Hi Mike,

Unfortunately I didn't save the MS word doc which caused the problem when I shut down last night and didn't get your message until this morning. I tried copying the original back from the thread and posting that but as you will have seen above, that worked OK. I will try creating a new document of about the same length and containing the same type fonts etc and see what that does. To other 'Catters please excuse the non sequiter that this will create, I'm afraid I'm not so dedicated as to write over a page on nothing off the top of my head, so I'll copy in another document.

Mike I'll make sure I save this just in case the test works ie it fails to post, repeat the procedure I used last time and try and do a compare and a hex dump on the two clipboard contents. Of course if it fails, the text posts, I suppose we will have to put it down to gremlins.

Perhaps that's worth mentioning though,

Test post unrelated to anything else follows.

Use of Radar

How it works

RADAR is an acronym for RAdio Direction And Ranging.

Radar sets operate by sending out a focused beam of radio waves, and capturing any signals reflected back to the set. The quality of the signal reflected will depend on the: Size, The bigger something is, the easier it is to see at a given distance Composition Something that reflects well, will be easier to see Orientation, Something square on to the observer is easier to see than something angled away, Texture A rough surface scatters light and radio signals whilst a smooth surface gives a good echo - ever seen a mirror with a rough surface? Shape A curved surface, or one which is at an angle to the beam, will reflect the beam away from the receiver.

Uses

1. Ranging

Radar uses the time taken for the return signal to be received to calculate the distance of the target, and since the speed of radio waves is precisely known, radar ranges are very accurate.

2. Direction finding

If you illuminate something, it is a fundamental law of physics that you cannot see anything smaller than the wavelength of the illuminating beam. Although there are still some sets around which operate in the metre wavelengths, most sets we are likely to come across operate in the centimetre band so definition is quite good. However there are a whole series of problems which degrade the signal to the extent that Radar bearings are nowhere near as accurate as ranges.

Display

As far as the radar set is concerned, it is the centre of the universe, and the screen reflects this by having the set (and the vessel it is mounted on) at the centre. Any echoes received are shown as a blob on the screen, the size and shape of the blob being determined by the factors noted above rather than the reality of the target.

The only information that we can deduce about a target from the radar screen is 1 It probably exists. 2 It's distance from us when the echo was received 3 Its bearing relative to the object on which the set is mounted.

These can be interpreted as follows:

1. Existence. If an echo is present for several sweeps, it is reasonable to presume that something solid is out there. What we cannot deduce is anything about its size or composition. We have been reminded recently that sails are not good reflectors of radio waves, and a 40,000 ton vessel will have a bigger echo than a yacht, but a 6000 ton destroyer may not!

2. Distance. This is obtained by moving the cursor or range ring to intersect the echo. It is very accurate, but only true for the time of that sweep!!

3. Bearing. This is also obtained by moving the cursor to intersect the target, is again only true for the time of the sweep, and it is relative to the transmitting vessel!!!

Interpretation of display

The first thing to remember is that radar is a decision making aid, and that the information it provides should be interpreted in conjunction with all other aids available to you, including the eyeball Mk1 and charts. A radar display, can only show you what has happened, a radar plot enables you to make an informed judgement about what will happen.

Range

It is impossible to tell from looking at a radar display whether a target shown is one cable or ten miles distant. If someone has changed the range setting since you looked at the set last, it is possible that the display will present the same picture, and a potentially dangerous situation be missed until it is too late. So, check the range setting before you do anything else!

A quick rule of thumb, is that an echo which remains on a constant bearing is on a collision course. This is fine for single, slow moving targets, at long range, but if any of these conditions are not met, a more rigorous approach to interpreting the data is required. Also, the converse, that an echo which does not remain on a constant bearing is not on a collision course is not true.

Plotting the tracks of the targets.

Relative bearings

It is essential to remember that ranges and bearings are relative. So the first thing which must be done is to establish your own vessels position and track.

To plot the position of the echo:

Establish your current position. (It is normal good practice to maintain a plot of the ships course, with fixes every 20 minutes, so at the very worst it will be possible to extrapolate your current track by the distance run since the last fix. Of course a new fix would be better.)

Establish your heading. This can be either derived from the track by plotting tidal influence, or converted from the compass bearing to true. Plot the echoes bearing, relative to your heading. Strike an arc at the range of the echo to intersect the bearing.

Example.

You are heading 094o T and the radar bearing of the echo is 054o R The true bearing of the echo is 094o + 054o = 148o T.

Plot all echo's of interest for that position. Repeat plot a suitable time later. Remembering to plot from your new position. For each echo, connect the points plotted and read off course and speed. Use this information to extrapolate the echo's, and your, future positions, and determine if any danger exists.

Remember to use ALL information available to you.

Example

You have three echoes on your screen.

position 1 at 0230 position 2 at 0245

Echo1 bears 083o R 3 miles bears 083o R 2.3 miles Echo2 bears 330o R 5 cables bears 290o R 6 cables Echo3 bears 315o R 2.2 miles bears 345o R 1.8 miles

Your speed is 6 knots

In the absence of any other information, what action, if any would you take?