What is wrong with Ground Stabilisation?

Captain Peter Starkey

NORWEGIAN DREAM& EVER DECENT

REPORT

To all Bahamian Shipowners:

To determine risk of collision using ARPA vectors, only the relative vectors should be used.

True vectors should be used to determine aspect

NORWEGIAN DREAM& EVER DECENT

REPORTTo all Bahamian Shipowners:

speed input for an anti-collision plot on radar/ARPA should always be:

speed through the water

not speed over the ground

The Relative Plot&

Relative Vector

Independent of input error

“Instant” assessment of risk of collision

CPA

TCPA

The True Plot&

True Vector

“ASPECT”

THE TRUE PICTURE – THE SITUATION

TRIAL MANOEUVRE

ASPECT

The International Regulations for the Prevention of Collisions at Sea

THE TRUE PICTURETHE TRUE SITUATION

Situation Awareness

TRIAL MANOEUVRE

Knowledge of Target Data required to calculate trial manoeuvre

But………

All true functions are dependent on sensor input and sensor accuracy

and are related to the stabilisation

Sea or Ground Stabilisation

Does the choice of stabilisation matter?

ASPECT

Visual aspect - heading

Radar aspect

sea stabilised - approximate visual aspect

ground stabilised - may be very different

Does radar aspect matter?

Clear visibility - look out of the window!!

Restricted visibility - vessels not in sight

Rule 19

What is aspect in Rule 19?

Relative Vector

Own Ship

RELATIVE VECTORS SEA OR GROUND STABILISED

Heading

Target

Current

Heading – Visual AspectTrue Vector – Radar Aspect

Current

Own Ship

Heading - True Vector

Target

SEA STABILISEDTRUE VECTORS

True Vector – Radar Aspect

Current

Own Ship

GROUND STABILISEDTRUE VECTORS

Heading

Target

True Vector – Radar Aspect

Own Ship

GROUND STABILISEDTRUE VECTORS

Heading

Target

The Practical Effect

Current

Own Ship

Heading

CHANNEL

Slow speed target

GROUND STABILISEDTRUE VECTORS

Current

Own Ship

CHANNEL

Slow speed target

RELATIVE VECTORS SEA OR GROUND STABILISED

Heading m

arker

Current

Own Ship

Heading

CHANNELSlow speed target

SEA STABILISEDTRUE VECTORS

Current

Own Ship

CHANNEL

Slow speed target

GROUND STABILISEDTRUE VECTORS

Current

Own Ship

Heading

CHANNEL

Slow speed target

SEA STABILISEDTRUE VECTORS

COLLISION AVOIDANCE

SEA OR GROUND

RELATIVE OR TRUE

SEA STABILISEDTRUE VECTORS

CURRENT

TARGET

OWN SHIP

SEA STABILISEDTRUE VECTORS

CURRENT

TARGET

OWN SHIP

RELATIVE VECTORS

CURRENT

TARGET

OWN SHIP

Heading marker

GROUND STABILISEDTRUE VECTORS

CURRENT

TARGET

OWN SHIP

Own Ship

RELATIVE VECTORS

North Up

RM (TT)

Ground Stabilised (GPS)

Heading 350°

CMG (GPS) 355°

Channel 355°/175°

HL

Current 090° 2 kts

Own Ship

TRUE VECTORS

North Up

RM (TT)

Ground Stabilised (GPS)

Heading 350°

CMG (GPS) 355°

Channel 355°/175°

HL

Current 090° 2 kts