motion sensor’s set-up software.
  • Accurately measure the location of the attitude sensor in three dimensions, relative to both the sonar transducers and the positioning system. It is necessary to account for the difference in position between the attitude system, position antenna and sonar transducers. This is called a ‘lever arm’ correction. In preference, use the attitude sensor’s internal lever-arm correction facilities, rather than the lever arm corrections in the Swath software. This is because the attitude system has more information at its disposal than the Swath system, and hence should be able to make a better correction. Make sure that the lever arm correction is not made in both systems. That is, if the attitude system can perform the lever arm correction and output attitude and position valid for the location of the sonar transducers, set the position offsets in the Swath system to zero.

See the User Guide topics on Calibration for further information on measuring and configuring the attitude sensor offsets.

Compass

A compass provides heading information.

Most gyrocompasses can be interfaced to Bathyswath. For smaller installations, a magnetic fluxgate compass can be used, but this is less accurate and more prone to disturbance from external metallic objects, including the ship itself.

Some attitude systems provide heading directly, either from a built-in compass, or because their gyros are accurate enough to detect the rate and axis of the Earth turning, and therefore are gyrocompasses in their own right. In this case, a separate compass feed is not required.

Alternatively, some heading sensors use a pair of GPS antennae, and measure the angle between them.

Bathyswath obtains heading through two main channels. Only one is required for processing, although comparing the two is a useful diagnostic check. The two channels are:

  • A direct connection to the compass;
  • Via the attitude sensor. In some cases, the attitude sensor takes in heading from the compass, combines it with ‘high-frequency’ yaw from its own gyros, and transmits the combined heading value back to Bathyswath.

Bathyswath reads the compass information in the standard NMEA format. As specified in the NMEA format, this is an RS232 serial link, typically running at 4800 baud, with 8 bits, no parity. Most attitude sensors also use this format, and some models will read other formats as well. Refer to the sensor’s own manual.

Sometimes the compass provides heading information in a format that can be read by the attitude sensor, but not by Bathyswath. In this case, the heading channel that passes through the attitude sensor is used in processing, and the direct link to the Bathyswath is ignored.

If a fluxgate compass is used as a heading reference, it should be mounted away from magnetic fields on the vessel (engine, generators and large ferrous objects). The masthead is often the best location.

The compass must be installed and aligned in accordance with its own Manual and good survey practice.

Heading information can also be obtained from course-made-good, which is the direction between successive positions from the GPS system. However, this is far less accurate than the heading from a compass, and should only be used as a last resort if heading from the compass or attitude system is not available.

See the User Guide topics on Calibration for further information on measuring heading and configuring heading offsets.

Position

The position of the vessel is obtained from a positioning sub-system. The position is usually derived from GPS, although several other types of system exist. Some motion sensors provide position data directly, or they read in position information and improve its accuracy using the motion sensor data.

The position interface is an RS232 serial port. A range of serial line parameters may be used. Bathyswath may be set-up to accept various settings of baud rate, number of bits, parity and stop bits. See the User Guide for how to set-up position inputs.

In order to maintain the accuracy of the survey, the positioning system should be accurate to 1 metre or better, with an update rate of 1 second or better. Surveys that require better depth accuracy may require better position accuracy than this.

Some positioning systems provide a measurement of height above datum. Bathyswath records this height information and it can use it instead of tide measurements. This feature is often very useful for improving the accuracy of a survey.

Be aware of the errors that can arise from differences in grid systems, geoids and datums. Bathyswath accepts position data as grid co-ordinated (easting and northing) or as latitude and longitude. In the latter case, the latitude and longitude are converted to grid positions when they are read into the system, using a range of conversion protocols, including Universal Transverse Mercator (UTM). A range of geoids and other conversion parameters are available in the Position input dialog. Post-processing is carried out using grid co-ordinates. Alternatively, use a proprietary software package to convert the position to grid co-ordinates on-line. Several excellent packages are available for this function; these also provide many useful features for planning and running the survey lines.

Some models of attitude sensor can accept information from the position system to help it correct for errors that arise from centripetal acceleration in turns. The position information required by the attitude sensor for this purpose must usually be of a specific NMEA format, but does not need to be as accurate in position as that used for locating the Bathyswath depth measurements. See the attitude sensor’s own manual for further details.

Height

The vertical height of the sensors can be obtained in several different ways:

  • Heave from the attitude sensor, combined with tide information. These are essentially high frequency and low frequency components of vertical position. If tide is used, the depth of the transducers below the waterline must also be measured.
  • GPS height: the altitude component of the GPS data. This is usually only accurate enough for surveying purposes if a high-accuracy GPS system is used, for example Real Time Kinematic (RTK).
  • Combining GPS height and heave. As these are both capable of recording relatively high frequency information, the GPS height needs to be filtered before combining. The length of the filter is selectable in the Attitude Derivation dialog. However, it is much better to allow the attitude system to perform this data merging if it has the capability to do so.
  • From a pressure sensor in an underwater vehicle; this can be further combined with heave from the motion sensor if necessary.

