Parallel indexing on the radar

What you do on the bridge

1. Set the PI line
2. Watch the echo
3. Regain the track and allow for the set

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. What is parallel indexing, and why use it?Often asked

   A good answer covers:

   • A line on the radar parallel to the planned track, offset by the planned passing distance of a radar conspicuous mark
   • If the mark's echo runs along the line, the ship is on track; if it moves off, she is being set
   • It shows cross-track error at a glance, continuously, without plotting fixes
   • It is independent of GNSS: a check on the ECDIS position, and a back-up if GNSS fails or is spoofed

   Show me on the bridgeThe coast is about two and a half miles to starboard. Where would you put the PI line?

2. How do you plan and set up PI lines for a passage?Often asked

   A good answer covers:

   • At the planning stage, pick a mark that paints well and is charted accurately: a headland, a light structure, a racon
   • Measure the beam distance from the track to the mark on the chart, and mark it on the plan
   • On the radar, set the line's bearing to the track course and its offset to that distance, on the right side
   • Use a north-up, true or relative motion, stabilised display; check the gyro error, since it turns the lines
   • Add wheel-over and abort lines, and limiting lines for the safe water each side
3. What are the limitations and traps of parallel indexing?

   A good answer covers:

   • Indexing off the wrong echo, especially on a featureless or low coast
   • Gyro error or a wrong heading input shifts every line
   • Lines set relative to the heading are wrong after an alteration unless they are reset
   • It shows where you are against one mark only: still cross-check by other means

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're heading south down the New Jersey shore at ten knots. The coast is about two and a half miles to starboard, and our track is due south.

   We'll put a parallel index line on the radar along the coast. If the coast runs along the line, we're on track. If it moves inside the line, we're being set towards the land.

2. Set the PI line

   Measure the range of the coast on the beam with the VRM. Then set PI line one: bearing 180, our course, offset that range to starboard. Or press Set and click on the coast.

   Use true bearings, so the line stays put if we alter course.

3. Watch the echo

   Now keep an eye on where the coast echo meets the line. It's creeping inside it: the land is getting closer than we planned. Something is setting us west.

4. Regain the track and allow for the set

   Alter to port to bring the coast back out to the line. Once it's back on the line, don't just steer 180 again, or you'll be set inside it once more.

   Steer a few degrees to port of the course, about 174, so the coast runs along the line. That is the set allowed for.

The rules and guidance behind it

• MGN 379 (M+F) Amendment 1: use of electronic navigational aids