
Pathfinder DVL Guide
April
2018
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Page 33
When operating at altitudes approaching the altitude capability of the DVL, the ping time should be:
Minimum Ping Time > 1.5 times the round trip time at the maximum specified operating altitude.
The long-term and short-term accuracy characteristics of the Water Track mode are discussed in the re-
spective accuracy sections of this document.
Altitude Performance
The altitude (distance above the bottom) capability of TRDI DVL’s is influenced by:
•
Bottom type
•
The average temperature and salinity of the water column from the DVL to the bottom
•
The voltage supplied to the system
•
The attitude of the DVL
•
Bottom slope
TRDI specifies this altitude capability for:
•
A typical bottom type being level
•
A temperature of 5° C, a salinity of 35ppt
•
An input voltage of 24v for Pathfinder DVLs
•
±5° vehicle pitch or roll
Changes in these values will affect the altitude capability as discussed below.
Bottom Type & Slope
Robert J. Urick (3
rd
edition, 1983) shows curves of backscatter strength vs. bottom type, frequency, and
grazing angle. This analysis shows that the scattering strength of bottoms can vary from approxi-
mately -10dB for bottoms of “sand and rock” to -40dB for “muddy” bottoms. A higher value (less nega-
tive) of backscatter strength means that the reflected signal from the bottom is larger and that the
DVL will operate to greater altitudes. TRDI uses a reasonably conservative value of -28dB for the pre-
diction of a DVL’s altitude performance. While our experience shows that this works in most environ-
ments, bottoms that are “bright” (some rocks and/or gravel) will allow the DVL to operate to
higher altitudes while bottoms that are “dull” (mud) will limit operation to somewhat lower altitudes
than are specified. In general, a 1dB change in backscatter strength will affect the altitude capability of a
DVL by approximately 4m and 2m for 300 kHz and 600 kHz Pathfinders respectively. Larger (less
negative) values increase the altitude capability. Bottom slope also strongly affects the return.
Temperature & Salinity
The transmitted acoustic pulse from a DVL is attenuated as a function of range due to a phenomenon
called absorption. Absorption is caused by the conversion of some of the transmitted acoustic energy be-
ing converted to heat while traveling through the water. This lowers the available energy for the detection
and processing of the reflected pulse from the bottom and affects the DVL’s altitude capability. This ab-
sorption loss is influenced through complex physics by the frequency and the average temperature and
salinity of the water column.
The 600 kHz systems are minimally affected by temperature and salinity changes. The 300 kHz system is
slightly affected by an increase in temperature and salinity. Warmer temperatures continue to degrade its
performance. Operation in fresh water enhances a DVL’s altitude capability; in general low-salinity waters
will increase the operating range of the unit.