Mechanical Installation | Electrical Installation | Operational

MECHANICAL INSTALLATION

• How do I mount the DVL on my vehicle and where?
• How do I align the DVL to my vehicle?
• What connector is used on the Navigator ?
• What is the depth rating for my DVL?
• How is the DVL protected against corrosion and erosion?
• What are the considerations when installing an acoustic window?
• How do I install, deploy and calibrate my DVL?

-top-

ELECTRICAL INSTALLATION

• How much power do I need?
• Which data format should I use and why?
• How do I correct for speed of sound?
• Can I externally trigger the DVL?
• Synchronization with another sensor?
• How can I interface with any other (acoustic positioning) system?
• Will my vehicle affect the accuracy of the DVL compass?
• How do I select RS232 or RS422 communication?
• Is the power line connected to the ground? Is the power floating?
• Is the data line floating with respect to ground? Is it Opto protected?
• Is the communication circuit protected from shorting or over voltage?
• Do I get more data if I increase communications speed?
• What frames of reference can I use for data (Data coordinate transformation)?
• Do I need to align the DVL with my vehicle?
• Does the Navigator output water velocity data?
• Can I apply an external time stamp on the data?
• What is the timing relationship between the DVL internal clock and the time of the Doppler bottom tracking measurement?

-top-

OPERATIONAL

• What interference might I get between the DVL and the other acoustic equipment on my vehicle?
• What are the acoustic source levels for the Workhorse Navigator DVL ? 
• What are the maximum and minimum altitudes that my DVL will operate?
• What are the maximum and minimum speeds that the DVL can detect?
• What happens to the DVL output if I don’t have bottom lock?
• How accurately can I navigate with my DVL?
• Can I use the DVL around offshore structures?
• How do I calibrate my DVL?
• Will it work in fresh water?
• What is the maximum seabed slope that I can still bottom track?
• Do sand waves affect my ability to bottom track and measure altitude?
• Do I need to reconfigure each time power is applied?
• Will Bottom Tracking be a problem over a pipe?
• What is the maximum rate that I can ping?
• What is the maximum pulse length?
• Is there an angular rate limit for the accuracy spec?

DVL Frequently Asked Questions

MECHANICAL INSTALLATION

How do I mount the DVL on my vehicle and where?

The DVL should be mounted from the end cap or by a clamp around the body. The mounting surface should be electrically isolated from the DVL (i.e. with rubber or other insulating material between the touching surfaces). You should mount the DVL as far from any thrusters and motors as possible. The DVL should be mounted in a position that allows a clear path for all the beams, plus a 15° conical ‘keep out zone’ around each beam because reflections from any solid object located in this cone can interfere with the DVL measurements (see Workhorse Navigator Installation guide for further details). The DVL has four beams at 90º azimuth and 30º from the vertical. Typical mountings place it at or near the bottom of the vehicle with the transducer beams pointing downward.

-top-

How do I align the DVL to my vehicle?

The DVL may be mounted with any azimuth orientation of the beams. The most common choices are:
Beam three pointed forward of the vehicle so that its altitude data may be used for obstacle avoidance.
Beam three rotated to 45° from forward so that all four beams are measuring both along ship and athwartship velocity for increased accuracy.

RD Instruments provides an optional alignment jig to help properly align the transducers. The EA command should be used to correct for any rotation angle of beam 3 relative to the vehicle axes. Also see the Electrical Installation section below on alignment.

-top-

What connector is used on the Navigator?

The I/O connector is LPMBH-7-MP from Impulse. The connector on the cable has to be an LPMIL-7-FS. It has 7 pins: two power lines, five for RS422, or three RS232 and two RS485.

-top-

What is the depth rating for my DVL?

The standard unit is currently rated to 3000m with a standard option for 6000m. Other non-standard depth ratings are available on request. If you already have a DVL and are uncertain as to its depth rating, please refer to the system configuration sheet that was shipped with the DVL.

