Page 58 of 68
Doc. P/N: WSP-009-002
Version: V2.3
Issue Date: July 2014
Installation Manual
www.wassp.com
Sea Trial - Commissioning Step 15 : GPS Time Delay (Required)
Most GPS sources used in conjunction with WASSP will have a significant delay
between when the ship passes through a position and that position is sent on the
serial port. This delay will be in the order of 1 second. This means a 10 knots an
object will move 10 metres if passed in opposite directions at this speed. This will
make mapping interesting objects very challenging.
For an SC50 this delay is about 1.2 seconds, and the SC30 appears to have a
value closer to 1 second. The procedure below explains how to calculate this
delay value.
To do the following tests you will need to find a distinct
feature
on the sea-floor
which is clearly identifiable, a large rock, a sharp bank, a pipe, a wreck or trench.
Test 1: For use with standard GPS
Approach this feature from one direction and then the exact opposite direction
at a constant known speed (SOG) that is fast but good for mapping e.g. 8 knots.
If the object moves then the GPS time delay value is incorrect, if not then the
delay value is good. Measure the distance the object has moved on the contour
display. It may help to disable the mapping until just after the object is passed to
prevent overwriting the first instance. Alternatively the two passes can be stored
in different databases using an external application like Olex or Navigator. If the
object appears earlier than the previous pass then the time delay is negative
(which will only happen if the value for GPS delay was positive at the time the test
was undertaken. The formula below outlines this process, the delay adjustment
is added to the current
GPS Time Delay
value on the
Ship Setup Sensor Values
Tab.
Some GPS sensors
can smooth the out-
put introducing more
than 15 seconds
delay! Remove GPS
Smoothing if the GPS
has this option.
Especially in shal-
low water calculating
these offsets will be
easier using WASSP
Navigator or another
External Package like
Olex or GECDIS due
to the 1m limit of the
mapping resolution
Test 12 and 13 can be done by recording a single pass of data for each test and then replaying the
recorded data files with modified parameters for GPS Delay, Heading Offset and Pitch Offset. It may
help to delete the Contour Maps created each time these tests are re run or the smoothing will probably
interfere with measurements. See Step 10 below for information on how to delete the Contour Maps.
(+ late / - early ) 2 x delta
(fast speed in knots) - (slow speed in knots)
Test 2: For use with DGPS
The advantage of this approach over the previous test is it will eliminate pitch
errors however the displacement of the object will be smaller and this will be
difficult to measure when using WASSP alone as the displacement will be smaller
and thus harder to measure on the 1m grid.
Approach the distinct feature from the same direction at two vastly different known
speeds, as close to zero and at the fastest mapping speed (this will eliminate any
Pitch errors). If the object moves by delta metres further along the vessel track
(+ve) at a faster speed the adjustment to the time delay will be:
Delay adjustment =
(+ late / - early ) change in position in metres
speed in knots used in both directions
19m at 21
knots gives
0.9s delay
delay adjustment =
