Navigation NAU 102 Lesson 21

Piloting Determining the position of the vessel by visual reference to landmarks, by measurements of depth or by radar. – Dutton’s Piloting involves navigating a vessel in restricted waters and fixing its position as precisely as possible at frequent intervals. – Bowditch

Piloting The task of piloting is to determine (“fix”) your position at a certain time. A Line of Position (LOP) is the building block of a fix. LOP – a line along which you know you are located.

Piloting

LOP’s can be obtained: Visually With Radar By taking Soundings Loran-C Radio Direction Finders

Visual LOP Using a bearing or azimuth circle or a pelorus. The bearing of a charted object is observed and the time noted. An LOP is drawn on the chart to the observed object.

Visual LOP Draw the line of the correct bearing to the object on the chart. Label the LOP with the time of the bearing. No need to draw a lengthy line

Visual LOP Visual Range Line Two charted objects are sighted in line. Draw an LOP through the objects, extended towards your probable position. No need to note bearing.

Visual LOP

Radar Range LOP Distance to a charted object is determined on radar. An arc of a circle centered on the object is drawn on the chart. Radius = distance

Radar Range LOP Arc is labeled with the time the range was determined. No need to draw the entire circle

Radar Range LOP

Radar ranges are more accurate than radar bearings. If possible, use visual bearings vice radar bearings.

Soundings Compare charted depth (corrected for height of tide) with echo sounder reading. A depth contour can be used as an LOP.

Soundings

The Fix A fix can be obtained where two or more LOP’s, taken at the same time, cross. Your Fixed Position (“Fix”) is your known location at a point on the earth at a specific time. Sometimes referred to as a “Simultaneous Fix”.

The Fix 1000

The Fix

Minimize positional errors caused by a bad LOP Three (or more) LOP’s are better than two.

The Fix Where am I?

The Fix Option 1 – In the center of the triangle..

The Fix Option 2 – In the most dangerous position.

The Fix Ideally, we’ll get a 3-point fix.

Fix Geometry If using 2 LOP’s, navigation aids should be approximately 90° apart. To ensure accuracy and minimize errors, we must choose our navigation aids carefully. Ideally, 3 navaids would be separated by 120°, or 60° if all within 180°.

Poor Fix Geometry

Good Fix Geometry

The Fix Unless you have several people on your navigation team, this is impossible. To be truly “simultaneous” all LOP’s (bearings and/or ranges) must be shot at exactly the same time. To minimize errors, first take the bearing or range that is changing the quickest.

The Fix Ranges – at fix time, mark the range of the navaid ahead or astern. Bearings – at fix time, take the bearing of the navaid closest to the beam.

Introduction to Navigation Questions?