-top-

How is the DVL protected against corrosion and erosion?

We typically hard anodize and then paint our standard DVLs. Sacrificial anodes are fitted to the DVL to prevent corrosion (See Workhorse Navigator manual for correct installation procedure of the anodes for the DVL). As for water ingress, our units are sealed with O-rings for operation at pressure. A desiccant bag is put inside the unit to remove any internal residual moisture or humidity.

-top-

What are the considerations when installing an acoustic window?

We supply a detailed document to cover this subject, which is available on request.

-top-

How do I install, deploy and calibrate my DVL?

Only a few bolts are required to install the DVL.
RDI performs a factory calibration of the transducers and no other calibration is normally required on the DVL itself unless the DVL is using an external heading sensor. (See question in Operational section of this document – How do I calibrate my DVL?).

If you are using the internal compass then you must calibrate the biases from the vehicle on the compass. This is done by rotating the entire vehicle in a circle after installing the DVL. It will be necessary to rotate the vehicle at 2 different roll angles when doing this calibration.

If you are using an external heading reference system (e.g. INS) it may be appropriate to calibrate the combined system against another independent system such as GPS.

-top-

ELECTRICAL INSTALLATION

How much power do I need?

Navigator Power Supply Requirements

The manual suggests that a 50-watt power supply is required. However, the average power drawn by a Navigator pinging at its maximum rate at maximum altitude is 3 Watts for a 1200kHz, 5 Watts for a 600kHz and 16 Watts for a 300kHz. This is based on an input voltage of 32 volts. There is a constant background power consumption of 2.2 watts for the processing electronics as long as the Navigator does not “go to sleep.”
The average power consumption depends upon the transmit duty cycle which is a function of many factors. For a Navigator pinging at its maximum rate, the maximum duty cycle is about 13%. By reducing the ping rate, the duty cycle is reduced and therefore overall power consumption may be reduced.

The Navigator contains a 10 mF capacitor, which provides filtering of the high power consumption during transmit. This means that the Navigator should not be adversely affected by reasonable variations in the power supply voltage. It also means that the Navigator should not unduly affect the power supply circuit.

Peak current drawn by the Navigator is never more than 3 amps due to a current limiting circuit that is provided to prevent blowing a 3 amp internal fuse. At start up this translates to 96 Watts at 32V input.

Note that the power supply must be able to provide at least 10 volts continuously at a current draw of 0.4 amps to get the processor started.

-top-

Which data format should I use and why?

Navigator can output data in several user selectable formats. Depending on the output format selected, data will be either binary or ASCII text. Individual parameters within a data string may be enabled / disabled. All binary output formats have the option of outputting data in HEX-ASCII instead of true binary. HEX-ASCII is an ASCII representation of the binary data. Binary output formats include PD0,3,4,and 5. Text output formats include PD6,11, and 13.

Example of the PD6, format:

The DVL comes with several different data formats that allow different levels of information. Deciding on which format to use depends on the needs of the vehicle operation. The following describes the basics of the formats available. For detailed information on these data formats please refer to the Workhorse Command and Data Output Format Guide.

The following formats allow the output of bottom track, speed through the water, and current profile data:

PD0 – This is RDI’s standard format. PD0 is a binary output format. It provides the most information possible including a header, fixed and variable leader, bottom track, and water profile information. The fixed and variable leader is a recording of time, DVL setup, orientation, heading, pitch, roll, temperature, pressure, and self test diagnostic results. Data fields to be output are user selectable.

File Size (If BT Only) is 211 bytes binary
File Size (if water profile included) varies depending on the number of bins selected.

PD3 – PD3 is a binary output format of bottom track speed over the bottom, speed through the water, range to bottom information, and 16 spare bytes with no definition.

File Size is 40 bytes binary.

PD4 – PD4 is a binary output format of bottom track speed over the bottom, speed through the water, and range to bottom information only.

File Size is 28 bytes binary.

