Torm Shipping chooses VSAT for entire fleet - Digital Ship

ELECTRONICS & NAVIGATION
Balancing technology and integrity
T
Improved technology can reduce the number of errors in navigational accuracy - but does it also reduce awareness
that the potential for errors still exists? Dr Andy Norris looks at the issue of navigational integrity
he phrase integrity monitoring is
increasingly used in the navigation
world. It has been commonly
applied to processes within advanced electronic
navigational equipment but is now
being used to describe essential aspects of
the human involvement in navigation.
Integrity monitoring within equipment
alerts the user if the navigational accuracy
has become suspect, and that the system
should be used with caution or not at all.
For instance, IMO requires all GPS
units to have Receiver Autonomous
Integrity Monitoring (RAIM) to immediately
detect and inform the user if the
positional accuracy has been compromised
by a signal from a faulty satellite.
Also, Integrated Navigation Systems
(INS) conforming to IMO requirements
monitor integrity by evaluating inputs
from several sources and giving timely
alerts if a problem is detected.
In the past, when navigation was a
mainly manual task, mistakes were easy to
make and were therefore common, constantly
reminding the mariner of the need
for validation of all navigational processes.
Today, because of the increased use of
electronics, navigational errors have significantly
reduced to a point on many
ships where there is no regular reminder
that things can go wrong.
The increasing reliability and effectiveness
of equipment tends to give a false
sense of security. However, we are a long
way from ship's navigation equipment
being able to be considered as intrinsically
reliable.
Because the human is becoming far less
involved with the detailed process of
establishing the navigation solution, it
perhaps becomes too easy to accept what
is seen on a display. As ECDIS replaces the
use of paper charts, this becomes an even
greater problem.
It is therefore essential that the OOW
continues to maintain a constant check on
the integrity of the displayed navigational
situation.
Manual checking
Manual checking of integrity firmly links
the user into the navigation process and
exposes problems before a failure leads to
a dangerous situation.
Integrity checking is not just a matter of
confirming position. It also needs to cover
all navigational data, including own ship's
speed, heading and course, the movement
and current position of other vessels significant
to navigation, and the proximity
of charted features and hazards.
Establishing integrity is all about checking
the perceived navigational situation
with reality. Fundamentally, therefore, it
acts to significantly improve overall
awareness.
Although manual integrity checking is
based on establishing whether there is a
fault in any equipment or system, its spinoff
benefit in improving situational awareness
is of even higher significance, simply
because most collisions and groundings
are caused by inadequate situational
awareness.
Positional integrity can be enhanced in
coastal waters by taking visual bearings
on conspicuous charted objects, transferring
the bearings to the chart and checking
that appropriate tie-up has been obtained.
It is essential to manually check the integrity
of data from navigational technology
Traditional three point fixes can be
taken but frequent single bearings can be
even more useful in many circumstances,
especially on ECDIS. The latency can be
very short and therefore even relatively
small errors in the electronically derived
position can become apparent from a good
visual fix.
Transferring radar conspicuous ground
fixed objects to the chart, again very easy
to do if using ECDIS, will identify positional
inaccuracies. These could be due to
problems in the positioning system, the
radar or the gyro. The most likely error
source is easy to determine from a series of
measurements if an initial discrepancy is
found.
The echo sounder output should also
be used as a consistency check with charted
data. Differences may indicate positional
problems, faulty equipment, an
inaccurate chart or a failure to compensate
properly for tidal difference.
For ocean waters, the options for monitoring
positional integrity have been quite
limited. However, a reasonably effective
method is to compare the current GPS
position with an estimated position based
on the GPS position of an earlier time.
It is necessary to include leeway, tidal
streams, ocean currents and surface drift
but the act of determining these adds
greatly to situational awareness.
Of course, the accuracy of such an estimated
position will not match that of GPS
Digital Ship Ship December 2008 page 30
but it will indicate gross errors and alert
the bridge team to the take appropriate
cautionary actions.
Celestial positioning is also a possibility
for those wanting to show off their skills
but in many areas of the world the poor
availability of sights makes this a very
unreliable method for regular verification
of GPS position.
GLONASS
Greatly improved positional integrity
monitoring of both ocean and coastal
route segments is possible using the
GLONASS satellite navigation system. It
now has 17 operational satellites, giving
good accuracy over a considerable proportion
of the globe.
The present coverage is certainly good
enough to justify fitting it to SOLAS vessels
as a position sensor independent of
GPS. Over half of the present satellites
have been launched since the beginning of
2007 and the expectation is that truly global
coverage is achievable in relatively
short timescales.
Fitted equipment should be certified to
meet IMO standards but these may be difficult
to procure at present. Non-approved
systems may interfere with other bridge
navigational equipment and may not meet
all IMO requirements.
Ideally, there should be an automatic
integrity monitoring process continuously
comparing the GPS and GLONASS positions
and generating an alert if the position
differs by more than a user-set
amount.
Despite automatic checks, the OOW
should still perform manual integrity
checks on the actual difference and, in
coastal and harbour waters, continue to
check position visually and with radar,
particularly to gain the benefits of
increased situational awareness.
By the middle of the next decade GPS
and GLONASS will be joined by other
satellite navigation systems, such as
Europe's Galileo and China's Compass.
It is also possible that eLoran will be in
use in some coastal areas by then. It has
the advantage that its failure mechanisms
are very different to that of satellite systems,
further increasing the quality of
integrity monitoring.
Detection of other
vessels
Comparison of visual, radar and AIS data
should be made on all vessels that are, or
may become, significant to navigation.
Consistency in all three methods gives
great confidence in the integrity of the
information concerning that target.
Differences should ring mental alarm
bells.
Correlation in two out of the three
methods, providing the prevailing circumstances
are consistent with any inaccuracy
or unavailability of the third, should also
give the OOW good confidence in the
integrity of the plot.
However, all three main methods of
detecting vessels and other floating objects
have common problems.
For instance: visual data is easily
impaired in poor conditions; radar data is
considerably affected by sea clutter and
rain; not all vessels have AIS fitted and on
those that do erroneously transmitted data
could affect the perceived position of the
target and create other anomalies.
If there is only one indication of a target
from the three possible sources, great care
is needed in assessing the necessary
required action, as the indication could be
erroneous. However, in general, it is likely
to be valid - almost certainly in many conditions,
if visual - and appropriate avoiding
action should be taken.
Consistently missed radar targets could
indicate a radar equipment fault or that it
is badly set up. If many ships are not
exhibiting AIS data or are wrongly displayed
it is likely to indicate a fault with
own equipment, which should be urgently
checked.
A poor antenna connection is a
common cause of problems with displayed
AIS targets. If so, it is likely to
mean that own-ship's transmissions will
be compromised.
Unfortunately, most ships cannot yet
display AIS data on radar, making proper
comparison an unwieldy task. However, a
number of specific checks during a watch
that compare specific AIS targets from the
MKD with radar data are still worthwhile,
at least to check for possible ownship
equipment errors.
As technology gradually improves
there is likely to be a time when the navigation
solution does not need to be
checked by the OOW for integrity.
However, until we enter an even more
distant point in time when technology
could completely take over, the human
OOW will always need to retain situational
awareness in order to make appropriate
navigational decisions. Continuing to
check integrity is an excellent way of helping
to achieve this.
DS
Dr Andy Norris has been well-known in the maritime navigation industry
for a number of years. He has spent much of his time managing high-tech
navigation companies but now he is working on broader issues within the
navigational world, providing both technical and business consultancy to
the industry, governmental bodies and maritime organizations.
Email: apnorris@globalnet.co.uk