April
2018
Pathfinder DVL Guide
Page 32
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Short Term Performance
Short-term errors are random errors with short correlation times that quickly average to zero. They are
characterized by the standard deviation. The short-term error in each beam is independent of that in
every other beam. Except at very slow speeds, the short-term error in each ping is independent of that of
every other ping. The correlation distances at very slow speeds are not accurately known. As the name in-
dicates, the short-term error will reduce the accuracy of instantaneous velocity readings but over larger
intervals of time, these errors will not affect the average velocity reading or navigation.
Bottom Track
The standard deviation of the bottom track velocity measurement varies with altitude and can vary with
velocity. The default modes have two primary sub modes, one for shallow water at low speed (
) and one for other cases (Bottom Mode 9).
The Pathfinder standard deviation performance depends on altitude. It generally decreases as the altitude
gets greater and increases as the altitude gets lower. At the lower altitudes, it increases inversely with alti-
tude. Below these altitudes, the systems attempt to change modes and if successful, the standard deviation
drops considerably as the Pathfinder flies below its deep switching altitude and then again when it flies
below its shallow switching altitudes.
Water Track
The standard deviation of the velocity measurement for Water Layer will vary as the size of the Water
Layer varies – see the descriptions for the
to determine the Water Layer size and the fol-
lowing equation:
meters
10
Size
Layer
Water
shown
Dev.
Std.
Dev.
Std.
=
Pathfinder DVLs have the capability to compute velocities off of a water track layer while Bottom Track-
ing. The commands for controlling this mode are
for Pathfinder DVLs. However, there are
some important consequences of enabling this mode that will be discussed.
First, the time required to complete a ping is significantly increased because two separate and inde-
pendent pings are involved. TRDI does not currently have ping time models for the Water Track ping.
If required, the times would have to be obtained experimentally.
Secondly, there is the opportunity for ping-to-ping interference when the Water Track is enabled. After
transmitting an acoustic pulse, the Pathfinder DVLs open their receivers and listen for a return (either
from the bottom for a Bottom Track ping or from the water column for a Water Track ping). The return
MUST be from the particular ping that is transmitted. If it is from a previous ping, interference results
and the system might not work at all, or worse, might give erroneous results.
For example, on a Bottom Track ping, a return from the bottom from the previous Water Track ping
might look like the return from the Bottom Track ping, but at the wrong altitude. The problem is usually
worse when using Water Track and the previous Bottom Track ping is the cause of the interference. The
return from the Bottom Track ping is usually much larger than that from water scattering and when the
operating altitude is low, that return can make several round trips from the bottom to water surface to the
bottom again and still be large enough to cause interference with the Water Track ping. TRDI recom-
mends that the time between pings be set so that there is enough time for each ping to travel four
round trips to the bottom and back when the altitude is significantly less than the specified operating alti-
tude. This time can be calculated by doubling the altitude and then dividing by the Cosine of the Janus
angle (normally 30°) and then dividing by the speed-of-sound and multiplying by four.
SoS
)
cos(
altitude
8
Time
Ping
Minimum
⋅
⋅
=
J