Tide data is not needed if GPS height can be used.

Tide information can be fed in as a real-time data stream into an Auxiliary serial port, if it is so available.

Combined attitude, heading and position systems

Some systems provide a combination of two or more of attitude, heading and position. In this case, it may be necessary to make more than one connection from the sensor to the Bathyswath computer.  Heading can be derived from the attitude string, but position needs to come in on a separate serial line. In any case, attitude and position are usually supplied in different formats, and need to be decoded differently.

The ‘Attitude Derivation’ dialog under the ‘Configuration’ section of the Swath program provides a range of options for deriving roll, pitch, heading and height.

Speed of Sound Sensors

Bathyswath can take inputs from speed of sound sensors in two ways:

1.     Regular vertical profiles (‘dips’) taken by the operator using a separate, stand-alone instrument. The profiles (measurements of speed of sound against water depth) are entered as tables into the Swath software.

2.     Continuous updates from a speed of sound sensor mounted next to the transducer heads. This measurement is used to perform the calculation of the angle of the sound wave at the transducer head. This may be necessary if the survey takes place in an area where the speed of sound at the surface changes significantly, for example, in an estuarine area.

The vertical profiles are always necessary where the speed of sound changes with depth. The continuous updates are important where the surface sound speed changes across the area. Frequently, both are needed, but sometimes one or neither of them is adequate for an accurate survey.

Echosounders

A single-beam echosounder can be mounted alongside the Bathyswath transducers. The echosounder does not have to be mounted close to the Bathyswath transducers, in order that the attitude system corrections apply, as a position offset can be entered into the Bathyswath software, and thus a ‘lever-arm’ correction made. It is likely that there would be acoustic cross-talk between the systems, and so the transmit pulse trigger output from the Bathyswath TEM (only available on later versions of the TEM) should be connected to the echosounder’s input trigger line, if available.

Bathyswath can read echosounder data on an RS232 serial line with formats including: Valeport, NMEA, AML, CSV, SVP16, WESTGEO and HYPACK.

Attitude system Ethernet interface

Some attitude and position systems provide data over an Ethernet interface. Ethernet interfaces are generally preferable to serial, because:

  • Latency is less of a problem
  • Modern laptop computers tend to be fitted with Ethernet, but not serial ports
  • The attitude and position systems can provide more data over Ethernet, which helps with decoding. For example, such systems generally provide data packets with time-tags on Ethernet, but not serial.

If possible, the attitude Ethernet input should be provided on a dedicated Ethernet line. Heavy traffic on a general-use Ethernet line can slow down the reception of data from the attitude system.

At the time of writing, the attitude Ethernet inputs supported by Bathyswath are those from the CodaOctopus F180 and the Applanix POS/MV. The instructions below are those for the F180. Other systems are similar.

1.     Set up the attitude system to send data by Ethernet. Typically, this is done from the Attitude system’s PC-based application, using a ‘UDP retransmit’ function. Tip: these attitude sensors are usually supplied with their own set-up program, using the same UDP port as Bathyswath. As it is not possible to configure the same port number to two applications running on the same computer, the attitude sensor’s own application program should be closed down before connecting it to the Swath program. If it is necessary to run the attitude sensor set-up program during the survey, perhaps to monitor the system’s performance, run the sensor’s application on a different computer on the network.

2.     Set up the parameters of the UDP output from the attitude sensor or its PC application in the initiation file.

3.     Open the Bathyswath initiation file in a text editor. This is generally found at:
‘C:\Program Files\Bathyswath\Bathyswath\ swathprocconfig.txt’.

4.     Find the section with entries that start ‘F180socket’. This section applies to other Ethernet input systems (such as POS/MV), too.

5.     Enable Ethernet attitude input by setting:
F180socket    enabled                     1

6.     If required for system timing, configure Swath to use the sensor time to set its own clock, using:
F180socket    timesyncEnable                                 1
For most applications, ‘0’ is the recommended setting; see §6for details.

7.     Run the Swath program.

8.     Click on ‘Attitude’, then ‘Network Settings’. Enter the port number that the sensor is broadcasting to (generally 3000 for F180 and 5602 for POS/MV).

9.     Click ‘OK’, and ‘Connect’ in the Attitude Sensor Settings dialog.

10.  If position is being obtained from the same source, open the Position dialog, and select the appropriate input format, e.g. ‘Coda MCOM’ or ‘POS/MV 102’.

11.  See here for advice on selecting timing from ‘Sensor Clock’ or ‘PC Clock’