March 2018
rapidCAST User’s Guide
Page
50
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
If Reject sentences with bad NMEA checksums is checked, the rapidCAST Interface will calculate
the checksums of each message received, and ignore sentences with invalid checksums, thus ensuring that
the reported depth readings have not been altered by communication noise.
However, it has been discovered that some depth sounders (which otherwise operate correctly in every
other respect) occasionally miscalculate the checksum value and provide bad checksums regardless. Some
transducers have also been encountered that do not provide checksums at all, in contravention to the
standard. If a user encounters this situation and wants to make use of the provided messages anyway, this
setting should be unchecked.
Message to use - Users should specify the expected message type (DBT, DPT, DBK, or DBS) to use for
extracting depth information. If more than one of these message types are present in the NMEA stream,
the user must choose one to use (ideally the message that gives the most reliable depth readings).
Auto Depth Status
Auto Depth Status informs the operator of Auto-Depth state.
Possible status messages are:
•
Channel Closed: The NMEA communication channel is closed and not receiving messages.
•
Channel Open: The NMEA communication channel is open and waiting for messages, but no
valid messages have yet been received.
•
Timeout: The allotted time to receive a new NMEA message has expired, causing Auto-Depth
and Auto-Repeat to be deactivated.
•
Ready: Valid NMEA messages are currently being received and can be used to update the Nomi-
nal Target Depth if the user chooses to enable Auto-Depth.
•
Paused for Deployment: During probe freefall, this indicates that Nominal Target Depth is
intentionally kept frozen to inform the user of the intended depth for the current cast.
•
Error: An unexpected communication error is preventing NMEA messages from being read.
•
Bad NMEA Checksum: Appears only when Reject sentences with bad NMEA checksums
is checked. This is reported if a checksum mismatch has been detected for the latest NMEA mes-
sage, causing it to be rejected.
•
Last Depth Timestamp reports the PC clock time when the last depth reading was received.
This should continually update while depth data is streaming. If this timestamp has stopped, valid
depth values are no longer being received, and if stopped for long enough, a Timeout condition
will result.
Using Coverage
In the ideal case, a properly-fitted dive table ensures that the probe will end up slightly shallower than its
target depth (i.e., the possibility of the probe impacting the seafloor is minimal). With an optimal dive ta-
ble, there is reasonably good agreement between the planned target depth and the actual depth achieved
by the probe during each cast. In this ideal situation, no adjustments are necessary.
If operating conditions or probe drop behavior changes drastically, the dive table may no longer be relia-
ble, and the probe may end up significantly deeper or shallower than the intended depth. The ideal rem-
edy is to redefine the dive table to match current conditions, but collecting data for a new dive table takes
time and effort.
As a crude alternative, adjustments can be applied to the Nominal Target Depth, increasing or decreasing
it as necessary, to reduce the gap between intended depth and actual depth.