PD5 – PD5 is a superset of PD4 and includes information on salinity, depth, pitch, roll, heading, and distance made good.

File Size is 74 bytes binary.

PD6 – PD6 is a text output format. Data is grouped into separate sentences containing system attitude data, timing and scaling, and speed through the water relative to the instrument, vehicle, and earth. Each sentence contains a unique starting delimiter and comma delimited fields

PD10 is similar to PD3 but with the addition of pressure and depth fields.

PD11 is a text output format. It complies with the NMEA 0183 version 2.30 standard.

PD13 – is a text output format similar to PD6 with the addition of information about range to bottom and raw pressure sensor data.

Data Format in RDI DVL / ADCP

The following rules are used in creating our BroadBand Data Format of PD0. Our recommended decoding sequence is presented next.

Rules for the BroadBand Data Format PD0:

1. All data types (i.e. fixed leader, variable leader, velocity, echo intensity, correlation, percent good, etc.) will be given a specific and unique ID number.
2. Once a data type has been given an ID number the format of the data inside that ID number will never change in units, order, or number of bytes.
3. Data may be added to an existing data type only by adding the bytes to the end of the data format. As an example, the variable leader data contains information on ensemble number, time, heading, pitch, roll, temperature, pressure, etc. The format for the bytes 1-53 are now specified by changes added in support to the WH ADCP. If additional sensor data is to be added to the variable leader data then it must be added to the end of the data string (bytes 54-x as an example).
4. The order of data types in an ensemble are not fixed. That is there is no guarantee that velocity data will always be output before correlation data.
5. The header data will include the number of data types in the files and the offset to each ID number for each data type.
6. The total number of the bytes in an ensemble minus the 2 byte checksum will be included in the header.

Recommended Data Decoding Sequence for BroadBand Data Format PD0:

1. Locate the header data by locating the header ID number (in the case of PD0 profile data that will be 7F7F).
2. Confirm that you have the correct header ID by:

   2a. Locating the total number of bytes (located in the header data) in the ensemble

   2b. Add 2 bytes to the value in 2a. This will be your offset to the next ensemble.

   2c. Read the 2 bytes following the offset to the next ensemble (calculated in step 2b).

   2d. Confirming that the next 2 bytes are the header ID number.
   If it is then you have located the Header ID. If not then go back to step 1 and search for the next header ID number occurrence.

3. Locate the number of data types (located in the header data).
4. Locate the offset to each data type (located in the header data).
5. Locate the data ID type you wish to decode by using the offset to each data type and confirm the data ID number at that offset matches the ID type you are looking for.
6. Once the proper ID type has been located use the DVL Technical Manual for the DVL you are using to understand what each byte represents in that particular data type.
   

-top-

How do I correct for speed of sound?

There are several options:

1. The Navigator has a built in temperature sensor and optionally a pressure sensor that can be used to aid the speed of sound calculation.
2. The Navigator may accept the RS485 input of an Applied Microsystems Limited Sound Velocity Smart Sensor (SVSS) to correct for speed of sound. This may be achieved by using the DVL in RS232 and accepting the data in RS485 from the SVSS. Customers are advised to contact RDI for further information on this interface as some modification to the DVL may be required.
3. Another option is to fix the Speed of Sound at a standard value for example 1500m/sec, and correct the velocities externally for the correct value.
4. Alternatively you can send the Navigator a fixed value from an external source.
5. Or send the Navigator Salinity, Temperature and Depth information and tell it to calculate using the EZ command.

-top-

Can I externally trigger the DVL?

There are now three methods of triggering the DVL. The third is a new development and is described here for completeness although it is only available on firmware V9.13 and above.

