$DVEXT: DVL Extended Data

The $DVEXT message is output at a variable rate between 5Hz and 20 Hz.

Format:

$DVEXT,v,g,abcd,r.r,p.p,h.h,k,u.uu,t.tt,n.nnn,e.eee,lat,long,e.eee,qw,qx,qy,qz,ga,gb,gc,gd,la,lb,lc,ld,va,vb,vc,vd,ra,rb,rc,rd,*hh

Field	Name	DVL Notes
v	DVL Lock	“T” DVL has lock on bottom, “F” DVL searching for bottom
g	GPS status	“A” fresh GPS fix, “V” invalid GPS fix, “X” stale GPS fix
abcd	Baseline IMU calibration status, undefined for IMU upgrade	a=system status digit, 0 to 3 b=gyro status digit, 0 to 3 c=accelerometer status digit, 0 to 3 d=magnetometer status digit, 0 to 3
r.r	Platform roll	Degrees, Vehicle Frame
p.p	Platform pitch	Degrees, Vehicle Frame
h.h	Platform heading	Degrees; range 0 - 360, Vehicle Frame
k	Data skips	How many attempts made since last successful ping returns
u.uu	Velocity up	Meters/second, positive up, do not rely on this number
t.tt	Altitude	Meters to target surface along sensor pointing axis
n.nnn	Velocity north	Meters/second, positive when heading north, expect noise
e.eee	Velocity east	Meters/second, positive when heading east, expect noise
lat	Latitude	Platform latitude in Decimal Digit (DD) format. If fresh GPS fix available, GPS value is used. If no GPS fix and DVL valid, DVL value is used. Otherwise, it’s the last computed position.
long	Longitude	Platform longitude in Decimal Digit (DD) format. If fresh GPS fix available, GPS value is used. If no GPS fix and DVL valid, DVL value is used. Otherwise, it’s the last computed position.
e.eee	Elapsed time	Seconds since previous EKF step
qw	Quaterion W	Relative to sensor head; range -1 to 1; east=0, level=0
qx	Quaterion X	Relative to sensor head; range -1 to 1; east=0, level=0
qy	Quaterion Y	Relative to sensor head; range -1 to 1; east=0, level=0
qz	Quaterion Z	Relative to sensor head; range -1 to 1; east=0, level=0
ga	Channel A gain	In dB, range 6 to 66 (Ch A is Port sensor)
gb	Channel B gain	In dB, range 6 to 66 (Ch B is Stern sensor)
gc	Channel C gain	In dB, range 6 to 66 (Ch C is Starboard sensor)
gd	Channel D gain	In dB, range 6 to 66 (Ch D is Bow sensor)
la	Channel A lock	T or F, getting valid return from the bottom
lb	Channel B lock	T or F, getting valid return from the bottom
lc	Channel C lock	T or F, getting valid return from the bottom
ld	Channel D lock	T or F, getting valid return from the bottom
va	Channel A velocity	In meters/second
vb	Channel B velocity	In meters/second
vc	Channel C velocity	In meters/second
vd	Channel D velocity	In meters/second
ra	Channel A range	In meters
rb	Channel B range	In meters
rc	Channel C range	In meters
rd	Channel D range	In meters
*	End of fields
hh	Checksum	NMEA standard

Notes on fields:

Look at DVL lock status before using altitudes, velocities, or ranges. If not locked, these values may be bogus.

See the section on baseline IMU calibration for an explanation of IMU statuses.

The GPS status field indicates if the GPS fix is fresh enough to use. A fix is “fresh” for 1.5 seconds after receipt AND it has a data status field of “A”. A GPS $GPRMC fix can come in the optional GPS serial port, or the Ethernet or serial command interface. A SET-POSITION command counts as a GPS fix and is fresh for 1.5 seconds. The GPS fix is invalid if the message’s data status field is “V”. If no fixes have been received for 10 seconds the GPS is considered unconnected.

See Appendix on coordinate frames for definitions of roll, pitch, heading, and quaternion.

The Velocity Up and Altitude fields are outputs of the sensor fusion process and are optimized for the DVL process and not for maneuvering or control. The Velocity Up is more like the velocity at which distance is changing between the DVL and the bottom, as the bottom undulates and the ROV shifts up and down, which of both affect Up/Altitude. Altitude is the approximate distance to the reflecting surface along the sensor’s pointing axis, even if the sensor is pointed sideways or upward. Altitude is also affected by the flatness of the bottom and other factors like submerged vegetation. The DVL altitude mechanism is optimized for finding coherent signal returns. Don’t rely on Altitude as a sounder substitute. For sounding, use something optimized for true bottom finding directly below.

Elapsed time is intended to be used if you are integrating your own positions.

Quaternion is defined in the Appendix on coordinate frames.

Sensor range and velocity are defined along the sensor beam axis for the four sensors. These values are unfiltered. See the DVL head mounting diagram for bow, stern, starboard, port. The channels are mapped as follows: Channel A = Port, Channel B = Stern, Channel C = Starboard, Channel D = Bow.