1. The system can be setup to wait for input before each ping. To setup the DVL in this fashion, clear the Auto Ping Cycle bit in the CF command by sending CFx0xxx, where the x’s represent the settings of the other parameters. Start the DVL pinging with the CS command. The DVL will output a ‘<’ before each ping and wait for input. Send any valid ASCII character to trigger the ping. The instrument will not enter sleep mode while it is waiting for the trigger.
2. Using RDS3, The instrument can be setup as a slave. In slave mode, the DVL can be commanded to remain awake at all times or it can be allowed to enter sleep mode between pings. If the DVL is allowed to sleep, then the latency from trigger to ping is greatly extended (on the order of 100 msec). To use you must have the DVL configured for RS-232 communications. Set the SM command to slave mode by sending SM2. Start the DVL pinging with the CS command. The DVL will then wait for a trigger before pinging. Setting the RS-485 lines to a break state for not less than 20 msec sends the trigger.
3. The trigger methods above all have latencies ranging from a few milliseconds to a few hundred milliseconds. Consequently RDI has recently developed a low-latency trigger method. To configure the DVL for low-latency triggering, set the CX command to enable trigger input by sending CX1. Start the DVL pinging with the CS command. The DVL will then wait for a trigger before each ping. Setting the RS-485 lines to a break state for not less than 10 microseconds sends the trigger. The DVL will then ping within 250 microseconds of the leading edge of the break pulse.

-top-

Synchronization with another sensor?

There are currently two ways to synchronize the Navigator with another sensor.
One: setup the system to ping on command through the RS232 or RS422 serial lines (send a carriage return, a TAB or a CS to ping).
Two: setup the system to be triggered via a short pulse through the RS485 serial lines, (this limits the DVL to RS232 communications).
Back to top

How can I interface with my acoustic positioning system?

The DVL communicates via a serial line. The protocol can be either RS232 or RS422. The output of data on this serial line will provide data that can be decoded by your positioning system. NMEA0183 output string is available and is detailed below.

Navigator NMEA Output Specification
Version 0.3.2

1.0 Introduction
This document is intended to show that the specification for output of navigational data from an RD Instruments Navigator DVL is compliant with the data format and structure provisions of the NMEA 0183 version 2.30 standard. Where possible an attempt has been made to conform to the structure of Approved Sentences (NMEA 0183, 5.3.1) although this document defines some Proprietary Sentences (NMEA 0183, 5.3.3).

2.0 Sentence Structure
There shall be at least three sentences containing sensor and navigational data. RDI may add additional sentences in the future so care should be taken to correctly identify the sentence by it’s ID.

2.1 G - Sensor Data
The sensor data sentence shall consist of heading, pitch, roll, and depth below surface. Each data field will be preceded by an identifier indicating the contents of the following field. All values are in SI units. All data fields are variable width. Empty data fields will indicate missing or invalid data. RDI may add additional fields in the future. Any such additional fields will be added after the last field in this specification and before the checksum.

$PRDIG,H,x.x,P,x.x,R,x.x,D,x.x*hh<CR><LF>
_ _ _ _ _ _ _ _ _ depth
_ _ _ _ _ _ _ _ depth ID
_ _ _ _ _ _ _ roll
_ _ _ _ _ _ roll ID
_ _ _ _ _ pitch
_ _ _ _ pitch ID
_ _ _ heading
_ _ heading ID
_ NMEA 0183 header

2.2 H - Bottom-Track Navigational Data
The bottom-track data sentence shall consist of range to the bottom, speed over ground, and course over ground. Each data field will be preceded by an identifier indicating the contents of the following field. All values are in SI units. All data fields are variable width. Empty data fields will indicate missing or invalid data. RDI may add additional fields in the future. Any such additional fields will be added after the last field in this specification and before the checksum.

$PRDIH,R,x.x,S,x.x,C,x.x*hh<CR><LF>
_ _ _ _ _ _ _ course over ground
_ _ _ _ _ _ course over ground ID
_ _ _ _ _ speed over ground
_ _ _ _ speed over ground ID
_ _ _ range to bottom
_ _ range to bottom ID
_ NMEA 0183 header

2.3 I – Water Referenced Navigational Data
The water referenced navigational data sentence shall consist of speed relative to the water current and course relative to the water current. Each data field will be preceded by an identifier indicating the contents of the following field. All values are in SI units. All data fields are variable width. Empty data fields will indicate missing or invalid data. RDI may add additional fields in the future. Any such additional fields will be added after the last field in this specification and before the checksum. A “water layer” ping is required in the ensemble sequence to output water referenced data. See BK and BL command of the command guide.

$PRDII,S,x.x,C,x.x*hh<CR><LF>
_ _ _ _ _ course relative to water
_ _ _ _ course relative to water ID
_ _ _ speed relative to water
_ _ speed relative to water ID
_ NMEA 0183 header

3.0 Usage Notes
The output format described in this document is enabled by setting the DVL output format to PD11. Data will continue to be recorded to the internal recorder in PD0 format if the recording bit is set in the CF command. Note that the DVL will ignore the serial output bit in the CF command when PD11 is set. The PD formats are discussed more fully in the “What data structure should I use and why?” section of the DVL FAQ.
Note: to get valid water-reference data, BK1 must be set in the DVL.

Below is an example of a valid sensor data sentence showing a heading of 197.34&Mac176;, a pitch angle of -10.2&Mac176;, a roll angle of -11.5&Mac176; and a depth of 122.7m.

$PRDIG,H,197.34,P,-10.2,R,-11.5,D,122.7*7E<CR><LF>

This example shows a valid bottom-track sentence that contains range to bottom of 143.2m, a speed over ground of 1.485 m/s, and a course over ground of 192.93&Mac176;.

$PRDIH,R,143.2,S,1.485,C,192.93*17<CR><LF>

Here is an example of a bottom-track sentence with invalid or missing data.

$PRDIH,R,,S,,C,*05<CR><LF>

This last example shows a water-reference sentence that contains speed relative to current of 1.503 m/s and a course relative to current of 203.5&Mac176;.

$PRDII,S,1.503,C,203.5*55<CR><LF>

Will my vehicle affect the accuracy of the DVL compass?

The DVL uses a flux gate compass. Flux gate compasses are biased by materials with magnetic properties (such as iron) or by induced magnetic fields (such as motors). It is possible to calibrate out the effects of magnetic materials that do not produce high magnetic fields and remain in a fixed position relative to the DVL. Motors and moving magnetic fields cannot be calibrated out.

-top-

How do I select RS232 or RS422 communication?

An internal switch makes selection of RS232 or RS422 communication. You have to open the system to modify the selection.

-top-

Is the power line connected to the ground? Is the power floating?

The power is isolated from the communication lines. The chassis is isolated from both of these. There is fuse protection for both input power and the communication lines in case the cable is cut or shorted. There is no protection for user equipment connected on the other end of the cable.

-top-

Is the data line floating with respect to ground? Is it Opto isolated?

With respect to power and chassis ground there is complete isolation. Therefore to maintain isolation users should not share data common and power common. Sharing lines defeats the isolation.

In RS232 mode data common to data in is >2.8kohm. In RS422 mode data common is >20kohm except to data out which is >2.8kohm.

-top-

Is the communication circuit protected from shorting or over voltage?

The serial line fuses actually open up when they heat up and then they close again as they cool.

-top-

Do I get more data if I increase communications speed?

Typically, DVL users set one ping per ensemble (ping cycle including data processing). Therefore, a ping and an ensemble are the same thing. A ping should be thought of as 3 distinct parts:

1. Measurement cycle - that is, transmit and receive sound (variable)
2. Data processing (fixed)
3. and another part that transfers out this information from the serial port. (variable)

The collection of the bottom track data varies based on the distance to the bottom. The further off the bottom you are, the longer the ping takes. The time it takes to transfer the data is fixed by the amount of data to be sent and the baud rate. The Navigator can typically collect a bottom track data point and transfer the data once per second at 9600-baud rate.

One way to increase this rate is to speed up the baud rate. A typical ensemble size is 195 bytes for bottom track only. It will take about 0.2 sec to transmit at 9600 Bauds/sec and 0.01 at 115000 bauds/sec.

Note, the Navigator will not transmit another ensemble until it has finished the previous ensemble.

-top-

What framees of reference can I use for data (Data coordinate transformation)?

The Navigator can output data in

1. Beam coordinates, output velocities are along the beams.
2. Instrument coordinates, output velocities are converted to XYZ relative to the instrument (X points to beam 3, Z is along the vertical axis)
3. Ship coordinates, correct for the orientation of the instrument relative to the platform. (uses the offset specified with the EA command and applies tilts).
4. Earth coordinates, apply heading and tilt information to rotate to East – North – Up.

-top-

Do I need to align the DVL with my vehicle?

If the output of the data is in Earth coordinates then the alignment is not a concern because the internal compass and tilt sensors are tied to the instrument frame.

If however, the output is to be ship relative (i.e. platform X, Y, Z) data, then the exact alignment of the transducer must be known. Beam 3 is the reference for the DVL and knowing its alignment to “forward” on the vehicle is critical. This angle is set in the DVL through the EA command. The accuracy of this alignment will directly relate to the accuracy in the velocity data.

It should be noted that if you are using external roll, pitch or heading then misalignment angles (between Beam 3 and the external sensors frame of reference) must be considered and appropriate calibrations performed.

-top-

Does the Navigator output water velocity data?

In standard configuration the Navigator can output a single reference layer of water velocity (Water layer reference). However, it is possible to purchase an upgrade that will enable a firmware option that will collect a profile of the water column.

-top-

Can I apply an external time stamp on the data?

The DVL has his own real time clock. The DVL does not accept a clock input. However, it is possible to periodically set the clock. During clock setting the DVL data collection would be momentarily halted.

What is the timing relationship between the DVL internal clock and the time of the Doppler bottom tracking measurement?

The actual measurement of velocity occurs when the transmitted sound reflects from the bottom. We take our measurement from the midpoint of the reflected signal, so the time of the DVL measurement for practical purposes is one half of the transmit pulse duration plus the one way travel time to the bottom. The length of the transmit pulse is typically 30% of the two way travel time to the sea bottom, however it can be limited by the BX command setting. The travel time in water is dependant on depth and speed of sound in water.

The DVL accepts external NMEA0183 heading data up to 10Hz or uses an internal compass. The DVL takes the latest heading sensor data from the buffer just prior to time tagging the ensemble and transmitting a pulse. Therefore external heading data used in the output data string typically occurs some time before the DVL time tag.

The Doppler measurement occurs after the time tag as shown below:

1/ C Cos J ms/meter x (Altitude in meters + min (0.3 Altitude in meters, 0.015 of BX command in decimeters)).

Example 1: The BX command limits the size of the transmit pulse.

Altitude = 20 m,
BX command set to 30 m,
C (speed of sound) = 1471 m/s
J (Beam angle) = 30°

0.78 x (20 + min (0.3 x 20, 0.015 x 300))

0.78 x (20 + min (6, 4.5)) = 19 ms

Example 2: The transmit pulse duration reaches 30% of the two-way travel time before the BX limit is reached.

Altitude = 12 m, BX command set to 30 m, C= 1471, J (Beam angle)= 30°

0.78 x (12 + min (0.3 x 12, 0.015 x 300))

0.78 x (12 + min (3.6, 4.5)) = 12 ms

-top-

OPERATIONAL

What interference might I get between the DVL and the other acoustic equipment on my vehicle?

Interference from other acoustic devices can cause velocity errors. In extreme cases it can prevent the DVL from operating.

The DVL transmit bandwidth is 25% and the front end receive bandwidth (determined by the transducer) is 40% about the carrier frequency.

It is possible to avoid most interference by having an acoustic management scheme. If the primary, harmonics or sub-harmonics coincide between the DVL and other acoustic device, you will need to consider either synchronous or asynchronous triggering of these devices. Your individual scenario will determine whether to use synchronous or asynchronous triggering.

Consideration should be given to:

1. Primary frequency
2. Harmonics
3. Bandwidth
4. Length of Transmit
5. Repetition rate

-top-

What are the acoustic source levels for the Workhorse Navigator DVL ? 

The general formula for source level is:

SL = 170.8 + DI + 10*Log10(Acoustic Power).

For our systems, DI = 20*Log10(pi*(Diameter of the transducer ceramic)/wavelength).

Acoustic Power = Electrical Power * Transducer Efficiency

Below is a table of these values.

-top-

What are the maximum and minimum altitudes that my DVL will operate?

This is different for each frequency. It should be noted that maximum power output is only achieved at 60vdc input. However, the DVL is designed to achieve its specified performance (shown below) at 32vdc, 35ppt @10°C.

-top-

What are the maximum and minimum speeds that the DVL can detect?

Maximum velocity for the Workhorse Navigator is 10m/s (19.4 knots) with beam 3 pointing forward. However, by rotating the transducer head by 45° this can be increased to 14m/s (27.2 knots). RDI can provide custom units capable of detecting higher velocities on request.

Minimum velocity is zero, which allows you to use it for station keeping (hovering). The DVL is able to provide high-resolution data at very low velocities. At altitudes of less than 4m from the seabed a special low altitude bottom tracking mode turns on automatically.

-top-

What happens to the DVL output if I don’t have bottom lock?

For output data formats PD3, PD4, PD5 and PD6 the velocities will go to -32768 when bottom lock is lost. Please note that we have four beams, but only require three beams for a solution. Therefore if we lose one beam we can still calculate a solution. Also, note that the altitude data will go to zero should we lose bottom lock. The DVL will continue to operate and report data.

The DVL has the capability to track a water reference layer as well as the bottom. This capability allows the user to continue to navigate during limited bottom tracking outages. The BK and BL commands found in the Navigator technical manual control this facility.

-top-

How accurately can I navigate with my DVL?

Specifications are stated on the Workhorse Navigator data sheet. 

There are two parameters stated for velocity - Precision and Accuracy. 

• Precision (random noise) and
• Accuracy (repeatable bias, corrected by calibration).

The Precision figure is a statement of random error that averages out over longer periods.

The Accuracy figure can be interpreted as consisting of two independently varying components, and therefore the maximum error can be expressed as the RMS combination of the two components. Positional error, therefore, is the total RMS error value of the components of (velocity error * elapsed time).

Position Error = DVL Accuracy x Velocity x Travel Time ( = DVL Accuracy x Distance Traveled)
At zero speed, the velocity drift is ±2 mm/s

For example: DVL Accuracy = ± 0.4%, Velocity = 1 m/s and Travel Time = 1 hr

Position Error = ±0.4% x 1 m/s x 3600 s
So, after one hour,
Distance Traveled = 3.6 km
Position Error = ±14 m

-top-

Can I use the DVL around offshore structures?

Yes, a DVL operated around a structure is OK until the structure actually is illuminated by the sound transmission. Most times a vertical structure that is taller than the depth of the water being measured will not return a signal that can be used for the bottom tracking pulse and so will be ignored. This is because of the angle of incidence to the structure. However, if the structure is not vertical then it can return a signal. As long as the structure is not moving there should be no problems with the received signal during the bottom track detection.

A structure that is illuminated for the water reference layer (or speed through the water layer) will typically be ignored when it interferes with 2 or less beams. If 2 beams are ignored by the DVL then no data will be collected. However, if the structure interferes with 3 or 4 beams then it will cause the DVL to measure its speed relative to the structure.

-top-

How do I calibrate my DVL?

The only calibration that is required in a DVL is the internal heading sensor. This calibration is started through a RDI’s WINDOWS program and done while the DVL is installed in the vehicle. The vehicle will have to be turned a full 360 degrees at 2 different roll angles.

See also - How do I install, deploy and calibrate my DVL? in Mechanical section of this document.

-top-

Will it work in fresh water?

The DVL bottom track will work very well in fresh water, provided the bottom is within range of the DVL. Indeed bottom track range is increased in fresh water. The water reference layer (speed through the water measurement) will not work in fresh water that is completely without suspended sediment. The DVL requires that there be objects in the water reflecting its transmitted sound in order for it to make a measurement. Fresh water (drinking water quality) can be too clear for the water reference layer to succeed. However the 1200 kHz DVL work well in fresh water lakes and rivers. It is sometimes necessary to set the salinity correctly for speed of sound calibration.

-top-

What is the maximum seabed slope that I can still bottom track?

The Navigator is designed to cope with seabed slopes of up to 20º assuming the DVL is level.

-top-

Do sand waves affect my ability to bottom track and measure altitude?

The DVL locates and then confirms the bottom by looking for a sharp rise in the returned signal strength. Water typically returns very weak signals compared to the bottom and so this rise is easily detected.

The DVL will not necessarily know the difference between the true bottom or a sand wave. Since both will cause a sharp rise in the returned signal. Therefore, the DVL will measure the range to bottom as the range to the top (or bottom) of the sand wave. When this happens, the DVL will also measure the speed over the bottom as the speed of the vehicle less the speed of the sand wave in the direction of DVL motion.

-top-

Do I need to reconfigure each time power is applied?

The Navigator can store a user configuration (CK command) and will restart with the same configuration every time power is cycled. (If the Navigator is stopped by removing the power while pinging, it will restart pinging and output data next time power is applied.)

-top-

Will Bottom Tracking be a problem over a pipe?

Depending on how the beams strike the pipe there may be problems. When bottom tracking over a pipe we believe it would be best to have the DVL aligned so that beam 3 is forward. This means that 2 beams would be right over the pipe and the spread of the beams should make the other 2 beams hit the bottom. Of course this will depend on the shape of the pipe, size of the pipe, and the distance off the pipe. If the pipe is small or the system is farther off the pipe then beams 1 and 2 would hit the bottom. If the pipe is large or the system is close to the pipe then beams 1 and 2 could be hitting the pipe at an angle causing the signal to be reflected away from the DVL.

-top-

What is the maximum rate that I can ping?

Maximum Update Rate = 1/Minimum Ensemble Time

Minimum Ensemble Time = Total no. pings per ensemble x time per ping x data transfer time

where: Total no. pings per ensemble = (Water pings + Bottom Track Pings) per ensemble
Minimum time per ping = Nominal (vertical) range x 1.57 ms/m
Data transfer rate = Baud rate/(10 x number of bytes per ensemble)

-top-

What is the maximum pulse length?

The pulse length is 30% of the range to bottom.

For example, 30% of 200m = 60m yields a maximum pulse length of
(60*1.57 = 94.2 ms).
It should be noted that when the range to bottom equals or exceeds 50% of the BX command value entered then the transmit pulse length is limited to 15% BX.

The pulse length may be modified if necessary. However reductions in pulse length may effect the Navigators ability to reliably bottom track some seabed materials / conditions.

-top-

Is there an angular rate limit for the accuracy spec?

The effect of increased angular motion is to increase the standard deviation of each individual ping rather than effecting long term accuracy. Our broadband systems have extremely good performance on single ping precision due to our patented broadband processing and therefore this is not typically a concern.

In addition to the above, you should note that considerable levels of roll or pitch (>20 degrees) may result in the decorrelation of some pings. As we use narrow acoustic beams to achieve the highest possible accuracy, the transmitted signal may not be received back at the transducer if the angular motion is high (roll or pitch rate) combined with high roll / pitch angles. This effectively would reduce the number of bottom tracking pings that would be reported from the DVL. We use the BC command to set a correlation threshold in our DVL. We set a default (recommended by RDI), however this threshold may be overridden (reduced) if you expect increased motion.

-top-